2_multicomponent_data_acquisition.pdf

Chapter 2

Multicomponent Data Acquisition Introduction When acquiring multicomponent seismic data, careful attention must be given to the vector motions associated associated with P and S seismic displac displacements. ements. For example, when acquiring onshore data with a vertical-displacement source, it is not necessary to be concerned about the azimuth orientation of the source at a source station. In contras contrast, t, when a horizo horizonntal-displacement source is used to generate S-wave data, it is essential to know the azimuth orientation of the source baseplate at each source station and the direction of first motion of that baseplate and to create consistent baseplate azimuth orientations at all source stations across a survey area. Likewise, it is mandatory to know the positive-polarity ends of the two horizontal sensor elements in a three-component (3C) receiver and to orient the horizontal sensors so that the positive-polarity ends point in consistent azimuths at all receiver stations. Such caution is not required when deploying vertical sensors used to acquire one-component (1C) P-wave data. If it is not possible to orient horizontal sensors in a consistent vector azimuth, as can be the case when four-component four-component (4C) receiver nodes are deploy deployed ed in de deep ep wat water er,, a da datata-pr proc ocess essin ing g pr proce ocedu dure re mu must st be imp implem lemen ented ted to de deter termin minee sen senso sorr or orien ien-tations at every receiver station. Analysis of the vector motion induced in seafloor sensors by first-arrival wavelets traveling from a large number of surface source-station coordinates is a common method used to determine 4C sensor orientation. This orientation information then can be used to mathematically transform data to a new coordinate system that describes data that would be acquired with sensors oriented at consistent azimuths at all stations. Users of vertical-seismic-profile (VSP) technology have defined sensor orientation through analysis of first-arrival wavelets for three decades. Those are only a few of the conditions that distinguish multicomponent seismic data acquisition from conventional P-wave data acquisition. Every procedure that is implemented in the field when acquiring multicomponent seismic data has to be based on one fundamental guiding principle — the need to understand the vector displacement associated with each mode of an elastic wavefield. Only then will sources and receivers be positioned in ways that result in data that have accurate and consistent vector properties. If multicomponent seismic data are acquired in a manner that causes vector properties of wave modes to vary in unknown ways across a survey area because of inconsistent orientati ta tion onss of ve vect ctor or so sour urce cess an and d re rece ceiv iver ers, s, it wi will ll no nott be po poss ssib ible le to re retr trie ieve ve va vali lid d ro rock ck an and d flu fluid id information from the data.

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Multicomponent Seismic Technology M on opol e

Dipole

Source classifications

Quadrupole B

A

Sources that generate P and S wavefields can be categorized as monopoles, dipoles, or quadrupoles. pol es. A mo mono nopol polee so sour urce ce ra radi diate atess A B energy equally in all azimuth directions (Figure 1) and typically B is considered to be only a P-wave source sou rce.. How However ever,, a ver vertica tical-d l-disisA A placemen plac ementt sou source rce cau causes ses an SV shear mode to radiate uniformly B in all azimuth directions directly Figure 1. Plan views of earth-displacement vectors refrom the source position, as illuslated to wave propagation away from monopole, dipole, trated in Figure 28 of Chapter 1. and quadrup quadrupole ole sources sources.. Arrow Arrowss indicat indicatee vectorvector-disdisThe P-SV mode creat ateed by a placement directions produced by each source. downgoing P-wave, such as created by the marine air-gun scenario illustrated in Figure 24 of Chapter 1, also radiates equally in all azimuths. Both of those S-wave propagation behaviors are representative of S-waves produced by a monopole source in the context that monopole implies equal radiation in all azimuth directions. The cla class ssica icall co conc ncept ept of an SS-wa wave ve so sour urce ce is a di dipo pole, le, wh which ich ge gene nerat rates es an ear earth th di disp splac laceement oriented in a specific direction (Figure 1). In multicomponent seismic data acquisition, siti on, an S-w S-wave ave dipo dipole le sou source rce create createss an ori oriente ented d disp displace lacemen mentt vector vector tha thatt is hor horizon izontal tal to the earth surface at the source station, as depicted in the radiation patterns depicted in Figures 16 and 17 of Chapter 1. As shown in those diagrams, a dipole source radiates variable amplitudes in different azimuth directions and does not create uniform-azimuth target illumination, as does radiation from a monopole source. Quadrupole Quadru pole sources produce two orthog orthogonal onal pairs of oriente oriented d force vectors vectors,, labeled A and B in Figure 1. Each vector-force pair creates S-waves. Small quadrupole sources have been used as high-frequency borehole sources in some applications, but quadrupole-type sources have not been used as large-scale, surface-based S-wave sources for imaging deep geology. Multicomponent seismic applications involving P and S radiation from only monopole and dipole sources will be considered in this book. An exa example mple of thre threee vect vectoror-typ typee vib vibrato ratorr sou source rcess pos positio itioned ned to crea create te ort orthog hogona onall source-displacement vectors is shown in Figure 2. In this example, a single vibrator is used use d to pro produc ducee each of the thr three ee ort orthog hogona onall sou source rce-di -displa splaceme cement nt vect vectors ors illu illustr strated ated for the vector source described in Figure 1 of Chapter 1. In large-scale surface-based seismic programs, arrays of vibrators might be needed to produce good-quality sourcedisplacement vectors where deep targets have to be illuminated from large offset distances. For example, 12 vibrators assembled for a nine-component (9C) 3D seismic survey are shown in Figure 3. In that instance, an array of four vertical vibrators produced a vertical-displacement source vector, an array of four horizontal vibrators produced an inline horizontal-displacement source vector, and a second array of four horizontal vibrators produced a crossl crossline ine horizo horizontal-di ntal-displaceme splacement nt sourc sourcee vector vector..

Chapter 2: Multi Multicompon component ent Data Acquis Acquisition ition

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orthogonal nal vector sources working to produce 9C vertical-seismicvertical-seismicFigure 2. Example of three orthogo profile data. The three vibrators create the three source-displace source-displacement ment vectors illustrated for vector seismic sources in Figure 1 of Chapter 1 in a time-sequence manner rather than simultaneously. A vertical array of 3C geophones is positioned in the well shown in the background to acquire 9C VSP data. F is the force vector produced by each vibrator when actuator A of the hydraulic system is oriented as labeled.

Figure 3. Twelve vector sources are ready to be deployed across a large 9C 3D seismic survey. In this instance, four vibrators work in an array to produce a vertical source-displacement vector, four work in an array to produce an inline horizontal source-displacement vector, and four work in an array to produce a crossline horizontal source-displacement vector. Because of S-wave signalto-noise to-noi se concer concerns, ns, these three source-displacemen source-displacementt vectors should be produc produced ed in a time-s time-sequence equence manner at each source station rather than using simultaneous-sweep technology.

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Multicomponent Seismic Technology

If the sources do not create those three orthogonal source-displacement vectors, some seismic wave modes of the full-elastic wavefield will not propagate into the earth. Rather than tha n usi using ng simu simultan ltaneou eous-s s-swee weep p pro proced cedure ures, s, the illu illumina minating ting wav wavefiel efields ds associa associated ted with the three thr ee ort orthog hogona onall dis displac placemen ementt vec vector torss are pro produc duced ed and reco recorde rded d in a time time-se -seque quence nce mann ma nner er to en enha hanc ncee SS-wa wave ve si sign gnal al to no nois ise, e, wi with th ti time me de dela lays ys of mi minu nute tess to ho hour urss am amon ong g ge genneration of the vertical vertical-displa -displacement cement vector, the inline horizontal-displac horizontal-displacement ement vector, and the crossline horizontal-displacement vector at each source station.

Earth coupling of S-wave sources Dipole sources have to apply a horizontal force vector to the earth to produce SH and SV shear modes. To produce the required horizontal force vector, some physical part of the source has to project into the earth and be moved laterally to create a horizontal earth displacement. This fundamental principle is satisfied by using S-wave sources that have baseplates with metal cleats. The cleats are forced into the ground and are moved horizontally by source source-gener -generated ated energy energy.. Horizontal vibrators are one of the best S-wave dipole sources developed to date. The baseplate design of one type of horizontal vibrator is shown in Figure 4. The three rows of metal cleats on each side of the baseplate are pressed into the ground by the weight of the vehicle and are moved laterally by a hydraulic-drive system to create horizontal force

Horizontal air bags

Rotating mechanism Baseplate accelerometer Reaction-mass accelerometer

Reaction mass

Baseplate

Vertical air bags

F

Baseplate cleats

Figure 4. Baseplate mechanism used in a horizontal vibrator manufactured by Sercel. A hydraulic system operates between the baseplate and the reaction mass to create horizontal movement of the baseplate. When the metal cleats are forced into the ground, the horizontal movement creates horizontal force vector F.


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vector F, indicated in the figure. The resulting earth-displacement vector is identical to that associated at ed wi with th th thee id idea eali lize zed d di dipo pole le source defined in Figure 1. Photog Pho tograp raphs hs of the gro ground und bene be neath ath a so sour urce ce sta statio tion n oc occu cu-pied by a vibrator that has that type of baseplate are displayed in Figure 5. The earth indentations created by the metal cleats aree mo ar mode dest st an and d ca caus usee min minim imal al ground gro und damage. damage. A writ writing ing pen is placed in one ground indentation to serve as a scale to judge inde in dent ntati ation on de depth pth.. At th thee tim timee these the se pho photog tograp raphs hs wer weree take taken, n, thee vi th vibr brat ator or ha had d ex execu ecuted ted 20 200 0 sweeps during a 9C VSP survey with wi thou outt mo movi ving ng th thee bas basep eplat late. e. Alll 20 Al 200 0 SV an and d SH wav avel elet etss recorded downhole were identicall fo ca forr al alll pr prac acti tica call pu purp rpos oses es,, showing that the source-to-earth coupling at this site was consistent during during a 200-sweep 200-sweep test. This base ba se cl clea eatt de desi sign gn se seem emss to be Figure 5. The horizontal baseplate described in Figure 4 quite qui te goo good d for gen genera erating ting con con-was used in this field test. A writing pen has been placed in sistent-quality sistentquality dipole data. one cleat indentation to serve as a scale. (a) F indicates the A second type of source that orient orientation ation of the forceforce-displa displacement cement vector. (b) The doublecan be used to generate S-waves headed white arrow indicates the direction of baseplate is an inclined-impact source. A motion. gene ge nera raliz lized ed de descr script iptio ion n of the baseplate concept used by that class of sources is illustrated in Figure 6. The baseplate used by an inclined-impact source also has cleats that project into the ground, and the size, number, and distribution of the cleats depend on the manufacturer’s concept of how to optimize earth-to-source coupling. A heavy weight impacts the baseplate at a nonvertical incidence to move the plate both vertically and laterally to create force vector F, shown in the earth. Depending on manufacturer design, this impacting weight accelerates downward by gravity or by a combination of gravity and compressed gas. Thee ho Th hori rizo zont ntal al co comp mpon onen entt of ve vect ctor or F ca caus uses es th thee ba base sepl plat atee to be beha have ve as a di dipo pole le so sour urce ce,, resulting in S-wave radiation patterns such as those described in Figure 17 of Chapter 1. However, the vertical component of F causes the plate to behave as the vertical-displacement source described in Figure 28 of Chapter 1, which produces monopole-type

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Baseplate ate concept associated with Figure 6. Basepl inclined-impact sources. The inclined force vector F produced by this type of S-wave source causes the wavefield radiation to have the characteristics of a dipole source (the horizontal component of F) and of a monopole source source (the vertic vertical al component of F).

uniform-azimuth radiation of the SV mode in all azimuth directions. Thus, an inclined impactor is a complicated S-wave source that combines elements of dipole and monopole wavefield radiation. This source behavior can be viewed as either an unwelcome complication or a great benefit, depending on one’s point of view.

Dipole-source polarity Dipole sou Dipole sources rces pro produc ducee eart earth-d h-disp isplace lacemen mentt vect vectors ors ori oriente ented d in a spe specific cific azim azimuth uth (Figure 1). This vector property of the illuminating S wavefields radiating away from a dipole source requires that a consistent dipole orientation be used at all source stations to avoid the possibility of data processors assigning false static corrections to 180 phase changes induced in field data when the orientation of a dipole source is reversed at a source station during data acquisition. A field test that demonstrates how dipole orientation affects data phase is summarized in Figure 7. In this test, a horizontal vibrator was positioned at the orientations shown in the figure at a selected source station on a 2D receiver profile. The response of the inline geophone at a receiver station offset 2200 ft (670 m) from the vibrator is used to demonstrate data polarity. For the vibrator used in the test, when the baseplate is perpendicular to the long axis of the vehicle, the first motion of the baseplate is to the right if you are sitting in the driver’s seat. This first-motion direction is indicated by displacement vector F. The positive-polarity end of the dipole is represented by the positive sign (+ ) written on the baseplate. When the baseplate is rotated to align with the long axis of the vehicle, the first motio mo tion n of th thee pla plate te is to towa ward rd th thee re rear ar of the ve vehi hicle cle.. Th Thee tw two o vi vibr brato atorr or orien ientat tatio ions ns shown to the left of the wiggle traces produce identical-phase data (traces 1 and 2). The two vibrator orientations shown to the right of the wiggle traces are opposite-polarity dipoles compared with the dipoles on the left and produce identical traces 3 and 4, with opposite polarities to those in traces 1 and 2. 8

Maintaining consistent source and receiver polarities The vector nature of the source and receiver elements involved in multicomponent seism se ismic ic da data ta acq acquis uisiti ition on is st stre resse ssed d in Ch Chap apter ter 1. Be Becau cause se ve vecto ctorr-ba based sed sen senso sors rs and


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Figure 7. Demonstration of relationships between dipole-source orientation and data phase. In this test te st,, a ho hori rizo zont ntal al vi vibr brato atorr wa wass of offs fset et 22 2200 00 ft (6 (670 70 m) in inli line ne fr from om a re rece ceive iverr sta stati tion on.. Th Thee wig wiggl glee tr trac aces es are the responses of the inline horizontal geophone that points to the vibrator station. This test vibrator had a rotating baseplate. Vector F defines the first-motion direction of the baseplate. The (+ ) sign on the baseplate defines the positive-polarity end of the dipole source.

sources are used to acquire multicomponent seismic data and because data polarity is affected by the orientations of those vector elements, it is essential to maintain consistent source and receiver orientations (polarities) across a seismic data-acquisition area. For example, when the positive-polarity end of either the source or sensor is reversed at any station, the polarity reversal of the data associated with that station leads to incorrect static corrections and velocity analyses during data processing. The result will be suboptimal images. Ideally, the positive-polarity ends of both sources and receivers should be oriented in the same direction, as illustrated in Figure 8. The critical requirements are that the orientations of horizontal dipole sources must be identical at all source stations and the orientations of horizontal sensors must be consistent at all receiver stations. Preferably, multicomponent seismic data should be acquired in a right-handed xyz coordinate system

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Recommended nded S-wave polari polarity ty conventions for multic multicomponent omponent seismic sources and Figure 8. Recomme receivers. Geophones should be deployed so that inline and crossline sensors have consistent polarities polari ties at all stations on all receive receiverr lines. Examples of senso sensorr positive polarities at two arbitr arbitrary ary receiver stations on two adjacent receiver lines are shown by the arrowheads on the geophone cases. The first motion of the S-wave source in the inline and crossline directions should be in the same direction direct ion as the posit positive-pol ive-polarity arity direction of the inline and crossline horizontal sensors. sensors. Basepl Baseplate ate orientations and first-motion directions are shown at two arbitrary source stations. The ( + ) sign on each baseplate defines the positive-polarity end of the dipole — the direction of first motion. F is a force vector defining the direction of earth displacement produced by the first motion. The preferred data-acquisition coordinate system is illustrated, which is a right-handed xyz system with positive z pointing down.

in which the positive z-axis points downward. Those requirements involve the following field procedures: †

Pointt th Poin thee he head adli ligh ghts ts of a ho hori rizo zont ntal al vi vibr brat ator or in th thee sa same me di dire rect ctio ion n at ev ever ery y so sour urce ce st stat atio ion. n. A vertical vibrator vibrator used to acquir acquiree single-component single-component P-wave data can be oriented in any convenient direction at a source station; a horizontal vibrator used to acquire S-wave data cannot. Consistent headlight orientation is not too difficult to achieve when acquiring multicomponent seismic data along a 2D seismic profile. However, when acquiring 3D mu multi ltico comp mpon onen entt sei seism smic ic dat dataa wh wher eree th there ere ar aree sev severa erall pa paral rallel lel so sour urce ce lin lines es,, a ho hori rizo zonntall vi ta vib bra rato torr ha hass to st star artt it itss mo move veme men nt al alon ong g ea each ch so sour urce ce li line ne fr from om th thee sa same me en end d of th thee li line ne to ensure that a common headlight orientation occurs at every source station on all source lines.


†

†

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Confirm the directio Confirm direction n of first motion of the the basepl baseplate ate for for each horizo horizontal ntal vibrato vibratorr used used in a multicomponent seismic project. The first-motion directions shown in Figures 7 and 8 aree sp ar speci ecific fic fo forr th thee vi vibr brato atorr us used ed in th thee tes tests ts do docu cumen mente ted d by tho those se fig figur ures es.. Oth Other er vibrators might have different first-motion behavior. If several vibrators are used in a project and one of them has a first motion to the left whereas the others have a first motion to the right, that behavior must be known and accounted for or the result will be suboptimal data. Do not trust the sensor polarity polarity indicated by the arrows imprinted imprinted on a geophone case. Usee re Us reli liab able le te test st eq equi uipm pmen entt to co confi nfirm rm th thee po pola lari rity ty of al alll 3C ge geop opho hone nes. s. In so some me su surv rvey eys, s, the numbers of sensors that have polarities opposite to that implied by the markings on geophone cases are surprising.

Linear versus nonlinear vibrator sweeps If a vibrator operates with a linear sweep rate, the baseplate vibrates the same length of time at each frequency component of a vibroseis sweep. That constant dwell time at each frequency component is assumed to cause the same amount of energy to be input into the earth for each frequency component (Figure 9). In contrast, if a vibrator operates with a nonlinear sweep rate, the length of time a baseplate vibrates and hence the amount of energy that is input into the earth increases as frequency increases. Common nonlinear sweep rates are 3 dB/octave and 6 dB/octave, meaning the dwell time of a vibrator at frequency 2 f is is a factor of either 2 or 4 longer, respectively, than the dwell time at frequency f . The reason for using nonlinear sweep rates stems from a desire to ensure that the highfrequency portion of an illuminating wavelet has an optimal amount of energy after the

Figure 9. Contrasts between linear and nonlinear vibrator sweep rates. During a linear sweep, a vibrator baseplate vibrates the same length of time at each frequency component of a vibrator sweep range. During a nonlinear sweep, a baseplate vibrates for increasingly longer intervals of time at each frequency component as frequency increases. The type of sweep rate that is used affects aff ects the amount of low-f low-frequenc requency y energy that propagates in the earth, which impacts the quality of S-waves.

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wavelet has propagated a long distance. By putting a higher level of energy into high frequencies than into low frequencies at a source station, the amount of high-frequency energy arriving at a distant receiver station should be approximately the same magnitude as the amount of low-frequency energy. This frequency-balancing strategy works well forr Pfo P-P P ill illum umina inatio tion n of geo geolog logy, y, an and d no nonl nline inear ar vib vibro rose seis is sw swee eeps ps ar aree us used ed wi wide dely ly in single-compo singlecomponent nent P-P data acquisit acquisition. ion. However, the use of nonlinear sweeps needs to be rethought when acquiring S-wave data because of the import importance ance of low frequ frequencies encies in all S-wave modes, including including the convert ve rted ed PP-SV SV mo mode de.. Lo Low w fr freq eque uenc ncie iess ar aree im impo port rtan antt fo forr bo both th P an and d S da data ta,, bu butt as il illu lust stra rate ted d in Figure 9, a robust amount of low-frequency energy is particularly important for goodquality S-waves. Nonlinear sweeps that rush through the lower frequency range of a vibroseis sweep can result in inferior-quality S-waves compared with what is achieved with a linear sweep rate. Much research and field testing remain to be done to compare the quality of SH-SH and SV-SV data acquired with linear and nonlinear sweep rates of horizontal vibrators. For vertical vibrators, although a nonlinear sweep can produce better-quality P-P data than a linear sweep does, the P-SV mode might suffer if a nonlinear sweep is used. As a result, a compromise might have to be made to use a linear sweep when the objective is to acquire both P-P and P-SV data. No rules concerning the use of linear and nonlinear sweep rates can be stated because ground-sur ground-surface face conditions vary too much from prospect to prospect and between wet and dry seasons. It is advisable to always conduct a rigorous wave test at each prospect to determine how linear and nonlinear sweeps affect the quality of P and S data reflecting from targeted geology.

Inclined-impact sources Inclined-impact sources (Figure 6) have been used to generate S-waves for decades. In the 1970s and 1980s, Bolt Technology manufactured and leased its Omni Pulse source, a tilted-impact tilted-i mpact source source powered powered by an air gun firing firing in a truck-mounted truck-mounted,, water-filled water-filled chamber. chamber. The air-gun discharge imparted a robust impulsive movement to a baseplate in a way that produced an inclined force vector to the earth. The source was a unique application of marine air-gun technology to create P and S data onshore. Several valuable S-wave studies were done using Omni Pulse sources. Also in the 1980s, Arco Research introduced its ARIS inclined-impact source, which dropped a heavy weight down a tilted guideway onto a baseplate. The inclined impact applied a tilted force vector to the earth that produced a mixture of P, SH, and SV modes. The ARIS source was not used widely to acquire surface seismic profiles because the source vehicle was rather large, heavy, and unwieldy. However, ARIS often was used as a static source at a fixed offset from a well to collect multicomponent VSP data. Omnii Puls Omn Pulsee and ARI ARIS S sou sources rces no lon longer ger exi exist. st. The onl only y cur curren rentt inc incline lined-i d-impac mpactt sou source rce available in the United States is illustrated in Figure 10. This source, named VSXTM , has been developed by Vecta Technology and is manufactured by United Services Alliance in Texas City, Texas. The vehicle shown in this photograph photograph is a thirdthird-genera generation tion version that


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Figure 10. VSXTM inclined-impact source provided by Vecta Technology and United Services Alliance. An inclined force vector can be applied to the earth in any azimuth direction and at any incident angle between 0 and 45 without moving the vehicle. 8

8

benefits bene fits fr from om tes testin ting g pr prog ogram ramss wi with th tw two o pr prece ecedi ding ng models. The source is quite powerful because it uses high-pressure nitrogen gas to accelerate a heavy weight that impacts a baseplate at angles as large as 45 from vertical and from any azimuth direction. Figure 11 illustrates force vectors and P and S displaceme pla cement nt vec vector torss asso associat ciated ed with an incl inclined ined-im -impact pact source. In operation, weight-driven force vectors F(+ ) and F(2) inclined at angle u impact impact the baseplate from two opposing directions. Assuming F(+ ) and F(2) have equiva equ ivalent lent mag magnitu nitude de and incl inclinat ination ion,, vec vector tor add additio ition n yields 8

FV = F( +) + F( −),

(1)

where FV is a vertical force vector representing the sum of th thee tw two o ve vert rtic ical al co comp mpon onen ents ts of F (+ ) and F(2). Vector subtraction of F(+ ) and and F (2) creates horizontal force vector FH, which is the sum of the two horizontal components of F(+ ) and F(2), FH = F( +) − F( −).

Figure 11. Baseplate operation for an inclin inclined-impac ed-impactt source source.. F (+ ) and F(2 ) are inclined-force impacts from opposing directions. FH is the direction of a positivepolarity S-wave displacement. FV is the direction of a positivepolarity P-wave displacement.

(2)

Vector FV creates monopole P and SV radiation patterns such as those illustrated in Figure 28 of Chapter 1. Vector F H creates dipole SV and SH radiation patterns similar

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to those diagrammed in Figure 17 of Chapter 1. In data processing, equation 1 above is implemented by simple arithmetical addition of the recorded F(+ ) and F(2) data, and equation 2 is satisfied by simple arithmetical subtraction of the recorded F(2) data from the recorded F(+ ) data.

Marine S-wave sources Shear waves cannot propagate in water or in any media in which the shear modulus, m, has a value of zero. For 9C multicomponent multicomponent seismic data to be acquired in marine environment me nts, s, so sour urce cess ne need ed to be on th thee se seafl afloo oor, r, whe herre th they ey ar aree in co cont ntac actt wi with th se sedi dime ment nt th that at ha hass a m. S-wave sources made for special geomechanical studies of the seafloor nonzero value of m apply a horizontal force vector directly to the seafloor. One approach uses a seafloor sled with one or more air guns or water guns mounted in horizontal orientations. When the air gun or water gun is pulsed, the base cleats of the sled impart a horizontal force vector to the seafloor. To date, this type of source has been rather lightweight and has been used primarily for seafloor geomechanical geomechanical studies. No contrac contractor tor or manufa manufacturer cturer offers offers such a source at the time of this writing. A sec secon ond d mar marine ine SS-wa wave ve so sour urce ce de desig sign, n, cal called led Vib Vibro roPil PileeTM , ap appl plies ies a ho hori rizo zont ntal al vi vibr bratating in g mo moti tion on to th thee se seafl afloo oorr mu much ch li like ke a ho hori rizo zont ntal al vi vibr brat ator or do does es in on onsh shor oree ap appl plic icat atio ions ns.. Th This is source concept is illustrated in Figure 12. When used in marine data acquisition, the base column of the source is embedded in the seafloor (similar to a support piling) by vibrating thee so th sour urce, ce, wi with th th thee bas basee co colu lumn mn in co cont ntact act wi with th se seaflo afloor or se sedim dimen ent, t, un until til the co colu lumn mn is bu buri ried ed to a dep depth th tha thatt pr prov ovid ides es ad adeq equa uate te so sour urcece-to to-s -seafl eafloo oorr co coup uplin ling. g. Th Thee so sour urce ce the then n is vi vibr brate ated d to generate illuminating wavefields. Interestingly, onshore (not offshore) VSP data have been acqu ac quir ired ed wi with th th thee so sour urce ce to de demo mons nstr trat atee so sour urce ce pe perf rfor orma manc ncee an and d to do docu cume ment nt th thee qu qual alit ity y of thee P an th and d S wa wave vefie field ldss th that at ar aree pr prod oduc uced ed.. Th Thos osee te test stss ha have ve co confi nfirm rmed ed th that at th thee so sour urce ce is ro robu bust st and that it produces good-quality elastic wavefields (Figure 13). Although either of those seafloor S-wave source concepts can be used to generate SH and SV modes directly at the seafloor, more testing is required to demonstrate how the sources can be moved safely and efficiently to sequential source stations, to document their mechanical reliability, and to define practical operational water depths. In addition, envi en viro ronm nmen ental tal-im -impa pact ct is issu sues es mig might ht re restr strict ict th thee us usee of an any y typ typee of sea seaflo floor or-p -pos ositi ition oned ed so sour urce ce until regulatory agencies decide how such sources affect seafloor-dwelling biota. For those reasons, reasons, the only source option option for acquiring multicomponent multicomponent seismic data in marine environments is an air-gun array suspended or towed in the water column. Such air-gun sources produce only P-wave seismic wavefields. Because the downgoing illuminati na ting ng wa wave vefie field ld is li limi mite ted d to th thee P mo mode de,, th thee on only ly re refle flect cted ed wa wave vefie field ldss th that at ca can n be re reco cord rded ed are the P-P mode and the P-SV mode.

Scholte wave During marine data acquisition, an interface wave might propagate across the seafloor During at the water-sediment contact just as an interface wave propagates along the air-sediment


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Figure 12. (a) Concept of a seafloor VibroPileTM source, a vibrator that generates both P and S modes. In operation, the base column is embedded in seafloor sediment to transfer vibrator energy into in to su subs bseafl eafloo oorr st stra rata ta.. (b (b)) Th Thee vi vibr brat ator or un unit it in inse sert rted ed in into to th thee bas basee pi pili ling ng is pow powere ered d by an el elec ectr tric icall ally y driven hydraulic system system..

contact at the earth surface during onshore data acquisition. In onshore data, the air-earth interface wave is a Rayleigh wave, but it often is called ground roll. By analogy, some geophysicists refer to a seafloor interface wave as mud roll, but the correct terminology for this wave wa ve mo mode de is eit either her Sch Schol olte te wa wave ve or Ra Rayl yleig eigh h wav wave, e, de depe pend ndin ing g on th thee re relat latio ions nshi hip p between P-wave velocity in the water layer and the propagation velocity of the interface wave. Referring to Figure 14, if V IW IW, the velocity of the interface wave, is less than V W, the P-wave velocity in the water layer, the interface wave is a Scholte wave. When V IW IW is larger than V W, the interface wave is a Rayleigh wave (Park et al., 2005). As illustrated in Figure 15, the propagation velocity of a seafloor interface wave is relat re lated ed clo close sely ly to the SS-wa wave ve ve velo locit city y of th thee sh shall allow owest est se seafl afloo oorr lay layer er,, wh wheth ether er th thee in inter terfa face ce

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Figure 13. Onshore VSP data acquired with a marine VibroPile TM source. To function as an onshoree VSP source, the base column of the vibrator was embedded in the earth at a surfa onshor surface-sour ce-source ce station offset a short distance from the receiver well.

Figure 14. Seafloor interface wave. When a seismic source is on or close to the seafloor, it creates an interface wave that travels horizontally along the watersediment contact with velocity V IW V IW IW. If V IW is less than V W, the interface wave is a Scholte wave. If V V IW IW is larger than V W, the interface wave is a Rayleigh wave.

Sea level V W

V W

= P-wave velocity in water

V P

= P-wave velocity in seafloor medium

V S

= S-wave velocity in seafloor medium

D Source

Seafloor IW

IW V IW

V P V S

V IW = velocity of interface IW = interface wave wave = Scholte wave if V IW < V W = generalized Rayleigh wave if V IW > V W

mode is a sl mode slow ower er-v -velo elocit city y Sch Scholt oltee wa wave ve or a fa faste sterr-ve velo locit city y Ra Rayl yleig eigh h wav wave. e. Ac Acro ross ss sea seaflo floor or areas in most marine hydrocarbon hydrocarbon basins, V S in the near-seafloor layer is less than V W, the propagation velocity in seawater. Thus, the Scholte wave will be the dominant seafloor interfa inte rface ce wav wavee enco encount untere ered d in mos mostt mari marine ne 4C data data-acq -acquisi uisition tion pro project jects, s, par particu ticularl larly y across oil and gas prospects in deepwater areas. Some investigators investigators deliberately create Scholte waves so as to evaluate geomechanical geomechanical properties prope rties and elastic constants of riverbed, lake-bottom, lake-bottom, and seafloor strata (Ewing et al., 1992; Stoll et al., 1994; Muyzert, 2000). In shallow-water, small-scale studies, a static


45

Sea level seismic source is placed on or close 1.0 0.9 to the water-sediment interface or a V W S D V 0.8 mobile source is towed a short dis / W Seafloor I 0.7 tance above the interfa interface. ce. Similarly, V Rayleigh IW 0.6 Scholte sensors can be planted in the sediV P V S V IW 0.5 ment layer or can be towed a short 0.01 0. 1 1 10 100 1000 Ratio: wavelength/water depth distance above the seafloor. V IW = velocity of interface wave IW Publ Pu blis ishe hed d st stud udie iess im impl ply y th that at only low frequencies (typically less Figure 15. Relationship between V iw iw, the propagation than 10 Hz) propagate as an intervelocity of a seafloor interface wave, and V S, the Sface fa ce mo mode de wh when en th thee V S sediment wave velocity in the shallowest seafloor layer. After velocity is low (typically less than Park et al., 2005, Figure 1. 100 m/s) and that the shortest propagating wavelength wavelength lengthens as the source height above the interface increases (Kugler et al., 2005). For sediments with slow S-wave velocities, a source needs to be close to the water-sediment interface to create a reasonably robust Scholte wave. Scholte waves rarely are observed in marine 4C data acquired across conventional oil and gas prospects because Scholte-wave propagation velocity is too slow to extend across many offset receiver stations during the trace-length recording time used for most seismic field records. In addition, the resonant frequencies used in most seafloor geophones attenuate the dominan dominant, t, low-fr low-frequenc equency y compon components ents of a Scholte wave, and high-f high-frequen requency cy components attenuate rather rapidly with propagation distance. However, when 4C data are acquired in shallow water where there is a hard (fast-velocity) seafloor, data-acquisition and data-processing procedures should be implemented that assume a Scholte wave will be embedded in the data in much the same way that a Rayleigh wave is embedded in onshore data.

Vector sensors Even whe Even when n thre threee ort orthog hogona onal-v l-vecto ectorr sou source rcess pro produc ducee illu illumina minating ting wav wavefie efields lds at a source station, if there are not three orthogonal vector sensors at all receiver stations, then some wave modes produced by the illuminating wavefields will not be recorded in an optimal manner. Three-component geophones are the oldest and most common type of vector sensor used to acquire multicomponent seismic data across onshore seismic prospects. A typical 3C geophone is illustrated in Figure 16. This sensor package has one vertical moving-coil geophone element and two orthogonal and horizontal movingcoil elements. A second vector-sensor technology based on solid-state accelerometers also can be used in multicomponent surveys. These sensors are called microelectromechanical system (MEMS) devices. The MEMS technology developed by ION is illustrated in Figure 17. Serc Se rcel el als also o of offe fers rs MEM MEMS S 3C ve vecto ctorr se sens nsor ors. s. Ser Sercel cel’s ’s co conc ncep eptt fo forr pa packa ckagi ging ng ME MEMS MS vector-based sensors is illustrated in Figure 18. MEMS sensors measure acceleration, whereas geophones measure particle velocity. The output of a moving-coil geophone is

46


a)

b)

Inline horizontal geophone

Crossline horizontal geophone

Vertical geophone

Level bubble


Standard d threethree-compone component nt moving-coil geophone. geophone. (a) Side view. (b) Top view. Figure 16. Standar

Sensor Height: 13.5 cm Diameter: 5.4 cm

Accelerometer (MEMS) (1.2 cm × 1.2 cm)

Accelerometer (X)

Accelerometer (Z)

Y accelerometer behind circuit board

Figure 17. Microelectromechanical system (MEMS) three-compone threecomponent nt seism seismic ic senso sensorr availab available le from ION. The basic sensor element is a solidsolid-state state accelerometer. accelerometer.

an analo log g sig ign nal th that at has to be di digi giti tize zed d af afte terr it le leav aves es th thee geophone. In contrast, a MEMS device digitizes data internal to the sensor package, and its output is a digital signal, not an analog signal.

Geophones versus accelerometers A pr prin incip cipal al dis distin tincti ction on be be-tween tw een mo movi ving ng-c -coi oill geo geoph phon ones es and soli solid-s d-state tate acce accelero leromete meters rs rela re late tess to th thee am ampl plit itud udee an and d phase ph ase res respo pons nses es of the sen senso sors rs at low freque frequencies. ncies. Comparisons betw tweeen th thee gai ain n and phase resp re spon onse sess of Se Serc rcel el’s ’s ME MEMS MS sensor and a standard 10-Hz geophone taken from Sercel literature are illustrated in Figure 19. Thes Th esee id idea eali lize zed d cu curv rves es im impl ply y that th at ac acce cele lero rome mete ters rs ex exhi hibi bitt a constan antt gai ain n ac acrross a wide range ran ge of fre freque quencie ncies, s, incl includi uding ng freq fr equ uen enci cies es be belo low w 10 Hz Hz,, an impor imp ortan tantt fr freq eque uenc ncy y ra rang ngee fo forr


47

S-waves. In contrast, the gain of a movingcoil co il geo geoph phon onee de decr creas eases es at a lin linear ear deci decibe bell/ octave rate below its resonant frequency of 10 Hz. Likewise, the phase response of a solid so lid-s -stat tatee ac accel celer erom omete eterr is sh show own n to be flatt be fla belo low w 10 Hz Hz,, wh wher erea eass th thee ph phas asee re re-sponse spo nse of a mov moving ing-co -coil il geo geopho phone ne shi shifts fts by more than 60 between 10 and 3 Hz. Depending on the importance of lowfrequency information (less than 10 Hz) in P and S data, the distinction between these two types of sensors can be important in multicomponen multico mponentt seismic data acquisi acquisition. tion. Private communication with sensor manufacturers indicates that some MEMS accelerometers sometimes might have undesirable noise and/or distortion below 10 Hz rather than the pristine behavior illustrated in Figure 19. In any application, a rigorous appr ap proac oach h to sen senso sorr se selec lectio tion n is to rec recor ord d data with several sensor options and then Figure 18. MEMS three-component sensor use numerical analyses to compare sensor package available from Sercel. responses. respo nses. Accelero Accelerometer meter data should should be integrated and converted to particle-velocity (geophone) data or geophone data should be differentiated and converted to acceleration (MEMS) data so MEMS and moving-coil geophone responses can be compared on equal footings. Valuable S-wave data can be acquired with either type of sensor — MEMS accelerometers or moving-coil geophones. To date, far more S-wave data have been acquired with geophones than with accelerometers. Usually, phase and amplitude variations that appear in low-frequency S-wave data acquired with either sensor type are simply ignored. If there is concern about low-frequency irregularities, numerical operators can be applied to S-wave field records to create data that appear to have been recorded with sensors that have equal gain and phase at all frequencies. A third type of 3C sensor based on fiber-optics technology might become a sensor of choice in some applications after sufficient tests are done to demonstrate the performance of optical-fiber technology in a variety of environments. Several types of multicomponent, vector-based sensors can be deployed on the seafloor. One popular option is illustrated in Figure 20. As shown in this illustration, marine seafloor sensors contain three orthogonal, vector-sensing geophones and a scalar-sensing hydrophone. Marine multicomponent seismic data are called 4C data because the three comp co mpon onent entss of ge geop opho hone ne da data ta ar aree co comb mbin ined ed wit with h th thee pr press essur uree da data ta (s (scal calar ar da data) ta) pr prov ovid ided ed by the hydrophone. The fourth data component, pressure, is important because water-column 8

48


multiple multi pless can be su supp ppre ress ssed ed be bette tterr by co comb mbin inin ing g th thee ve vert rtica icall ge geop opho hone ne re resp spon onse se an and d hy hydr droophone response. Seafloor sensor packages are available in which the three orthogonal moving-coil geophones are replaced with three orthogonal MEMS accelerometers similar to those used in the onshore sensors illustrated in Figures 17 and 18.

a)

10 0

) B d ( n i a–20 G

DSU (acceleration)

Resonant frequency

Geophone (velocity)

–40 1 .0

3.0

10

30

100

200

30

100

2 00

Frequency (Hz)

b) 180 150 ) ° ( e100 s a h P 50

Resonant frequency

Geophone (velocity)

DSU (acceleration)

0 1 .0

3.0

10 Frequency (Hz)

Figure 19. Comparison between the (a) gain and (b) phase responses of Sercel’s solid-state accelerometer acceler ometer and a 10-Hz moving-coil moving-coil geophon geophone. e.

Crossline horizontal geophone


Vertical geophone

Hydrophone 0

2 i n.

0

5 cm

Figure 20. One type of multicomponent seismic sensor that can be deployed on the seafloor to record multicomponent multicomponent marine seismic data.


49

Point arrays Most proponents proponents of S-wave data acquisit acquisition ion seem to agree that S-wave receiver arrays shou sh ould ld be po poin intt re rece ceiv iver ers, s, wh whic ich h me mean anss ea each ch re rece ceiv iver er st stat atio ion n is oc occu cupi pied ed by on only ly on onee 3C (o (orr 4C) sensor package. If a geophone string has several 3C geophones, this philosophy means all geophones geophones in a string are clustered clustered to span only 1 or 2 m at each receiver station. station. That is because studies have shown that S-wave static corrections vary over shorter distances than do PP-wav wavee st stati atics cs,, wh which ich in intr trod oduc uces es th thee po poss ssib ibili ility ty th that at int intra ra-ar -array ray SS-wa wave ve st stati atics cs can occ occur ur within distances over which standard P-wave geophones are deployed at a receiver station. Such an earth condition is depicted in Figure 21. In this hypothetical situation, the responses of the geophones distributed about the receiver flag have differing S-wave staticinduced time delays, and the sum of their outputs is not the same as it would be if all geophones geoph ones had identical static time delays. Evidence that intra-array S-wave statics are a serious issue has been provided by Hoffe et al al.. (2002 (2002). ). Th Thee da data ta show shown n in Fi Figu gure re 22 do docu cume ment nt a fie field ld te test st in wh whic ich h im imag ages es were were ma made de using three receiver arrays of different lengths and with differing numbers of geophone elements per station. The P-wave images are identical regardless of the number of geophones per station and the distance over which the geophones are distributed. In contrast, thee Pth P-SV SV im imag ages es de dete teri rior orat atee as th thee nu numb mber er of ge geop opho hone ness is in incr crea ease sed d an and d th thee ge geop opho hone ness ar aree distributed distrib uted over a greater distance. The optimal P-SV image is the one made when data are recorded with only one geophone deployed at the receiver stations. To confirm that the variation in P-SV image quality is contro controlled lled by intraintra-array array statics, the data were processed as single-sensor data with a receiver-station spacing of 4 m. Data spaced at 4 m then were summed to replicate data acquired by a 10-geophone array spanning 40 m. The effects of each static-correction static-correction step are docume documented nted in Figure 23. The last image on the right is identical to the point-receiver image shown on the left of Figure 22b. Thus, for P-SV imaging, extend ext endeded-arr array ay data can L replica rep licate te poi pointnt-arr array ay data Receiver flag only if each geophone in the long array is treated as an individual point receive ce iver. r. Th Thee ev evid iden ence ce is rather compelling compelling that S1 2 3 wavestaticscanvaryover 1, 2, 3 = distinct S-wave static corrections distances less than those L = length of receiver array = 3C geophone used to deploy strings of P-wave geophones. Figure 21. A hypothetical surface condition at a receiver station. Geo eop pho hone ne strrin st ings gs Within distance L spanned by a single geophone string, different that th at ha have ve se seve vera rall 3C ge geooS-wave static-correction regimes are labeled 1, 2, and 3. The Sphones can be deployed wave output signal — the sum of the six geophones — is a mixas point-receiver arrays. ture of different static delay, which creates undesired wavelet An example of a threedistortions. P-wave statics do not vary as much with distance as geopho geo phone ne stri string ng dep deploy loyed ed S-wavee static S-wav staticss do.

50


a)

One-element array 20-m spacing

Five-element array 20-m span

10-element array 40-m span

Five-element array 20-m span

10-element array 40-m span

1.0

) 1.1 s ( e m i T

1.2

1.3

b)

1000 m One-element array 20-m spacing

1.5

) s 1.6 ( e m i T

1.7

1.8

1000 m

Figure 22. (a) P-wave images created using receiver receiver arrays with a dif different ferent number of geophones distributed over different distances at receiver stations. (b) P-SV images created using the same receiver-station conditions. P-wave imaging is affected minimally by the variation in the number of geophones and the length of the receiver array. In contrast, the quality of the P-SV images decreases as more geophones are distributed over longer array distances. From Hoffe et al., 2002, Figures 11 and 12.

Chapter 2: Multi Multicompon component ent Data Acquis Acquisition ition None 1.4

Refraction

Refraction + residual

51

Refraction + residual trim

1.5

) s ( e m1.6 i T

1.7

1.8

1000 m

Figure 23. P-SV data processed processed as single-receiver single-receiver data with a receiver receiver-stat -station ion spacing of 4 m and then summed to simulate data acquired with a 10-geophone array distributed over 40 m. This processing strategy eliminates intra-array statics. Note how image quality improves with each static correction. Compare the image on the right with the single-receiver images on the left and right of Figure 22b. From Hoffe et al., 2002, Figure 18.

in th that at ma mann nner er is il illu lust stra rate ted d in Fi Figu gure re 24 24.. Pr Prud uden entt fie field ld pr prac acti tice ce is to co cons nstr trai ain n th thee pl plac acem emen entt of the geophones so that they occupy an area that has a diameter of no more than 1 m. No intra-array statics should affect geophones deployed in such tight clusters. The philosophy that receiver arrays used to acquire S-wave data should approximate point arrays to minimize the possibility of intra-array variations in S-wave statics also applies to S-wave source arrays. The possibility that S-wave static corrections can vary over a distance equal to the dimensions of some source arrays is an issue only in onshore multicomponent data acquisition. Source-side S-wave statics are not involved in marine data acquisition in which source stations involve towed air-gun arrays. When Wh en th thee ob obje ject ctiv ivee is to us usee an SS-wa wave ve ge gen ner erat ated ed di dire rect ctly ly at a so sour urce ce st stat atio ion n (n (not ot an SV mode produced produced by P-wave conversion conversion at a subsurface interface), interface), the source array that produce du cess th that at S wa wave ve sh shou ould ld be as sm smal alll as pr prac acti tica cal. l. Fo Forr an ex expl plos osiv ivee so sour urce ce,, a si sing ngle le sh shot ot ho hole le is ideal. If several several shot holes have to be used at a source station, station, the distance spanned spanned by the shot-hole spread should be constrained to the shortest dimension allowed by charge size, hole depth, and local ground conditions. If an inclined-impact source is used and several impacts have to be summed at a source station to achieve a desired signal-to-noise condition, the area spanned by the repeated impacts should be limited to length and width dimensions of only 3 or 4 m or less, if possible.

52


Inline direction Polarity arrows

Inline direction 3C geophone

0 0

1m

Figure 24. Example of a three-element string of 3C geophones deployed as a point receiver. A suggested guideline is that geophones should be clustered so that they are planted in an area that has a diameter of no more than 1 m. The inset shows how each geophone at the receiver station is oriented.

When multiple vibrators are used in a source array, it can be challenging to arrange the vibrators into a reasonable point array. Configurations used to position vertical and horizontal vibrators at source-station flags in one 9C 3D project are illustrated in Figure 25. Three vertical vibrators were positioned inline and bumper to bumper to form a 60-ft (18-m) array at each source flag, with some stations occupied by only two inline vibrators when one vibrator was out of service time (Figure 25a). This source-array length was acceptable in this instance because no effort was expended to use the SV mode produced by these vertical-displacement sources. The vertical vibrators were used only to generate downgoing P waves. If the objective had been to analyze the SV mode created by the vertical vibrators, then they should have been configured to have a smaller array dimension, similar to the strategy used for horizontal vibrators (Figure 25b). Because the exploration target was relatively deep, four horizontal vibrators were used to generate SV and SH data. If the vibrators were positioned positioned inline and bumper to bumper, the array would span 90 ft (27 m), which is larger than an S-wave source array should be. Consequently, the vibrators were positioned in a box pattern with side dimensions of 30 ft (9 m) at ea each ch so sour urce ce fla flag g (F (Fig igur uree 25 25c) c).. Wh When en on onee of th thee ho hori rizo zont ntal al vi vibr brat ator orss ha had d me mech chan anic ical al or electronic problems, a three-vibrator array that had a maximum dimension of 30 ft (9 m) was used as shown until the fourth vibrator could return to service (Figure 25b and 25c). The vibrators used in this project were Mertz Model 26. The vertical vibrators used a drive force of 49,600 lbs (221,000 newtons), and the horizontal vibrators used a drive force of 24,000 lbs (107,000 newtons). Drive force is defined as x percent of the total weight of a vibrat vib rator or veh vehicle icle.. Usu Usually ally,, x is 80 80% % or 90 90% % fo forr ver erti tica call dri riv ve for orce cess an and d 50 50% % or le less ss fo forr ho hori ri-zont zo ntal al dr driv ivee fo forc rces es.. Be Becau cause se of mec mecha hanic nical al co cons nstr train aints, ts, a ho horiz rizon onta tall vi vibr brato atorr is op oper erate ated d at a drive force that is smaller by a factor of 0.3 to 0.5 than the drive force used for an equalweight vertical vibrato vibrator. r.


a)

53

Vertical displacement Vertical (Mertz Model 26; 49,600-lb drive) N t f 0 6

t f 0 2 2

Source flag Vibrator pad Vibrator movement

t f 0 3

Source line Receiver line

b) Crossline displacement

c) Inline displacement

(Mertz Model 18; 24,000-lb drive)

(Mertz Model 26; 24,000-lb drive)

N t f 0 2 2

N t f 0 2 2

Source flag Vibrator pad Vibrator movement t f 0 3

Source line Receiver line

t f 0 3

Source displacement

Configurations rations and dimensi dimensions ons of vertic vertical-vibr al-vibrator ator arrays used in one 9C 9C 3D 3D survey. survey. Figure 25. (a) Configu (b and c) Configurations and dimensions of horizontal-vibrator arrays used in the same 9C 3D survey.. Inline is the direct survey direction ion in which receivers receivers are deploye deployed. d. Crossline is perpen perpendicular dicular to inline.

Seafloorr senso Seafloo sensorr techno technology logy Because shear waves cannot propagate in water, elastic wavefield data can be acquired in ma mari rine ne en envi viro ronm nmen ents ts on only ly if P an and d S se sens nsor orss ar aree de depl ploy oyed ed on th thee se seafl afloo oorr an and d ar aree co coup uple led d physically to earth strata. Two types of seafloor receiver technologies are used — oceanbottom cable (OBC) and modular ocean-bottom sensors (OBS). Data acquisition with OBC technology is illustrated in Figure 26. Ocean-bottom Oceanbottom cables can be made that can functi function on at great water depths. However, However, in practice, practic e, the maximum water depth D in which cable can be deploy deployed, ed, retrieved, and repositioned in an efficient manner is usually no more than 1000 m. Cable lengths are variable, but a typical cable is 3000 to 5000 m long. Sensor spacing DX varies among seismic contractors but is usually in the range of 15 to 25 m, which allows good lateral sampling of P and S wavefields. Each sensor package is heavy, typically weighing 30 kg or more, so that earth gravity can create an efficient sensor-to-earth coupling. Even though OBC

54


receivers and cables are heavy, some sensors still might not have good seafloor contact across rough seafloor topography. One ineffective seafloor contact is shown for the second receiver station from the left in Figure 26. Once an ocean-bottom cable is deployed, it remains stationary while a source boat makes the required traverses across the receiver spread. When data acquisition is completed, the cable is retrieved and redeployed in a new position. In large-scale OBC operations, several cable boats can be used. As data are recorded across one cable layout, a second boat is deploying (or has deployed) the next active cable, and a third boat is retrieving the cable over which data acquisition acquisition just has been completed. The sensor sensor elements in a seafloor cable can be two component (2C), consisting of a hydrophone and a vertical geophone, or they can be 4C, consisting of a hydrophone and three orthogonal vector sensors. Four-compon Fourcomponent ent sensors are prefer preferred. red. The intern internal al sensor elements at each OBC receiver station can be a combination of hydrophones hydrophones and 3C geoph geophones, ones, as shown in Figure 20, or a combination of hydrophones and MEMS sensors, as shown in Figure 17. When multicomponen multicomponentt data need to be acquired in water depths exceeding exceeding 1000 m, the preferred procedure is to deploy individual sensor nodes using a remotely operated vehicle (ROV), as shown in Figure 27. That type of receiver packaging often is called ocean-botSource boat

Cable boat Sea level

Air gun D 4C sensors

ΔX

Ocean-bottom cable Seafloor

L

Figure 26. Acquiring multicomponent seismic data with ocean-bottom-cable (OBC) technology. A cable of length L is deployed on the seafloor. Multicomponent sensor packages are distributed along the cable at intervals of DX. Water depth D usually does not exceed 1000 m.

Acquiring ng multicomponent multicomponent marine data with ocean-b ocean-bottom-s ottom-sensor ensor (OBS) nodes. This Figure 27. Acquiri technology is preferred when water depth D exceeds 1000 m. A remotely operated vehicle (ROV) is used to transport the nodes to the seafloor and to retrieve them for data collection and receiver repositioning.


55

tom-sensor technology. OBS technology has been used to acquire multicomponent data in water depths of 3000 m and more. The number of receiver stations deployed is considerably less than the number deploy dep loyed ed in OBC ope operat ration ions. s. Thu Thus, s, the spacing DX be betw twee een n OB OBS S st stat atio ions ns is often an order of magnitude greater than thee sta th statio tion n sp spaci acing ng us used ed wit with h OB OBC C tec techn hnol ol-ogy. og y. Th Thee geo geomet metry ry in wh which ich no nodes des ar aree positioned is variable, depending on the imaging objectives. An example of one contractor’s OBS technology is shown in Figure 28. OBS node no dess ha have ve in inter terna nall ba batte tteri ries es an and d da datatastorage capabilities that allow the nodes to remain on the seafloor for 30 days or more before they need to be retrieved for dataa dow dat downlo nloadi ading ng and bat battery tery cha chargi rging. ng. Node retrieval is done with an ROV.

Responses of seafloor sensors Multicomponent sensors stationed on the seafloor record both pressure and particle-v ticl e-velo elocity city wav wavefiel efields. ds. As the wav waveefield fie ldss re rever verber berate ate in th thee wa water ter col colum umn, n, they th ey ind induc ucee dif diffe fere rent nt re resp spon onses ses in sea sea-floor-p floo r-posi osition tioned ed hyd hydrop rophon hones es and geo geo-phon ph ones es be becau cause se pr press essur uree an and d pa parti rticle cle-velocity wavefields have reflection coefficients with opposing algebraic signs at the top and base of the water column. The water-column reverberations have a profound impact on seafloor seafloor-senso -sensorr data, and it is imp impor ortan tantt to un unde ders rstan tand d the wa wave ve physics that is involved. An effective way to visu visualiz alizee the evo evoluti lution on of wav wavefiel efields ds reco re cord rded ed by se seaflo afloor or hy hydr drop opho hone ness an and d geophones is to †

†

†

Figure 28. (a) Ocean-bottom-sensor nodes available from CGGVeritas. (b) Underwater photograph of a remotely operated vehicle carryin ry ing g a nod nodee to a se seafl afloor oor st stat atio ion. n. (c (c)) Ph Phot otog ogra raph ph of a node deployed on the seafloor. Photographs courtesy courtes y of CGGV CGGVeritas. eritas. Used by permis permission. sion.

construct the upgoin construct upgoing g events that aff affect ect seafloor-positioned seafloor-positioned sensors sensors construct constr uct the downgoing events recorded recorded by those same sensors sum upgoing and downgoing downgoing events to create the total-wavefield responses for a hydrophone (pressure data) and geophone (particle-velocity data)

56


The upgoing events that affect seafloor sensors can be defined using the model exhibited ite d in Fig Figur uree 29 29.. The ob objec jectiv tivee is to co cons nstr truc uctt a reflec reflectiv tivity ity ser series ies that that re repr pres esen ents ts the ar arriv rival al of an upg upgoin oing g refl reflecti ection on at the seafl seafloor oor and its asso associat ciated ed wate water-c r-colu olumn mn rev reverb erberat eration ions. s. The refle re flecti ction on co coef effic ficien ientt at the se seaflo afloor or is ass assig igne ned d a mag magnit nitud udee R, an and d the re refle flecti ction on co coef effic ficien ientt at the water-a water-air ir interface has a magnitu magnitude de of 1.0. The algebraic signs of the reflectio reflection n coefficie fic ient ntss fo forr a pr pres essu sure re wa wave vefie field ld at th thos osee tw two o in inte terf rfac aces es ar aree de defin fined ed by th thee po posi siti tive ve an and d ne nega ga-tive symbols imposed on the model on the top left. The algebraic signs associated with the reflection of a particle-velocity wavefield are described on the top right. The wave-physics princ pr incip iple le de demo mons nstr trate ated d by th this is mo mode dell is th that at a se seaflo afloor or hy hydr drop opho hone ne an and d a se seaflo afloor or ge geop opho hone ne record the same upgoing wavefield. However, the responses of a hydrophone and a geophone differ significantly when downgoing events are constructed and added to the upgoing wavefield to create a totalwavefield response of each sensor. To illustrate this wave physics, the reflectivity series recorded record ed by a seafloor seafloor geophone geophone is constructed constructed in Figure 30. 30. The correspond corresponding ing reflectivity reflectivity series recorded by a seafloor hydrophone is illustrated in Figure 31. In those displays, each downgoing event is time delayed by 2Dt, the two-way traveltime through the water column, compared to its time coordinate position in the upgoing wavefield. Each downgoin go ing g ev even entt als also o is as assig signed ned a po polar larity ity di dicta ctated ted by th thee alg algeb ebra raic ic si sign gn of th thee wav wavefie efield ld re refle flecction coefficient at the water-air interface. The total-wavefield response is displayed as the reflectivity series in Figures 30c and 31c. The wave-physics principle demonstrated by

Figure 29. Earth model used to determine the upgoing wavefields measured by a seafloor-positioned hydrophone and geophone. R is the reflection coefficient at the seafloor. A is an upgoing reflection reflect ion event. B and C are upgoing water-column water-column reverberations reverberations of A. The (+ ) and (2 ) notation defines the algebraic sign of the reflection coefficient at the top and base of the water column. Note that reflection coef coefficients ficients for press pressure ure (hydro (hydrophone phone data) and particl particle-velo e-velocity city (geopho (geophone ne data) have opposite algebraic signs signs.. Dt is the one-way traveltime through the water column. The bottom display displ ay shows that the upgoing upgoing press pressure ure wavefield wavefield (hydropho (hydrophone ne respon response) se) is identic identical al to the the upgoing particle-velocity wavefield (geophone response).


57

Figure 30. Reflectivity series recorded by a vertical geophone positioned on the seafloor. (a) Upgoing events are defined in Figure 29 with upgoing reflection A scaled to an amplitude of 1.0. (b) Downgoing Downgoing events are a time-d time-delayed elayed copy of upgoing events because the reflection coefficient coefficient for a particle-velocity wavefield is positive at the water-air interface. (c) The resulting geophone response is the sum of upgoing and downgoing events.

these analyses is that water-column reverberations have higher amplitudes on a hydrophone response than on a geophone response. Because water-column reverberations have stronger amplitudes on a hydrophone response than on a geophone response, the magnitude of seafloor reflectivity has a pronounced effect on the data acquired by those two seafloor sensors. Responses for a soft seafloor with a reflection coefficient R 0.2 and for a hard seafloor with R 0.7 are illustrated in Figure 32. For both seafloor reflectivity conditions, the geophone response is dominated less by water-column reverberation than the hydrophone response is. A surprising result is that as the reflectivity of the seafloor increases from 0.2 to 0.7, the amplitudes of the reverb reverberating erating events decrease on the geoph geophone one response. In contrast, the amplitu amplitudes des of the water-column reverberations recorded by a hydrophone on a hard seafloor (R 0.7) 0. 7) exc exceed eed th thee amp amplit litud udee of th thee in initi itial al up upgo goin ing g re reflec flectio tion n ev even entt A th that at cr creat eated ed th thee reverberations. ¼

¼

¼

58


Figure 31. Reflectivity series recorded by a seafloor hydrophone. (a) Upgoing events are defined in Figure 29 with event A scaled to a magnitude of 1.0. (b) Downgoing events are an inverted and time-delayed copy of upgoing events because the reflection coefficient for a pressure wavefield is negative at the water-air interface. (c) The total-wavefield response for a hydrophone is the sum of upgoing and downgoing events.

Thesee an Thes analy alyse sess ass assum umee th that at th thee so sour urce ce ev even entt is a sin single gle pu pulse lse.. Ad Addi ditio tiona nall co comp mplic licati ation onss arise when the source has trailing water-column reverberations, which is a more realistic representation of real-earth wave propagation. Two analyses involving a reverberating source wavelet are presented in Figure 33. One analysis examines the effect of a soft seafloor (R 0.3), and the second depicts the response across a hard seafloor (R 0.7). The total-wavefield responses for a hydrophone and a geophone are dramatically different. For a hard seafloor, the amplitudes of water-column reverberations recorded by a hydrophone exceed the amplitude of upgoing reflection event A by a factor of 2 to 3, whereas the reverberations recorded by a geophone diminish to less than 20% of the amplitude of reflection event A. ¼

¼


Responses es for seafloo seafloorr hydrophones and vertical geophones when seafloor reflecFigure 32. Respons tivity is (a) R 0.2 and (b) R 0.7. The source wavelet (top trace of each display) has no reverberations. ¼

¼

59

60


Responses es for seafloor hydrophones hydrophones and vertica verticall geophones when seafloo seafloorr reflecti reflectivity vity Figure 33. Respons is (a) R 0.3 and (b) R 0.7. The source wavelet has a reverberation period of 2Dt and a reverberation amplitude that is controlled by seafloor reflectivity R. ¼

¼


61

The wavefield behavior predicted by those models can be used now to evaluate data acquired with seafloor sensors. A 4C field record acquired in a water depth of 40 m is displayed in Figure 34. At first glance, the hydrophone data appear to define reflection events better than the vertical-geophone data do. However, the events that dominate the hydrophone hydro phone response response are water-column reverberations reverberations that are genera generated ted as demons demonstrated trated in the models displayed in Figures 32 and 33. The amplitudes of the reverberations are reduced significantly on the geophone response. As water depth increases, the reverberation period will increase, and hydrophone data will show near-seafloor reflections that have no overlap overlapping ping water-c water-column olumn reverb reverberation erations. s.

Designing onshore P and S surveys In onshore 3D seismic data acquisition, considerable effort can be expended in surveying and setting flags at regularly spaced earth coordinates where source-station and receiver-station flags are to be placed. Placement of the flags will eventually instruct field personnel exactly where to plant geophones and vibrator drivers and shot-hole drillers exactly where to position their vehicles. Sometimes there is a long delay (perhaps weeks or months) between the deployment of station flags and the arrival of a seismic crew. In such instances, a station-surveying crew might visit the prospect a second time and invest

Figure 34. Four-component data records acquired in a water depth of approximately 40 m. Hydrophone response is dominated by water-column reverberations, as demonstrated by the models exhibi hi bite ted d in Fi Figu gure ress 32 an and d 33 33.. The reverberations are attenuated on the verticalgeophonee data. geophon

62


additional time and expense to reset missing station flags at exact, regular spacings across a prospect. The justification for setting station flags at precise, regular intervals is based partly on tradition and partly on seismic data-processing requirements. Numerous data-processing algorithms require seismic data to be sampled at regularly spaced intervals in x,y space. Thus Th us,, so some me ex expl plor orati ation onis ists ts be belie lieve ve it is ju justi stified fied to ex exer ertt a ser seriou iouss ef effo fort rt to po posi sitio tion n source-station and receiver-station flags at precise, regularly spaced intervals so dataacquisition activity will be done in an optimal manner. However, powerful algorithms exist to convert irregularly sampled data to regularly sampled wavefields, and the practice of setting station flags at exact regular intervals along source and receiver lines might be overemphasized. In fact, some advanta advantages ges accrue by positio positioning ning source and receiver stations with some amount of randomness. For example, consider the two seismic data-acquisition concepts illustrated in Figures 35 and 36. Great care and expense were taken so that the acquisition geom ge ometr etry y of Fig Figur uree 35 ha hass so sour urce ce/receive receiverr stations stations at precise precise,, regular interva intervals. ls. In contras contrast, t, the stations shown in Figure 36 are erratic, with the constraint that no station can be more than one-half of a station interval from where a regularly spaced station would be. The stacking folds displayed in Figures 35 and 36 are calculated for common-midpoint (CMP) reflection points and thus describe illumination properties associated with P-P, SH-SH, SHSH, or SVSV-SV SV data data.. Exa Examina mination tion of the plo plots ts of stac stacking king fol fold, d, sou source rce-to-to-rece receiver iver offse of fsets, ts, and sou sourcerce-toto-rece receiver iver azim azimuth uthss tha thatt acco accompa mpany ny each acqu acquisit isition ion geo geometr metry y shows that that the rando random-statio m-station n geometry geometry increas increases es stacking stacking fold in some bins bins and decreases decreases fold in other bins (Figures 35b and 36b). Although randomness in station positions does intr in trod oduc ucee bin bin-t -too-bin bin va vari riati ation onss in fo fold ld,, th thee mag magnit nitud udes es of the var variat iatio ions ns are ra rare rely ly a serious problem. The average stacking fold across the image space is unchanged in these two analyses because the same number of source-receiver pairs is used in each calculation, and the size of the stacking bins is the same in each case. More important, randomness creates more uniform distributions of offset and azimuth than the regular-station geometry does (compare Figure 35c and 35d with Figure 36c and 36d). The off offset set and azimuth behavio behaviors rs created by random-station random-station geometr geometry y are prefer preferred red for studies of amplitude variation with offset and attribute versus azimuth. Positioning source and receiver stations at exact, regular intervals is not necessary, and introducing a reas reasona onable ble amou amount nt of ran random domness ness into acqu acquisit isition ion geo geometr metry y can be adv advanta antageo geous. us. A modest amount of random station positioning does considerable good; too much can be a disaster. The preceding discussion illustrates some of the principles involved in acquiring CMP data with orthogonal source and receiver lines. The results illustrated in Figures 35 and 36 apply with equal validity to P-P, SH-SH, or SV-SV data acquisition because each of those mode mo dess in invo volve lvess do down wngo goin ing g an and d up upgo goin ing g wa wave vefiel fields ds th that at pr prop opag agate ate wit with h eq equi uiva valen lentt velocities, which by definition produces CMP imaging. In CMP data acquisition, there is rather wide latitude in the values chosen for the spacings among receiver lines, source lines, receiver stations, and source stations. However, when the objective is to acquire both P-P and P-SV data with the same 3D source-receiver geometry, careful attention has to be given to source and receiver line spacings and station spacings because the


63

seismic ic data-acquisition data-acquisition grid consisting of source and receive receiverr stations Figure 35. (a) 3D seism positioned at precise regular intervals. Line spacing is 1320 ft (402 m). Station spacing is 220 ft (67 m). (b) CMP stacking fold across the acquisition template associated with P-P, SH-SH, or SV-SV illumination. (c) Azimuth and (d) offset distributions have large outliers that create undesirable oscillations of bin statistics. Data constraints A, B, and C labeled in parts (c) and (d) are discussed in the caption for Figure 36. From Alkan, 2007. Courtesy of Engin Alkan. Used by permission.

same data-acquisition template has to honor the requirements of both CMP binning and common-conversion-point (CCP) binning. The relationship between CMP and asymptotic conversion-point (ACP) coordinates forr a so fo sour urcece-re recei ceiver ver pai pairr se separ parate ated d at dis distan tance ce X is di diag agra ramme mmed d in Fig Figur uree 37 37.. Th This is simple, straight-raypath straight-raypath (constant-velocity) (constant-velocity) model defines the relation relationship ship X = XACP (1 + [tan(u S )/ tan(u P )]),

(3)

wheree XACP is th wher thee di dista stanc ncee between between th thee asy asymp mptot totic ic co conv nvers ersio ion n po point int an and d th thee so sour urce ce statio station. n. Cordsen et al. (2000) rewrite this equation as XACP =

X

(1 + [V S /V P ])

,

(4)

64


Figure 36. (a) The same data-acquisition grid as in Figure 35 but with source and receiver stations positioned at irregular, random coordinates. (b) CMP stacking fold associated with P-P, SHSH, or SV-SV illumination. (c) Azimuth and (d) offset distributions have minor outliers and are less erratic than the distributions shown in Figure 35. The data of part (c) fit between values A and B labeled in Figure 35c. The data of part (d) fit below line C labeled in Figure 35d. From Alkan, 2007. Courtesy of Engin Alkan. Used by permission.

where V S/V P is th thee av aver erag agee V S/V P va valu luee do down wn to th thee de dept pth h wh wher eree th thee CC CCP P co coor ordi dina nate te cu curv rvee becomes approximately vertical and continues downward as a constant-coordinate function.. Thi tion Thiss qua quasi-v si-verti ertical cal por portion tion of the CCP coordinate coordinate curve defi defines nes the ACP dep depth th domai do main. n. By Sn Snell ell’s ’s law law,, V S/V P sin(u S)/sin(u P); th thus us,, eq equa uati tion on 4 is va vali lid d on only ly fo forr angles u where, where, to first order, sin(u ) tan(u ). ). For incident angles of 30 and 35 , tan(u ) and sin(u ) differ by 15% and 22%, respectively. However, those differences between tan(u ) and sin(u ) tend to divide out in the V S/V P ratio term, making equation 4 applicable over a surprisingly large range of offsets. Equation 4 or an equation with similar assumptions and simplifications is embedded in commercial data-acquisition design software and has proved to be valuable for designing 3D source-receiver geometries appropriate for acquiring P-SV data. ¼

¼

8

8


The CC The CCP P da data ta-a -acq cqui ui-sition example displayed in Figu Fi gure re 38 il illu lust stra rate tess th thee principle that a 3D geometry that works well for P-P dataa acqu dat acquisit isition ion migh mightt not be des desir irab able le fo forr acq acqui uiri ring ng P-SV P-S V data data.. This par particu ticular lar design involved orthogonal sour so urce ce an and d re rece ceiv iver er li line ness spac sp aced ed 48 480 0 m ap apar art, t, wi with th source and receiver stations spaced 60 m apart along the line li nes. s. Th Thee si size ze of th thee re reco cord rd-ing patch is 12 × 96 (12 receiver lines of 96 stations). The resulting resulting CMP CMP fold (not

65

Figure 37. Position of an asymptotic conversion point (ACP) relative to a common midpoint (CMP).

Figure 38. Errati Erraticc CCP stacki stacking ng fold for a source source-receiv -receiver er geometry that produce producess an attract attractive, ive, uniform CMP fold. In contrast to the constant bin-to-bin CMP fold produced by this grid, CCP fold between adjacent bins varies by a factor of 2, and some bins have a zero fold value. Line spacing is 480 m. Station spacing is 60 m.

66


shown) was uniform across the image space, and all other CMP data-acquisition properties were acceptable. However, the CCP fold illustrated here is unacceptable, with fold values in adjacent bins oscillating between 32 and 64 and some bins having a fold of zero. To minimize this type of erratic behavior in orthogonal survey designs, Cordsen et al. (2000) recommend that the spacing between source lines not be an integer multiple of receiver stations and that the spacing between receiver lines not be an integer multiple of source stations. Instead, a line spacing should be a distance L (N + 0.5)D or L (N + 0.25)D, where N is an integer and D is the station spacing. Other options that help to reduce oscillations in stacking fold are the use of a nonorthogonal grid or the implementatio tat ion n of ran rando dom m sta statio tion n sp spac acing ings. s. Th Thee ad adva vanta ntage ge of sta statio tion n ra rand ndomn omness ess in or orth thog ogon onal al PP-SV SV data acquisition is illustrated in Figure 39, which shows the P-SV fold for the two survey geometries discussed in Figures 35 and 36. A second principle of CCP data-acquisition design is that the V P/V S velocity ratio of the propagation medium has to be known or assumed so as to calculate where CCP coordinates are located (equation 4). That requirement is fundamentally different from CMP data-acquisition design, in which it is not necessary to know the propagation velocity of a wavefield to calculate CMP coordinates. CCP fold maps are quite sensitive to the value of V S/V P used in equation 4, as demonstrated by the fold maps displayed in Figure 40. In Figure 40, the source and receiver station spacings were 60 m. Receiver lines were sepa se parat rated ed 43 435 5 m (6 (6.2 .25 5 so sour urce ce sta statio tions ns), ), an and d so sour urce ce lin lines es we were re sep separa arated ted 45 450 0 m (6 (6.5 .5 re recei ceive verr stations). Those noninteger relationships between line spacing and station spacing follow the recommendations of Cordsen et al. (2000). The result is a uniform CMP fold of 36 (Figure 40a) and a uniform CCP fold of 36 (Figure 40b), using the assumption that the V P/V S velocity ratio is 2. If the V P/V S ratio of the rocks is really 1.8 at the depth of asymptotic CCP binning (Figure 37), this source-receiver geometry causes the CCP fold to undergo large swings between values of 15 and 56 (Figure 40c). If the V P/V S ratio is 2.2, a second erratic CCP fold pattern is created that varies from 16 to 70 (Figure 40d). Thus, an optimal design of a CCP data-acquisition grid occurs when the V P/V S ratio is known with reasonable accuracy at the depth of asymptotic binning. Thee CC Th CCP P fo fold ld pl plot otss di disc scus usse sed d in th this is se sect ctio ion n ar aree ba base sed d on ge geom omet etri ricc pa para rame mete ters rs of da data ta-acquisition grids. If P-SV data are processed by prestack time (depth) migration, much of the concern expressed about erratic CCP fold behavior is removed. However, even when such data-processing strategy is implemented, it is still wise to use an acquisition geometry that minimizes large variations in CCP fold across an image area. ¼

¼

Marine data acquisition Deploying 4C seafloor sensors requires careful execution, particularly in deep water. An example of 4C OBC sensors deployed along a 2D profile in a water depth of 1400 m is illustrated in Figure 41. This ocean-bottom cable was 5000 m long, and the cable had to be deployed three times to create a 15-km receiver line. The position of each cable deployment is defined on the plot. Note the difference in the distance scales used for east-west coordinates and north-south coordinates. The positions of the end-point receiver stations from cable lay to cable lay are amazingly close, considering the 1400-m water


67

Figure 39. (a) Erratic and undesi undesirable rable CCP stacking fold for an orthogonal data-acquisitio data-acquisition n grid. Source and receiver line spacing is 1320 ft (402 m). Source and receiver station spacing is 220 ft (67 m). (b) Improved CCP stacking fold when source and receiver stations are positioned randomly with the constraint that no station is displaced more than one-half of the regular station spacing of 220 ft (67 m). These CCP analyses are companion plots for the two CMP analyses presented in Figures 35 and 36. From Alkan, 2007. Courtesy of Engin Alkan. Used by permission.

68


a)

Model 2 42 Patch 12 × 90 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2

4800

5000

b)

Fold

West-east (m) Fold

5200

5400

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West-east (m)

36

4600

4800

5900

28 26

5800

24

) m ( 5700 h t u o s h t r 5600 o N

22 20

Receiver

18 16 14

5800 ) m ( h t 5700 u o s h t r o5600 N

Source

Receiver

12

5500

10 Source

5500

8

5400

6

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4 2

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5300 Model 2

2400

Fold

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2800

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3200

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68

5600

56

56

2800

3000

3200

3400

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48 44 )

5400 Source

m (

40 h t 5300 36 u o

24

40

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) m5500 ( h t u o s5400 h t r o N 5300

32 28

s -

32 t h r 5200 28 o

N

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44

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24 20

Receiver

5100

Source Receiver

16 12

16

5000

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8

8

0

2 Patch 12 × 90

West-east (m)

Fold

48

62

4

d)

V P / V VS =

52

64

12

5400

30

0

20

5200

6000

32

5900

0

c)

5000

34

4

4900

0

Model 2

V P / V VS =

1.8

Patch 12 × 90

5100 5000 Model 2

V P / V VS =

2.2

Patch 12 × 90

Figure 40. CMP and CCP fold maps for an orthogonal data-acquisition geometry based on source and receiver station spacings of 60 m, receiver line spacing of 435 m, source line spacing of 450 m, and a 12 × 90 (1080 station) recording patch. (a) CMP fold has a uniform fold of 36. (b) CCP fold has a uniform fold of 36 if V V P/V S is 2. However, CCP fold is erratic if (c) V P/V S is 1.8 or if (d) V P/V S is 2.2.

depth in which the cable had to be retrieved and repositioned. Likewise, each cable lay is oriented correctly and is a reasonable approximation of a 2D profile. The OBC cable used in this data acquisition had 400 receiver stations spaced 12.5 m apart. The details of the sensor configurations along the cable are defined in Figure 42. Each receiver station consisted of seven 4C sensor elements separated by 1.5 m, resulting in a receiver array spanning 9 m. That receiver-array dimension might be rather large for purists who insist on a point-receiver array for acquiring P-SV data, but it is an acceptable array dimension for most users of multicomponent seismic technology. For each cable lay, a source boat moved inline with the 5-km seafloor cable to create sourcee stations spaced 25 m apart and extending sourc extending to offset offset distances of 10 km from each end of the receiver cable. The source boat had to occupy most source stations three times to create the final data set representing a single 15-km receiver line traversed by 25-m source stations extending to offsets of 10 km. When 3D data have to be acquired with OBC 4C sensors, more than one OBC cable need ne edss to be de depl ploy oyed ed to ach achiev ievee ac accep ceptab table le da data ta pr prod oduc uctio tion. n. Of Often ten,, on only ly tw two o ca cable bless are used to form an x-y distribution of receivers. In some instances, three cables might be dep deploy loyed. ed. The sou sourcerce-rec receive eiverr geo geometr metries ies illu illustr strated ated in Fig Figure ure 43 desc describ ribee 3D 4C


69

data-acquisition projects that have been used across some marine prospects. These geometries are general in nature, and no dimensions are defined for source-line and receiver-line separations. Specific receiver-line separations, source-line spacings, and receiver/source stat st atio ion n in inte terv rval alss ha have ve to be de desi sign gned ed to im imag agee ge geol olog ogy y at a ta targ rget eted ed de dept pth h fo forr ea each ch pr pros ospe pect ct..

Figure 41. Seafloor receiver stations created along a deepwater 2D profile. The receiver cable was 5 km long and had 400 receiver stations spaced 12.5 m apart. Three inline deployments of the cable, labeled 1, 2, and 3, created a 15-km receiver profile. The north-south and east-west distance scales differ by a factor of 10. The most significant digit of the easting and northing coordinates is not defined to protect the location of the study.

Figure 42. Sensor arrangement at each seafloor receiver station.

70


Figure 43. Source-receiver geometries such as this often are used to acquire 3D 4C data with ocean-bottom ocean-b ottom cables. (a) Parall Parallel el geometry geometry.. (b) Orthogonal geometry. Source lines could traver traverse se the seafloor cables obliquely if preferred.

Most OBC cables are 4 or 5 km long. Thus, to acquire 3D data over an extensive area, cables must be moved repeatedly, similar to the movement illustrated in Figure 41. For each cable deployment, a source boat traverses several source lines to create long-offset data. Many of the source stations are occupied numerous times to create a continuous source-receiver grid over a large 3D area. Where water depth exceeds 1000 m, OBS sensor technology is used to acquire 4C data rather than OBC sensor technology. In fact, several OBC cable options fail to operate in hydrostatic pressure conditions for water depths that exceed 1000 m. In OBS operations, only a small number of sensor stations is used because of the cost and effort of deploying sensor stations with a remotely operated vehicle, as discussed in association with Figures 27 and 28. As a result, deepwater 4C data acquisition is achieved by using acquisition


71

Δx

Δy

ΔX

Sparse source stations

ΔY

Dense source stations

L

Figure 44. Acquiring 4C data in deep water requires OBS sensor technology. The cost and difficulty of deploying OBS stations cause projects to be done using the least number of OBS units possible. possi ble. To compens compensate ate for the small small number of receive receiverr stations, stations, data-acquisi data-acquisition tion procedures procedures use use a large number of source stations. DX and DY define receiv receiver-stat er-station ion interv intervals. als. Dx and Dy define source-station intervals. L is the width of the survey fringe.

geometries based on grids of sparse receiver stations and deployments of dense source stations. A simplified source-receiver geometry based on that principle is illustrated in Figure 44. In OBC cables, sensor stations are spaced at intervals of 25 m or less. In contrast, separations DX and DY between OBS sensors are larger by a factor of 10 or 20. To create a high number of source-receiver pairs across seismic image space, intervals Dx and Dy between source stations are as small as possible, often on the order of 10 or 20 m. Thee sp Th speci ecific fic va value luess of DX, DY, Dx, Dy an and d th thee wid idth th of th thee su surv rvey ey fr frin inge ge L (F (Fig igur uree 44) us used ed in a survey have to be selected so that targeted geologic units at specific depths are illuminated properly.

Downhole receivers and repeat surface-seismic surveys An important application area for multicomponent seismic technology is time-lapse (4D) seismic surveying across reservoir systems to optimize development and recovery of hyd hydroc rocarb arbons ons res resour ources ces or to mon monito itorr sequ sequeste estered red CO2. In 4D ap appli plicat catio ions ns,, the the ob objec jectiv tivee is to detect subtle changes in porepore-fluid fluid distributions distributions within a reserv reservoir oir interval over calendar time. To detect pore-fluid movements, it is essential to illuminate a reservoir target with identical basic wavelets in each repeat seismic survey. Otherwise, it is difficult to

72


determine whether a variation in a seismic attribute across a reservoir compartment is caused by an alteration in pore fluid within that compartment or by a difference in illuminating wavelets wavelets.. To ensure that identical imaging wavelets exist in 4D surveys, great effort is taken to use the same sources and identical receivers in each seismic survey and to position the sources and sensors at precisely the same source-station and receiver-station coordinates in ea each ch de deplo ploym yment ent.. Th That at str strate ategy gy en ensu sure ress th that at wa wave velet let pr prop opert erties ies re relat lated ed to so sour urcece-re recei ceive verr geometry are equivalent from survey to survey. However, because the time between repeat surveys is typically several months, a complication for onshore projects that is not encountered in marine environments environments is that ground-surface ground-surface conditions, particularly particularly soil moisture, might differ from survey to survey. Soil moisture affects source-static and receiver-static corrections, source-to-earth coupling, and receiver-to-earth coupling. All those factors combine to potentially cause differences between basic imaging wavelets involved in repeat surveys. More research is needed to analyze how soil moisture affects seismic wavelet properties across onshore 4D project areas. One technique technique for quantifying quantifying amplitude amplitude and phase proper properties ties of 4D seismic illuminatilluminating in g wa wave vele lets ts fr from om su surv rvey ey to su surv rvey ey is to po posi siti tion on a pe perm rman anen entt fa farr-fie field ld do down wnho hole le re rece ceiv iver er at a de dept pth h wh wher eree sen senso sorr re resp spon onse se is no nott infl influe uenc nced ed by va varia riatio tions ns in ea earth rth-s -sur urfa face ce co cond nditi ition ons. s. In practice, it is wiser to deploy an array of downhole receivers rather than a single receiver. To ensure that variations in downhole receiver-to-earth coupling do not affect wavelet character from survey to survey, the sensor or sensors should not be uncoupled over the time period of a 4D study. A data-acquisition geometry implementing a downhole single-sensor wavelet-monitoring strategy is illustrated in Figure 45. The objective of the project was to collect 4D P and S data to evaluate the effectiveness of injecting CO2 to enhance secondary oil production. In this study, a single 3C far-field receiver was positioned at a depth of 3300 ft (1006 m) in a well approximately in the center of a seismic data-acquisition grid. Offsets from the vertical receiver well to the farthest source stations were approximately the same as the depth of the downhole receiver. Two 9C 3D data volumes were acquired across this prospect with an interval of two months between surveys. Examples of downgoing illuminating wavelets produced by vertical and horizontal vibrators at repeat source stations for the two surveys are shown in Figure 46. The stations were chosen to create a profile that travers traversed ed a signific significant ant variation in soil type — playa lake bed and local, normal soil. The data demonstrate two advantages provided by a far-field, wavelet-monitoring receiver station. First, the equivalence of the trace pairs confirms for this project area that there are no major differences in P or S wavelets produced at any source station for the baseline survey and wavelets produced at the same stations in the repeat survey. In other projects, significant differences might be observed from survey to survey, depending on what field practices were implemented. Numerical analyses of the wavelet pairs can be reviewed in Roche (1997). Second, the data define how P and S source wavelets vary as vib vibrat rators ors mov movee acro across ss dif differ ferent ent soi soill typ types. es. The mon monitor itor-re -receiv ceiver er res respon ponses ses allo allow w wavelet-equalization operators to be calculated to create uniform illuminating wavelets at all source stations. This application can be an essential data-processing step because


73

Figure 45. Map view of a 9C 3D data-acquisition grid used to acquire time-lapse (4D) P and S data across a producing oil reservoir. reservoir. A 3C geophon geophonee was positioned at a depth of 3300 ft (1006 m) in the labeled receiver well to capture downgoing wavelets produced by vertical and horizontal vibrators. After Roche, 1997. Courtesy of Steve Roche. Used by permission.

wavelet variations existed across different soil types in this study area even though good ground-force phase-locking technology was used on all vibrators. Wavelet Wav elet alte alterati rations ons cau caused sed by var variati iations ons in sur surfac face-r e-recei eceiver ver cou couplin pling g betw between een rep repeate eated d surveys cannot be analyzed with remote downhole-sensor data. However, remote-sensor techniques ensure that downgoing illuminating wavelets can be equalized in 4D data, and for that reason alone, remote-monitoring receivers should be deployed in all 4D projects. Other strategies need to be implemented to minimize surface-receiver coupling effects, even for permanent sensors, when soil moisture content varies from survey to survey.

Permanent sensors The example illustrated in Figures 45 and 46 involves 3D surface-based sensors that are deployed anew at the same earth coordinates as repeat surveys are done at selected calendar-time intervals across a target area. The illuminating wavelets acquired by the single permanent downhole sensor used in this study are invaluable for adjusting source wavelets so that illuminating wavelets are identical between repeat surveys. However,

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Figure 46. (a) Downgoing vertical-vibrator wavelets produced along a source line of the dataacquisition grid (Figure 45) and recorded by a downhole sensor in the central receiver well. Illuminating wavelets are shown in pairs, with the left wavelet generated during the baseline survey and the right wavelet produced during the repeat survey. (b) Downgoing S-wavelets produced by horizontal horizo ntal vibrators oriented north-south north-south and recorded by a northnorth-south south horizontal geophone. (c) Downgoing S-wavelets produced by horizontal vibrators oriented east-west and recorded by a easteast-west west horizontal geophone. Significant differences differences exist in wavele wavelets ts associ associated ated with source stations positioned in the playa lake bed and those associated with stations outside the playa area. Wavelets produced at all source stations during the repeat survey are close matches with wavelets produced during the baseline survey. After Roche, 1997. Courtesy of Steve Roche. Used by permission.


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those source-calibration wavelets do not provide information that allows variations in surface sur face-ba -based sed rec receive eiverr cou couplin plings gs to be rec recogn ognized ized and equ equaliz alized ed bet between ween succ successi essive ve survey sur veys. s. An incr increasi easing ng req requir uiremen ementt for opt optimal imal-qu -qualit ality y rep repeat eat seis seismic mic sur survey veyss is the deployment of permanent sensors that minimize problems related to variations in sensor couplings and receiver positioning between surveys. Such permanent sensors should be buried at shallow depths as point receivers or cemented in place as reasonable-length vertical arrays in holes extending 50 m or more below the earth surface. The depths at which vertical arrays are deployed depend on near-surface conditions across a target area. Multicomponent seismic data acquired as repeat surveys are beginning to be available and should be a rich source of information in future updates of this book.

2_multicomponent_data_acquisition.pdf

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