Masw Sheet

Nonreflection Nonref lection seismic and inversion of surface and guided waves waves

MASW for geotechnical site investigation HOON P ARK , Park Seismic LLC C HOON P ARK

/ g r o . g e s . y r a r b i l / / : p t t h t a e s U f o s m r e T e e s ; t h g i r y p o c r o e s n e c i l G E S o t t c e j b u s n o i t u b i r t s i d e R . 5 5 . 5 5 . 8 5 1 . 0 9 1 o t 3 1 / 0 2 / 9 0 d e d a o l n w o D

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ultichannel anaylsis of surface waves (MASW) is a seismic surface-wave technique developed specifically for near-surface applications at depths usually shallower than a few tens of meters (Park et al., 1999). Since its introduction in the late 1990s, use of the technique has rapidly increased for two reasons: (1) it provides the shear-wave velocity ( V S) of ground materials, which is one of the most important geotechnical parameters in civil engineering, and (2) it is easier to use than other common seismic approaches (e.g., refraction, reflection, and surface-wave surveys). Elastic moduli are commonly used in geotechnical engineering to describe the behavior of Earth materials under stress, which is ultimately related to such tasks as properly designing earthworks and structural foundations, risk assessment under specific site conditions, and monitoring various types of existing infrastructures for public safety. Among three primary types of modulus—Young’s (E), shear (µ), and bulk () moduli—the first two are most commonly used because of what they represent. Young’s modulus simply describes the deformation tendency along the axis of stress, whereas the shear modulus describes the tendency of shape deformation (“shearing”) that, in turn, is related to the viscosity of material. Young’ Y oung’ss and shear moduli are determined from the parameters of density (), shear-wave velocity (V S), and Poisson’s ratio () (Figure 1). From the two defining equations shown in the figure, it is obvious that V S plays the most important role as it is included as squared terms. In addition, V S in reality changes through a broader range than density and Poisson’s ratio. Terefore, accurate evaluation of V S can be extremely valuable in geotechnical engineering. As shown in the equations, the shear modulus can be determined fairly accurately once V S is known. On the th e other hand, han d, Young’s Young’s modulus requires Poisson’s ratio to obtain a comparable accuracy. MASW provides V S information of ground materials by processing Rayleigh-type surface waves that are dispersive when travelling through a layered media (different frequencies travel at different speeds). Tis dispersion property is determined from a material’s shear-wave velocity ( V S) (by more than 95%), P-wave velocity (V P) (≤ 3%), and density () (≤ 2%). By analyzing dispersion properties, we can therefore determine V S fairly accurately by assuming some realistic values for V P and . Te accurate evaluation of the dispersion property is most important with any surface-wave method in this sense. By using a 2D wavefield transformation (for example, f-k transformation), the MASW method converts raw field data in a time-offset (t-x ) domain directly into a frequency-phase f-v ) domain in which dispersion patterns are evident velocity ( f-v through the wavefield maxima. Te remaining procedure extracts a dispersion curve of, usually, fundamental mode that will be used in a subsequent process in search for the onedimensional (1D) V S profile. An accurate dispersion analysis is obviously an important part of data processing, and this is 656

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Figure 1. Defining equations for Young’s and shear moduli showing relationship relationsh ip with shear-wave velocity ( V S ) and other other parameters. parameters.

Figure 2. A diagram showing the relative amplitude change change with offset among surface waves, body waves, and ambient noise indicating that the most commonly used offset range for MASW data acquisition is usually shorter than 100 m. Tis almost always falls into the “optimum offset” due to the strong energy of surface waves. Site Class

S-velocity (V S) (ft/s)

S-velocity (V S) (m/s)

A (Hard rock)

> 5000

> 1500

B (Rock)

2500−5000

760−1500

C (Very (Very dense soil and soft rock)

1200−2500

360−760

D (Stiff soil)

600−1200

180−360

E (Soft clay soil)

< 600

< 180

F < 600, and meeting (Soils requiring some additional additional response) conditions

< 180, and meeting some additional conditions

Table 1. NEHRP seismic site classification based on shear-wave velocity ( V S ) ranges. ranges.

Nonreflection seismic and inversion of surface and guided waves


why it is often the signal-to-noise ratio (SNR) of dispersion as liquefaction evaluation, is related to the elastic property image that directly influences reliability of MASW results. of V S that is closely linked to the viscosity of material; the A high SNR is required in all types of wave-based tech- lower the V S, the more viscous is the material. On the other niques to achieve highly accurate results. Te surface-wave hand, ground amplification for a given earthquake magmethod utilizes Rayleigh waves as signal—the most trouble- nitude, which causes most earthquake-related damages, some source-generated noise in the history of exploration changes with ground stiffness at relatively shallow depths. seismology, commonly known as ground roll. Surface waves Based on the premise established from empirical studies that provide the highest SNR possible in any type of seismic ap- the top 30 m influences the most, and also from the fact that proach. As a consequence, the field operation for data acqui- the shear-wave velocity (V S) is the best indicator of stiffness, sition and subsequent data analysis become extremely simple the average V S in the top 30 m (usually denoted as Vs30m) and effective, almost always ensuring the most reliable results. is used as an important criterion in the design of building MASW is the most advanced surface-wave method be- structures. In general, a site with a lower Vs30m would be cause of its full adaptation of the multichannel principles subject to a greater ground amplification (and suffer more long used in seismic exploration for natural resources. Figure damage from an earthquake). 2 illustrates the tolerance in data acquisition with MASW by Te National Earthquake Hazard Reduction Program showing that the common range of source-receiver offsets re- (NEHRP) established by the U.S. Congress in 1977 adopts quired for most geotechnical projects—usually shorter than this criterion and classifies a site into one of several categories 100 m—is optimal within which a high SNR is almost always (able 1). Te International Building Code (IBC) published guaranteed. Te area too close to the source (for example, ≤ 5 the same classification designations in 2000 as one of the pam) is usually avoided because of the near-field effects that pre- rameters that should be accounted for in structural design. Calculation of the average V S for a certain depth range vent full development of surface waves. On the other hand, an excessively far offset (for example, ≥ 100 m) is also avoided (for example, the top 30 m) can be accomplished in two because of far-field effects that can make the energy level of ways: (1) based on relative thickness-contribution of each surface waves drop below that of ambient noise. layer (method 1 in Figure 3), and (2) based on the definiBecause shear-wave velocity (V S) information is a good tion of velocity—total distance (∑di) divided by total travel indicator of the material stiffness, MASW is often applied in time (∑ti) that is calculated by summation of thickness (di) civil engineering to deal with mechanical aspects of ground divided by velocity (Vsi) of each layer (method 2 in Figure materials (for example, assessment of load-bearing capacity, 3). Both methods can yield significantly different results for ground behavior under continuous and prolonged vibration, the same V S profile as illustrated by using a simple two-layer and ground amplification and liquefaction potential under V S profile. Vs30m as defined in International Building Code earthquake). MASW also finds application in mapping the (IBC 2000 and later editions) uses the second method, which soil/bedrock interface, which is often more usefully and re- tends to put a heavier weight on the lower V S: alistically defined from the stiffness concept than any other characteristics (Miller et al., 1999). Vs30m = ∑di / ∑ti = 30 / ∑(di/Vsi) (m/s) (1) Seismic site classificationVs30m One application of MASW in earthquake engineering, such

One of the most demanding applications for Vs30m evaluation occurs in wind-turbine site characterization (Park and Miller, 2005). In this case, the V S value provided by MASW is important to account in the foundation design not only for the potential earthquake hazard, but also for the continuous and prolonged vibration of the ground produced by rotating blades. Vs30m values and corresponding site classes presented in Figure 4 are selected from sites at several different wind farms in the midwest and the northeast. Tey are presented in the typical format to deliver the results to the engineers.

Figure 3. wo possible ways to calculate an average shear-wave velocity ( V S ). Te second method used for V s30m tends to put a heavier weight on the lower V S .

Site characterization of a potential nuclear power plant Another example of the application of MASW for 1D (depth) site characterization comes from the seismic hazard assessment of potential nuclear power June 2013

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plants that are routinely subjected to machinery vibration and potential ground motion from earthquake. An instance recently implemented at Tyspunt, South Africa, is presented. o meet the increasing demands for electricity generation, the government of South Africa is committed to the construction of several new nuclear power plants, with the coastal site at Tyspunt, west of Port Elizabeth, being considered as one of the sites for characterization (Figure 5) (Bommer et al., 2013). Although South Africa is not a region of elevated seismicity, destructive earthquakes have occurred. Te most recent had a magnitude 6.2 and occurred in 1969. Commissioned by the state-owned energy utility (Eskom), the Council for Geoscience (CGS), one of the National Science Councils of South Africa, conducted a seismic hazard analysis following the most stringent international standards. Te MASW survey, adopted as one of the several Figure 4. Shear-wave velocity ( VS ) profiles selected from five different wind-turbine sites that fall into each different class in seismic site classification (as defined in able 1). approaches for this comprehensive analysis, was conducted at six different locations in the area (Figure 5). Te purpose was to evaluate V S structure to depths as deep as possible, preferably down to 100 m. Because of the unusually deep investigation depth being sought, both active and passive surveys were conducted using a 48-channel seismic acquisition system and 4.5-Hz geophones as receivers. Since all the sites were in remote coastal areas without strong vibration sources available, such as traffic, passive surveys relied on ocean activities for lower frequency surface waves (for example, 10 Hz or lower). In addition, two different active surveys were conducted at each site: one with relatively short receiver spacing (dx) of 1 m and a 5-kg sledge Figure 5. Site map of a potential nuclear power plant in Tyspunt, South Africa, that shows six hammer source, and another with MASW sites and deep borehole sites. a longer dx of 4 m and a rock-drop source facilitated by a tracked hoe (Figure 6). Te former generated sufficient energy at frequencies as low as 10 Hz and survey setup was designed to investigate relatively shallow lower at some sites (Figure 7). Te passive survey adopted a depths (for example, ≤ 30 m) and the latter was designed to two-dimensional cross-receiver array with a 10-m separation investigate deeper depths (≤ 50 m). between receivers (Figure 6). Most sites had soft sandy overburden of varying thickDispersion imaging results from these passive surveys also nesses, thereby attenuating surface waves quite rapidly, showed remarkable energy at the lowest frequencies in the especially in the short-spread surveys using a sledge hammer range of 4−20 Hz (Figure 7). Te results from the long-spread source. Te long spread surveys with the rock-drop source active surveys were quite similar, with differences mainly in 658

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the higher-frequency content and resolution. All three types of dispersion images were stacked on top of each other to extend the usable bandwidth and increase the overall SNR of images. Tis stacking also enhanced higher-mode patterns that existed in different frequency bands on different images (Figure 7). Te inversion process to produce a 1D V S profile at each site consisted of two phases. Te first phase used only the fundamental-mode (M0) curve to produce the first approximation of the velocity profile. Ten, using this as an initial model in the second phase, fundamental and higher-mode dispersion curves were generated through the forward modeling process. Tese multimodal dispersion curves were then examined against observed patterns in the stacked dispersion image. Tis second phase of multimodal inversion was carried out and repeated after manually changing the velocity (V S) and thickness models until satisfactory matches were found. Figure 8 shows the final V S profiles at all six sites obtained through this two-phase inversion approach. Teoretical bounds for 50% change in dispersion curves are also indicated in the profile. Borehole data from PS-suspension logging are also presented in Figure 9 with their locations marked on the map in Figure 5. No borehole sites were close enough to any MASW site to allow a meaningful direct comparison. Nonetheless, borehole data can show possible V S ranges of overburden and bedrock in the area. Tey show bedrock depths change significantly from one site to another in an unpredictable manner. Tey indicate V S of overburden at about 200 m/s and that of bedrock at about 1500−3500 m/s with fluctuations between the two. MASW results also show V S values of bedrock in a similar but slightly lower range and almost the same V S of overburden (Figure 8). Depths of bedrock are also observed changing without any predictable pattern. Underground mine investigation Figure 7. Dispersion images obtained from passive and active data Another common application area of MASW is mapping sets acquired at site 1 in Tyspunt, South Africa. Te image created bedrock in depth and relative competence related to stress from combining (stacking) the two images is shown at the bottom.

Figure 6. A costal view (left) from an MASW site at the potential nuclear power-plant location in Tyspunt, South Africa. Ocean activities generated surface waves for passive surveys that used a 2D receiver array (center). A rock-drop source using a tracked hoe (right) was used for the active survey. June 2013

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from overburden and cultural activities. Te interface be- these newly constructed highway segments as a means to tween overburden and underlying bedrock can be a sharp monitor the general condition of the bedrock. MASW surboundary such as soil over competent basement rock, or a veys were conducted as one of the approaches at the four gradational transition such as the buried bedrock influenced target locations marked on the map in Figure 10. Te main by a severe weathering process with no physically distinct purpose of the MASW surveying was to map the general toboundary. From a perspective of elastic property, the inter- pography of bedrock and any other noteworthy subsurface face is also a sharp boundary in the former case, whereas it is features that could be linked to potential progression of beda gradational change in the latter case because the weathered rock weakening or vertical migration of collapse structures. top portion would consist of varying degree of rock stiffness. o simultaneously survey two 12-ft wide lanes (both drivTis suggests the stiffness mapping by MASW would show ing and passing), a specially built double land streamer was the interface from a highly realistic standpoint. MASW is known to provide highly effective and accurate information about bedrock depth, especially at depths shallower than 20 m or so. Tis is because surface-wave dispersion properties are highly sensitive to change in this depth range. Although the shear-wave velocity (V S) of overburden can be accurately estimated, V S of the bedrock tends to be slightly underestimated as depth increases beyond the most sensitive range of 20 m unless special care is taken during the initial model creation at the beginning of inversion process. Naturally, a common application would be the bedrock mapping in association with public safety where a potential hazard of bedrock collapse exists due to man-made or natu- Figure 8. MASW results of five-layer shear-wave velocity ( V ) profiles at six sites in Tyspunt, S ral causes in the subsurface such as South Africa. mining and karst sinkhole development. Mapping bedrock topography can delineate the collapsed features, whereas a zone of bedrock with significantly lower V S than adjacent areas may indicate a potential for vertical migration of a void. In 2009, the Minnesota Department of ransportation (MnDO) built a special type of pavement called CRCP (continuously reinforced concrete pavement) along several segments of runk Highway (H) 169 in Chisholm, Minnesota (Figure 10). Tis construction followed several surface collapse features in the area near H169 that were deemed to be related to previous mining activities for more than 100 years that left a subsurface maze of abandoned mine shafts and tunnels (Figure 10). In 2011, the Office of MnDO Materials launched a project that included geophysical approaches to in- Figure 9. Deep borehole data from PS-suspension logging at six locations in Tyspunt, South vestigate subsurface conditions below Africa. 660

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Figure 10. Aerial map showing locations of four MASW survey lines on runk Highway (H) 169 near Chisholm, Minnesota. Locations of mine properties and workings are also shown.

Figure 11. Te double land streamers (24-channel acquisition each) built at the Minnesota Department of ransportation (MnDO) for the simultaneous MASW surveys over two (driving and passing) lanes on H169. Te weight-drop source shown in the inset used a polyethylene impact plate that tends to increase surface-wave energy at lower frequencies. o minimize traffic-generated noise and the burden of traffic control, surveys took place during the night.

used to collect surface-wave data by using a 48-channel acquisition system with each land streamer equipped with 24 4.5-Hz geophones installed at 1-m spacing (Figure 11). Te left- and right-side land streamers (facing from source) were connected to channels 1−24 and 25−48 of the seismograph, surveying on driving and passing lanes, respectively. A powerful weight-drop source specially designed and built at the University of Saskatchewan in Canada was used to generate surface waves 6 m ahead and at the midpoint between the two streamers (Figure 11). o minimize traffic control and to avoid traffic-generated noise as much as possible, surveying took place during the night. Figure 12 shows typical field records from each land streamer and corresponding images of fundamental-mode dispersion patterns that possess an almost ideal SNR (i.e., 100% signal) in a broad frequency band of approximately 5−40 Hz. Figure 13 shows analyzed 2D shear-wave velocity

Figure 12. ypical field records from MASW surveys on H169, and their corresponding dispersion images from each land streamer that show almost ideal signal-to-noise ratio of 100% signal.

(V S) maps for the longest survey line on the eastbound lanes (line 3) that were obtained with a maximum analysis depth of 25 m. Te bedrock surface is denoted by a relatively sharp transition boundary of velocities from approximately 200 m/s to 500 m/s. Te bedrock depth is shown to gradually increase from about 7 m on the western end to the maximum depth of about 20 m on the eastern end, and this general trend conformed to the boring results from several locations along or near the surveyed line. Interoverburden layers of higher velocity materials are probably lenses of gravels and boulders. Tey can be identified on both maps of driving and passing lanes, appearing as localized lenses and continuous layers. Tis interpretation is consistent with the general geology of the area as confirmed from borings and other sources. Although the two maps from each lane look identical at a regional scale, differences are noticeable when examined from a local perspective. For example, bedrock is slightly deeper on the eastern half of the passing-lane map, and interoverburden layers have slightly different depths and lateral extent. Considering the identical and consistent acquisition conditions June 2013

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Figure 13. MASW results of 2D shear-wave velocity ( V S ) maps for line 3 from the surveys on H169. Results from left (channels 1–24, driving lane) and right (25–48, passing lane) land streamers are shown.

the two land streamers were subjected to, it is reasonable to generosity in allowing H169 data to be used for this publication as well as sharing other related information. attribute these differences to subtle subsurface realities. Corresponding author: [email protected]

References Bommer, J. J., K. J. Coppersmith, E. Hattingh, and A. P. Nel, 2013, An application of the SSHAC level 3 process to the probabilistic seismic hazard assessment for the Tyspunt nuclear site in South Africa: Proceedings, 22nd Internationa l Conference on Struct ural Mechanics in Reactor echnology (SMiR22). Miller, R. D., J. Xia, C. B. Park, a nd J. M. Ivanov, 1999, Multichan nel analysis of surface waves to map bedrock: Te Leading Edge, 18, no. 12, 1392–1396, http://dx.doi.org/10.1190/1.1438226. Park, C. B., R. D. Miller, and J. Xia, 1999, Multichannel analysis of surface waves: Geophysics, 64, no. 3, 800–808, http://dx.doi. org/10.1190/1.1444590. Park, C. B. and R. D. Miller, 2005, Seismic characterization of wind turbine sites near Lawton, Oklahoma, by the MASW method: Kansas Geological Survey Open-file Report 2005-22.

Acknowledgments: I thank officials at Eskom in South Africa for permission to use the data sets in this article. Julian J. Bommer at Imperial College, London, UK, and Artur Cichowicz at the Council for Geosciences (CGS) in South Africa played critical roles in getting permissions. I also acknowledge all those actively involved in the field operation during the MASW surveys at the Tyspunt nuclear site. Cichowicz and Denver Birch from CGS made major contributions to the MASW work. Henni de Beer of ESKOM facilitated access to the site and provided assistance in clearing the MASW test locations. Vincent Jele, Robert Kometsi, and Leonard abane of the CGS assisted Birch and two I team members, Ellen Rathje and Adrian Rodriguez-Marek, with the MASW field work. Wits University provided some equipment for use in the active MASW testing. Institute of Mining Seismology (IMS) performed the passive MASW experiments. Special thanks to Jason Richter at Minnesota Department of ransportation (MnDO) for the

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Masw Sheet

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