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Part One
and February 1997 saw the publication in EPE in EPE of of Robert Beck’s electronic ronic Earth Resistivity Meter Meter , an elect tool to assist amateur archaeological societies “see beneath the soil” in their search for ruins and other hidden features. The design presented here is based upon the same concept as used in Robert’s circuit, but it has been been considerably considerably simplisimplified in terms of the components count and their ready-a ready-availabi vailability. lity. Significa Significantly, ntly, it has also been put under the command of a PIC microcontroller and provided with data logging facilities. The principal features of this design are outlined in Table 1.
J
ANUARY
Before goin Before going g any any furt further her,, thou though, gh, the author wishes to “put his cards on the survey grid”. He is not an archaeologist and has approached this design purely as an electronic problem to be solved – transmit a signal, retrieve it at a distance and store it for later analysis. Along Alon g the path to this this end, end, he has has researched resear ched a fair fair bit, chatted with with a local local archaeological society and with EPE read EPE readers who have knowledge in this field. Most importa impo rtantly ntly,, Nic Nick k Tile Tile,, EPE reader and friend frien d of the author author,, has spent spent severa severall
connected across connected across them, curre current nt will flow between betwe en them, them, just as it does does through through an ordinary resistor. The amount of current that flows depends on how much resistance the soil interposes between the two electrodes. The value depends on several several factors, the soil’s water content and chemical make-up (i.e. the impurities impurities the water contains), contains), and the presence (or absence) of non-conductive objects. objec ts. The relationship relationship is complex, complex, and will not be discussed in detail here, although some experiments which should give an insight into it are suggested in the text file supplied with the software. It is discussed more fully by Anthony Clark in his book. The current flow through soil is also complicated by the fact that it is not flowing in a straight straight line, as it does (in effect effect)) through an ordinary resistor. The current can simultaneously flow through a multitude of paths, paths, not only only horiz horizonta ontally lly,, but three-dim thre e-dimensio ensionally nally,, as illus illustrat trated ed in in Fig.1. It also radiates outwards beyond the
PROBES GROUND LEVEL
SECTION THROUGH SOIL
PLAN VIEW
Fig.1. Curren Currentt paths s array.
main field, field, as you will see p Fig.2. The overall current flow probes is thus not just gov resistance of one direct horiz by the total resistance of paths effectively in parallel w volume of soil, and each ferent values of resistance. Sign up to votecomple on thisxity, title complexity , though, as far a useful current meter current meter is c Not Useful on answerr is a single value, answe value, assessment of the s density can be
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What is being looked for in an electronic survey is reliably monitored variations in readings across a site, the pattern of which indicates where different sub-soil features exist.
A problem arises, however, in that not only does the soil have resistance, but it also has capacitance and additionally exhibits various electrolysis effects as the d.c. current continues to flow, and most significantly, a polarization process takes place, resulting in progressively changing values on the meter. To be able to take meaningful readings it is necessary to counteract the polarization effect. This can be done by passing an alternating current through the soil instead of a direct one. With each of the a.c. current’s phases, the polarizing effects of the preceding phase are reversed, thus causing a more consistent current flow to occur in both directions. Whilst the soil’s electrolysis process will not be reversed, its effect is likely to be so minute in relation to the polarization effect, that it can be ignored during the relatively brief time during which current flow readings are taken. The capacitance effects are also largely overcome by using an alternating current at a suitable frequency.
The question then arises: at what frequency should the current direction be repeatedly reversed? Too high a frequency will cause the soil’s capacitance effects to “mop-up” and attenuate the alternating signal’s amplitude. Too low a frequency will again cause variation in the monitored readings, albeit smaller than would occur through using a d.c. signal. It appears that the optimum rate at which the signal phases must be changed has been established at around 137Hz (Anthony Clark quotes 137·5Hz but also says that 67Hz is used in some equipment). These frequencies assist in not only the elimination of the polarizing effects, but also in reducing the affect of other alternating electrical fields which might be present in the site being surveyed, such as a 50Hz mains frequency, for instance.
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TABLE 1. WHAT IT DOES
The PIC microcontroller performs the following functions: Generates 137Hz square wave ground-penetrating transmission signal Converts the received and amplified analogue signal to a 10-bit digital va Stores each converted value to user-specified non-volatile (EEPROM) me address representing specific site plotting coordinates Continually displays immediate real-time data and coordinates on alphan liquid crystal display (l.c.d.) On request, outputs stored data via serial link to Windows 95/98/ME PC f to disk and subsequent analysis
Other features of the logger include: Switchable output resistance to vary transmission current Switchable amplifier gain, x1, x10, x100 Pushswitch selection of survey site row and column coordinates allocatio Memory capacity for 16384 10-bit samples, representing a survey site gri 128 x 128 squares Data storage action under complete user control Data locations may be overwritten with fresh data if required Sampled data stays in memory indefinitely, even after power switch-off Recall of last used survey coordinate when next switched on, allowing su spread over several days or weeks Individually stepped push-button recall and display of recorded samples a coordinates Total clearance of memor y to zero value upon request, with security featu prevent erroneous use Operable from any d.c. supply between about 9V and 15V, consuming ab It is equally suited for use with a 9V PP9-size battery (rechargeable types available), or a 12V car battery (see later)
Software features for the downloaded memory samples include: Program written in Visual Basic 6 (VB6) Disk storage under unique dated and timed file name Graphical display of data on PC screen as waveform graphs and value-re coloured or grey-scale grid squares Four screen slider controls allow data to be processed for best visual con to aid analysis You're Reading a Preview Facility to invert data values for viewing as “valleys” or “peaks” Main screen display as 20 x 20 samples block, with vertical and horizonta Unlock withgrid a free trial. across full full access 128 x 128 Secondary screen displays of separate grid or graph data for full 128 x 12 block Download Zoom facility for closer examination With Free Trial of separate graph and grid data Reloading of previous survey files via dedicated file selection screen Downloaded files stored in format suited for analysis and graphical displa Microsoft Excel (found on most PCs) Data may be downloaded to PC as often as required without disrupting its on-board storage (allowing on-going visual display of site progress across periods) Suited to survey monitoring using any of the standard probing techniques Schlumberger, Twin-Probe, etc).
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Useful
EPE contributor Aubrey this latter as reported the presence of ma quencies in some locations ined, some emanating fro
into Not useful researched
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electrically probing the soil in their search for minerals and oil deposits (since 1946 says Robert Beck), it has been found that there are better probing techniques than just using two probes. Some of these have been adopted by archaeologists. Most of the favoured ones all use four probes – two for transmission (TX), and two for reception (RX). The righthand section of Fig.2 shows one way in which the second pair of probes can be used. Anthony Clark says that there are also some techniques that use five probes – with push-pull TX across two and the fifth becoming a grounded reference perhaps?
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IN
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There are several ways in which four probes are used in relation to each other, and each with its own merits. Their use is outlined later, but no quality judgement is offered here on their appropriateness to
_ Inside Circuits _ Simple
C5 22µ+
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Fig.3. Power supply and transmission interface circuit for the Earth Resis
a wire attached will the other TX probe is connec do. The probes don’t power line. IC3 is configured MEASURED POTENTIAL even need to be tor whose inverting input (pi inserted very far, just the potential divider chain for enough to penetrate value resistors R1 and R2. Th the soil to make connected across the +5V and electrical contact the voltage at their junction is with its moistness. The non-inverting input (p It will be obvious, connected to one of the PI of course, that dry troller’s output pins (RA2) a LINES OF soil will be less a 137Hz square wave, gen EQUAL POTENTIAL capable of passing a software, and which alt CURRENT FLOW current than moist +5V and 0V. As this square w You're Reading a Preview LINES soil. Keep in mind ly crosses above and below t A) B) that the surface of ence voltage, IC3’s comp Unlock full access with a free trial. the soil can dry out takes place and its output (pi Fig.2. How current flowing between two probes is detected by faster than that between the device’s upper an a second pair. age limits, i.e. swinging b below it, and so a Download With Free Trial reasonable amount +4V and –4V. various survey situations – but it is worth Note that the op.amp to w of penetration should be allowed. Robert noting that Clark considers the Twin-Probe Beck allows 200mm with his probe strucprobes are connected (IC3) i technique to be the most favoured for tures discussed in Part 2. protected internally and is un archaeological surveying, although the With some sites it may be necessary to fer if the probes accidental Wenner technique is said to provide more evenly damp the soil with water before contact with each other whil detailed results. Nick in his extensive use adequate probing can begin. switched on. However, do no of the prototype adopted the Twin-Probe contact since it could cause technique. to get hot, and it will shor The Twin-Probe and Wenner techniques The PIC-controlled processing circuit is charge life. Sign up to vote on this title were outlined in Robert Beck’s article and almost irrelevant to the main aspects of soil were used in the author’s garden tests with monitoring! So first let’s look at theUseful power Not useful this Logger. They will be discussed in Part supply requirements, and the simple transDepending on the probi 2 in a bit more detail. Suffice to say for the mission circuit, both illustrated in Fig.3. used, experienced geophy moment, both involve placing in the soil a As said in Table 1, the power can origimine not only the subterrane mA
CURRENT SOURCE
V
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+5V R7 1k
Sheet Music TO SK3 (P1, YELLOW)
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Fig.4. Differential amplifier that receives, amplifies and conditions the RX probes signal prior to sending to the AD PIC microcontroller. the author and Nick Tile were carried out via the direct TX path (Nick says he has not found the switchable resistance facility to be useful). In this role, the signal amplitude across the TX probes is picked up by the RX probes simply as an alternating signal whose amplitude varies according to the soil density it has to pass through.
C6 to the amplifying stage around IC4d. Here the gain can be switched by S3 between ×1, ×10 and ×100. In the prototype’s garden tests, the ×1 gain was satisfactory across the maximum probe separation distance that the dense garden flower beds would allow (11 metres)! Nick says he prefers the ×10 setting. At this stage the signal is swinging above and below 0V. It has to be shifted so that it only swings between 0V and +5V at the maximum extremes, to suit the PIC microcontroller’s limits.aThis is achieved You're Reading Preview by a.c. coupling the signal via capacitor C7 to the level-shifting potential divider Unlock full access with a free trial. formed by resistors R22 and R23. Diodes D4 and D5 limit the maximum voltage swing then fed to the PIC, preventing it With fromDownload swinging above or Free belowTrial the PIC’s limits of acceptance. It will be seen that two additional signal paths are provided from the output of IC4a/b and consist of resistors R16 and
The receiving circuit is shown in Fig.4. The twin RX probes and their received d.c. coupled signals are connected via buffering resistors R7 and R8 to the respective inputs of the differential amplifier, formed initially around op.amps IC4a and IC4b and having a gain of three. The outputs from these op.amps are summed, still as d.c. signals, by op.amp IC4c, which provides unity gain. The resulting signal represents the difference between the two input signal levels. It is now a.c. coupled via capacitor
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The PIC repeatedly converts the input signal to a 10-bit binary value which it outputs for display on the 2-line × 16-character l.c.d. X2, as a decimal number. As usual with the author’s designs, the l.c.d. is controlled in 4-bit mode (and its pinouts on the printed circuit board are in his standard order). Its screen contrast is adjustable by preset VR1. Pressing switch S8 causes the PIC to store (Save) the ADC’s 10-bit binary output value to the 32 kilobyte (32768 bytes) serial EEPROM chip, IC6, at the address set by the user via switches S4 to S6. This chip is another Microchip device, and was first demonstrated by the author in his PIC16F87x Data Logger of Aug/Sep ’99. Its device number, 24LC256, indicates that it has 256K single-bit memory locations. These are accessed as 8-bit bytes. In other applications, the 24LC256 is capable of being multiplexed with seven others of its type, using its A0 to A2 inputs to set each device’s multiplexed address. In this application they are left unconnected, leaving them biased internally. Resistor R31 is essential to the correct reading of the device’s retrieved data output value. The 24LC256 data sheet can be downloaded from Microchip’s web site (www.microchip.com ). Data stored in the 24LC256 can be retrieved and downloaded serially to a PC via the RS-232 interface device (IC7) and socket SK5, in Fig.6. Transfer is initiated by pressing switch S7. Once started, all 32K bytes are sent to the PC in consecutive address order.
The software controls the output of a train of square wave pulses at the 137Hz rate. Data sampling takes place on each phase of the output pulse (high and low). On each complete cycle, the minimum value received is subtracted from the maximum (to establish the received signal’s amplitude) and the result stored to a 32byte temporary memory block. So that maximum peak-to-peak values of the received square wave have stabilised, the synchronous sampling takes place at the end of each peak. About once a second, the pulse train stops while the 32 sample values are averaged, and the l.c.d. display updated. The pulse train then recommences for another second. This gives the soil time to respond
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Resistors See R1, R2, R9 to R15, R20, R22, R23 100k (12 off) R3 10 page R4 100 R5, R7, R8, R25 1k (4 off) R6, R16 to R19, R24, R26 to R31 10k (12 off) R21 1M All 0·25W 5% carbon film or better
SHOP TALK
Potentiometer VR1 10k min. preset, round Capacitors C1, C4 to C6 C2, C3
22 radial elect.25V (4 off) 100n ceramic, 5mm pitch (2 off) C7 470n ceramic, 5mm pitch C8, C9 10p ceramic, 5mm pitch (2 off) C10, C11 1 radial elect. 16V (2 off) C12 to C14 10 radial elect 16V (3 off)
Semiconductors D1 1N4001 rectifier diode D2 to D6 1N4148 signal diode (5 off) IC1 78L05 +5V 100mA voltage regulator IC2 ICL7660 voltage inverter IC3 TL071 f.e.t. op.amp IC4 TL074 quad f.e.t. op.amp IC5 PIC16F876 microcontroller, You're Reading a Preview preprogrammed (see text)
Approx. Cost Guidance Only exc IC6 IC7
24LC256 2 serial EEPR MAX232 R interface dr
Miscellaneous S1, S9 s.p.s.t. min (2 off) S2 2-pole 6-w switch S3 4-pole 3-w switch S4 to S8 min. pushswitch (5 of SK1 to SK4 4mm single1 each blac yellow, gree text) SK5 9-pin D-typ connector, chassis mo TB1, TB2 pin-header 1mm termin X1 3·2768MH X2 2-line, 16-c (per line) al numeric l.c.
Printed circuit board, availa code 388 with see-through lid, 190mm 90mm (see text); 8-pin d.i 14-pin d.i.l. socket; 28-p knobs (2 off); 4mm plugs, colo 4mm sockets (4 off); heavy-d clips, with coloured covers to sockets (4 off); robust cabl (see text); 9V PP3 battery a text); p.c.b. supports (4 off); to suit l.c.d. mounting style internal connecting wire; sold EPE PCB Service ,
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Software, including sou for the PIC unit and PC inte Resistors R16 and R17, mentioned Download With Free Trial preable on 3·5-inch disk from viously, allow the PIC to monitor the voltoffice (a small handling cha age on the outputs of IC4a/IC4b for test see EPE PCB Service page) purposes, via its ADC inputs RA0/RA1. downloaded free from the Diodes D2 and D3 prevent the PIC from The latter is accessible via t receiving damaging negative voltages. home page of the main EP Originally, these outputs were intended www.epemag.wimborne.co purely for development use. However, their “FTP Site (downloads)”, use has also proved beneficial in the outPUB and PICS, in which pag door monitoring environment and has been EarthRe in the folder named retained. The monitored valuesSign are up disto vote on this title This month’s ShopTalk played in decimal on the l.c.d. and provide information Useful Not useful about obtain indication of relative probe signal grammed PICs. strengths, and of the loss of connection to The PIC program (ASM) w one or more probes. TASM, although the run-tim In relation to this test-motivated option,
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TO PROGRAMMER (SEE TEXT)
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S2
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_ Inside Circuits _ Simple
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pinouts for the latter are show will probably be necessary contrast using VR1 before a d seen. With power switched on that +5V and –5V are still p they should be. Switch off im they are not, and correct malfunction. On line 1 of the l.c.d., “SOIL RESISTIVITY” will briefly before being replac numerical values, with more
L.C.D. display following
The final prototype board prior to installation. It is recommended that a case of at least 50 per cent larger than used in the prototype should be employed to allow a large 9V to 12V battery to be adequately housed. Probe sockets were 2mm types on the prototype, simply because the author had them in stock. It is recommended that 4mm types should be used. These provide greater robustness of the plugged connections and allow them to be removed readily. Nick recommends the use of restraints near the sockets to prevent the connections pulling out during a survey. The probe sockets should be colour coded, as should their respective plugs. Colour suggestions are shown in the circuit diagrams of Fig.3 and Fig.4, but may be changed to suit availability. It is important NOT to duplicate the colours – doing so could result in leads being incorrectly allocated to probes. The use of crocodile clips with colourcoded plastic covers was found to facilitate the connection of leads to the probes themselves. Heavy-duty crocodile clips are recommended for ease of use (especially in cooler or wet weather!). When testing the prototype, it did not appear to matter whether the probe leads were screened or not. Consequently, standard lighting or low current cable could be used. Twin-core mains cable was used by
differing lengths and cores. Obviously the thicker it is, the lower the loss over long lengths, but 50m (say) of such cable is expensive, and heavy to drag about. Details of constructing customised probes are given in Part 2, but in simple applications four thin metal rods of the type used in gardens as flower supports can be used.
Having established +5V is present You're Readingthat a Preview on the output of regulator IC1, plug in the voltage inverter chip, IC6, and check that Unlock full access with a free trial. around –5V is present on its output. Naturally, always disconnect power before making component changes. Free i.c.s Trialcan be If Download all is well, theWith remaining inserted and the l.c.d. connected. Typical
Example display when car monitoring with S9 switch mode.
With Test switch S9 switch two values on line 1 show the m ues present at the outputs of detected by the PIC’s ADC Respectively, they are suffix B and A, indicating the op.amp refer (as given in the circuit di With S9 off, the values are lower peak values resulting f conversion of the output of IC suffixed by the letters H and Low). Any value between 0 a appear at this time for all fou
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At the top right of line 1 is another number, suffixed by a hash symbol (#). This is the processed value that, when Save switch S8 is pressed, is stored to the serial memory as a grid value for the coordinates on line 2. Switching between gain settings using S3, the value will change. (During a survey always keep S3 at the same setting.) Note that if too strong an input signal is amplified, the op.amp’s output may saturate (reach its maximum obtainable level). In practice, keep the value at the right of line 1 well below about 500. A value of 1023 is the maximum that can result from an ADC conversion, indicating that the ADC has received an input voltage equal to the power line voltage of +5V. This is an improbable event as the op.amp output is unlikely to swing that high.
At the left of line 2 are shown the column and row values which represent the survey grid coordinates, and thus the location in the serial memory at which the processed IC4 value is stored. They are suffixed C and R respectively. An asterisk symbol (*) will be seen to the right of one or the other of these coordinate values (more on setting coordinates in a moment). At the right of line 2 is shown the value that is currently stored at the specified memory address. During the survey it will normally show 0 as each new coordinate is selected. When the Save switch S8 is pressed the display will change to repeat the number that has just been saved to the memory as a 2-byte value. At any time during the survey, the coordinate switches may be used to recall the values that are stored for each grid location. There are three switches for coordinate setting. Two of them, S4 and S5, respectively increment or decrement the value beside which is shown the asterisk. The range is 0 to 127, rolling over to 0 after
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incrementing beyond 127, or rolling over to 127 after decrementing below 0. Pressing Mode switch S6 changes the position of the asterisk, thus allocating the +/– switches to that aspect of the grid, i.e. vertical (column) or horizontal (row).
Pressing Download switch S7 causes the PIC to send the contents of the serial memory to the PC at a rate of 9600 baud. As previously said, the values for each of the 16384 possible grid coordinates are stored as two bytes – the MSB and LSB of the 10bit ADC values. No attempt has been made to be selective about which set of values is sent to the PC. All 32768 values are sent on each occasion that S7 is pressed. The transfer takes about 30 seconds. During transfer, the top l.c.d. line shows the message “SENDING TO PC”, with line 2 blank. Upon completion of the transfer, line 2 shows “SENDING FINISHED”, and line 1 briefly displays the “SOIL RESISTIVITY” message again, before clearing to once more show the values being sampled. Line 2 remains with its last message shown until the asterisk (Mode) switch S6 is again pressed, to once more show the coordinate values.
You're Reading a Preview Unlock display full accesswhen with a downloading free trial. Example stored data to a PC-compatible computer has been completed.
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Check that all the switches perform as intended. It is not necessary to have probes connected at this time, and it does not matter that the serial download will not be destined anywhere – the PC’s data reception side of things will be covered in Part 2.
Example of display when Save switch S8 is pressed. In this case saving 28 to EEPROM location 41.
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ability to display values as d sity grey-scales was found to ed to justify the extra expe another £30) and so the dropped. Had the result been a PIC16F877 would have been screen, in a manner similar t Using Graphics L.C.D.s with of Jan ’01.
The contents of the seri can be reset to zero when re security measure (to avoid r propriately!), the reset rou be called at the moment tha being switched on. With th press and hold down Sav then switch on the power. W sage CLEARING EEPRO release S8.
Example display during se resetting.
On line 2 will be a progre play as the software writes 32768 EEPROM data loca somewhat lengthy process, three and half minutes. Th numerous essential delays into the writing procedure. The software for the EEP and reading was originally from Microchip’s CD-ROM PIC16F877 Data Logger er. It is recommended that attempt to modify Microchi speed the resetting process! On completion of the res also resets the column and ro screen briefly shows the SOI ITY message and proceeds way as described earlier.
Incidentally, experiments were made Sign up to vote on this title using a graphics l.c.d. instead of an alphanumeric one, to see if survey data Not In the final part next mo useful Useful could be illustrated by the unit as an incompatible Windows softwar built 20 × 20 grid display. However, the and probing methods discuss
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Part One last month we discussed the principles of earth resistivity monitoring and described the construction of a circuit through which this could readily be accomplished and the data stored for computer analysis. This month we detail the software that can help in this analysis, and then examine some of the soil probing techniques. The latest updates to the software are then discussed, followed by briefly considering the ethics of surveying and some practical advice and a list of further reading.
I
The Earth Resistivity Logger’s PC software is written in Visual Basic 6 (VB6). It has been proved under Windows 95, 98 and ME. It has not been tested with Windows NT, XP or 2000 as the author does not have these systems. Readers who wish to try running the software under the last three systems may find benefit from reading Mark Jones’ article Running TK3 under Windows XP and 2000, published in Oct ’02.
Part Two
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There are six screens associated with the Logger’s VB6 program:
Main screen as shown below, through which sectional analysis of the survey data is performed Full graph screen on which all 128 × 128 download amplitude values are displayed graphically, in oscilloscope fashion (bottom photo on next page) Full grid screen on which all 128 × 128 download are displayed as grid You're values Reading a Preview squares having amplitude-related hues orUnlock greys (top photo on next page) full access with a free trial. Download screen through which data retrieval from the Logger is initiated Directory screen through which previDownload With Free Trial ously recorded survey logging files can be loaded for on-screen analysis Error Message screen – which hopefully you will never see! This comes into action if the VB6 software detects various types of error (such as trying to load a named file which does not exist). It does not intercept errors which occur outside VB6’s specified
error interception reperto grammed by the author – t will report any such unlike
The main screen offers sev to enable you to analyse the d from the Logger. It must be s that the facilities offered t Windows Excel software su most PCs probably exceed screen can offer – more on Ex There are two main areas on as seen in its screen-dump pho To the right is a 20 × 20 g squares, arranged so that the represents the survey site colu horizontal axis the site rows. T values determine the colour o appearance of each of these s scroll bars are provided whic grid data coordinates to be p cally and horizontally so tha values of a 128 × 128 survey viewed in 400-sample blocks joined. The range of coordinates fr matrix displayed is stated b know the precise coordinate of add the values (numbered 0 to ed alongside the edges of the m
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Useful
There is a choice of four
useful shade range, ingNot the colour
left of the screen. The leftha display shows the grey-scale able, from white to black, 36 s
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Note that if an original sam 0 is found, a dash line (–) is sh of a numerical value. This allo tion of any survey site squares sample has not been taken.
Example of the full screen grid display, using a zoom value of x9. With zoom at x1 all 16,384 grid squares are shown. The contrast will show more clearly on screen than it may on this printed page. practice, the greyscale and monochrome bargraph provide the clearest indication of sample value relationships. The values which are actually obtained from the survey site could, as said previously, fall into the range 0 to 1023. Two slider controls are provided so that the values logged can be suitably displayed as comparative values within the grid squares. They are to the left of the grid squares, jointly captioned Graph, with sub-captions of / (forward slash symbol) and minus. Clicking the sub-captions with the mouse cursor toggles them to show X (multiply) and add , and back again on the next click. With the lefthand control, moving the slider causes the basic sample values to be either multiplied or divided by the slider’s value, according to its sub-caption mode. Similarly with the second slider, adding or subtracting the slider’s value. Multiplication/division take place first in the software routine, followed by add/minus.
These two controls allow the optimum shades or colour to be shown that best illustrate the sample value relationships. Even seemingly similar readings can have their values manipulated to increase the contrast. Above the two sliders is a Show Values tick box. When unticked, just the colour shades are shown. When ticked, the equivalent numerical value of the scale shade, a Preview from You're 0 to 35, Reading or 0 to 7 as appropriate, is displayed within the squares as well. Clicking the full boxaccess alternates twotrial. modes. Unlock withthe a free If a particular shade is too dark to read the value, move the mouse cursor over it and a Download “Tool Tip Text” box Free will appear, With Trialstating the value. Tool Tip Text box messages appear for various functions on screen if you move the cursor over them. To the bottom of the screen below the grid are two text lines. The first shows the actual range of the sample values, the second shows the range after correction.
The large vertical display a the left of the screen shows the ues plotted as graph waveform 20 lines (each numbered) repr numbered grid rows to Horizontally, the lefthand end corresponds with the lefthand grid row. The two sliders to the left o area allow the plot values to the same way as with the gr same multiply/divide and options. Thus the display ca manipulated to show the surv tures to best degree, in this ca ing amplitude waveforms. The range of sample grap changed at the same time a coordinate range is set via its sc sliders. Below the graph area is anot Fill Graph Blanks. When unticked, any zero values in (unmodified) samples are no screen, indicating any surv that have not been sampled. W ticked, the zero values are plot continuous graph line is show the box alternates the modes. To the left of the graph dis numbered tick boxes. These al graph lines to be hidden (no ti the viewing of the data in the clearer. As with all tick box them again alternates between
Below the Show Values another tick box, Invert Values veying, less-dense soil prod values than dense soil. High duce darker shades on the grid lower troughs on the graph lin The Invert Values tick box value relationships to be s becoming low, low becoming allows denser Sign up to vote on this title soil conditions played more darkly on the g Not useful Useful less-dense soil, and the graph l peaks rather than troughs Clicking the box alternates b two modes. The default is fo
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inversion are available as on the main screen. So too is colour mode selection. Above and to the left of the grid and graph areas are two sliders. When clicked, these display the survey site grid coordinates to which their arrows point. Their position is also used by the zoom slider facility at the top left. There are 10 values of zoom selectable according to the ratio Zoom / 2 + 0·5, with a range of ×1 to ×5·5. The slider arrow positions determine the origin point on which the enlargement is made. Intercepts are included in the program to keep the display within the bounds of its frame.
You require a standard serial cable, of the type used with normal modems (Drange 9-way male to female, straight through), for data transfer from the Logger. It should have a connector suited to your PC at one end, and a 9-pin male plug at the other. Adaptors (25-pin to 9-pin) are available if an existing modem lead has a 25-pin male plug.
Download option screen. To download data serially from the Logger, first click on the Download Data button at the bottom of the main screen. This causes a message screen to be displayed, asking if you want to continue with the download, or cancel the call and reshow the full main screen. If the OK button is clicked the small Download screen is displayed. As advised on the previous message you now have about 30 seconds in which to press the Logger’s Download switch S7. During this 30 seconds or so, a bargraph shows the
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elapsing time before a time-out error occurs. If the time-out occurs before data is received, you are offered the options to cancel the download, for the PC to try downloading via its other COM port address (there are two allowed for, COM1 and COM2, at addresses 2F8h and 3F8h), or to retry downloading from the same COM port address. If you choose that the other COM port address should be tried, this address is stored to the EarthResSettings.txt file, which resides in the same folder as the rest of the Earth Resistivity software. It is then recalled next time you run the program. It is permissible to change the COM port address within this file if you wish (via Windows Notepad for instance) – it is the first entry in the file. Take care not to upset the positions of the other lines in the file. These lines set various parameters for the program each time it is loaded and run. When the Logger starts to send data before the time-out ends, and the PC begins to receive it, the countdown
Screen displayed if synchronisation is You're Reading a Preview not correct.
offering the option to try aga the download. Occasionally, the PC soft that data is arriving immediate the click of OK in the Downlo box, even before the Logger switch S7 has been pressed. Th not be found. It is a rare situat and in this event the PC soft immediately experiences a tim does not continue to arrive, you the option to try again. It is worth waiting a couple after OK has been clicked bef switch S7, in case this situatio occur. Once the Logger has sta data the process must run its and cannot be halted. The sam the PC routine, it too cannot be and will continue until a timeexperienced. It had been hoped to provid to graphically show the prog download. Regrettably, it wa on slower PCs the software is simultaneously updating the b other visual forms of timing) inputting the serial data. Conse option has not been provid about 30 seconds to downlo 32768 bytes. A starting time on screen below the prima bargraph. When the download has be fully finished, a routine co double-byte values into single ry values. These are converted and combined into lines of tex value separated by a comma holds the data for one survey s values). There are 128 lines, the number of survey columns This data is then output to d whose unique name is in the following example:
Unlock full access a free trial.that bargraph halts and a with confirmation data is being received is displayed. The full 32K block of Logger data (16384 Download WithatFree samples) is downloaded 9600 Trial baud, and initially stored into temporary memory locations. During this process, another time-out EarthRes 12JAN03 10-27countdown of about one second per data byte is monitored. If this period is exceedin which the date and time (h ed the program assumes that the download that applying at the moment th is complete (the PIC has stopped sending data), or that the serial link has been broken. Sign up to vote on this title Because the down Useful Not useful load is asynchronous (i.e. no handshaking), an error checking routine has been includ-
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created. (The Logger itself has not been provided with date or location recording options – it is up to you to record this information in some other way.) The file is held in the same folder that holds the rest of the Earth Resistivity software. Having saved the file, the software splits the recombined values into a matrix of registers whose coordinates correspond with those used during the site sampling. It is these values that are used for display via the main screen’s graph and grid areas. They are plotted there immediately the Download screen closes. Simultaneously, the grid matrix location coordinate sliders are reset to zero. The value correction sliders are left as previously set, allowing various sets of file data to be recalled from disk for viewing under the same corrective conditions. On return to the main screen after the Download, the name of the current file loaded (in this instance that just saved) is displayed in bold towards the screen’s bottom right.
To load the program with data from a previously saved file, click on the Directory button. This displays a multifunction screen through which files in any folder on any installed disk can be selected. It is a modified version of the Directory screen originally designed for use with the author’s PIC Toolkit TK3 software (Nov ’01), and since used in modified form with other VB6 programs as well. It will not be discussed in detail here as the screen has a NOTES button which calls up a Windows Notepad text window through which you can read the detail of the Directory screen’s use. In brief, you can change drives and folder paths, set a “filter” option to only show files having a specified prefix in their name, and recall previously selected paths through a History box. To select a file, double click on its name in the righthand display area. This causes it to be loaded and split for grid matrix allocation in the same way that the downloaded file just discussed was split and displayed. One of the author’s files is included with the software (but with fewer that 400 samples), plus a much longer one produced by Nick during his survey work. Experiment with them and the screen’s manipulative controls.
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Example of using Windows Excel to display data graphically The formatting simply entails using commas to separate the sample values, which are expressed as normal text characters (e.g. 1234). Inevitably, there are many versions of Excel and specific use details that apply to all of them cannot be given. The chances are, though, that the use will be similar to that on the author’s main PCs. The following is the procedure he uses for Excel 97: Load Excel, using Windows’ Find button to locate it if necessary – on the author’s PCs it is at
Move this cursor anywhe darkened area and left click t Chart Wizard – Step 1 of 5 w darkened area reverts to norm white, surrounded by a dotted bly “shimmering”. Ignore the options offered a Next, to show the Chart Wizar 5 window. Select (left click) graph type options offer Column” option is suggested. Click Next to show the Cha Step 3 of 5 window. Select one type variants on offer, the one You're Reading a Preview perhaps. C:\MSOffice\Excel\EXCEL.EXE Click Next to show the Cha Unlock full the access trial.toolNow follow pathwith Filea free (in top Step 4 of 5 window, and an il bar), Open, Select folder, set File Type to the Sample Chart selected w Text Files, then double click on the Ignore the right hand option Download With Free required EarthRes file name to Trial load it. A click Next, to show the Cha Text Import Wizard – Step 1 of 3 window Step 5 of 5 window. Now ju is now shown, with the first several importFinish. ed values on display. Select the Delimited The graph type selected will option as the active “radio” button. played on the main Excel scre Click Next to show the Text Import value boxes still visible behind Wizard – Step 2 of 3 window. Click the moved around the screen and “Delimited Comma” box to reveal a tick. usual Windows style. A sm Click other ticks to become unticked (if Chart selection window will necessary). Ignore the “Text Qualifier” played, allowing different opt box. play be selected and manipu Sign up to vote ontothis title Click Next to show the Text Import Save the file and its graphi Useful Notthan useful Wizard – Step 3 of 3 window. Ignore the (more one can be gener options offered, but click Finish. screen at any time and placed The main Excel screen will now be positions) as an Excel-type fi shown, with the survey values allocated to name of your choosing. Altern
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ROW
COLUMN 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
7 8 9
C2 P2 AT COORDINATES R19/C3 THESE PROBES ARE MOVED BETWEEN COORDINATE NODES
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11 12 13 14 15
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19
BASIC GRID LAYOUT, ALSO SHOWING TWIN-PROBE EXAMPLE POSITIONS
PROBES C1 AND P1 REMAIN IN FIXED POSITION. PROBES C2 AND P2 TRAVERSE THE GRID.
C1
P1
C2
P2
WENNER ARRAY PROBES EQUIDISTANT
SEE TEXT FOR DISTANCE C1
C2
P1
P2
C1 P1 THESE PROBES REMAIN IN FIXED POSITION A GRID MATRIX OF SQUARES IS COMMON TO ALL NORMALSURVEY METHODS. THE ABOVE ALSO SHOWS EXAMPLE POSITIONS FOR PROBES IN THE TWIN-PROBE TECHNIQUE.
SQUARE ARRAY
PLACING PROBES AT THE GRID INTERSECTIONS ASSISTS REGIMENTATION OF THE SURVEY TECHNIQUE. SQUARES TYPICALLY HAVE SIDES MEASURING 1 METRE.
Fig.8. A 20 x 20 grid layout with Twin-Probe example positions, and the positioning of the probes in Wenner and Square arrays. There are numerous archaeological web sites with bulletin boards and “chat-zones” on-line if you search through the excellent www.google.com, or other quality internet search engines. It is worth noting, though, that Anthony Clark considers the TwinProbe technique to be that most suitable to archaeological work, and is the one used by Nick with his surveys. With all techniques, the area to be surveyed is first marked out as a grid with tapes or similar, to form squares having sides of, say, one metre in length (this is a commonly quoted distance in this context), and probably forming a 20 × 20 matrixed area, see Fig.8. Anthony Clark comments that plastic covered clothes line is also useful for setting out a grid matrix. He cautions, though, that it can be difficult to untangle and on one site he knows of, it had to be “guarded in the presence of sheep, by whom it was regarded as a rare delicacy”!
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finished with, putting it in the n opposite top corner, and swa leads to retain the correct orde The author surveyed his ga times in different ways du development, and on ea became faster at doing so. On vey, on an 11 × 7 grid (77 sa about an hour and half. Of course, during the proce the test surveys, several method ing it were imagined. For a soli or, perhaps the most efficient speed would be to insert separ each corner of the matrix pri readings. It would then only be repeatedly change the lead con seemingly much faster “con system. No doubt, though, tant would probably make the just two probes a speedy altern A perhaps less practical m (bizarrely?) thought up too motorised vehicle like a golf probes attached to the wheels fashion. This would then be and forth across the grid, the matically inserting themselves, ing the storing of each rea correct coordinates! (Well – dream, can’t he?!)
Another seemingly useful known as the Wenner configur are then connected to the probes, using method the four probes are ar heavy duty crocodile clips seems the straight line, equally spac easiest method. metre between them. Fig.8b TheYou're Logger’s storage coordinates are set Reading a Preview order of arrangement. to suit the square number, i.e. to R0/C0 in This method is apparently b this case, and a reading saved to the Unlock full access with a by free pressing trial. to doing a more detailed su Logger’s serial EEPROM matrixed grid site. The princip switch S8. TX probes are the outer tw The C2/P2 probes are then moved to the Download With FreetoTrial probes are in line between the top corners of the next square, the right rent flows across the TX pair a for example, to be monitored and its coorup across the RX pair, the rec dinates set into the Logger, in this case value varying with the resistan R0/C1. Again a reading is stored to the with the probes in a more di EEPROM. than with the twin-probe techn The process continues fully across horiA variant of this tec zontally for the width of the marked survey Schlumberger, in which the p area, e.g. R0/C19 (the final column of this equally spaced, is discussed row in a 20 × 20 grid). The probes are then Clark’s book. But he regards i moved down by one row, and the process allyon suited to archaeologica repeated, to the left this time,Sign backuptoto vote this title R1/C0. And then down by another row, and Not useful Useful so on for all 400 squares. Another method is known a Array in Anthony Clark’s boo method, the TX and RX pairs Note that the relative order of all probe
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It is evident, however, that balanced (zero) readings, when the two input values are equal, would only indicate the uniformity of the terrain in that grid. It would not indicate whether that uniformity was due to a highly resistive feature or a highly conductive one. Nonetheless, the detection of only outlines might in itself be a desirable situation. A variant of this technique would be to place the two TX probes at one end of a column, and the RX pair at the other, taking a reading and moving both pairs to the next column, still at the top and bottom points. This could perhaps yield initial information about whether or not a site is worth examining more closely. Not being an archaeologist, though, the author cannot comment on the validity of this. Anthony Clark discusses the above named probing techniques in more detail, and describes others.
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assemblies described 100mm 170mm by Robert Beck should be considered. Schematics of the original figures illustrating these probes 330mm TUFNOL have been redrawn COLLAR 250mm and are repeated here. Other than the folB) lowing details, no additional information can be offered. Robert’s rigid 930mm frame assembly for BAND OF two probes is shown PAINT 330mm TUFNOL TAPE in Fig.9. Details of COLLAR 250mm his single probe are given in Fig.10. 15mm B) His original text states that the TwinHOLE Probe assembly was TO PAS 250mm TUFNOL specially developed A) A) COLLAR and that its top mem ber is a wooden batten, 30mm × 50mm × During garden tests with the prototype 1050mm, the ends of Logger, individual metal rods measuring which are bound with about one metre long by 5mm diameter, self-amalgamating 120 D) and with a right-angled bend at the top tape to form hand 0BA TAPPED HOLES were used as the probes. These were purgrips. TO RECEIVE SOCKET HEAD GRUB SCREWS chased inexpensively from a garden cenAn aluminium tre, their intended use being to support platform is attached plants. to the centre of this Fig.10. Construction details of Robert Beck’s A recently observed, but not tried, batten to carry the possibility was in the form of long inexcase that holds the either dry and coat the batten w pensive barbecue skewers – seen in a local electronics, secured by rubber bands. The or devise insulating collars o supermarket. bottom member is a similar wooden batsimilar material, and fit them If you wish to construct purpose-built ten, but this piece must have good insulatYou're Reading a Preview probes go through the batten. probe structures of more durability, and ing properties (to prevent current leakage The top and bottom batte perhaps greater ease of use, the probe between the probes). He suggests that you together by metal conduit pip Unlock full access with a free trial. at each end and secured by loc In describing the construc 1050mm other probes, he says that n Download With Free Trial L-SHAPED dimensions are critical and m ALUMINIUM BRACKET ed by what is to hand. In Fig.1 30mm a substantial probe made out steel tubing with a brazed on T THIS AREA THIS AREA tip which assist soil penetr TAPED TO TAPED TO FORM FORM probe is designed to be used b HANDLE HANDLE tor in the standing position. A smaller version of Fig.10a 4mm PLUGS TO CONNECT TO RESISTIVITYMETER Fig.10b. This has a 4mm scr meth added, an alternative NOTE: THE UPPER AND LOWER HORIZONTAL RAILS Sign up to vote on this title connection. ARE OF WOOD. tHE LOWER RAILSHOULD BE DRIED AND VARNISHED. tHE L-SHAPED ALUMINIUM BRACKET IS TO may be constructed Not useful Useful Probes SUPPORTTHE RESISTIVITYMETER. other than stainless steel, whi 1000mm sive and a little difficult to ob (provided it is corrosion r
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This option has previously only bee n available to readers who have a registered version of MSCOMM (as Robert Penfold has discussed many times through his Interface series). Joe’s serial OCX facility will be published in full at a later date – probably the September issue. However, a section of Joe’s program has been built into this Earth Resistivity (ER) program and is available to readers who are using the EarthResist.EXE standalone version. To use Joe’s option, though, several changes need to be made to ER’s p.c.b., without which the facility cannot function. They are: 1. Cut the track (0V) connecting to IC7 (MAX232) pin 13. 2. Connect IC7 (MAX232) pin 13 to 9pin serial socket SK3 pin 3. 3. Connect IC7 (MAX232) pin 12 to IC5 (PIC) pin 18 (RC7). This action allows the PIC to receive handshake data from the PC. To set the PIC program to respond to the correct serial data transmission routine, initiate the following procedure: 1. Before switching on power, press and hold down the Mode switch, S6. 2. The screen will go into serial path change mode, alternating at about one-second intervals between a display saying “SERIAL PORT NORM” (original version) and “SERIAL PORT OCX “ (Joe’s OCX). 3. Release switch S6 when the mode you require is shown. This mode becomes the active path mode and is also stored into the PIC’s data EEPROM, to be recalled next time the program is run. 4. On release of switch S6, normal running of the PIC program resumes. The serial path mode may be changed whenever you choose.
Ensure that the PC is also set for the chosen mode, as follows: Click the on-screen button labelled Please Read. Accept the option that then follows to read the text. Having read it, exit the text reading screen to reveal another options screen. This allows you to choose between the new OCX option and the original (normal) serial mode. Click YES for Joe’s OCX, NO if you want to use the nor-
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is reported on screen, allowing you to redownload if you prefer, although minor “first aid” is provided by the program to regain sync after that point. It is rare, though, for more than one loss of sync to occur. Such loss should not occur with Joe’s OCX program. It should be noted that readers who wish to make their own changes to the ER source code cannot make use of Joe’s OCX input option. For that to be used, the installation of Joe’s full OCX facility is required. For copyright reasons this will not become available to readers until its publication. Attempting to examine the ER source code will generate an error condition because of the presence of Joe’s program. Until Joe’s full serial program becomes available, the ER program can only be recompiled if Joe’s sub-program (EarthResOCX) and all references to it in the main program are removed. Also be aware that this version of ER with Joe’s OCX has not yet been proved on a wide variety machines. If it will not work on your PC, revert to using the normal serial download option on PIC and PC. Please advise us at HQ if this is necessary, telling us the PC and its operating system type.
A. The first value shown (foll is that monitored from IC4 described in Part One. The o IC4a is no longer monitor l.c.d. If current values greater t encountered, they are limited t word MAX is displayed o Switch S2 may be used to sele other resistors (R3 to R6) in th the site being surveyed has gre er resistance than appropriate t resistor value. Do not change value during a survey. The PC program stores the survey value to disk. On re-in rent value is extracted from th the MSB is then limited to on (bit 0). The range of survey va from 0 to 511. During survey setting via switch S3 should b keep the values below 511, middle range centred on 256 greater than 511 is encountered it is limited to 511 and the w shown on the l.c.d.
All three display screens of gram now have an extra tick Current. When it is ticked, Another feature added to this version is value is multiplied by its assoc the ability to monitor the current flowing value divided by 10. The th between the transmission (TX) probes. It slight differences in the transm too requires a small change to the PCB: rent value at each survey grid s the actual value of the received 1. Cut the track between resistor R16 nal from the receiving probes. You're Reading a Preview and pin 7 of IC4. these voltages to their prevaili sion current, compensatio 2. Connect the now-open end of R16 to Unlock full access variations in the latter. The cu the pole of switch S2. with a free trial. are not actual milliamp values With switch S2 in the R5 (1k resistor) numbers representing the rela position, current flows switch Download Withfrom Freethe Trial flowing. pole through the 1k resistor and to 0V via It is suspected, however, the resistance of the soil. These two resisthe variations make little diffe tances form a potential divider. The square interpretation of the displayed wave voltage at their junction is buffered repeat the statement made in by R16 and half-wave rectified by diode aim of this Logger is to show D2. The resulting peak positive voltage is ferences in signal amplitude a monitored via PIC pin RA0 operating in being surveyed. It is the diff analogue (ADC mode). The peak voltage then indicate different sub-soil depends on the resistance of the soil, and If there are significant diffe from this voltage value the equivalent relaareon worth physical investigati Sign up to vote this title tive current through the resistance path can are no significant differences, be calculated. Not useful Useful is probably not worth examin To establish an initial reference value unless such techniques as magn prior to any survey, switch on the unit. ground-penetrating radar revea Then set switch S2 to the setting that A magnetometer design is cur
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A few other “tweaks” have also been added since Part One. The text and demo circuit for some experiments referred to on Part One page 1 are now accessible via buttons at the bottom left of the screen. Two other buttons allow you to examine the survey data as text files, one showing the twin-byte values separately, the other as the full combined value. These values include the current values as well. All three display screens have also been given a “pre-subtract” box, allowing you to subtract, say, the minimum value received from all other values, enabling relevant data to be extracted from any overall bias levels. Because this PC software will be used with the Magnetometer currently under development, two “radio” buttons allow selection of whether Earth Resistivity or Magnetometer data will be processed. Ensure that the Resistivity button is the one selected.
It was said in the opening paragraph in Part One that the original Earth Resistivity Meter published in EPE was an electronic tool to assist amateur archaeological societies. So too is this Logger design. Whilst there is nothing to stop anyone from carrying out surveys on their own property, there are considerable ethical issues regarding the surveying of other land. First, other land is not your land, and so any surveying of it requires the permission of those who own it. Remember that all land in the British Isles is owned by someone. Find out who it is and gain their permission before you proceed. Secondly, do not dig without an archaeologist’s involvement . If you have located through your earth resistivity survey something that proves to be a site of any importance, your unsupervised digging will certainly destroy information that is necessary to fully interpret the site. Earth resistivity surveying is essentially non-invasive except for the slight intrusion made by the probes just into the surface. Many landowners could well be as interested as an archaeological society in knowing what history might lie beneath their land, especially if they are approached in a
Nick’s survey was done not far from where John Constable paint Building Near Flatford Mill scene. The contours in the full graph sc earlier clearly indicate a trench comparable to that in this painting. www.excelsiordirect.com/constable.htm.
To find a local society, look in the teleThe primary area covered phone directory, or ask at the library. The is approximately 16m × 26m author’s local library building even has a dimensions. Most of it was c You're a Preview display of the Reading artefacts found by the society day, but then rain “sto in his area. It is a region once heavily popseveral weeks. ulated by the Romans, withamany artifacts Unlock full access with free trial. that have been found on display, and even the ruins of two Roman villas (but left From his experience with th whereDownload they were found!). this Logger, and from his gen With Free Trial Only a few hundred yards from the activity, Nick offers the follow author’s house a Roman corn drier was For extensive survey wor recently found by his local society. 400 yards further on are the ruins of a Roman needs to be bigger than PP bath house. It is quite probable that his The case should be large garden is on a site where Roman’s once prototype and a better sh trod. about Do not use small plugs an Although his survey graphics did not show anything other than known modern The sockets need to be features, and probably includingSign builder’s possibly up to vote on this titleon a metal ba rubble of recent decades, perhaps he’ll one description, and include Useful useful how hard you then day do a more detailed survey and call Not surprising in the archaeologists to uncover an amaz50m of cable laying on ing find – one way to get the garden dug Lay the survey out accura for him! a 3, 4, 5 triangle to get the
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Buy high visibility cable in case someone tries to trip over it! Colour code the probes – you need to be consistent Ask permission, most people will be chuffed to bits that you want to do the survey – but not everybody, and make sure that you are not somewhere where you should not be Make contact with your local archeology group, they will be very helpful and interested, and may well bite your arm off to get you helping them Be prepared to talk to people, you will cause interest if you are somewhere public, and they will be surprisingly knowledgeable – and probably have all been watching Time Team Keep your ears open for local stories of old ruins, you might be the one that rediscovers something lost to history because you happened to take the time to listen to the ramblings of the old guy in the pub You can do a survey on your own, but it is much easier with two of you Keep a note book that notes the time, place, date, etc of the survey and things like weather conditions which could explain odd results, for example if it started to rain half way through the work Any survey must have a repeatable base point, or base line so that if you do find something interesting, you can be sure where it was without having to repeat the survey!
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Use compass bearings, fixed physical features, corners of buildings, drain covers etc, or triangulate from fixed points if the survey is in an open area. Most archaeologists work north to south.
The author offers very grateful and appreciative thanks to Nick Tile for carrying out extensive field tests with the prototype, for discussing at length many aspects of its use, for lending Seeing Beneath the Soil and vetting the script. The author also thanks those EPE readers who provided him with information during the development of this design (in alphabetical order!): Dave Allen for sending an ancient issue of ETI containing a rudimentary ER circuit using d.c. probing (and yes Dave, this design could be used for monitoring relative impurity content levels in water). Peter Barnes, for vetting the script and for several useful email exchanges of thoughts and circuits, plus comments from his archaeologist acquaintance Derek about using Robert Beck’s design. Robert Beck, for the original inspiration. Aubrey Scoon, for comments about stray electrical currents in the soil. ODAS, the Orpington and District Archaeological Society, and Alan Hart in particular.
Applied Geophysics, W.M. Telford, L.P. Geldart, K.E. Sheriff, D.A.a Keys. Cambridge You're Reading Preview University Press. ISBN 0521-20670-7.
Applied Geophysics, Grif Pergamon Press. 1965. (ISBN Seeing Beneath the Soil, Methods in Archaeology. Ant Routledge. 2000. ISBN 0-4 This is a revised edition of th enced in EPE Feb ’97, and hav ent ISBN and publisher. It informative source used by the ing the design of this Logger. It additionally covers other e ing techniques, including ma and provides several furthe sources. It is known to be avai line ordering from www.Amaz www.BOL.com, current price
www.archaeology.co.uk . Va of the subject, including furthe to the magazine Current Archae the Council for Independent Ar www.geop.ubc.ca. Source mathematical tutorial on eart and a link to a site called Int Exploration Geophysics. www.google.com. Excell engine.
The ER software placed on site on 17 March ’03, was v Look in on the site occasionally further updates have been intro
CORRECTION Crystal X1 should be 3·6864 Fig.5), not 3·2768MHz as in nents list.
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