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Earth resistivity meter part 2 [john m. stanley]
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Earth resistivity meter part 2 [john m. stanley]

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  • 1. Earth resistivity meter John Stanley From gold to archaeological remains — this simply constructed instrument will assist your prospecting.ETI: How To Build Gold & Treasure Detectors, 1981 — Copyright © Murray/Modern Magazines, reproduce for personal use only An earth resistivity meter can be used to identify the composition of various earth strata and the depth at which each strata occurs and by detecting changes in earth composition, to point to the existence of buried objects. An earth resistivity meter may be used to locate archaeological objects to assist in finding conditions favourable for alluvial gold or gemstones, or even for such prosaic duties as determining where to locate a septic tank! These instruments are not expensive compared with most electronic instru- mentation. Nevertheless at $1000 or so they are way above the budget of most amateur archaeologists or rock-hounds. But for such people all is not lost — it is possible to construct a simple dc operated resistivity meter for a mere fraction of the price of commercial units. For this to be possible we have to accept a few operating limitations — primarily of operating depth — for whereas a commercial unit may be used ties — a centre position of the switch to depths of 100-200 metres, our unit is (SW2) meanwhile short-circuits the two limited to 15 metres or so. But unless centre electrodes between readings to you are hoping to locate oil bearing discharge the galvanic potential. deposits in your backyard the limitation Figure 1 shows the circuit diagram of on operating depth should not be a the instrument. A connection diagram is problem. reproduced in Figure 2. The basic instrument is extremely We have not provided any mechani- simple — four equally spaced elec- cal assembly drawings, for this will trodes are placed in line in the earth. An depend almost entirely upon the meters accurately known current is caused to used. A pair of cheap multimeters are flow from one outer electrode to the ideal but if these are not available then a ent when the wiring diagram is other and a measurement is taken of the voltmeter and a milliameter with swit- compared with the switch. voltage between the two inner chable ranges should be used. The The ground probes should ideally be electrodes. milliameter should be capable of mea- made of copper coated steel or brass — Having measured both voltage and suring from microamps to a maximum however electrodes made from 25 mm current, a simple formula is used to of 100 milliamps or so, the voltmeter to 50 mm steel tubing or rod will work establish depth and composition of the should cover a range from approxi- quite well as long as they are kept clean. strata. mately 100 microvolts to three volts or It is of course essential that they make Professional earth resistivity meters so and should have a sensitivity of the best possible contact with the sur- use alternating current across the earth about 20 000 ohms per volt. rounding earth. Electrode cable electrodes in order to eliminate the Switch SW2 is a three-pole four-way connections must be securely made effects of the small galvanic voltages wafer switch. All switching contacts are using proper terminals — remember caused by the earth. located on one wafer. Each of the four that you are looking for fairly minor This effect cannot be totally elimi- segments shown in the wiring diagram changes in earth resistance. nated with dc instruments but it can be (i.e: SW1 SW2 etc) consists of a wip- Operating voltage is not critical — a minimised by switching the battery ing contact and three fixed contacts — six or twelve volt dry cell is adequate across the electrodes in alternate polari- the connections will be readily appar- for most applications. Geotech Page 1
  • 2. ETI: How To Build Gold & Treasure Detectors, 1981 — Copyright © Murray/Modern Magazines, reproduce for personal use only Measuring earth resistivity of their separation. This is important if We can determine the depth ‘d’ with There are several methods of measur- standard curves are to be used for the the aid of ‘standard curves’. The proce- ing soil resistivities, mostly variations interpretation of the experimental data. dure is to measure the resistivity of the of the original method devised by Wen- Having inserted the four electrodes an ground each time the electrode separa- ner. This consists of driving four metal average value for both V and I must be tion ‘a’ is increased about a central spikes (commonly called electrodes), determined for both polarities of the point. To use the standard curves pro- into the ground, at equal intervals along battery. Reversing the polarity removes vided it is necessary to plot the a straight line as shown in Figure 3. the possibility that the earth may have measured resistivity (p) on the vertical A current is passed through the outer its own potential due to galvanic reac- axis, against the electrode separation electrodes C1 and C2 and the resulting tions underground. From these distance on log/log graph paper. voltage drop across the earth resistance measurements the resistivity p can be The standard curves provided (Figure is measured across the inner pair p1 and calculated. 4), are also constructed on log/log graph paper i.e: graph paper that is ruled in p2. Resistivity depth sounding both directions at logarithmic intervals. If the ground has a uniform resistiv- Consider for example the problem of Each major division on the paper corre- ity p then measuring the depth beneath the ground sponds to a power of 10 and is therefore p = 2πa V/I = 2πa R of the water table or perhaps the thick- called a decade. We suggest that for where ‘R’ is the apparent resistance ness of soil overlying the bedrock. This plotting your data you purchase measured between the inner potential type of situation is by far the most com- semi-transparent paper that has three electrodes. mon — where a layer of resistivity p1 decades on either axis and a decade sep- Generally the current will flow in an and thickness ‘d’ is overlying a layer of aration of 2½ inches. The 2½ inch arc between the electrodes and hence different resistivity p2. decade separation is most important as the depth penetrated will increase as the electrode separation is increased. The effective depth at which R is measured is usually taken as 0.6 times the separa- tion ‘a’. For the greatest accuracy in determin- ing the ratio V/I it is desirable that the current flow I be maximised and hence in dry surface conditions it is common to moisten the soil about the electrodes to reduce the contact resistance. The depth to which the electrodes are inserted must not exceed one-twentieth Geotech Page 2
  • 3. ETI: How To Build Gold & Treasure Detectors, 1981 — Copyright © Murray/Modern Magazines, reproduce for personal use only paper having other decade separations curve. metre and the standard curve that is a will not allow your plotted results to be When the best matching curve has best match shows a p2/p1 ratio of one overlaid on the standard curves. This been found, note where the vertical axis tenth, that is p2 equals 0.1 p1. paper should be readily available from of the standard curve intersects the ‘ab’ Thus p2 is approximately 100 ohms/ major stationary suppliers such as John curve of your plotted data. This line metre. Relating these figures to Table 1 Sands or Dymocks. extended vertically downwards to inter- we see that the most likely strata forma- Figure 5 shows a typical plot of field sect the ‘electrode separation’ axis of tion is two layers of sandstone of data overlaid onto the standard curve. your plotted data will show the depth of different densities or a top layer of To do this, place your plotted curve the first layer — in our example this is sandstone and a lower layer of over the standard curve and slide it hori- 4.25 metres. limestone. zontally until you find the standard We know from our plotted data that From the section bc it is possible to curve that best matches your plotted the resistivity p2 is about 1000 ohms/ calculate the resistivity and depth of the Geotech Page 3
  • 4. menter should refer to these for further than half the dimension of the object to information. be located; in fact the closer the read- ings are taken, the greater will be the Resistivity trenching resolution. Another common application of the If it is desired to follow the depth of resistivity meter is in searching for bur- bedrock beneath the surface, it is best to ied objects such as large water mains, first carry out a vertical depth sounding buried stream beds or underground sew- to locate the bedrock. Then divide this depth by 0.6 to give the most suitableETI: How To Build Gold & Treasure Detectors, 1981 — Copyright © Murray/Modern Magazines, reproduce for personal use only erage tunnels. The method used is simply to decide approximately at what electrode separation. The depth sound depth the object is likely to be found, will also tell you whether the bedrock and divide the distance by 0.6 to give a has a higher or lower resistivity (from suitable electrode separation. Maintain- the ratio p2/p1). If p2 is greater than p1 ing this same separation, the array of all then an increase in your measured resis- four electrodes should be progressively tivity will tell you that the basement is second layer but this requires the use of getting shallower and vice versa. Alter- a second set of auxiliary standard moved in a line over the ground being explored. Readings of resistivity should natively, if p2 is less than p1 an increase curves. These are very complex and in resistivity will indicate that the base- beyond the scope of this article. Simi- be made at each point and the value plotted against distance moved. (See ment is becoming deeper. This method larly section cd provides data on the is most suitable for looking for alluvial third layer and so on. There are a num- Figure 6 in our feature on Exploration Archaeology). The distance between gold or heavy gemstones which tend to ber of standard texts on such be concentrated in the hollows of the measurement and the interested experi- each reading point should be no greater bedrock along alluvial creekbeds. Earth electrodes should not be inserted into the ground to a depth greater than one- twentieth of the probe separation. Because of this, poor electrode/ground contact may result at close spacings. This problem can be reduced by using porous pots filled with copper sulphate solution. Electrodes specifically intended for such work are available from geophysical supply houses. Geotech Page 4