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some additional methods for resistivity testing

Some studies ave indicated tat eart potentialgradients inside or near an electrode (currentsreturning to te source) are primarily a functionof te resistivity of te surface layer, except in

extremely ig resistivity soils, wile electroderesistance is more a function of deep soil.Transmission-line impedances are sensitiveto layers of dierent resistivities at powerfrequency, wile ig frequencies, includingsurge frequencies, establis impedance only inte top few meters. erefore, bot surface andlayered resistivities to considerable depts areof concern to grounding and ground resistancemeasurement.

A common practice is te testing of soils in

sample boxes in laboratories. is can be doneby connecting an eart tester to a specially madebox containing a soil sample. A volumetricmeasurement can tus be obtained. however,tis tecnique is more applicable to abstractresearc tan practical eld work suc asgrounding design. It is dicult to obtain a usefulapproximation of resistivity by tis metod forseveral reasons. Obtaining a truly representative

sample of site conditions in a small volumecommensurate wit te dimensions of a testbox is a callenge. In addition, it is dicult toaccurately replicate moisture content and in

particular compaction in a laboratory sample.Anoter igly limited but sometimes usefulmetod is an adaptation of te resistancemetod (3-point test) to resistivity testing.is metod is adapted to small areas werespace limitations preclude te larger distancesacross te test probes tat te standardenner Metod requires, and can be called avariation of dept metod. epeated resistancemeasurements are made of a test rod drivento increasing depts, using familiar metods

derived from fall of potential. esistance wouldbe expected to diminis wit increased dept,so interpretation of results is dicult and makesgood use of experience. ut an uncaracteristicor unexpected cange can indicate a cange inresistivity, as wit anoter layer of dierent soiltype. Suc data isnt detailed enoug for practicaluse in soware design programs, but can providea useful indication of on-site conditions wit

by Je f f Jowet t Megger

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s o m e A d d i t i o n A l m e t h o d s f o r

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some additional methods for resistivity testing

respect to soil layering. e vicinity of the testrod aected by this method is about ve to tentimes rod length. arger sites can also be testedfor lateral changes in resistivity by relocating therod to a number of points, but where sucientspace is available, this application is better served

by the traditional enner Method.

ariation of epth method can be enhancedby the use of some mathematical formulae. ebasic resistance formulae involved are:

= [/2l]ln[2l/r]

and

= [/2l][ln(4l/r) 1]

Tese rearrange to:

a = [2l] / [ln(4l/r) 1]

here = measured resistance at various depths

a = resistivity calculated

l = test rod depth

r = test rod radius

Accordingly, a series of resistivities can becalculated from resistance readings at various

depths and plotted against those depths. If thegraph resembled Fig. 1A, it could reasonablybe concluded that the tested soil consists oftwo layers, one at a shallow depth of relativelyhigh resistivity and one at greater depth that ismuch more conductive. It might reasonably beconcluded, then, that the additional investmentin a deeper-driven permanent rod would bejustied. In Fig. 1B, however, the shallowresistivity can be determined to be relativelyconductive, but the deeper layer cannot beclearly determined.

e traditional enner Method uses fourequally-spaced probes, which thereby measureto a depth equivalent to the spacing betweenany pair. e current probes are positionedon the outside. is is so that the current willhave escaped the localizing eects of the probe-soil interface and established a uniform eldacross the inner probes which sense potential.is arrangement is conducive to maximumaccuracy. An alternative method, called theSchlumberger Palmer Method, utilizes unequalspacing. is method is designed to addressa problem that can arise with enner (equalprobe spacing) where spacing has increasedto considerably large distances: the drop inmagnitude of potential between the voltage(inner) probes to a degree that the instrumentcannot adequately measure. To counteractthis problem, Schlumberger-Palmer brings thepotential probes closer to the current probes(Fig. 2). e formula describing the resistivity

measurement in this conguration is:

= c(c + d)/d

where c = spacing between currentand potential probe

d = spacing between potential probes

= resistance reading from tester

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figure 1A

figure 1b

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Te dept of penetration of te test probes isassumed to be small; enner calls for a 1/20 ratiobetween dept and spacing. uried conductivesources like power lines and pipes could becausing interference wit te measurement,or lateral canges in resistivity could render

a specic location nonrepresentative of tegeneral area. Terefore, it is recommendedto take additional measurements at dierentlocations on te same site, or at least to make tesame measurement 90 relative to te rst.

Four-point metods can provide data to beplotted similarly to te tree-point metodsdescribed above. Measured resistivities areplotted against dept, as as been sown. Clarityof interpretation as to te dept of various layersand te attendant soil structure, owever, isoen not so well dened as in Fig. 1A. arious

autors ave oered rules for determiningte dept of individual layers. One guidelineis tat any break in te curvature of te grapindicates a separation of layers at te deptcorresponding to tat probe spacing. Anoterprefers 2/3 te probe spacing at wic a pointof inexion occurs as representing te separationbetween layers. Similarly, it as been suggestedtat cange in apparent, or calculated, resistivityalways occurs at probe spacings larger tan tedept of te actual cange; ence, te grap ofapparent resistivity is always to te rigt of te

actual. Tis interpretation furter suggests tatsuc graps do not correspond to te actualdepts or magnitudes of cange in soil layers.however, tey can be used as models of relativedierences and so provide a guideline for moreexaustive testing and calculation tat will bedescribed in a later edition.

Always remain aware tat anomalies in testresults can occur from sources of interference.Tese interference sources can be bot passiveconducting bodies and active electricalelements. Passive sources can be metallic fences,buried conductive objects suc as water pipes,

building foundations, and pole grounds, as wellas oters tat may be completely unexpected.Parallel transmission and distribution lines andcommunication services can act as live sourcesof interference. Passive conductive objects canprovide a sort circuit tat distorts te eartpotentials tat are being measured as part ofte testing process. Similarly, active sources canprovide current tat is added to, or subtractedfrom, te test current. emember tat an earttester employs two test circuits, current and

potential, in order to make te measurement.ot of tese can, terefore, be subject todistortion by interfering sources. Finally, te testleads, wic can be stretced out to considerablelengts, can develop a serious sock potentialby inductive coupling wit parallel current-carrying lines.

So many dierent metods, oen witconicting interpretations, can be a source ofconfusion. however, teir presence indicateste wide degree to wic variables aect ground

measurements, including te enormous span ofte resistivity scale, te problems associated witspatial variations, te diculty in recognizingdistinct soil layers, and te possible inuence ofcomplicating factors like interference. ookedat anoter way, te numerous metods givete operator a cance to examine a situationfrom several perspectives and look for temost pertinent consistencies. Soil layering andmodeling will be explored in a future edition.

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figure 2

some additional methods for resistivity testing

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