TECH TIPS
some AdditionAl methods for
RESISTIvITy TESTING il resistivity istheppetythatdenestheailityfsiltcnductcuentand ilresistivity istheppetythatdenestheailityfsiltcnductcuentand hencewhetheagivensiltypecaneexpectedtpvidea good good a a poor poor gund(high gund(high lwesistance).bycntast,eath resistance denestheelatinshipfagunding denestheelatinshipfagunding elec el ect td de e t it its s i ie edi diat ate e en envi vi n nen ent. t. Tey a ae e tw tw di die ee ent nt e eas asu ue een ents ts, , pef pe fed ed in tw di diee eent nt way ways, s, and exp expes essed sed in tw di diee eent nt uni units. ts. re resis sistiv tivity ity easu ea suee eents nts a ae e use useful ful dat data af f the des desig ign n and ins instal tallat latin in f fthe the ele elect ctde de. .Te Te pincipalethdfeasuingesistivityisknwnastheWennemethdandhas eenpeviuslydesciedinthisseies.otheethdsaealspacticedinsituatins wheeWenneisntthe wheeW enneisntthestsuitale,andse stsuitale,andsefthesewilledescied fthesewilledesciednext. next. by Jeff Jowett Megger
Some studies ave indicated tat eart potential gradients inside or near an electrode (currents returning to te source) are primarily a function of te resistivity of te surface layer, except in extremely ig resistivity soils, wile electrode resistance is more a function of deep soil. Transmission-line impedances are sensitive to layers of dierent resistivities at power frequency, wile ig frequencies, including surge frequencies, establis impedance only in te top few meters. erefore, bot surface and layered resistivities to considerable depts are of concern to grounding and ground resistance measurement. A common practice is te testing of soils in sample boxes in laboratories. is can be done by connecting an eart tester to a specially made box containing a soil sample. A volumetric measurement can tus be obtained. however, tis tecnique is more applicable to abstract researc tan practical eld work suc as grounding design. It It is dicult to obtain a useful approximation of resistivity by tis metod for several reasons. Obtaining a truly representative
sample of site conditions in a small volume commensurate wit te dimensions of a test box is a callenge. In addition, it is dicult to accurately replicate moisture content and in particular compaction compaction in a laboratory sample. Anoter igly limited but sometimes useful metod is an adaptation of te resistance metod (3-point test) to resistivity testing. is metod is adapted to small areas were space limitations preclude te larger distances across te test probes tat te standard enner enner Metod requires, and can be called a variation of dept metod. epeated resistance measurements are made of a test rod driven to increasing depts, using familiar metods derived from fall of potential. esistance would be expected to diminis wit increased dept, so interpretation of results is dicult and makes good use of experience. ut an uncaracteristic or unexpected cange can indicate a cange in resistivity, as wit anoter layer of dierent soil type. Suc data isn’t detailed enoug for practical use in soware design programs, but can provide a useful indication of on-site conditions wit
TECH TIPS respect to soil layering. e vicinity of the test rod aected by this method is about ve to ten times rod length. arger sites can also be tested for lateral changes in resistivity by relocating the rod to a number of points, but where sucient space is available, this application is better served by the traditional enner Method.
figure 1A
ariation of epth method can be enhanced by the use of some mathematical formulae. e basic resistance formulae involved are: = [ρ/2πl]ln[2l/r] figure 1b
and = [ρ/2πl][ln(4l/r) – 1] Tese rearrange to:
Ρa = [2πl] / [ln(4l/r) – 1] here = measured resistance at various depths Ρa = resistivity calculated l = test rod depth r = test rod radius Accordingly, a series of resistivities can be calculated from resistance readings at various depths and plotted against those depths. If the graph resembled Fig. 1A , it could reasonably be concluded that the tested soil consists of two layers, one at a shallow depth of relatively high resistivity and one at greater depth that is much more conductive. It might reasonably be concluded, then, that the additional investment in a deeper-driven permanent rod would be justied. In Fig. 1B, however, the shallow resistivity can be determined to be relatively conductive, but the deeper layer cannot be clearly determined.
e traditional enner Method uses four equally-spaced probes, which thereby measure to a depth equivalent to the spacing between any pair. e current probes are positioned on the outside. is is so that the current will have escaped the localizing eects of the probesoil interface and established a uniform eld across the inner probes which sense potential. is arrangement is conducive to maximum accuracy. An alternative method, called the Schlumberger Palmer Method, utilizes unequal spacing. is method is designed to address a problem that can arise with enner (equal probe spacing) where spacing has increased to considerably large distances: the drop in magnitude of potential between the voltage (inner) probes to a degree that the instrument cannot adequately measure. To counteract this problem, Schlumberger-Palmer brings the potential probes closer to the current probes (Fig. 2). e formula describing the resistivity measurement in this conguration is: ρ = πc(c + d)/d where c = spacing between current and potential probe d = spacing between potential probes = resistance reading from tester
TECH TIPS Te dept of penetration of te test probes is assumed to be small; enner calls for a 1/20 ratio between dept and spacing. uried conductive sources like power lines and pipes could be causing interference wit te measurement, or lateral canges in resistivity could render a specic location nonrepresentative of te general area. Terefore, it is recommended to take additional measurements at dierent locations on te same site, or at least to make te same measurement 90° relative to te rst.
figure 2
Four-point metods can provide data to be plotted similarly to te tree-point metods described above. Measured resistivities are plotted against dept, as as been sown. Clarity of interpretation as to te dept of various layers and te attendant soil structure, owever, is oen not so well dened as in Fig. 1A . arious autors ave oered rules for determining te dept of individual layers. One guideline is tat any break in te curvature of te grap indicates a separation of layers at te dept corresponding to tat probe spacing. Anoter prefers 2/3 te probe spacing at wic a point of inexion occurs as representing te separation between layers. Similarly, it as been suggested tat cange in apparent, or calculated, resistivity always occurs at probe spacings larger tan te dept of te actual cange; ence, te grap of apparent resistivity is always to te rigt of te actual. Tis interpretation furter suggests tat suc graps do not correspond to te actual depts or magnitudes of cange in soil layers. however, tey can be used as models of relative dierences and so provide a guideline for more exaustive testing and calculation tat will be described in a later edition.
Always remain aware tat anomalies in test results can occur from sources of interference. Tese interference sources can be bot passive conducting bodies and active electrical elements. Passive sources can be metallic fences, buried conductive objects suc as water pipes, building foundations, and pole grounds, as well as oters tat may be completely unexpected. Parallel transmission and distribution lines and communication services can act as live sources of interference. Passive conductive objects can provide a sort circuit tat distorts te eart potentials tat are being measured as part of te testing process. Similarly, active sources can provide current tat is added to, or subtracted from, te test current. emember tat an eart tester employs two test circuits, current and potential, in order to make te measurement. ot of tese can, terefore, be subject to distortion by interfering sources. Finally, te test leads, wic can be stretced out to considerable lengts, can develop a serious sock potential by inductive coupling wit parallel currentcarrying lines. So many dierent metods, oen wit conicting interpretations, can be a source of confusion. however, teir presence indicates te wide degree to wic variables aect ground measurements, including te enormous span of te resistivity scale, te problems associated wit spatial variations, te diculty in recognizing distinct soil layers, and te possible inuence of complicating factors like interference. ooked at anoter way, te numerous metods give te operator a cance to examine a situation from several perspectives and look for te most pertinent consistencies. Soil layering and modeling will be explored in a future edition.
