An Empirical Method of Interpretation of Earth-resistivity Measurements

A graphical method of analyzing the data obtained from shallowearth-resistivity depth tests is presented. The method is based upon empiricalresults and has no theoretical basis. The usual apparent resistivity-electrodespacing curve is used together with a cumulative resistivity-electrode spacingcurve plotted on the same sheet. The greatly reduced scale required forplotting the cumulative values of resistivity together with the effect of thesummation of the individual resistivity values serves to minimize the effect ofpurely local surface anomalies and inadvertent errors of measurement. The pointof intersection of tangents or straight lines drawn to intersect at zones ofmaximum curvature in the cumulative curve indicates the depth to the underlyingmaterial. Numerous figures are presented in which data from published reportsand from recent field studies are analyzed and the results compared with actualdepths established by borings or with depth values obtained by the use oftheoretical methods of analysis. Smoothly rounded curves of apparentresistivity such as are often obtained in the field, and which have been aserious drawback to attempts to analyze the data empirically heretofore, appearto be susceptible to rather accurate analysis by the method described. The method is best suited to analyses involving shallow two-layerformations. It has been applied successfully, however, in analyzing the dataobtained from tests made over shallow three-layer formations. As with mostempirical methods, its chief advantage is its simplicity and ease ofapplication. Introduction There have been published many papers that discuss the interpretation ofdata obtained from earth-resistivity tests when using the four-terminal methodof electrode spacing developed by Wenner. The majority of these have dealt withtheoretical analyses for two-layer and three-layer formations. Some sets of"master curves" have been presented for use in analyzing field data todetermine the depth to the first and possibly the second horizon below theearth's surface. Although practically all of these theoretical methods ofanalysis have appeared to have particular merit and some have been usedsuccessfully in practice, they have been found to be of little value where thelocal conditions surrounding the test failed to conform to those assumed in thetheory. In certain fields, particularly in civil engineering, relatively shallowexplorations are often involved and geophysical methods of test must competewith the direct methods of exploration ordinarily used. Only when it can bedemonstrated that geophysical methods of test can materially reduce the timeand cost of a given exploration project will the civil engineer abandon directmethods in favor of the interpretations of geophysical exploration data. T.P. 1743