A comparison of three geoelectric methods in the presence of shallow 2-D inhomogeneities: A case study

Abstract

Shallow variations of electrical resistivity often interfere with the interpretation of data from electric and electromagnetic surveys in terms of a 1–D subsurface. We interpret data of two resistivity (vertical electric sounding, VES, and dipole–dipole profiling) and two electromagnetic methods (very low frequency–resistivity profiling, VLF–R, and transient electromagnetic, TEM, soundings) over three shallow inhomogeneities: a metallic pipe, an outcropping granite wall, and a barbed–wire fence. By applying several of 1–D, 2–D, and 2.5–D numerical algorithms we estimate the subsurface resistivity structure, and the perturbing effect of the lateral inhomogeneities. Only in the case of the granitic wall a need for a 3–D algorithm to model the TEM data was apparent. The VLF–R method was useful as it detected the presence of inhomogeneities that might have remained unnoticed by the other methods. The VES and TEM methods turned out to be complementary in several aspects. The VES data were not affected by the conductive inhomogeneities and the shallow resistivity structure was better resolved. On the other hand, the TEM method was not affected by high contact resistances and was immune to the equivalence problem affecting the VES data. Despite its impervious paint cover, the pipe perturbed the galvanic current flow. The presence of the fence with metallic posts produced a strong VLF–R anomaly and affected the neighboring TEM soundings. The fences with wooden posts produced no anomaly.