Physical Modeling Results on Modified Pseudodepth Sections in Exploration of Highly Resistive Targets

Abstract.

—Resistivity sounding curves obtained by plotting (ρ _{a max} in a profile versus its spacing (L)over an infinitely resistive vertical sheet model with Wenner, dipole-dipole and three-electrode arrays reveal that the maximum anomaly in a sounding is obtained at a spacing which when multiplied by 0.5 times the depth of investigation of the array for homogeneous ground (Roy and Apparao, 1971) gives rise to the depth of the target model. But in case of resistive horizontal cylinder and sphere models with any of the arrays, the multiplication factor is 1.5 to arrive at the target depth.¶Using the new depths of investigation of the arrays in the presence of a highly resistive target, the modified pseudodepth sections so prepared with the arrays show-up that the maximum anomaly contour falls, in the case of the Wenner array, right on the target cross section irrespective of its size and depth. This distinct feature would help an exploration geophysicist to easily locate resistive targets with the array. In the case of an axial dipole array the maximum anomaly contour falls on either side of the target. The midcenter of the two highs locates the position of the target. With three-electrode array (+IV ₁  V ₂ ), the maximum anomaly contour falls on one side of the target. However, with combination profiling (+IV ₁  V ₂ )and V ₁  V ₂+I) using different spacings, the position of the target could easily be located at the midcenter of the maximum anomalies.