Geoelectric profiling

Abstract

The response of the focused surface laterolog system using seven electrodes, for conducting as well as for resistive targets, is equivalent or nearly equivalent to that of the modified unipole having only three electrodes, which in turn can be computed from simple two-electrode measurements. Thus focusing the current down towards the target does not necessarily improve the response measured on the ground surface.

As far as conducting vein-shaped targets are concerned, the simplest unfocused two-electrode array has overwhelming advantages over the Wenner, the Schlumberger and the focused systems like the unipole, modified unipole and the surface laterolog in shape and amplitude of anomalies, in depth of detection and in cost of operation. For resistive targets, not one system seems distinctly better than the others, except for cost of operations which would be lowest for the two-electrode array.

In comparison with the dipole-dipole array, the two-electrode array spacing to spacing (L) gives again better response in regard to amplitude and shape of anomaly, depth of detection and cost of operation. But, if the spacing (L) between the farthest moving active electrodes in an array is not considered as a yardstick for comparison, and the availability of the source power is not a problem in the field, then the dipole array appears better in shape and amplitude. It requires less cable and does not need the infinite cable lay-out.

Defining the depth of investigation of an electrode array as the depth of a thin horizontal layer of a homogeneous ground that contributes maximum to the total signal measured on the ground surface, the two-electrode array is found to have the largest depth of investigation. The theoretical analysis on depths of investigation of different electrode arrays has once again brought out the superiority of the two-electrode array over the others, even focused systems. However, the advantage of the two-electrode array in having a high depth of investigation is counterbalanced by its low vertical resolution.

It is a matter of intuition that a buried target at the depth of investigation of an electrode array gives more response on the ground surface than when the target is above or below that depth. A modified pseudo-depth section was suggested to obtain by plotting the apparent resistivity and/or apparent polarisability values at the maximum contribution depth of investigation of the array. Model and field studies demonstrate that the pseudo-depth section serves as a convenient tool in prospecting for conducting minerals and in the location of the boreholes. The tool was successfully tested in a virgin area. The results of the field survey were described.

For some reason or the other, it is still not uncommon to find that the term, depth of detection, is loosely used by field geophysicists for the depth of investigation of an electrode array. Depth of detection of a target with a given electrode array is defined as the limiting depth below which the target cannot be detected with the array. Following this definition, the dipole array is found to have the greatest depth of detection and the two-electrode array has the least, as far as detection of a sandwiched layer in a horizontally three-layered ground is concerned. The Wenner and/or Schlumberger array is found to have a depth of detection in between those of dipole and the two electrode arrays.