THE 3-DIMENSIONAL DC SENSITIVITY FOR SURFACE AND SUBSURFACE SOURCES

Four different methods of determining the DC sensitivity in three dime nsions are presented: three numerical approaches for arbitrary conduct ivity structures and an analytical one for a homogeneous case using th e sensitivity theorem. Since the sensitivity is a very important and i ndicative property in any interpretation process, its spatial distribu tion is shown as an overview for commonly used pole-pole, pole-dipole, and dipole-dipole arrangements at the surface and subsurface. Distinc t regions of negative sensitivities appear for any configuration. For horizontal subsurface pole-pole configurations, they assume tube-like, cylindrical shapes stretching from the electrode locations towards th e surface and yielding a circular sign-reversal pattern at the surface . These shapes and sign reversals occur as long as the electrodes are located at a finite depth. Similar forms occur for subsurface pole-dip ole and dipole-dipole arrangements. A series of model studies are carr ied out to examine the validity of the homogeneous responses for more realistic inhomogeneous media. Generally, the spatial sensitivity patt erns for homogeneous environments are good approximations for moderate conductivity contrasts not exceeding 1:10 if the source is located wi thin conductive material. If the source is buried within a resistor, c onductive structures perturb the homogeneous pattern more significantl y. Finally, a crosshole model study reveals significant differences be tween 2-D and 3-D conductive bodies, suggesting a need to examine the target very carefully before approximating some structures in two dime nsions.