ASSESSMENT OF 2-D RESISTIVITY STRUCTURES USING 1-D INVERSION

Schlumberger and Wenner array resistivity soundings over 2-D resistivity structures are interpreted using apparent resistivity pseudosections and cross-sections constructed from 1-D inversions in order to determine the effectiveness of 1-D interpretations over such structures. Cross-sections contoured from resistivities of inverted "layers" show distinct differences from the apparent resistivity pseudosections and may be used as interpretational aids. Contour lines in the cross-sections locate the horizontal interfaces of the 2-D structures quite well. The vertically oriented segments of the cross-section contours are relatively undistorted in the inversion process and are similar to the vertically oriented portions of contours in the apparent resistivity pseudosection. A simple, empirically determined formula is used to separate the sections into resistive and conductive zones and helps to define the geometry of the anomaly. In order to apply the formula, it is necessary to know whether the target is a relative conductor or a relative resistor. Except for the case of a square prism, the Schlumberger array appears to hold advantages over the Wenner in qualitatively assessing an anomaly. The primary drawback of the Wenner array is that its expanding potential electrodes create false anomalous zones and complicate interpretation. As might be expected, structures with long horizontal interfaces, i.e. those more nearly 1-D, yield the most accurate interpretations.