SUBSURFACE DC RESISTIVITY MAPPING - APPROXIMATE 1-D INTERPRETATION

Resistivity prospecting is the main tool used to investigate the shall ow structure of the ground. A series of new techniques for determining the 2-D and 3-D geometry of the ground is now finding increasing use, but the light and simple Wenner prospecting technique remains a pract ical and efficient tool for rapidly mapping lateral variations in resi stivity. When the resistivity changes are smooth, 1-D modelling can be used to interpret the data, and the criteria governing this approxima tion can be defined from synthetic data generated by a 3-D slab-model. For a Wenner array, two quadripole configurations can be used, Normal and Dipole-Dipole. For these two configurations the width of the tran sition zone, the apparent anisotropy effect and the precision of the r esistivity values recovered from 1-D inversion differ. However the sim ultaneous inversion of both sets of data gives better results than for either configuration by itself. Two examples illustrate that in geolo gical contexts where the thickness of the weathered zone causes the ch anges in the apparent resistivity value, this parameter can be recover ed from 1-D inversion.