Combined Schlumberger and dipole-dipole array for hydrogeologic applications

Abstract

A novel approach to 2D resistivity sounding, called combined Schlumberger and dipole-dipole (CSDD) sounding, probes the subsurface both vertically and diagonally. It transforms the dipole-dipole values to Schlumberger values using the Patella method. Gridding then creates an apparent-resistivity section in which vertical variations are inferred from the Schlumberger values and lateral variations are inferred from the dipole-dipole values. The section is inverted by the Zohdy 1D method to vertical strips. The resulting 2D resistivity model of the subsurface identifies the layers and their dips. We tested the ability of the CSDD method to delineate a layered structure by comparing the method’s response to a model and then by a field test at a site where we have some degree of subsurface information from a resistivity tomography profile. Finally, we used this new CSDD sounding for hydrogeologic applications in the Agia Valley on Crete, Xylokastron City, and Skyros Island in Greece. The CSDD soundings map formations and interfaces at various depths and dips, including a phyllite-limestone interface and marl with overlying conglomerate. It also maps circulation, supply, and salinization of groundwater. In all cases, we find that CSDD sounding can detect the layers and their dips in sedimentary basins.