Resistivity modelling of fracture zones and horizontal layers in bedrock

Abstract

2D resistivity modelling was done in order to examine the imaging resolution of 4 different configurations for mapping of fracture zones in bedrock. The studied arrays include Dipole-dipole, Gradient, Pole-dipole and Wenner. A variety of geological models were tested and the imaging possibilities and limitations of the different arrays were analysed. Apart from imaging fracture zones with various depth, width, contrast and dip, some models for horizontal layers were also examined. The forward modelling was done with the program RES2DMOD whereas the inversion was carried out with RES2DINV. The protocol files simulate the measuring procedure with an ABEM Lund system which is based on a layout of 4 cables with total 81 electrode positions.\rGenerally it can be noted that the best results were achieved with Gradient and Dipole-dipole, especially for mapping fracture zones with various depth and width. The Gradient configuration gives the best response for mapping steeply dipping structures and different contrasts, whereas Wenner is good at illustrating horizontal layers. Dipole-dipole and Pole-dipole show the most accurate results for situations where a low resistivity top layer is present.\rAll in all it can be said that the vertical\/horizontal filter value 2 clearly improves the image of vertical structures whereas a filter value of 0.5 increases the image quality of horizontal layers.