The borehole transient electromagnetic response of a three‐dimensional fracture zone in a conductive half‐space

Abstract

Borehole geophysical methods can be useful in detecting subsurface fracture zones and mineral deposits which are nearby, but not intersected by boreholes. One electrical borehole technique which can be applied to this problem is the surface‐to‐borehole transient electromagnetic (TEM) method. In this method a transmitting loop is deployed on the surface while a receiving coil is moved down a borehole. A conductive, horizontal, tabular body in a homogeneous half space was chosen to simulate a 3-D fracture zone or mineral deposit within the earth. Theoretical borehole TEM responses for several models of practical interest were computed using a direct integral‐equation formulation. The anomalous TEM response (secondary response) is the result of a complex interaction between vortex and galvanic currents within the body. Distortion of the secondary response by the conductive host does not affect the estimate of the depth to the horizontal body but it does lead to erroneous estimates of the conductivity and size of the body. Increasing the resistivity of the host decreases the host effects and increases the peak response of the body. Decreasing the separation between the body and borehole or decreasing the depth of the body increases the secondary response. The decrease in the vortex response due to the decreased coupling when a transmitting loop is offset from the body is nearly countered by an increase in the galvanic response at late times; however, this phenomenon is model‐dependent. This study indicates promise for the borehole TEM method, but the application of the technique is limited by the hardware and modest modeling capabilities presently available.