Non-invasive characterization of fractured crystalline rocks using a combined multicomponent transient electromagnetic, resistivity and seismic approach

Abstract

AbstractDC resistivity and seismic-refraction methods are well established in basement studies, and the multicomponent transient electromagnetic (TEM) method is also emerging as an effective tool for locating electrically conductive zones in crystalline rock underneath thick overburden. It can be expected that combining these three techniques will furnish a powerful non-invasive approach for characterizing fractured crystalline rocks. The aims of this paper are twofold, namely: (1) to establish the in situ geophysical signature of fracture-zones in weathered crystalline rock masses using the TEM profiling method, and (2) determine the resistivity and seismic-velocity relationship in the zones of fractured crystalline rock evinced by TEM profiling. Multicomponent TEM data from terrains with varying degrees of weathering are examined in this paper. The TEM response over a deeply weathered fracture zone in granite is found to be band-limited, with a consistent pattern of vertical voltage response (Vz) at early times enabling accurate location of the wet fracture zone which manifests as a steep conductive feature in 2D DC resistivity imaging. Depending on the thickness of the weathered mantle and the measurement bandwidth, the composite Vz signature consists of three parts: a high amplitude response with a single peak centred on the target fracture zone at very early times; a near invariant or slowly decreasing amplitude response across the fracture zone at some intermediate time; and a marked low-amplitude response at the centre of the fracture zone with higher amplitudes near the flanks (i.e. twin peaks) at later times. The across-strike voltage response (Vx) is diagnostic of conductive fracture zones and exhibits a marked sign reversal. The result of 2D inversion of resistivity, magnetotelluric and seismic-refraction recordings over an intensively fractured granodiorite suggests that resistivity (ρ) and compressional-wave velocity (VP) can be related in the form log10 ρ = m log10VP + c, where m and c are constants.