Sparse Borehole Radar Array Imaging of Orebodies in Three Dimensions

Abstract

Broadband very high frequency (VHF) borehole radars (BHR) can be used as tactical tools to map orebodies, faults and marker horizons; to identify hazards well in advance of mining and to optimize mine development. In principle, radar profiles and cross-hole scans from fanned arrays of boreholes can be used to synthesize three-dimensional images interferometrically if: borehole trajectories are accurately known; signal-to-noise ratios are adequate; the spatial sampling rate is sufficient; target echoes are well correlated, and if the target space is sufficiently uncluttered to limit the formation of mirages during reconstruction. However, automatic methods of projecting data into three-dimensional image space make stringent demands upon rock homogeneity, translucence and the accuracy of borehole trajectories. These demands can be relaxed by reconstructing objects from parts of geologically plausible 3D primitives such as cylinders, planes and spheres. In this paper we show that a number of useful primitives derive from a common element—a hoop (with semi-circular shell cross-section) of finite radius, centered on the borehole. The parameters defining the hoop’s plane, radius and thickness can be recovered sequentially from borehole radar data to determine the shape of curved 3D reflectors. We show how kinematic mapping can be applied in practice by using data acquired in a Western Australian mine. This survey was shot using [Formula: see text] borehole radars from a fanned array of [Formula: see text] long boreholes. The holes were drilled from a single station at relative angles of [Formula: see text]. Both cross-hole transmission and single-hole reflection surveys were performed. Reflection ranges of [Formula: see text] were achieved in the rocks hosting the deposit. Echo patterns correlated by using kinematic primitives were used to establish the geometry of an arched interface that was located approximately [Formula: see text] above the uppermost edge of the borehole array.