Weak elastic‐wave scattering from massive sulfide orebodies

Abstract

Massive sulfide ore deposits, traditionally the target of electromagnetic or potential‐field geophysical investigations, can potentially be recognized and mapped using seismic methods based on their scattering response. To characterize the seismic expression of massive sulfide orebodies, I review formulas that describe elastic‐wave scattering from isolated inclusions in the far‐field and weak‐scattering limits (the Born approximation) and conduct a series of numerical tests. The minerals pyrite, sphalerite, and galena are used as the basis for these numerical modeling studies because they are relatively abundant and collectively span the full range of observed velocities and densities for ore rocks. The assumption of weak scattering, on which the Born approximation rests, is verified empirically in a representative example, by examining in‐situ wavefield measurements from a vertical seismic profiling experiment in northern Québec, Canada. For simplicity, orebodies are modeled as ellipsoidal inclusions in a homogeneous medium. The mathematical formalism leads to decoupling of factors for “composition” and “shape.” At seismic frequencies (∼50 Hz) and for ore deposits of sufficient size to represent economically viable targets (more than 109 kg), the shape factor effectively controls the scattering response. In this regime, negligible backscattering from spherical inclusions is predicted by the theory. In the case of flattened ellipsoids, which represent a more realistic orebody morphology, the shape‐factor effect leads to significant backscattering that is focused in the direction of specular reflection. For noisy 2-D data, numerical modeling indicates that these characteristics can cause scattering from an isolated dipping orebody to resemble reflections from a planar interface. For 3-D seismic surveys, however, isolated scattering bodies will produce a diagnostic pattern of concentric circular diffractions in unmigrated time‐slice sections.