Seismic reflection for hardrock mineral exploration: lessons from numerical modeling

Abstract

The seismic reflection technique is an essential tool that is extensively applied in hydrocarbon exploration and development, but in hardrock mining and mineral exploration its use has been restricted. Ore bodies present quite elusive targets, in that they are often of limited lateral extent, have steep boundaries and complex shape, lie within high velocity undifferentiated rock and are characterized by small reflection signatures. Additional complications are that they occur at relatively shallow depth and the weak diffracted signals from the mineralization are frequently masked by many overlapping and interfering arrivals, especially mode conversions and reverberations within the weathered surface layer. Conventional CDP (common depth point) reflection profiling and even pre-stack migration reflection approaches do not favor the imaging of such structures. We have undertaken numerical model studies to determine the response of ore bodies and related geological structures (contacts, intrusions) with a view to detection and recognition on actual seismic sections. One type of modeling performed is 3-D acoustic, based on the Kirchhoff integral, to highlight the influence of out-of-plane reflections and the complicated diffraction patterns produced. The other type of modeling is full 2-D finite difference time-domain elastic simulation to not only study the reflection behavior of steeply dipping structures, but also to investigate the influence of mode conversions and to identify the most troublesome forms of noise. It is clear that ore bodies and associated geology present detectable targets, but the high velocity contrast at the base of weathering produces strong surface waves and reverberations which obscure the events of interest. The modeling can help in designing field layouts, which require long apertures and close detector spacing to image steep dips and avoid spatial aliasing problems. The classes of models investigated are quite limited, but the results nevertheless serve to illustrate the difficulties.