Controlled source electromagnetic inversion for resource exploration

Abstract

lectrical conductivity is a diagnostic physical property for a variety of problems in mineral and hydrocarbon exploration. In mineral exploration, it can sometimes be related directly to the sought mineralization or it can be used to delineate geologic structure associated with the deposit. In oil exploration, conductivity can be a direct hydrocarbon indicator. One of the main impediments to using electromagnetic (EM) surveys is the complexity of the data. Unlike gravity or magnetic surveys, where the data can sometimes be used to infer geology or possibly to identify drill targets, EM data images are complex and are not connected with geology in a simple way. Over the last few years, however, we have developed computer algorithms to invert controlled source EM (CSEM) data. This opens the door for enhanced applications and for re-assessment of how data are collected and interpreted. The purpose of this article is to illustrate these ideas by applying our inversion techniques to synthetic and field data relevant to resource exploration. We begin with some background about EM surveys and our forward modelling. In an EM survey, the transmitter, or source, can be a grounded dipole or a loop which carries a time varying current. The measurements can be an electric field (E), a magnetic field (H) or its time derivative (dH/dt). The governing equations are those of Maxwell and the data are dependent upon three physical properties: electrical conductivity, magnetic permeability, and electrical permittivity. In this article we neglect the effects of electrical permittivity and assume that magnetic permeability is equal to that of free space. We concentrate upon the electrical conductivity, σ (or its reciprocal, resistivity, ρ), which, as illustrated by Table 1, varies by many orders of magnitude in earth rocks and minerals.