Novel electromagnetic surveying and interpretation methods for improved near surface characterization and deep exploration

Abstract

Inductive electromagnetic (EM) geophysical methods are often applied and their data processed for two different purposes. One is to create images of the distribution of electrical conductivity of the subsurface while the second is to derive simple robust physical estimates of the location, size and attributes of discrete conductors. In order to improve both the ability to efficiently image the subsurface and to derive simple robust physical properties of discrete conductors, several surveying methods and interpretation algorithms have been developed. To improve the lateral near-surface resolution of thin conductors, such as overburden, nickel laterites and mine waste, two interpretation methods were developed which rely on the measurement of the vertical spatial derivative of the time-varying magnetic field. The first method does not require grid or line data, but, is less accurate than the second inversion method when the spatial gradient of the resistance is strong and/or when the horizontal magnetic fields are large. When applied to data collected over an old mine tailings pond, the two methods produced similar high resolution maps of the conductance. For both shallow and deep exploration a simple and robust conductance estimation method for borehole EM data was developed. The method relies on the calculation of the vertical spatial derivative of the magnitude of the magnetic field using adjacent down-hole stations. In a field trial, a reliable conductance was calculated for two deep sulfide bodies. To improve the resolution of EM surveying, a multi-transmitter surveying processing method was developed. Individual targets are highlighted by calculating a weighted-sum of the multiple transmitter data and the method is shown to produce high signal-to-noise ratio data for high finesse surveys in complex conductor environments where many transmitter-to-target coupling angles are required and/or for deep focused exploration. A field example over an offset dyke located in the Sudbury Basin showcased the ability of the surveying and processing method to determine the location and orientation of a sulfide body. To provide geoscientists with low-cost and efficient interpretation tools, a fast approximate 3D inversion for fixed-loop surface data was developed. The method solves for the causative subsurface current system which is approximated with a 3D subsurface grid of 3D magnetic (closed loop current) or electric (current element) dipoles. Ground data from a deep massive sulfide body was inverted and the results were consistent with existing interpretation and a second example over a near-surface mine tailings pond highlighted the strength of being able to invert magnetic field data using either magnetic or electric dipoles.