Using spatial derivatives of electromagnetic data to map lateral conductance variations in thin-sheet models: Applications over mine tailings ponds

Abstract

Mine waste, variable overburden, and the saprolite associated with nickel laterites have conductivity thicknesses (conductances) that vary laterally. In order for electromagnetic methods to be used to easily map lateral changes in conductance over thin-sheet-like bodies such as these, a simple conductance estimation method has been developed from Price’s equation. Through forward modeling, we found that assuming a uniform conductance and solving for an apparent conductance was sensitive enough to identify lateral conductance changes. The method was independent of the transmitter location, and each measurement provided a direct estimate of the apparent conductance below that station. The receiver can be moved around quickly allowing for lateral variations in apparent conductance to be determined efficiently. However, one of the required terms in the equation used is the vertical derivative of the secondary vertical magnetic field ([Formula: see text]). The accurate measurement of spatial electromagnetic derivatives requires a good signal-to-noise ratio (S/N), which can be hampered by low derivative signal values. Field studies performed over a dry tailings pond in Sudbury, Ontario, Canada, showed that an S/N greater than three was achievable even with [Formula: see text] values of less than [Formula: see text]. Apparent conductance estimates revealed that the tailings had a large resistive zone associated with surface vegetation, which may be correlated with favorable growing conditions and/or less conductive or thinner tailing material. Larger apparent conductances in other areas may be related to zones of thicker tailings and/or more conductive material (possibly due to increased metal content). Further drilling and sampling work is required to answer these ambiguities. Regardless, mapping the conductance of a thin sheet is an important step toward assessing if there are leftover metals in mine waste. However, the developed method is general and can be used in many other situations involving laterally varying thin bodies.