Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance – the Gaussian variation applied to a field example

Abstract

Formulae for the moments of the magnetic field response can be derived for simple models which have conductivities that vary suddenly as a function of depth (thin and thick sheets) or not at all (half space). In a companion paper we have derived expressions for the moments of a conductivity-depth profile that varies smoothly, taking the form of a Gaussian function. In this paper we apply the Gaussian model to data from Russell South, an area in the Athabasca Basin of Canada. The low signal-to-noise ratio in this area means that estimating the overburden thickness is a challenging problem, so this dataset is a good candidate for demonstrating the applicability of our approach. The estimated thicknesses can be compared with drill information, also somewhat problematic as a reliable source of information. If we constrain the Gaussian model to be similar to a thin sheet or a thick sheet at surface, we get estimates of the overburden thickness which are much greater than what is inferred from drill information. However, if the overburden is allowed to vary gradually and the depth and value of the maximum conductivity can vary, then we find that the depth of the most conductive part of the overburden is realistic as it is generally above the base of overburden as determined from drilling. Features of geological interest that are not apparent on the original data can be identified on the derived images.