Enhancing the resolution of the subsurface by joint inversion of x- and z-component SkyTEM data

Abstract

Numerical modelling shows that the resolution of the subsurface resistivity structures is significantly enhanced if transient electromagnetic (TEM) x- and z-component data are inverted jointly. The enhancement is most pronounced in the upper part of the model as the sensitivity kernel for the x-component is more ``condensed' compared to the corresponding kernel for the z-component. Best results are obtained if the ground is relatively conductive and therefore the method is best fitted for surveys where the average resistivities of the ground are below approximately 100 ohmm, e.g. mapping of saline layers and the layers above. In the field x-component data can be measured with only a few extra costs. However, whereas the method is simple in theory it is complicated in practice where a number of problems have to be addressed. Tilt of the x- and zcomponent receiver coils must be measured with an accuracy of better than 1. degree as even a small tilt adds a significantly amount of z-signal to the x-signal. This ?contamination? must be modelled in the forward response of the inversion algorithm and the tilt of the coils must be added as extra (constrained) inversion parameters. It is necessary to low-pass filter the x-component data in order to efficiently suppress the high frequency background noise. Finally the timing of the instrument must be better than 200 nanoseconds to be able to model the first time gate at 11 micro seconds. In the presentation we discuss the method, present a parameter sensitivity study and a field example using SkyTEM data collected at the Toolibin test line.