Conductance-depth imaging of airborne TEM data

Abstract

Recently a number of conductivity-depth imaging techniques have been developed for processing transient EM data. These techniques require the magnetic field data due to a step current source. However, current commercial airborne TEM systems employ a half-sine pulse source. To apply the conductivity-depth imaging techniques to airborne data, deconvolution is needed to transform the data into the required step function response. This deconvolution may be unstable with noise in the data. Furthermore, in areas of conductive overburden, the system response is mainly a function of the conductance of the overburden. The conductivity and thickness of the overburden can not be resolved uniquely. The system response may also be so high that "halfspace apparent conductivity" is undefined or the conductivity image has to be unphysically extended above the earth surface to fit the data. To overcome the above problems, a conductance-depth imaging method is introduced. The method is based on the horizontal thin-sheet model which has a simple analytical solution. By fitting the data measured at two adjacent time channels, the conductance and the depth of a thin sheet are found using the iterative least-squares method. Transient electromagnetic profile data can then be presented as a conductance-depth section for preliminary interpretation. The technique has been tested and found to be robust on model and field data. An example of processing field data is used to demonstrate the capabilities of the conductance-depth imaging technique in minerals exploration.