Improving the accuracy of shallow depth determinations in AEM sounding

Conductivity-depth images (CDIs) are finding application in salinity, groundwater, and environmental mapping. Hydrological modelling demands are for a much higher vertical resolution than the 10+m accuracy that was adequate in CDIs used for mineral exploration. Contractors are increasingly confident of system waveform, geometry, and some provide corrections for factors such as pitch, roll, and yaw. This increased system accuracy is the trigger for efforts in increasing the accuracy of processing. The CDI process makes a number of approximations in order to increase the speed of processing. One of the most critical in program EMFlow is an assumption that the transmitter and receiver are entirely within the current system induced in the ground at all delay times. This assumption equates to all components of the secondary field decaying monotonically with time. For typical fixed-wing AEM geometries, this assumption is poor for the z component of the response, and in fact on a CDI, z component data may predict the top of a surficial conductor to be several metres in the air. Allowing for part of the induced currents to lie between the transmitter and receiver, when coupled with an Inductive Limit constraint, leads to an accurate prediction of surficial conductors to lie at or below surface.