Estimation of transmitter-receiver separation for mobile ground-based transient electromagnetic systems using on-time gates

Reliable hydrogeological interpretations based on electromagnetic (EM) imaging hinge on accurate forward modelling of the EM measurement. Modelling inaccuracies may lead to the introduction of imaging artifacts or an inability to fit observed data; both scenarios that degrade the value of EM imaging tools for hydrogeological studies.<br/>In this work, we present an approach to constrain the offset separating the transmitter and receiver coils in mobile EM systems, where variations in the offset may occur during data collection. Errors in the estimated offset lead to biased forward modelling thus degrading image reliability. We consider a subset of offset geometries based on ground-based modalities where transmitter/receiver coils are restricted to the plane of the ground surface as is common for man-carried or towable platforms. We demonstrate that a simplified version of airborne EM receiver localization schemes, where the primary field produced by the transmitter coil is measured by the receiver in order to determine the relative positions of the two coils, can be adapted to ground-based systems employing only two single-orientation coils (e.g. a +z receiver, and a +z transmitter) in contrast to airborne schemes requiring multi-moment transmitters and multi-component receivers.<br/><br/>An overly simplified assumption, where the observed primary field at the receiver is attributed solely to a lateral (+x) displacement of the two coils is shown to reliably deliver centimeter to decimeter accuracy for offsets ranging from several to several tens of meters. We demonstrate that deliberate neglection of small rotations and/or +y/+z displacements introduces only minimal centimeter-scale biases in the estimated offset. As such, a simple solution to offset quantification of mobile ground-based platforms can be realized using the primary field.