On the Advantage of B-Field Sensor Arrays in TEM: From Minerals to UXO and Back Again

Abstract

EM receiver systems which measure magnetic field (B-field detectors), not time rate of change of magnetic field, are in frequent use in mineral exploration, and are subject of a current development for detection of unexploded ordnance (UXO). The advantages of B-field detectors stem from both logistical and target discrimination capabilities. This paper quantifies the advantages of the B-field measurement using simple mathematical models for the response of highly-conductive mineral targets in a conducting earth, and for unexploded large-calibre steel munitions in the presence of scrap metal. The small size of B-field detectors greatly facilitates the use of arrays of sensors. A series of model studies was conducted to establish what improvement in target orientation/shape information could be achieved with single or multiple B-field sensors compared with conventional moving-loop surveys conducted using single-component single-sensor in-loop systems. The model studies show that when the target size is known (as may be the case in munitions search), the use of a single vector sensor delivers an order of magnitude greater accuracy in location in 3D space and orientation of the target, compared with the use of vertical-component only data in conventional metal detectors. When the target size is unknown, a single vector sensor can produce accurate location, but is likely to produce unstable orientation/shape estimates. We conclude that an array of receiver sensors is preferable. Comparison of a single vector sensor versus an array of three vector sensors in model studies shows that the array of three vector sensors has the advantage of delivering accurate and stable estimates of both location and orientation/shape of the target. These finding are currently being implemented in a new design of metal detector designed for location and discrimination of unexploded munitions. The results are equally applicable to the design of multiple-receiver arrays for moving-loop EM surveys in mineral exploration.