Feasibility of In-Phase Airborne TEM Utilising Gradient Measurements

Abstract

Detection of highly conductive ore bodies with airborne TEM is difficult or impossible as a result of the response of the target being largely in-phase with the transmitter signal. Measurement of magnetic field spatial gradients, in addition to the usual field measurements is proposed as a means to allow in-phase measurements in a non-rigid TEM system. With in-phase measurements the most conductive targets are as detectable as any geometrically equivalent target with lower time constant, greatly extending the utility of TEM. A simplified, proof-of-concept, analytical system model, in which only coaxial transmitter, receiver and target are considered, is used to demonstrate the compensation concept and to quantify the expected performance of such a system. The model demonstrates that a compensated signal preserving (or even enhancing) the target response can be generated. An assessment of noise in this model shows that noise from the gradient measurements significantly increases the system noise, over that of an equivalent receiver, by ~ 100-fold. However this does not extinguish the benefit of in-phase measurement and only results in halving of the depth of detection for the most conductive target, in comparison with an equivalent target with optimum time constant, in a system measuring the off-time response. A numerical model of a system with 7 degrees of freedom is then used to confirm the results can be extended to physically realistic TEM systems.