Wire-loop surface conductor for airborne EM system testing

The known mineral deposits used to test and compare airborne electromagnetic systems are often difficult to model because of nonideal geology, and may also be inconvenient or costly to survey. A simple wire-loop conductor has the advantage of being easily transported to the survey location and can be tuned to deliver a range of responses that will closely match the theoretical response, particularly on resistive ground. We calculated the response for such a tuned loop laid out at the surface of conductive ground and compared that response to field data. For an AEM system flown over a surface loop, when neglecting the second order of mutual induction, the receiver signal can be divided into three parts: (1) transmitter-earth-receiver (TER) signal; (2) transmitter-loop-receiver (TLR) signal; and (3) loop-earth-receiver (LER) signal. While the TER response has been extensively addressed in the literature, we modeled the more complex case of TLR and LER system responses. We first calculated the mutual inductance between the transmitter and the surface loop, taking the loop impedance (resistance and inductance) into account, and then we calculated the signal in the receiver by calculating the mutual inductance between the loop and the receiver. To calculate the LER response, we divided the surface loop into a series of transmitting dipoles, with the "transmitting" current in the dipoles obtained by analyzing the mutual induction between the transmitter and loop. The numerical experiments show that the free-space wire-loop response (TLR system) displays the standard exponential decay for a conductor. The loop resistance and inductance define the decay constant and initial signal value. While the TER system largely influences the receiver signal at early time channels for conductive earth, the LER system influences very late time channels. The more resistive the test ground is, the longer the time window is for the free-space wire-loop signal to dominate. For the survey over a test loop on conductive ground in the Dominican Republic, using the Fugro Heli-GEOTEM system, the predicted response compares well to the collected data.