Physics of airborne EM systems

We investigate the basic physics of electromagnetic exploration with the aid of a simple model of a thin tabular target in a conductive host. The variables are host conductivity, system dimensions, and frequency of operation. We measure the system effectiveness by using the ratio of the target response to the host response. In terms of this quantity we find that there is an optimum window, in frequency and in time, for target detection which is defined by the target parameters. Within this window, the amplitude of the target signal is controlled by the host conductivity and by the system dimensions. For all system configurations considered, the ratio of target-to-host response is maximised for the smallest separation of transmitter and receiver. For the simple model used here a short separation transient coaxial system flown at a height of 30 m over a 50 m deep target in a 10 μS/m half-space has a detection ratio that is about five times that of the conventional INPUT configuration which flies at a higher altitude.