Effect of transmitter current waveform on airborne TEM response

Abstract

This paper investigates the effect of different transmitter waveforms on the airborne EM system response. For a wire loop target with an exponential decay, the voltage response is derived for waveforms of half-sine, square, triangular and trapezoidal shape. It is found that, for pulses of the same width and amplitude, the maximum response is generated by a square pulse. The optimum pulse width for a half-sine pulse is 2.5 times the target time constant. The square pulse does not have an optimum width; the greater the pulse width, the greater the response. A square pulse with a width of twice the target time constant would generate 85% of the response as an 'infinite width' pulse. For targets of large time constants, the ratio of the target responses due to two pulses of different shape is approximately equal to the ratio of the areas that these pulses enclose with the time axis. In areas of conductive overburden, the shape of the transmitter pulse appears to have little effect on the target-to-overburden response ratio (TOR). A larger pulse width, however, can significantly enhance the differentiation of target signal from the overburden response. As a practical consideration, a full-duty-cycle square waveform with an exponential rise is also studied. It is found that, for a 25 Hz system, the time constant of the exponential rise must be sufficiently small for the square waveform to be effective in detecting targets in areas of conductive overburden. In such conductive areas, it is also demonstrated that the 25 Hz, 4 ms half-sine waveform is significantly better for the detection of targets than the 75 Hz, 1 ms half-sine waveform.