Modelling the time-domain response of AEM systems

The past decade has seen considerable growth in methods to compute the frequency-domain electromagnetic response of a multiplicity of 2.5D and 3D model types. Converting the output of such programs to accurate time-domain responses for real airborne electromagnetic (AEM) systems is not a trivial process. Initially, the geoelectric model and the AEM system being modelled will determine the required range and density of frequency responses. A Hankel digital filter technique is used to convert the frequency-domain responses to step-function time domain responses out to several pulse lengths. Accuracy is enhanced by selecting frequencies and times to correspond to filter abscissae. The result is splined and folded into a single pulse length, which is re-splined before convolution with either the transmitter current or its time-derivative. Since the first few receiver channels for real AEM time-domain systems are often within the on-time of the transmitter waveform, the zero time response must be computed carefully. Large errors may result if the transmitter tilt is not modelled accurately. Improved accuracy can be achieved through the use of the receiver waveform rather than the transmitter waveform if the former is sampled sufficiently.