A simple time-domain EM modelling system

It is generally recognised that the broad-band nature of time-domain electromagnetic systems offers definite advantages in comparison with frequency-domain systems. A balanced view is given by Parasnis (1979), who also indicates the problems involved in the analysis. The high price of available time-domain field instruments has not contributed towards making it a widely used method and only a few coil configurations have been investigated in depth with an eye to interpretation procedures. Both the coil configurations and the field procedures are determined by the manufacturer of the particular equipment in use, who will also have provided his own method of data-processing and interpretation. The latter is of course determined by the equipment's specifications of pulse-shape, time-windows, sampling rates and frequency range. In this regard there are no universally accepted standards. A short case-history by Staples (1984) shows the approach used in one particular combination of instrument and problem. The geological structure could be approximated by a dipping tabular conductor below a conducting overburden. Both the geology and the model were very simple and yet direct inversion was not possible. Mathematical modelling in this case led to a structure which corresponded to the known ore body. In situations of greater complexity the mathematical modelling is not easy. Mathematical modelling can be supplemented or replaced by physical modelling. This requires a straightforward modelling set-up which is flexible enough to use any desired coil configuration as well as spanning a wide range of other variables. We describe a system which uses existing laboratory facilities and involves no exotic know-how or specialised tools.