Scale model studies are often used to determine the distribution of secondary magnetic fields when the numerical approach is unreasonably difficult. Reduction in size (L) is compensated by an increase in conductivity (o) or frequency (f), or both. In most cases the results are expressed in a way whereby they are independent of the absolute dimensions of the model and the general modelling relation oufL^2 = constant, is valid. However in some applications such as transient electromagnetic (TEM) modelling it is necessary to determine the power level of the full-scale system. An expression relating the voltage levels of the model and the full-scale system is: Vm/V = LmT/LTm where the subscript m represents the model system. This expression must he used in conjunction with the general modelling relation in order to compensate for changes in the modelling parameters. It is possible to scale time, as well as dimensions and conductivity and, using the above relationships, many geological situations may be simulated with a single model. By varying either one of the parameters conductivity or dimensions while keeping the other constant, the same transient decay curve can be obtained by transposing the individual curves (plotted on a log-log scale) along the time and response axes. An example illustrates that by modelling field cases absolute quantities such as conductivity can be estimated. Modelling of the Woodlawn orebody gave bulk conductivities of 5 S/m and 20 S/m. The analysis of TEM data using early and late time responses must be done with care. The response typical of a late time may be similar to an early time response if the electrical or dimensional properties of the conductor are varied. More correctly one should analyse responses in terms of large and small values of the parameter oufL^2/T.
