The distance to sources of magnetic field anomalies of a known structural index can be determined by using ratios of the analytic signal amplitudes (As) of different orders, and this can be performed in several different ways. Local minima of the distance correspond to the source depth. If an incorrect structural index has been used, then the different methods will yield different depths. Hence, a comparison of the results obtained from the different methods can help us to differentiate between valid and invalid source depths. These methods are computationally straightforward, and in some of the methods, the orders of the As that are used can be chosen based on the noise level of the data. Some approaches that do not require the a priori specification of the structural index of the source are introduced, including methods that use data from two different altitudes (obtained by vertical continuation). We have applied the methods to aeromagnetic data sets from the Giyani and Kuruman regions of South Africa with plausible results.

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