The determination of the depth of magnetic anomaly sources

Magnetic prospecting is used mainly for horizontal mapping, whereas the determination of the depth of features receives only limited attention. The inverse problem has an infinity of solutions and it is not possible to define precisely either the exact shape or the exact depth of a given feature. On the one hand, the depth of the source of an anomaly is limited and minimal information about contrast, horizontal location and depth of the centre can be extracted from the data. On the other hand, comparison between results obtained from different methods or by the simultaneous interpretation of their results necessitates in the first place an evaluation of the correspondence between the depth of the sources indicated by the different methods. Details of a series of four techniques that deliver information about the depth or the range of depths of magnetic anomaly sources are presented here. The search for the equivalent dipole (i.e. a spherical body) is of very limited interest owing to the difference that exists between the actual archaeological features and a sphere. In contrast, Euler deconvolution provides interesting results when using a structural index approaching 2. The depth of the centre of magnetization and the total magnetic moment also can be determined. The latter allows an estimation of the product of the volume and the susceptibility contrast (the possible presence of non-induced magnetizations being considered). The downward continuation of the equivalent stratum susceptibility needs more empirical tests to define divergence criteria, but it delivers another estimation of the susceptibility changes and information about its depth variation.