Inversion of aeromagnetic data using digital terrain models

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Although draped magnetic surveys contain more information about the magnetization of the rocks near the surface of the earth than surveys at constant elevation, allowance for the effects of the terrain is critical for their correct interpretation. A new method for calculating the magnetic effect of the topography from a digital terrain model by integrating analytically in the vertical direction and then numerically in the horizontal plane is presented. This method lends itself to the calculation of anomalies when the magnetization of the rocks varies with position and thus is well suited to the inversion of draped aeromagnetic surveys to obtain the apparent magnetization of the surficial rocks. This inversion is achieved by repeated use of an approximate inverse function in the form of a two‐dimensional (2-D) filter that is applied to gridded data. An example, using draped magnetic data collected over White Island, an active volcanic island of high relief, shows that although the anomaly pattern is dominated by topographic effects, the distribution of near‐surface magnetic bodies can be determined by a joint inversion of the data and the topography. One of the highly magnetized areas of White Island is interestingly in the vicinity of the active crater, with another near the inner wall of the caldera where there are numerous fumaroles. It may be expected that the higher temperatures in these areas would reduce the magnetization. However, it appears that an explanation for the higher magnetization can be found in the stability field of the mineral magnetite.