INVERSION OF AEROMAGNETIC DATA USING DIGITAL TERRAIN MODELS

Although draped magnetic surveys contain more information about the ma gnetization of the rocks near the surface of the earth than surveys at constant elevation, allowance for the effects of the terrain is criti cal for their correct interpretation. A new method for calculating the magnetic effect of the topography from a digital terrain model by int egrating analytically in the vertical direction and then numerically i n the horizontal plane is presented. This method lends itself to the c alculation of anomalies when the magnetization of the rocks varies wit h position and thus is well suited to the inversion of draped aeromagn etic surveys to obtain the apparent magnetization of the surficial roc ks. This inversion is achieved by repeated use of an approximate inver se function in the form of a two-dimensional (2-D) filter that is appl ied to gridded data. An example, using draped magnetic data collected over White Island, an active volcanic island of high relief, shows tha t although the anomaly pattern is dominated by topographic effects, th e distribution of near-surface magnetic bodies can be determined by a joint inversion of the data and the topography. One of the highly magn etized areas of White Island is interestingly in the vicinity of the a ctive crater, with another near the inner wall of the caldera where th ere are numerous fumaroles. It may be expected that the higher tempera tures in these areas would reduce the magnetization. However, it appea rs that an explanation for the higher magnetization can be found in th e stability field of the mineral magnetite.