Simultaneous estimation of total magnetization direction and 3-D spatial orientation

摘要

Magnetic interpretations are usually carried out either by assuming induced magnetization and estimating the model geometry, or by presuming a known source spatial orientation to estimate the total magnetization. We present a 3-D magnetic interpretation method that estimates simultaneously the total magnetization direction and the spatial orientation of the source. It is based on the approximation of the anomaly by the series derived from expanding the magnetic potential into multipoles and retaining source moments up to second order. The moments and linear combinations of moments appearing in the series are then inverted from the magnetic anomaly. The total magnetization is assumed constant in direction but not in magnitude. It is also presumed implicitly that the anomalous distribution of magnetization intensity has nonzero values in a finite-volume region, is far from the observation points, and presents three othogonal planes of symmetry intersecting at the center of the dipole moment. The method is essentially linear and requires no a priori explicit assumption of a fixed geometry for the sources. The method is particularly suited to interpret compact, isolated or disjoint, but spatially correlated sources. If the source satisfies all assumptions presumed by the method, it is possible to obtain accurate, stable estimates of the total dipole moment vector, the position of the center of dipole moment, and the directions of all three principal axes of symmetry. If the source is not far from the observation plane and/or if the total magnetization direction is not constant, it is still possible to obtain accurate and stable estimates of the direction of the mean total magnetization and the projection, on the observation plane, of the center of dipole moment. The method is applied to magnetic data from the Gulf of Guinea Seamount. The estimated magnetic palaeopole is at 50°48′S and 74°54′E which is in good agreement with estimates published by other authors.