The magnetic gradient tensor: Its properties and uses in source characterization

Abstract

Airborne magnetic surveys have improved dramatically over the past three decades with advances in both data acquisition and image processing techniques. Magnetic surveys form an integral part of exploration programs and are now routinely undertaken before geological mapping programs. These advances have been made despite treating the magnetic field as a scalar, wherein various processing procedures that assume a potential field are compromised. If the vector information could be retrieved, either by direct measurement or by mathematical manipulation, magnetic surveys could be improved even further. For instance, the total magnetic intensity (TMI) could be corrected so it represents a true potential field. Vector surveys, where the direct measurement of vector components has been attempted, have met with mixed success. The accuracy of direct measurement of the field vector is largely governed by orientation errors, which for airborne platforms are so large that the theoretical derivation of the components from the TMI is actually preferable. For this reason, and others listed below, it is desirable to measure the field gradient(s) rather than the field vector. We discussed the calculation of vector components and magnetic moments from the TMI in previous articles (Schmidt and Clark, 1997; Schmidt and Clark, 1998) which compare theoretical derivations with laboratory measurements and demonstrate the validity of the approach. Phillips (2005) has since taken these ideas further by using a moving window to generalize the technique to tackle larger areas, and also to search for sources with specified directions of magnetization. This article is largely drawn from, and updates, an earlier contribution of ours (Schmidt and Clark, 2000). Gradient measurements are relatively insensitive to orientation. This is because gradients arise largely from anomalous sources, and the background gradient is low. This contrasts with the field vector, which is dominated by the background field, i.e., arising from the earth's core. Gradient measurements are therefore most appropriate for airborne applications. Another advantage is they obviate the need for base stations and corrections for diurnal variations. They also greatly reduce the need for regional corrections, which are required by TMI surveys because of deeper crustal fields that are normally not of exploration interest, or the normal (quasi-) latitudinal intensity variation of the global field. Gradient measurements also provide valuable additional information, compared to conventional total field measurements, when the field is undersampled. Undersampling is common perpendicular to flight lines in airborne surveys, is usual in ground surveys, and always applies in downhole surveys. Conditions under which calculation is preferable to measurement of vectors and gradient tensors have yet to be characterized by modeling and case studies. Synergistic interpretation of calculated vectors and measured gradients may allow significantly more information to be extracted from airborne su...

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