Abstract

Gravity and magnetic data provide complementary information on sedimentary basins and basement/cover relations. Magnetic anomalies are usually dominated by magnetic sources in the basement and, to a lesser extent, by volcanics and associated intrusives in the sedimentary sequence. In contrast, gravity anomalies reflect changes within the sedimentary sequence as well as basement.

Where the 'basement' residual gravity anomaly can be extracted, the complementary nature of gravity and magnetic data can be used to recognize composite basement geophysical domains based on uniform magnetization/density characteristics. Comparison of the vertically integrated, 3D analytic signal with reduction to the pole or pseudo-gravity data helps to overcome some of the ambiguity.

Pre-processing of magnetic and gravity anomaly data is essential so that a direct comparison of the two anomalous fields is meaningful. Total magnetic intensity anomalies are related to the gravity gradient by Poissons Relation. Magnetic anomalies are differentiated one order further than gravity anomalies, therefore magnetic anomalies should be compared with the gravity gradient. It is important to try to center magnetic anomalies over the source either by reduction to the pole or analysis of integrated 3D analytic signal. It is also important that the two anomalous fields have similar wavelength contents, therefore, pre-filtering is usually required.

Separation or layer filtering allows the effects of shallow sources to be removed, therefore the magnetic and gravity signatures of deeper layers can be recognized.

There are a number of ways to make a direct comparison of reduced-to-the-pole magnetic data and gravity gradient data, including global correlation, local window correlation and coherency analysis. These techniques have been applied to the northern margin of the Canning Basin in NW Australia. This is a difficult area for potential field interpretation as the relationships between magnetic anomalies, gravity anomalies and basin structures are unclear. Gravity interpretation is complicated by a high degree of isostatic compensation as a result of crustal thinning. Pre-processing of the magnetic and gravity data was effective, resulting in a high degree of correlation of the two anomalous fields over much of the area, but also significant differences. The global correlation method of comparing the two anomalous fields gave the best results and improved recognition of subtle trends. The magnetic/gravity correlation data provided good resolution of the major fault systems bounding the Fitzroy Trough, the main depocentre. The data also show internal, east-west structures in the Fitzroy Trough, parallel to the major folds, of Jurassic age. A number of cross-cutting trends are interpreted as basement faults and some of these appear to be previously unrecognised.