Exploration Geophysics - Volume 8, Issue 4, 1977

The necessary mathematics for describing the potential field has been developed for a considerable time. Recent papers in gravity interpretation continue to emphasise the mathematics of the potential field rather than the nature of its causative geology.

This paper argues the case for pragmatic or ingenuous interpretation. While many ingenious methods have a role to play in gravity interpretation, the inherent ability of the human interpreter should not be overlooked. An illustration of the argument is given for some Murray Basin gravity data.

The collection, processing and modelling of geophysical data are three important phases in obtaining and interpreting geophysical information. They form part of a feed-back loop in which lessons learned during the later phases are used to modify procedures used in the earlier phases.

Models of subsurface structure derived from gravity data are inherently non-unique. The problem of non-uniqueness can be partly overcome by the rigorous application of all known physical constraints and by the introduction of some artificial constraints. The use of these constraints is illustrated by models along three profiles across the Papuan Ultramafic Belt. The resulting models tend to be uncomplicated thus avoiding overinterpretation and emphasizing regional features.

A non-linear optimisation procedure has been applied to the interpretation of two-dimensional gravity anomalies. In comparison with linear methods, the non-linear approach is more difficult to control but can give more realistic solutions. Successful application of the non-linear approach depends upon the selection of a reasonable starting model from rules of thumb. The model is then changed iteratively so that the difference between the calculated and observed values is minimised. The method of changing the model is dependent upon the optimisation procedure used. Three basic procedures are available: gradient descent methods, the simplex method and the Monte Carlo method. In the authors’ experience, the simplex method is a relatively efficient optimisation procedure in this application.

As an example of the application of the non-linear method an interpretation of the gravity anomaly over the Wentworth Trough (western New South Wales) is presented in terms of quadrilateral models of fixed density contrast. A range of possible models is presented, all of which fit the observed data equally well.

In a paper read at the 25th International Geological Congress, Sydney, 1976 Stanley described a simplified method for interpreting gravity anomalies due to contact and dyke-like structures. The method involved the assumptions of considerable depth extent relative to thickness of overburden, infinite strike length, and in the case of a dyke, a depth of burial much greater than the width. In real situations these assumptions may not always be justified. We now evaluate the limits to which the approximations may be extended before unsatisfactory errors are introduced to the interpreted structural parameters.

An ideal two-dimensional mass distribution is in fact a three-dimensional distribution which has an infinitely long dimension and does not change its configuration or density along this dimension. The condition that the long dimension be horizontal and the sphericity of the earth do not exclude the assumption of two dimensionality provided an end correction is applied in appropriate cases.

HALF-WIDTH and HALF-LENGTH of an anomaly associated with a line source of finite length are defined respectively as the depth and length estimators. Errors in these estimators are discussed and a method proposed for the determination of true depth and length. The method facilitates the making of an end correction.

Vast areas of Australia’s sedimentary basins have not been properly explored for oil and gas. The areas are generally remote from-population centres so that exploration, particularly seismic, is expensive. In these circumstnaces gravity exploration if properly applied has a great deal to offer. It is therefore appropriate to attempt to disseminate an understanding of gravity interpretation to encourage its proper application.

The basic theme of this workshop manual is the nature of gravity anomalies amenable to interpretation and their causative geology in a sedimentary basin environment. An interpretation procedure that is known to work well and an example of its application in the Perth basin are described to complete this work.