Exploration Geophysics - Volume 8, Issue 3, 1977

This paper describes one simple procedure which can be applied to the interpretation of gravity or magnetic profiles measured perpendicular to the strike of contact or dykelike structures or to structures which can be considered as a combination of both. The method is very easy and rapid to apply, is unambiguous within the assumed contact or dyke-like model, is applicable to airborne or ground level data and does not necessarily require computing facilities but may be automated if desired. As the interpretation requires only a relatively short length of traverse (usually less than 3 depth units) in close proximity to the structure, the procedure results in a very high rejection of neighbouring disturbances. In the gravity case the method has the particular advantage of freedom from regional, topographic and latitude corrections and in most applications only a minimum of elevation surveying is necessary.

Two methods for the construction of apparent chargeability functions over horizontally layered media are presented. Both combine the theory of Seigel (1959) with the more recent linear filter applications to geoelectric soundings originally proposed by Ghosh (1971). An example is given to illustrate the usefulness of these methods in the interpretation of field data.

A Bouguer anomaly map of India prepared from some 30,000 point gravity values brings out a number of zones of gravity “highs” and “lows”. These are well correlated with the regional geology of the country. Significantly some of the “highs” or their slopes are associated with the well known mineralized districts of Aravallis in western India, the Hazari-bagh Plateau of eastern India, the Panna-Narmada region in Central India, and the greenstone associated mineralization in Karnataka in South India.

About one hundred and seventy line kilometres of integrated airborne geophysical surveys were conducted in India during 1967-1971. A comparison of the areas flown with the recently prepared Bouguer anomaly map shows that almost all the areas fall over regions of gravity “highs” or off their slopes on steep gradients. Apart from its tectonic significance this correlation bears a useful approach to mineral search.

A comparative study of gravity material with tectono-stratigraphy and mineralization suggests that gravity surveys can be used as a reconnaissance tool in seeking extensions of some litho-stratigraphic horizons of economic interest beneath covers.

This approach was utilised in selecting areas for an integrated airborne geophysical survey. Interestingly the surveys in the two selected regions have picked aeromagnetic as well as electromagnetic anomalies. A ground check coupled with earlier photogeologic mapping has revealed some zones of possible mineralization associated with the geophysical indications.

A regional magnetic (vertical force) anomaly map based on field observations at a spacing of about 30 kms by the Survey of India also brings out some major magnetic lineations which compare well in some cases with the regional gravity anomalies. However in some areas they depart substantially from gravity trends. In such areas the basement seems to be different magnetically while having more or less the same density. An interesting zone of an EIME orientation is found to occur from the India-Pakistan

SI ROTEM, a new portable multichannel instrument for transient electromagnetic ITEM) measurements, has been developed by CSIRO with the assistance of the Australian Mineral Industries Research Association and collaborating mining companies. The specifications of SIROTEM greatly exceed those of the earlier generation of TEM instruments. The incorporation of a microprocessor to control the operations, measurement and output functions of the instrument provides SIROTEM with great versatility, e.g. data can be transformed by the instrument into an apparent resistivity cross-section form. The instrument has been tested in eleven separate field locations, and results from two of these test sites are presented.