Exploration-risk parameters evaluated through the geologic integration of gravity and magnetic data during the prospect definition stage include:
At the prospect definition stage, a company has completed the fundamental basin reconnaissance work, has selected a prospect (or prospects), and is doing detailed evaluation of the prospect before deciding whether to commit drilling money. Gravity and magnetic methods can be applied again at this point to contribute to the evaluation of various risk parameters.
The most common application of gravity and magnetics at this stage of exploration is to better define geologic structure in areas of poor seismic data quality. For autochthonous sedimentary structures, magnetic data allow the geometry and depth of underlying basement structures to be determined. This is useful in structurally complex areas involving overthrusting and folding, such as that occurring in the Carpathian Mountains of Romania and the Dinaride Mountains of Yugoslavia. Potential-field methods often permit deep structure to be determined where the deep seismic image is poor because of the often high acoustic impedances and steep dips of the contorted structure within the overthrust units.
Similarly, where intrasedimentary igneous material occurs, as with the basalt flows of the Chaco-Parana Basin of Argentina, potential-field methods can be used to peer beneath the seismically reflective barrier presented by the basalt. Gravity and magnetic data also can be used to resolve seismic no-data zones caused by igneous intrusions or fossil volcanoes, such as those offshore the Netherlands in the North Sea (see also the St. George Basin study by Chapin et al. in this volume).