Abstract The exploration industry is moving broadly in unison toward the use of increasingly more rigorous, quantitative, and consistent techniques for assessing exploration risk, a movement facilitated in part by the equally widespread and parallel effort to compile and maintain large, globally comprehensive, historical/ technical exploration databases. Probabilistic models of a prospect's chance of succeeding (or failing) are constructed routinely now, in part on the basis of various geologic and geophysical data interpretation inputs to the exploration-risk model. As this note will discuss, the geologic integration of gravity and magnetic data has a definite impact on the evaluation of certain of the exploration-risk model parameters. The geologic integration of gravity and magnetic data can be used to reduce risk at two key stages of the exploration process. The first stage, often referred to as basin reconnaissance, is the role automatically relegated by most explorers to gravity and magnetic methods. Contrary to this widespread notion, however, gravity and magnetic methods are equally effective for reducing risk at the more local scale typified by the prospect itself. In cases in which the application of gravity and magnetic analysis negatively impacts a prospect's viability, valuable exploration resources can be redirected at finding different and, it is hoped, more economic prospects. This short philosophical note will summarize some of the chief ways in which risk is reduced by an exploration philosophy embracing the use of gravity and magnetic methods during the two main stages of the exploration process.

Abstract 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).