Estimating optimum density for regional Bouguer reduction by morphological correlation of gravity and bathymetric maps: examples from offshore south-eastern Brazil

Free-air gravity and bathymetric maps are routinely used for large-scale geological studies but rugged topography and varying shallow-depth conditions can complicate their usefulness in geological interpretations. It is crucial to reduce the gravity anomaly map to a common datum for regional correlation studies so that Bouguer anomalies can be more confidently interpreted to reveal subsurface bodies of exploration interest. However, Bouguer anomaly computation requires a precise knowledge of the shallow-depth density variation, while direct measurement of the near-surface density at every observation station is not feasible. A simple, objective indirect method for estimating the optimum density for Bouguer reduction is desirable. Using data from the Hotspur seamount and the Santos Basin offshore south-eastern Brazil, it is shown that correlating bathymetry and computed Bouguer anomaly for a range of densities provides a simple means for selecting the optimal density for Bouguer reduction. Unlike previous approaches, the use of angular differences between the gradients of bathymetric and Bouguer gravity maps is proposed to select the optimum density for reduction. Their standard deviations clearly define a minimum when the maps are least correlated, providing an objective metric for determining the optimum density for large-scale regional Bouguer reduction. The results improve the delineation of major features such as the Santos hinge line and the landward limit of the salt province in the Santos Basin.