FA2BOUG-A FORTRAN 90 code to compute Bouguer gravity anomalies from gridded free-air anomalies: Application to the Atlantic-Mediterranean transition zone

In this paper we present a computer program written in FORTRAN 90 specifically designed to determine the Bouguer anomaly from publicly available global gridded free-air anomaly and elevation database sets. FA2BOUG computes the complete Bouguer correction (i.e. Bullard A, B and C corrections) for both land and sea points in several spatial domains according to the distance between the topography and the calculation point. In each zone a different algorithm is used. In a distant zone we consider the harmonic spherical expansion of the potential of each right rectangular prism representing an elevation grid point. In an intermediate zone we compute the gravitational attraction produced by each prism using the analytic formula. Finally, an inner zone contribution is divided into two parts: a flat-topped prism with a height equal to the elevation of the calculation point, and four quadrants of a conic prism sloping continuously from each square of the inner zone to the calculation point. The program has been applied to the Atlantic-Mediterranean transition zone to obtain a complete Bouguer anomaly map of the area, integrating available onshore Bouguer anomaly with satellite-derived free-air anomaly data. Positive Bouguer anomalies are found in the Atlantic oceanic domain (240-300 mGal), central and eastern Alboran Basin (40-160 mGal) and SW Iberian Peninsula (>40 mGal). Major negative Bouguer anomalies are located beneath the west Alboran Basin (<-40 mGal), the Rif, the Rharb Basin and the Atlas Mountains (<-120 mGal). An isostatic residual anomaly map of the study area has been computed and compared with the crustal and lithospheric structure inferred from previous work.