Interpretation of Borehole Gravity in the Presence of Anomalous Vertical Gradients

The first borehole gravity surveys to be carried out in Brazil were done as part of a joint program between the Federal University of Pará, the USGS and Petrobras (CENPES / DEPEX / DENEST) in the state of Sergipe, in parts of the Sergipe-Alagoas Basin, during October and November of 1990 (Gonçalves, 1991). A total of three producing wells were surveyed to investigate the formation density properties of a producing basal conglomerate zone of Cretaceous age overlying a fractured pre-Cambrian Basement. The basement is shallow, ranging from about 500 to 800 meters below sea level. The interpretation of the borehole data is complicated by the fact that the basement complex, and an overlying evaporite sequence of Cretaceous age, is highly folded and faulted. As the borehole gravimeter actually penetrated both the evaporites and the basement, gravitational effects caused by the differential relief of the basement terrain and evaporite layers contributed a significant amount to the downhole gravity signal. The irregular surfaces of the both the basement and evaporite sequences were modelled using a tesellation of triangular facets and a L1 norm on the borehole gravity data. Computation of the gravitational field of each facet employed an algorithm for a polyhedral body (Pohanka, 1988). The tesselation was automatically generated (Watson, 1982). The resultant three dimensional model was used to compute the borehole Bouguer field, and this was subtracted from the observed borehole data, to yield the anomalous Bouguer field which was used to correct densities estimates.