Abstract: Analysis of Integrated Satellite, Land, Marine and Airborne Gravity Surveys using Euler Deconvolution and Horizontal Derivative Processes

William G Dickson - Union Texas Petroleum, and J. Derek Fairhead - Getech

Integration of satellite, land, marine and airborne gravity surveys from the entire South Atlantic region has been used to study and compare tectonic features on both African and South American continental margins. A seamless gravity grid, comprising hundreds of individual surveys was carefully processed and merged onto a common datum, allowing full utilization of Euler deconvolution and horizontal derivative (HD) techniques on a regional/tectonic scale. Observations from West Africa include: better definition of transverse faults and lineaments such as the Mayumba Spur; improved understanding of the limit of continental crust; better imaging of the roots of the onshore Congolide foldbelt; and an inference that the Congo Canyon is erosional rather than tectonic. While most anomalies follow published shapes and locations, our more extensive and uniform coverage provides much more detail. As our working area expanded, we continued to refer to the Cameroon to Angola offshore since other areas generally had less-well defined anomalies, possibly due to poorer data coverage or bathymetry.

A first HD map in West Africa showed two distinct domains: one full of high- amplitude lineaments interpreted as continental and another, quiet zone, of oceanic crust. The dividing line is sharp from the Walvis Ridge to the Niger Delta. One domain is hot (with high amplitudes), dissected and full of linear features corresponding to the continental syn-rift fabric. The seaward domain is cool and undisturbed, especially in the magnetically quiet area of Cretaceous crust (R D Muellar et al, 1993). Lineaments in the continental crust reveal a syn-rift pattern of half-graben development. Offsets correlate with known oceanic transforms such as the Fang and N?Komi Fractures. While these familiar anomalies were confirmed, the maps show the Mayumba Spur trend is east-west, probably with pre-rift origins, rather than the commonly published ne-sw syn-rift trend.

Euler Deconvolution results from the Bouguer gravity (EDB) were plotted in depth slices of 0-4; 4-8; 8-10; 10-15 and >15km to source depth, using an SI of 0.5. These show the Congo (Zaire) River is not basement controlled offshore where it trends nw-se rather than its ne-sw trend onshore. EDB shows computed source depths of the river on only the 3 shallowest slices (0-10km) while the crust is at least 20km thick. The explanation is that mislocated bathymetry causes an erroneous Bouguer correction, affecting all subsequent calculations. For the same reason, seamounts including the Cameroon Line volcanoes show spectacular artifacts on the HD maps. EDB also behaves intuitively showing deepening to at least 20km from the flanks into the center of an onshore anomaly associated with the Congolide Fold Belt.

From a confident interpretation on the African side, it was natural to extend observations to the South American margin. Since the two continents fit at breakup, a clear definition of one passive margin dictates the shape of the other. The HD fit of the Brazilian continental shelf extends seaward the previously published limit of the syn-rift Campos and Espirito Santo Basins, enlarging the area of inferred lacustrine source potential. This limit is not clearly defined just from the HD map on the Brazilian margin, partly due to masking by Tertiary to Recent sedimentation. Similarly, the Amazon Cone shows the signature of the drift age sequence clearly on the Isostatic Residual but the thickness of these younger rocks diminishes the HD response of the underlying syn-rift section, requiring both tools to image syn-rift and post-rift sections.

These observations derive from improved shelfal gravity coverage where the resolution limit of current satellite-derived data is for features greater than 15 kilometers in size. Poor bathymetry, with spatial resolution locally only 20km, degrades results. Future work will include gravity inversion at the water-sediment interface to improve bathymetry, continued research to improve satellite gravity resolution, and means to separate surface and sub-surface anomalies.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil