Satellite Gravity Gradients Complementing Seismology for Imaging of the Lithosphere

Abstract:

Satellite gravity gradients as for example derived from the recent GOCE satellite mission can be used to improve modeling of the Earth’s lithosphere and thereby contribute to a better understanding of the Earth’s dynamic processes. In general, gravity gradient data are sensitive to shallower structures than the gravity field itself and provide information about the variations in both the horizontal and vertical plane.

Validation of satellite data in different orbit heights show that the gradients in different heights have a significantly different sensitivity, which can be exploited to construct the most reasonable lithospheric setting.

To explore the benefit by using gravity gradients in addition to conventional gravity data, a case example from the well-explored and understood North-East Atlantic Margin will be shown. Here, a 3D model of the lithosphere preexisted that incorporates a wealth of geophysical data sets, e.g. seismics, magnetics and borehole information. The model is initially optimized for near-surface gravity data. However, in the NE Atlantic the gravity field is affected by a regional trend, which is reflected as well in the geoid, and associated to sub-lithospheric density domains.

Using this model for sensitivity analysis shows that the satellite gravity gradients are little affected by the sub-lithospheric field, but are especially sensitive to the density contrasts from the lower crust to 100 km depth.

Another important observation is that modeling of the gravity gradients requires depth-dependent crustal densities and temperature dependent upper mantle densities. A too simplified use of average densities leads to clear misfit to the observed data.

Satellite gravity gradients are as well sensitive to compositional changes in the upper mantle, but for to decipher the thermal structure and composition integration with information from seismic tomography is needed. In summary, the sensitivity analysis in the NE Atlantic region shows, that satellite gravity gradients are a valuable addition to image the lithosphere in 3D and consequently to distinguish the lithospheric and sub-lithospheric components in the gravity field.