Impact of salt diapir geometry and caprock composition on gravity survey results

With respect to difficulties seismic and magnetotellurics may have in accurate definition of exact salt diapir geometry, we performed various gravity simulations and calculations to investigate such exploration targets. Our motivation included the fact that not much has been done on caprock impact on the gravity signal. A very significant favorable condition is the high contrast of rock density between salt and basin sedimentary formations, especially carbonates. However, this can be more complicate if caprock formation is present. Therefore we defined approximate bulk density of typical caprock formation based on its usual composition, resulting in density values 2.45–2.70 g/cm3.

We modelled by forward and inverse procedures the gravity signal Gz of salt diapirs with caprock of variable thickness to demonstrate to which extent the salt diapir negative gravity anomaly may be reduced by the impact of caprock formation. In the tested cases the gravity anomaly was reduced by more than 30% depending on respective caprock composition and thickness. Significant contribution to the delineation of salt diapirs themselves, as well as diapirs hidden under caprock, came from the application of horizontal gravity gradients Gzx. We showed the difference of Gzx spikes indicating the edges of density contacts according to the type of gravity survey – land or airborne. It was also proved by calculating the gravity effect of laboratory analogue models of salt deformation and extrusion.

We demonstrated that gravity is still a valuable and relatively cheap tool for in-detail investigations of the salt structures within exploration projects.