Modelling 3D subsurface structures using gravity and enhanced gravity gradient method

Keywords: salt structure, gravity inversion, density model

A gravity survey is a good choice to investigate various subsurface structures, including salt domes. We performed numerous gravity survey simulations based on synthetic and analogue geological models, but also on real survey data. We started with forward gravity/density modelling of various shapes of salt diapirs (intrusions), using not only usually measured gravity data, but also gravity gradients. The resulting data mixed with certain levels of noise was then used for the gravity inversion process. We found some limits of sensitivity to selected starting models and extreme significance of the realistic definition of starting models for geologically plausible inversion results.

We applied this experience to real data – we digitized published gravity maps with negative anomalies related to salt structures. Contrary to the publication, we developed a more complex 3D model of the principal salt structure.

Currently, we follow analogue modelling of a simulated salt intrusion process in a laboratory and perform gravity modelling according to the digitized shape of salt (special silicon) intruding homogenous sedimentary (sand) formations.

Besides other methods, we apply 3D deterministic inversion coupled with the estimation of the starting model parameters based on the gravity gradients analysis. These parameters are mainly the dip, depth, and lateral extent. The problem is defined on a discrete rectangular mesh with the possibility of localized refinement to increase or decrease the resolution in certain parts of the model. The results provide a detailed density model of the diapir allowing the estimation of the spatial extent of the salt sheet. The usage of gravity gradients leads to the construction of more reliable starting models of near-surface salt structures for gravity inversion. Our aim is also to achieve a suitable geometrical correlation with magnetotellurics (MT), as such a twin gravity-MT response for various types of salt structures may encourage the application of such twin geophysical methodology.