Experiences with a detailed estimation of the mass density contrasts and of the regional gravity field using geometrical information from seismograms

Abstract

In this paper a new method of geological stripping (Strykowski, 1997) is illustrated on real data. Complex information about the subsurface (seismograms, borehole data, surface gravity signal) is used. The setup includes many geological layers with varying mass density contrasts in different depths. The mathematical formulation of the problem makes use of all available information in a hierarchic way. The method is based on general considerations about the nature of ambiguity of the solution to the inverse gravimetric problem. The main idea is to avoid that the obtained model of the subsurface reflects implicitly mathematical assumptions. Such assumptions can ensure a unique model, but not necessarily a good approximation to the true physical conditions in the subsurface. Instead, the ambiguity of the modelling problem is exposed in the mathematical formulation. In order to obtain a particular earth's model, the interpreter has to make a conscious choice of how to weight different types of available information relative to each other. The test area is Jutland peninsula (Denmark). The size of the area is 256 km × 206 km. The geology of the area is known to a depth of 10000 m. The model of the subsurface has a horizontal resolution of 2 km × 2 km and a vertical resolution of 500 m. A linear system of equations is set up. The geometrical information about depths to various interfaces in the subsurface (interpolated from seismograms and boreholes) has been transformed to the equivalent surface gravity functions. The unknowns are the absolute mass density values associated with different geological layers in different depths and parameters describing the surface gravity signal generated from large depths (the regional gravity field).