Gravity Data Imaging of Subsurface Structures


The gravity method involves measuring the gravitational attraction exerted by the Earth at a measurement station on the surface. The strength of the gravitational field is directly proportional to the mass and therefore to the density of subsurface materials, so that the gravity method responds directly to a mass excess or deficit. Anomalies in the Earth's gravitational field result from lateral variations in the density of the materials and the distance to these bodies from the measuring equipment. Measured gravity anomalies may be order of 10-8 smaller compared to mean gravity reference, typically 9.81 m/s2.

The aim of this project is to provide a reliable and fast imaging tool to improve the understanding of the subsurface geology through the determination of the sediment thick, the basement depth and the type of material.

Starting from observations of gravity anomalies, imaging and characterization are made possible by reconstructing the mass density distribution of the investigated medium. This basic limitation is brought about from the unavoidable fact that the governing physics and the direct problem formulation lead to the solution of a difficult ill-posed inverse problem. While the direct problem for finding the gravity effects of a given mass density distribution, which is the induced scalar gravitational field, is perfectly unique, the inversion solution is notoriously non-unique since the large number of degrees of freedom exceeds the number of measurements. As a matter of fact, many different geologic configurations can reproduce the same gravity measurements, although, many of them are not of any geophysical interest. To mitigate this difficulty, the role of the interpreter becomes of paramount importance to provide reliable information to shrink the null space of the inverse problem.

These anomalies can then be interpreted by a variety of analytical and computational methods to determine empirically depth, geometry and density that cause the gravity field variations. The inverse problem based on a finite-element Poisson solver as the numerical engine for the solution of the direct and the adjoint problem constitutes an important step forward for the subsurface characterization from zero to 10-50 km.

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