The generalized gravity anomaly: Endoscopic microgravity

Abstract

Various underground 3-D gravity surveys have necessitated a generalization of the usual gravity corrections and of the Bouguer anomaly. The method presented here compares raw, time‐dependent gravity measurements, to a model’s total theoretical field, including known fields: moon, sun, 1967 Reference Ellipsoid, oceans; partially known fields: due to a single digital terrain model of known geometry but of unknown densities; and unknown fields due to underground structures of unknown shapes and of unknown densities. For a single‐density model, the corresponding first‐degree residual is close in concept to the Bouguer anomaly. To best determine underground structure, generalized inversion then leads to determination of the one or several densities and of one or several “regional” parameters, which minimize residuals. The suggested method is mainly advantageous in special types of gravity surveys, such as rugged terrain, or in the case of underground surveys, where conventional corrections, with a preset terrain density can possibly lead to substantial errors. Two field examples are developed (1) the Cheops pyramid survey, where the processing of gravity measurements inside, above, and around the pyramid led to an evaluation of the structure’s overall density and of density changes in the structure; and (2) the Coche hydroelectric tunnel in the Alps, where the method leads to a 3-D model explaining the very strong gravity anomalies observed in the tunnel and on the mountain above it.