Gravity disturbances in regions of negative heights: A reference quasi-ellipsoid approach

Compilation of the bathymetrically and topographically corrected gravity disturbance, the so called BT disturbance, for the purpose of gravity interpretation/inversion, is investigated from the numerical point of view, with special emphasis on regions of negative heights. In regions of negative ellipsoidal (geodetic) heights, such as the Dead Sea region onshore or offshore areas of negative geoidal heights, two issues complicate the compilation and subsequently the inversion of the BT disturbance. The first is associated with the evaluation of normal gravity below the surface of the reference ellipsoid (RE). The latter is tied to the legitimacy of the harmonic continuation of the BT disturbance in these regions. These two issues are proposed to be resolved by the so called reference quasi-ellipsoid (RQE) approach. New bathymetric and topographic corrections are derived based on the RQE and the inverse problem is formulated based on the RQE. The RQE approach enables the computation of normal gravity by means of the international gravity formula, and makes the harmonic continuation in the regions of negative heights of gravity stations legitimate. The gravimetric inversion is then transformed from the RQE approach back to the RE approach, following the now legitimate harmonic upward continuation of the gravity data to stations on or above the RE. Stripping, the removal of an effect of a known density contrast, is considered in the context of the RQE approach. A numerical case study is presented for the RQE approach in a region of NW Canada.