A Method to Constrain the Configuration of the Subsurface Structure in 3-D Gravity Inversion

Abstract

A three-dimensional inversion technique is developed to investigate the structure of the oceanic crust, using high quality offshore bathymetry, gravity and seismic data. The gravity signatures associated with variations in the thickness of the oceanic crust are isolated from the observed free-air anomaly by subtracting the gravitational effects of seafloor topography and the upper mantle thermal structure, downward continued to the mean depth of the crust/mantle interface and converted onto the relief on that surface. The thickness of the oceanic crust is then calculated by subtracting sea water depth from the depth of the gravity-inferred crust/mantle interface. Seismic refraction data was introduced directly as a constraint in the construction of the initial model for the configuration of the crust/mantle interface and the iterative process of the 3- D joint inversion to reduce the ambiguity in gravityinterpretation. This technique can be easily applied to the off-shore areas to interpret bathymetry, gravity and seismic data that have been routinely collected for the purpose of geophysical exploration. Compared to the unconstrained gravity inversion, this technique can predict a 3-D crustal model that fits better both gravity and seismic observation data of the study area. Introduction The non-uniqueness is an inherent problem in gravity inversion: the observed gravity field cannot be inverted to obtain a unique subsurface structure, unless some additional restrictions are imposed'. The ambiguity in gravity interpretation may he reduced by placing bounds on the density contrast and depth of the subsurface structure, which can be determined by applying certain physical principles. In the present study we propose a method to use seismic data to constrain the depth and configuration of a subsurface density discontinuity in a 3-D gravity modeling. The results of the seismically constrained gravity interpretation are in good agreement with seismic results, and the misfit of the predicted gravity field to the observed does not increase. Methodology The method is simple in principle: in a study area where both seismic and gravity data are available, we use seismic data to construct a framework for gravity modeling, and use gravity inversion technique to resolve the details of the configuration and depth of the subsurface density discontinuity. The implementation of the method may include the following steps. In the initial model for a subsurface interface, those points (nodes) that follow the projections of the seismic profiles are given the depths determined in seismic refraction (or reflection) surveys. Other points on the interface may be chosen at a constant depth (e.g., the average depth of the interface in the area, or the mean value of !he seismic depths). Doing so may introduce discontinuity, or the artificial impulse-shaped variations, to the configuration of the interface. To solve this problem we further CU1several narrow bands on that interface, These narrow hands are centered by the projections of the seismic profiles.