New Global Earth Model and Seafloor Gravity Analysis for Offshore Exploration

Abstract

ABSTRACT An improved seafloor gravity analysis for offshore exploration is developed using a new global Earth model (NEWGEM) based on exact solutions of the ellipsoidal layer model of the rotating Earth with local and global corrections. The new analysis is expected to be capable of extracting more reliable in situ mass-density profiles under the seafloor since it uses many available geophysical data such as sea surface gravity and seismic surveys as well as seafloor gravity and bathymetric surveys. INTRODUCTION Presently, it is customary to select offshore exploration drilling sites based mostly on seismic data analysis. Gravity surveys are not used as extensively as seismic surveys, primarily because gravity analysis is not developed to its full capabilities and sensitivities. In this report, a new gravity analysis method is described to remedy the above situation. The new analysis method is a hybrid interactive method involving both partial inversions of gravity survey data and forward modeling of density profiles and uses interactive graphics and expert systems. The new method is complementary to the existing seismic analysis methods, since it uses the results of the seismic and geological surveys. The new analysis is expected to give an improved in situ mass density profile under the seafloor from seafloor gravity surveys and other surveys. In particular, it will be extremely useful when a choice for the final site is to be made from several potential sites determined from seismic and other surveys. NEW GLOBAL EARTH MODEL (NEWGEM) Some knowledge of the mass distribution in the interior of the Earth is emerging from seismic studies4,5 and rather detailed knowledge of the Earth's crust and surface is becoming increasingly available in many places. Therefore it is desirable to introduce a new physical representation of the mass distribution for the entire Earth which can be refined to accommodate new and more accurate data as it becomes available and which can describe many different component properties and processes of the entire Earth system on both global and regional bases. A new global Earth model (NEWGEM)1.3 has been recently proposed to provide a unified description of the Earth's gravity field inside, on, and outside the Earth's surface using the Earth's mass density profile as deduced from (1) seismic studies,. (2) elevation and bathymetic information, and (3) local and global gravity data. Using NEWGEM, it is possible to determine the constraints on the mass distribution of the Earth imposed by gravity, topography, and seismic data. Seismic studies indicate that the Earth has a concentric shell structure with deeper shells having larger mass densities. This structure of the Earth is summarized in the Preliminary Reference Earth Model (PREM).4 PREM defines the Earth as consisting of eight principal layers. It specifies the thickness of each of these layers and give polynomial expressions for calculating, as a function of radius, the density in each layer.