6. Gravity Exploration Methods

Abstract

The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets.Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of global positioning systems (GPS) have led to a marked improvement in the quality of marine gravity and have transformed airborne gravity from a regional technique to a prospect-level exploration tool that is particularly applicable in remote areas or transition zones that are otherwise inaccessible. Recently, moving-platform gravity gradiometers have become available and promise to play an important role in future exploration.Data reduction, filtering, and visualization, together with low-cost, powerful personal computers and color graphics, have transformed the interpretation of gravity data. The state of the art is illustrated with three case histories: 3D modeling of gravity data to map aquifers in the Albuquerque Basin, the use of marine gravity gradiometry combined with 3D seismic data to map salt keels in the Gulf of Mexico, and the use of airborne gravity gradiometry in exploration for kimberlites in Canada.

On the drape and level flying aeromagnetic survey modes with terrain effects, and data reduction between arbitrary surfaces

“…However, there is a significant issue for this computation. Since standard wavenumber domain Fourier filtering techniques allow data reduction or continuation from only one level surface to another level surface (Nabighian et al, 2005), this process cannot be performed between arbitrary/uneven surfaces using standard wavenumber domain techniques. Therefore, Schwarz-Christoffel transformation (Parker and Klitgord, 1972), extended potential field theory (Syberg, 1972;Hansen and Miyazaki, 1984), chessboard technique (Cordel, 1985), Taylor-series expansion (e.g.…”