One of the main purposes of geophysical mapping is the identification of units that can be related to known geology. On a regional scale, aeromagnetic and gravity maps are the most useful tools presently available, although other techniques such as conductivity mapping (Palacky, 1986) or remote sensing (Watson, 1985) are very helpful in locating lithologic boundaries. Interpretation now makes extensive use of enhanced maps: susceptibility maps for magnetic data, density maps for gravity data, first and second vertical derivative, and horizontal gradient maps for both types of data. The objective of susceptibility and density mapping is to transform the potential field data into a physical property map. For physical property mapping, some hypotheses and simplifications are made. The earth model is assumed to consist of right rectangular prisms of finite (gravity) or infinite (magnetics) depth extent. For ease of data processing, the potential field is interpolated onto a regular rectangular array, so that each point in the array corresponds to one prism.