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This paper discusses an approach to determine the least‐squares optimum order of the regional surface which, when subtracted from the Bouguer gravity anomaly data, minimizes distortion of the residual component of the field. The least‐squares method was applied to theoretical composite gravity fields each consisting of a constant residual component (sphere or vertical cylinder) and a regional component of different order using successively increasing orders of polynomial regionals for residual determination. The overall similarity between each two successive residual maps was determined by computing the correlation factor between the mapped variables. Similarity between residual maps of the lowest orders, verified by good correlation, may generally be considered a criterion for determining the optimum order of the regional surface and consequently the least distorted residual component. The residual map of the lower order in this well‐correlated doublet is considered the most plausible one and may be used for gravity interpretation. This approach was successfully applied to the Bouguer gravity of Abu Roash dome, located west of Cairo in the Western Desert of Egypt.

Bouguer anomalies in the area tested are mainly found to be due to structures on the basement and within the sedimentary cover. Lithologic variations in the basement are the cause of Bouguer anomalies in too few areas. Detailed quantitative studies of these areas showed that lithologic variations only cannot explain the given anomalies. So, even in such areas anomalies were found to be due to both effects, structures, and lithologic variations (Othman, 1976; Sayed, 1977; Moustafa et al, 1977). The effect of lithologic variations and basic intrusions within the sedimentary cover was found to be very small (less than 1 mgal in all cases) (Tealeb, 1973) and consequently will not be reflected on a Bouguer map of the studied scale. Thus we still believe that most lineaments traced from the Bouguer anomalies are due to structures (faults).

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The Bouguer anomaly map for the northern part of Egypt was used for determining fault systems which are probably present in the area. These systems show the presence of a number of almost parallel shear zones, striking in a northwest‐southeast direction. Extrapolation of some of these zones is suggested in the Gulf of Suez area. The movement of the eastern side of each zone is thought to be right‐lateral to the southeast. The shear zones are probably related to the interaction between the European and African plates. They probably started developing in the Oligocene and are presently still active. The opening of the Gulf of Suez is thought to be mainly due to the action of these transcurrent faults.