The Bouguer Anomaly Map of Iraq According to a New Local Theoretical Gravity Equation and Its Geological Importance

Four hundred and seventy eight gravity base stations in Iraq were used to obtain a new local theoretical gravity equation. The obtained equation was used to construct a Bouguer anomaly map of Iraq depending on the available gravity base stations. This map was compared with the Bouguer map constructed for the same stations using the international formula (1930). Good similarity in shapes and locations of the anomaly were observed, while the gravity anomaly values in the new map were increased by about 30 mGal. The eastern zero gravity contour line of the new obtained gravity map coincides with the western boundary of the tectonic Mesopotamian zone, while the main negative gravity values coincide with the Mesopotamian area, which contains most oil fields in eastern Iraq. All negative gravity anomaly areas coincide with the deepest basement rocks (the sedimentary basins) in Iraq. The obtained results are very valuable in geological applications.

Reply by author to A. I. Bayoumi and E. M. Abdelrahman

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).

On: “A Method of Computing Residual Anomalies from Bouguer Gravity Map by Applying Relaxation Technique” by M. K. Paul (GEOPHYSICS, vol. 32, no. 4, p. 708–719, August, 1967)

On page 711 the author, after reference to previous users of the least‐squares method for estimating residuals in a Bouguer gravity map, states, “All of them have used the method for estimating the residual field over the entire area under consideration, while in this case the method will be applied to obtain the same on the boundaries only.” He then proceeds to compute residuals on the boundaries from the second degree polynomial [Formula: see text], (10) representing the regional field over the entire region. However, by this procedure the residuals on the boundaries are influenced by all the gravity observations inside the region, as implied by equation (16) where the vector A is a function of the Bouguer map values over the whole area. In fact, equation (12) could be solved for the vector b, and the condition [Formula: see text] arising from [Formula: see text] (A) could be introduced. The following expression for the regional over the entire area results: [Formula: see text], and there is no need for additional computations by the relaxation technique described.