USING GEOPHYSICAL MATHODS FOR DETAILED GEOLOGICAL SERVEY IN ALNAJAF, AL- NASRIYAH AND SUQ AL-SHUOKH AND AL- SALMAN

Abstract

The present work deals with geophysical surveys; gravity and radiometric, executed in the Southern Iraqi Desert. The gravity survey aims to solve some geological problems related to formation thicknesses that changed rapidly. However, it also aims to studying in more details some impressive anomalies appeared in Bouguer map. On the other hand, radiometric survey has different aims. It aims to measure the distributions of the U235, K40, Th and Dose rate, of some predefined anomalies and also to measure the level of cesium (Cs137) contamination. It is also aims to detect the differences in radiometric levels between the adjacent lithologies that may be useful in geological mapping. Five gravity profiles of lengths range (7 – 35) Km are situated on the boundary between two tectonic zones; the Outer and the Inner Platforms i.e. passing across Abu Jir Faults zone. However, a gravity profile (SL) of 70 Km length has been totally executed in the Inner Platform and passing nearby Al-Salman Depression. The results show that the gravity curves express high horizontal gradients of different magnitudes and shapes. The change in the gradients, from low to high, delineates the area affected by the fault. The source of the high gradient is either Basement deep seated faults or intrasedimentary faults at shallower levels in accordance with the two trends (regional and local) appeared in some of the gravity curves. The differences in the magnitudes of the high gradients are mainly related to density contrast and the depth of the fault whereas the differences in shapes may reflect the number of the effecting faults. Some morphotectonic phenomena existed in the studied areas like the alluvial fan in Sha'ib El Qusair and the Sulaibiyat Depression refer to surface extended fault(s) (neotectonics). The gravity profile SL shows gravity high which may be structurally important due to its possible interest in hydrocarbons exploration, and a relatively small and local negative anomaly interpreted as subsurface dissolution in the carbonate rocks. Radiometric survey generally shows low radiation level for both Quaternary sediments and pre Quaternary rocks. However, noticeable change in the radiation level has been recorded, between the carbonate rocks and the Quaternary Gypecrete and also between the upper and middle member of Dammam Formation, which refers to the difference in the radioelement contents. The results show that the studied area underwent high weathering processes according to the low K concentration (with average of 0.3%). Spectrum analysis shows a contamination with artificial isotopes, especially Cs-137, with low activity around 0.04 cps in most of the measured stations. It needs to get more attention for future studies.