Evolution the Geological View of the Southern Desert of Iraq Using Gravity and Magnetic Data

Abstract The study area is the Southern Desert (SD) which occupies the southwestern region of Iraq. The northeastern, northwestern, southwestern, and southeastern boundaries of the SD are the Mesopotamian Basin, Wadi Al-Khir, the Iraqi-Saudi borders, and the Iraqi-Kuwaiti borders, respectively. The SD is situated on the stable part of the Arabian Platform and is known for its intensive karst terrain, which is notoriously difficult to seismically image in the subsurface. The area is described by a thick sedimentary cover (not less than 5.5 km in the eastern part) in which the strata dip northeast with a dip angle estimated at 2°–3°, and the exposed sedimentary sequence ranges in age from Paleocene to Pleistocene. This study aims to evolve the geological view of the SD using essentially the potential (gravity and magnetic) data. This objective can be achieved by investigating a) karst system and the underlying geological structures, b) basement composition and structures, c) potential anomalies, d) edges of potential sources and e) Moho depth, heat flow, and effective elastic thickness of the lithosphere. Many processing, enhancement, and depth estimation techniques are applied to attain the aim of this study. Meanwhile, software and open-source codes, including MATLAB and Python scripts are utilized in the processing and interpretation. A simplified 3D forward magnetic susceptibility model is used to select the most relevant field separation method among four separation techniques. Upward continuation technique is employed to separate both the karst system and the underlying gravity highs. Parker-Oldenburg's method, which uses power spectrum analysis of gravity data to estimate depth, is applied to the basement and Moho surface. The method is suggested as an effective separation technique when adequate geological information about a certain surface are available. To enhance the subtle features in magnetic and gravity datasets, the Phase Preserving Dynamic Range Compression technique, which generates a series of III dynamic range compressed images at different scales,is utilized. The resultant magnetic images, at long wavelengths, show a possibility of dividing the basement into three blocks, separated by grabens, and the blocks into subblocks. A new structural element map is accordingly presented. Texture and lineament analyses are used to study gravity and magnetic grids’ textures and faults in the sedimentary cover. Further, seventeen edge-detection methods are applied to the potential data and the results are compared to determine the more convenient methods for the study area. In this context, a new filter nominated as Inverse Tilt Angle of Second-gradients (ITAS) is introduced. The filter is tested on the Bishop model test and the outputs are compared with three pertinent filters. The results show that the ITAS has good performance in detecting smallscale magnetic features and has a high sensitivity to the basement structural variations. The proposed filter, with three other filters, are employed to produce a new basement structural map containing regions that underwent reactivation during the geologic time. However, the quantitative interpretation is focused on utilizing Reliable Euler Deconvolution, Werner Deconvolution, Analytic Signal, and Tilt-depth methods to obtain depth to basement maps from the magnetic data. A 1D radially averaged power spectrum and Parker-Oldenburg's methods are used in gravity data to define depths to gravity structures of hydrocarbons interest and basement depth, respectively. Moreover, basement magnetic susceptibilities are estimated showing that the basement rocks are potentially composed of igneous rocks mostly granites “S” type and/or altered basic with volcano-sedimentary complexes and/or high-grade metamorphic rocks. In addition, two 2D forward modeling profiles give a reversed magnetization in two portions; Al-Ma'aniyah and Takhadid suggesting that these regions depict the northern projection of the Al Amar suture (~610 Ma) in Iraq. The estimated Moho depth is ~36 km which is deeperthan the Curie-point depth of ~32.5 km and the calculated heat flow using a window size of 300 km × 300 km is 54.67 mW/m 2 which is higher than the neighboring Mesopotamian Basin due to thick sedimentary cover.