Processing and interpretation of full tensor gravity anomalies of Southern Main Ethiopian Rift

Abstract

The study area is situated in the Southern Main Ethiopian Rift being bounded within the limit of 37 o 00″0′-38 o 50′00″E and 5 o 50′00″-7 o 00′00″N. It is well known that the complex geological structure of the Main Ethiopian Rift has attracted intense attention so far and numerous geophysical investigations have been performed using potential field data-sets in Central and Northern Main Ethiopian Rift with the exception of the Southern Main Ethiopian Rift which is poorly constrained. Analysis of Full Tensor Gravity anomalies helps in understanding of the nature of shallow subsurface structural features and has a paramount importance in building general understanding of subtle details about subsurface geology of the area. Separation of regional and residual gravity field is performed using upward continuation filtering technique. The residual gravity anomaly caused by local structures and anomalous body delineated four sub-basins with low amplitude response which is in agreement with the vertical gravity gradient anomaly (G zz ) and tilt derivative horizontal (TDX) that clearly outlined and characterize edges of the sub-basins. The sub-basins delineated are the northern and southern Abaya, Chamo and Gelana basins. The tilt angle method which is used to delineate major subsurface structures and determine the source depth results showed that the area was affected by different lineament trending NE-SW, N–S, NNE-SSW, NW-SE and E-W, directional analysis performed indicates that the dominant trend is in agreement with the regional fault orientations. The estimated depth to the top of the lineaments on average varies from 0.9 km to 3.1 km and it is relatively deeper in the basins compared to the surrounding areas giving clues to the amount of sediment infill.

Structural interpretation of Southern Main Ethiopian Rift basin using constrained full tensor gravity inversion of the basement morphology

“…Directional analysis performed on the measured structures and DEM extracted lineaments presented in a rose diagrams ( Figure 4 c) and ( Figure 4 d) generally showed that dominant trend tend to run in the NE–SW in Abaya and Chamo basins and N–S direction align with the axial portions of the Tertiary Ethiopian Rift which is also in agreement with fault block geometries identified by Invariance Tensor analysis ( Figure 8 b). These results have good correlation with the faults deduced from analysis and interpretation of Full Tensor Gravity ( Kebede and Mammo, 2021 ) and known faults deduced from geological studies in the area ( Asrat et al., 2001 ; Mengesha et al., 1996 ).…”

“…3D Euler deconvolution technique was performing on the Bouguer gravity anomaly and Full tensor gravity components , and in an attempt to find depth to basement of lineaments. The structural index is chosen according to a prior knowledge of the source geometry of the area ( Kebede and Mammo, 2021 ), contacts like faults have a structural index of in gravity field so identifications of basement blocks faults are performed using index . Euler depth solution ( Figure 9 ) show that the limits of the crystalline basement sources running between 2500 and 3500 m deep in rift basin area where Abaya Lake, Chamo Lake and Gelana basins resides.…”

“…A prior information is incorporated to address the nature of non-uniqueness in the inverse problem and the inversion process conducted as optimization problem ( Ellis and Oldenburg, 1994 ; Jackson, 1979 ; Mammo, 2012 , 2013 ; Mehanee and Essa, 2015 ). Since the subsurface model derived from independent gravity modeling is not unique, plausible or reasonable model is defined using Auxiliary (a prior) information's by incorporating the results from geological field work, Euler Depth solutions ( Figure 9 ) and Spectral depth estimations ( Figure 10 ), Tilt Angle depth estimation ( Kebede and Mammo, 2021 ), EAGLE controlled source wide-angle reflection/refraction P-Wave 2D seismic velocity models ( Maguire et al., 2006 ) and nearby well data ( Tullow Oil, 2014 ) to arrive at a more plausible geological model.…”

“…The crystalline basement interface density ( ∼2.6 gm/cm 3 ) and volcano-sedimentary rocks density ( ∼2.4 gm/cm 3 ) are also determined from density log of Gardim-1 wells of Chew Bahir Basin ( Tullow Oil, 2014 ). The average interface depth of the basement is derived from depth solutions of Euler deconvolution and Spectral depth estimations ( Figure 9 and Figure 10 ) and depth estimations using Tilt Angle by Kebede and Mammo (2021) .…”

“…In the absences of constrains in which performing inversion is difficult, depth estimations of the sediment infill of the basin is very crucial step, such results are used to produce best estimate initial depth to source that can act as initial models for any detailed quantitative modeling exercises using 2D/3D forward/inverse modeling techniques. Thus Euler Depth solutions and Spectral depth estimations performed in this study, depth estimations using Tilt Angle ( Kebede and Mammo, 2021 ) and EAGLE controlled source wide-angle reflection/refraction 2D seismic velocity models ( Maguire et al., 2006 ), geology and nearby well data ( Tullow Oil, 2014 ) have been used as a prior or axillary information's. To better understand the geology of the area and to obtain optimal value of basement density, geological field works were conducted.…”