The first task in quantitative interpretation of a gravity data is separation of the Bouguer anomaly into its regional and residual components which are respectively related to deep and shallow subsurface geology. The decomposition process is subjective and non-unique as there is no single best approach to approximate the low frequency signature. For example, the use of spectral analysis and upward continuation require the wise choice of slope change location and continuation height respectively, which could be chosen differently by different researchers. This requires a need to work on more than one method and select the best to be applied for a given study area. The "best" choice is made based on the anomaly signature of the underlying geology. In this research, the most frequently used methods such as upward continuation and trend surface analysis methods are used and compared to approximate the regional field in Central Main Ethiopian rift bounded between 38 0 00 0 -39 0 30 0 E and 7 0 00 0 -8 0 30 0 N. The upward continuation height and the order of trend polynomial surface are first chosen, to approximate the regional gravity field signal. Accordingly, an upward continuation height of 6km and first order polynomial trend surface are chosen to be appropriate. Comparison of the two methods shows that the upward continuation technique reflects the shallow source anomalies of the area better than that of the first order linear trend surface. This outcome is verified against the result obtained based on the first vertical derivative method, spectral analysis depth estimation method, well-log data and surface geology of the area. It is therefore recommended to consider the various existing filtering techniques and choose the best candidate for the separation of the regional and residual components of the observed field.
Quantitative analysis of potential field data are made in the Ziway-Shala lakes basin over an area bounded by 38
°
00′ E - 39
°
30′ E and 7
°
00′ N - 8
°
30’ N. Most previous geophysical studies in the region under consideration focus on mapping the deep crustal structures and undulation of the Moho depth. Only few studies are targeted at mapping the shallow subsurface structures. The main focus of this paper is mapping geometries of the major lithological and structural units of the shallow subsurface using gravity and magnetic data. The ultimate objective of the research is to understand the hydrogeological dynamics of the region through mapping interfaces geometries. Automatic inversions, 2D joint forward modeling and 3D inversion are the major techniques employed. The 2D Werner de-convolution based on both gravity and magnetic data along the rift axis showed source depths tending to deepen northwards. Source depths estimates determined by Source Parameter Imaging also showed similar tendency. This is further strengthened by the joint 2D forward modeling of gravity and magnetic data which showed the top of the basement is sloping northwards. The result of the 3D gravity interface inversion agrees with results of the above mentioned depth estimation techniques. Finally, the gravity power spectral analysis resulted in two depth estimates, 1.53 km and 2.87 km which approximate the positions of two density interfaces. The shallow depth interface is thought to presumably delineate the low density Fluvio-lacustrine sediments including the rift floor volcanic units and crystalline basement. Our investigation results agree with the results of previous seismic studies which identified low velocity (“sediment-volcanic”) horizon in the rift floor with low resolution. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model appears to reveal that the groundwater flow in the study region is controlled by subsurface structures, particularly, the mapped interface topographies.
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