Spatial interpolation techniques comparison and evaluation: The case of ground-based gravity and elevation datasets of the central Main Ethiopian rift.

The ground-based gravity data reveals diverse anomaly signatures in areas of the Main Ethiopian rift where active volcanic and tectonic activities are dominant. In such a region ground-based data collection is restricted to existing roads and relies on accessible stations. These resulted in gaps in data, either missing, uneven, or insufficient spatial coverage that must be estimated with proper interpolation techniques. Comparison and evaluations of the spatial interpolation methods that are commonly used in potential field geophysical data analysis were made for the terrestrial gravity and elevation data of the central Main Ethiopian rift. In this research, two widely used interpolation techniques, minimum curvature interpolation, and Ordinary Kriging were compared and assessed. A 10 % hold-out validation was employed, where 90 % of the data points were used to generate interpolated surfaces, which were then evaluated against the remaining 10 %. Following interpolation with each technique, the generated grid was converted into discrete data points (estimated values). These are then compared with the available gravity data, which were deliberately excluded from the gridding process (10 % remaining dataset). The accuracy of each method was assessed by evaluation metrics such as mean value, variance, Mean Absolute Error (MAE), Root Mean Square Error (RMSE), correlation coefficient (r), and R-squared. The results showed that the ordinary Kriging interpolation method outperformed the minimum curvature interpolants for gravity data with all performance metrics, while both interpolants seem to perform equally well for the elevation dataset. Therefore, it is proposed to use the Kriging interpolation method for potential field gravity studies conducted in the central Main Ethiopia rift.

Constrained 3D gravity interface inversion for layer structures: implications for assessment of hydrocarbon sources in the Ziway-Shala Lakes basin, Central Main Ethiopian rift.

Multi layer 3D gravity inversion for layered structures and density interfaces are performed in the Central Main Ethiopian rift bounded between 38000'-39030' E and 7000'-8030' N. The inversion is carried out in wave number domain using Parker-Oldenburg algorithm and is constrained with initial model information. The previous studies in the region focused on mapping crustal structures and Moho depths and least is known about the shallow earth. This study thus targets on mapping layers relief of shallow earth origin. Stacked horizons with depth to tops of density contrast are obtained from well log data and previous geophysical studies. These stacked grids represent major geological boundaries where density contrast exists. The model utilizes observed residual gravity anomaly and generates the structural relief maps of the respective layers with their corresponding gravity anomaly responses and the associated errors. Successive structural inversions are performed on three layers with their corresponding acceptable mean misfits' errors. The iteration process converges successively for each layer in each structural inversion and the result is validated against a priori information. In addition to the topography/thickness of each layers, this study for the first time identified a new Mesozoic horizon laying between a Tertiary ignimbrite layer and the crystalline basement at depths between -2499 m and -3060 m and having estimated maximum thickness of 561 m. The identified Mesozoic sediment formation underlies a thick volcanic cover of 2.5 km which might be a suitable geologic setting for the growth of hydrocarbon reserves in the area and could probably be the source of CO2 degassing.

Depth estimates of anomalous subsurface sources using 2D/3D modeling of potential field data: implications for groundwater dynamics in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift.

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.

Mapping geologic structures from Gravity and Digital Elevation Models in the Ziway-Shala Lakes basin; central Main Ethiopian rift.

This study attempts to delineate subsurface lineaments for the tectonically and volcanically active region of the Ziway-Shala Lakes basin, central Main Ethiopian rift. Most of the previously mapped subsurface structures in the region under consideration focus on delineating crustal structures thicknesses and Moho depths undulations. Moreover, surface structures in the same region were mapped using analysis of Digital Elevation Model image data. On the other hand, there are few studies that have targeted in mapping geologic structures lying at depth levels between the shallower and deeper subsurface. The objective of this research is thus to map the subsurface geologic structures/lineaments to an average depth of 3 km (crystalline basement layer depth) from surface using gravity data. These investigation results are validated by Digital Elevation Model extracted lineaments. Filtering techniques including derivative filters, upward-continuation and line module algorithm of PCI Geomatica are used to extract the gravity and topographic lineaments of the region. Orientation analyses of these subsurface and surface lineaments are made using line direction histogram of the QGIS software. Accordingly, the gravity subsurface lineaments mapped in this study are found to be dominantly oriented in the NNW-SSE to NW-SE and E-W direction on average. These results appear to be contrary to the NNE-SSW to NE-SW trending surface geologic structure mapped on the bases of actual field observation carried out by previous researchers and automatically extracted lineaments based on Digital Elevation Models data considered in this research. The subsurface lineaments mapped using gravity data are believed to govern groundwater dynamics within the basin and the adjacent basins in the area. These structural lineaments which are considered to be masked in the subsurface coincide with the orientation of the Mesozoic Ogaden rift as compared to the overlying surface structures which appear to coincide with the orientation of the Cenozoic Main Ethiopian rift.

Upward continuation and polynomial trend analysis as a gravity data decomposition, case study at Ziway-Shala basin, central Main Ethiopian rift.

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 38000'-39030'E and 7000'-8030'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.