Abstract
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.
Highlights
The Ziway-Shala lake basin is located in the central Main Ethiopian rift being bounded within the limit of 380000-390300 E and 70000-80300 N (Figure 1 (a)).The main Ethiopian rift (MER) which encompasses Ziway-Shala lakes basin is thought to be developed over a span from the Oligocene to the Quaternary (Woldegabriel et al, 1990)
One needs to work on whether the chosen regional approximation method best represent the deep geology which in turn should result in shallow earth of the area than the other methods
Numerous decomposition techniques are available in literature for approximating the low frequency signals, no specific information is available for the choice of a particular representative method
Summary
The Ziway-Shala lake basin is located in the central Main Ethiopian rift being bounded within the limit of 380000-390300 E and 70000-80300 N (Figure 1 (a)). The main Ethiopian rift (MER) which encompasses Ziway-Shala lakes basin is thought to be developed over a span from the Oligocene to the Quaternary (Woldegabriel et al, 1990). The compiled data set is reprocessed and homogenized with reference to the International Gravity Standardization Network 1971 (IGSN71). The 1967 international gravity formula, a reduction density of 2.67 g/cm and sea level as a datum are used. The computed complete Bouguer anomaly data are gridded and mapped (Figure 1 (c)) using the Geosoft Oasis montaj version 7.1. It is a known truth that, the computed complete Bouguer anomaly is a superposition of the effect of shallow and deep anomaly signatures which need to be separated in to their respective components before scheduling quantitative interpretation of the data
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