Geophysical Studies in Filwuha Geothermal Area: Depth estimation of the heat source and Mapping of Deep Seated Structures and Intrusions

ABSTRACT

On a large scale the Bouguer anomaly maps encompassing Filwuha show regional structures that are parallel to the rift. This is an indication that filwuha is inside a region where there are deeply seated regional structures. Moreover, no significant anomalous zone at Addis Ababa is obtained indicating the absence of a magma chamber as a major geothermal heat source for Filwuha.

The radially averaged power spectrum analysis assisted in obtaining cut-off wave numbers. Using the cut-off wave numbers anomaly maps are generated at different depths by using filters that work on wave number domain. The anomaly maps that are generated at different depths show structures that are parallel and sub-parallel to structures in the MER. It also helped to identify 7 gravi-stratigraphic layers at depths of 11km, 4.84km, 3km, 1.5km, 0.94km and 0.32km.

Gravity anomaly maps of Filwuha show anomaly sources found at different depths. Euler Deconvolution method taking a window size of 2.5km by 2.5km and structural index for vertical cylinder is used to estimate depths to anomaly source. Accordingly, four intrusive bodies are identified at depths 230m (near Filwuha Meda), 350m (at the Grand Palace and at the Municipality hall) and 641m just at Filwuha Meda.

In conclusion, the major causative body for the geothermal heat of Filwuha appears to be a cooling intrusive body found below half a kilometer depth and by other three anomaly sources found at shallow depths.

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