Integrated geophysical investigations to study thermal zones at Boku volcanic Centre, Main Ethiopian Rift

Abstract

This paper presents the results of electrical and magnetic surveys carried out over the Boku fumarole sites (Main Ethiopian Rift). On the basis of observed thermal manifestations, surface alterations and geophysical results we suggest that the Boku thermal field is a vapour-dominated, dry type geothermal system tapping heat from the shallow acidic magma chamber of Quaternary volcanic rocks of the Rift floor. The cap rock is formed by the self-sealing process through hydro thermal alterations of the Plio-Quaternary volcanic rocks in the area as witnessed by the surface manifestations of calcite and silica that are suggestive of such an environment. The aquifer system consists of the Quaternary Rift floor ignimbrites that are fractured intensively. The necessary supply for the replenishment of the reservoirs is derived from the closed basin system of the Nazareth watershed and/or from the large open irrigated grounds of the Wonji plantation. The investigations have clearly mapped the Quaternary faults that are the major tectonic structures for the passage of the deep-seated vapour to the surface, and the recharging of the geothermal reservoir.


Key words/phrases: Boku caldera, hydrothermal alterations, magnetic anomaly, vapour-dominated geothermal systems, vertical electrical sounding


SINET: Ethiopian Journal of Science Vol.25(2) 2002: 253-262