GEOPHYSICAL EXPLORATION OF A BURIED RIVER VALLEY FOR GROUND WATER IN THE SOUTHERN BORANA REGION

ABSTRACT

This report describes the gravity survey carried out in El-Gof area located some 38 km north of Moyale town in the southern Borana region, during the period 4 March to 10 April, 1996. The survey was conducted with the aim of delineating a buried river channel and evaluating the usefulness of the method in such geologic situations.

Buried river valleys usually contain sand and/or gravel which are potential aquifers. However, the buried channels have no surface expression and thus appropriate sitting techniques may be required.

The gravity method may help locate favorable areas for ground water development by delineating the extent and trend of buried channels.

The gravity survey consisted of 246 stations carried out on six lines each 1 km long at an average line spacing of 200m, and a station interval of 25m along the profiles.

Standard data reduction procedures were applied to the field data to obtain a Bouguer gravity map. A first order polynomial which is assumed to represent the regional trend was subtracted from the Bouguer gravity map. As a result, a residual gravity map was obtained, which shows a clearly defined gravity low which is wavy in shape and is interpreted as the buried channel.

The buried channel is characterized by undulations with shallows and deeps. Therefore borcholes should be sited at the closed gravity lows which probably correlate to the deeper sections on the buried channel so that maximum thickness of sand and gravel could be intercepted.

Because of the presence of favorable geological conditions the gravity method may be successfully used to help locate buried channel aquifers in other parts of the southern Borana region.

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