The hydrogeological investigation of fissure flow by borehole logging techniques

Abstract

The variety and sophistication of the equipment which the authors have at their disposal are impressive, as are the data obtained and the conclusions reached. It is opportune, however, to acquaint readers with an additional tool in the armoury of the hydrogeologist: radio active tracers. In Rhodesia the majority of boreholes are sited in the crystalline rocks (granites, gneisses and the highly metamorphosed volcanics and sedimentary rocks of the Basement Complex) where the aquifer consists of fissured or decomposed rock. The delineation of fissures and zones of flow and the estimation of natural through-flow is, at times, of considerable importance. Radio active tracer in the form of Iodine 131 is emplaced in the borehole, at 3 or 4 points in the water column, by a simple injection device whereby the isotope ampoule is shattered by a messenger sent along the wire holding the tracer ampoule. Either compressed air or the drill bailer is used to mix the tracer uniformly throughout the water column. Subsequent monitoring with a nuclear detector at time intervals reveals the zones of flow. We have used both the pulse/velocity method and the Total Count Technique, (which is uniquely applicable to radio tracers) to quantify the flow into or out of the borehole at the particular zone. Further, a quantitative measure of through-flow in the borehole as an entity can be obtained. The direction of flow can also be determined by use of a collimated detector which is rotated through 360° at the required horizon. The position