Hydrology: Current Research

Geoelectrical Investigation at Ain Al-Faras Spring Site of Gadames City, Libya

Abstract

Fathi Salem Elburas

Geoelectrical resistivity techniques are based on the response of the earth to the flow of electrical current. With an electrical current passed through the ground and two potential electrodes to record the resultant potential difference between them, we can obtain a direct measure of the electrical impedance of the subsurface material. The resistivity of the subsurface, a material constant, is then a function of the magnitude of the current, the recorded potential difference, and the geometry of the electrode array.

Depending upon the survey geometry, the data are plotted as 1-D sounding or profiling curves or in 2-D crosssection in order to look for anomalous regions. In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of resistivity is, in general, a measure of water saturation and connectivity of pore space. Resistivity measurements are associated with varying depths relative to the distance between the current and potential electrodes in the survey, and can be interpreted qualitatively and quantitatively in terms of a lithologic and/or geohydrologic model of the subsurface.

This technique conducted at fifteen vertical electrical sounding (V.E.S) locations; were carried out, employing the expanding Schlumberger configuration, covering a radial distance of 5km from Ain Al-Faras spring area, of Gadames city, Libya, as a part of UNDP sustainable Human Development for Rehabilitation of the Old City of Gadames.

The qualitative and quantitative results of the field curves and space sections together with the available
geological borehole information were used to construct geo-electrical sections for the study area. The most common features characterizing these sections were the appearance of a fault or barrier zone. clearly shows that some of these faults or barrier zones were laterally displaced for a few meters these faults can be divided in two systems oriented in different directions; the first one, trending North-east South- west, whilst, the other one trending nearly in East-west direction. The main obvious feature demonstrated was the intersection between those different systems, in which a horizontal displacement had been occurred, and the trend and delineation of the topographic features of the investigated site strongly reflect the presence of such fault system, and according to the mentioned horizontal displacement (i.e. the intersection) of the fault system, Ain Al Faras spring had been developed, together with Ain
Tala spring few tens of meters to the North-West.