Groundwater exploration in the granitic basement in the Assin District of the Central Region using the electrical resistivity method
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Date
2004-11-17
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Abstract
The electrical resistivity method of geophysical prospecting was used to delineate regions of fractured zones, faults, and the thickness of the top layer, conductivity and impermeable strata which might give clues to the presence of groundwater in some Communities of the Assin District of Central Region of Ghana.
The methodology involves, first of all, a preliminary reconnaissance study, followed by the use of the geo-electrical method of resistivity profiling and sounding. The reconnaissance study served the purpose of identifying and studying the outcrops in the area and also identifying suitable sites for the location of potential borehole to serve the communities in question.
The electrical resistivity technique was used for both profiling and sounding, based on the Schlumberger array. The profiling was first done along laid down traverses in the communities and a preliminary interpretation of the results made it possible to identify suitable points for sounding. The vertical electrical sounding (VES) method employed was used to investigate the variation of the resistivity with depth of the subsurface geological structure.
The sounding results were modeled using the software Sondel, to obtain the layer structure of each sounding point together with the resistivity values for the various layered Earth models. The results were then interpreted and the
groundwater potential of the various sounding points deduced. This was then followed by drilling of all the selected groundwater potential zones.
In all 20 communities were chosen in the Assin District for this work and a profile length of between 160 to 190 m was chosen for each community. The minimum depth of first struck of water was 9 m and the maximum 51 m. The maximum and minimum depths drilled within the 20 communities were 76.4 m and 31.17 m respectively. Below is a selection of some of the twenty (20) communities studied:
At ABASE, the resistivity profiling covered 180 m with apparent resistivity values ranging from 395.82 to 2099.35Ωm. The sounding indicated three layered subsurface structures. The first layer presumably is laterite with a mean thickness of 1.3 m and apparent resistivity of 41Ωm. The second layer, which is highly to moderately weathered granitic zone, has its thickness varying between 3 to 35 m with a mean of 3.8 m and an apparent resistivity of 15Ωm. The basement rock has a mean apparent resistivity of 4961Ωm. The depth drilled was 55 m and the yield was 1.2 m3/h.
At Briskoe Camp, the resistivity profiling covered 170 m with apparent resistivity values ranging from 834.92 to 1252.38 Ωm. The sounding indicated four layered subsurface structures. The first layer presumably is laterite with a mean thickness of 1.9 m and apparent resistivity of 111 Ωm. The second layer, which is completely weathered material, has its thickness varying between 3 to 18 m with a mean of 6.0 m and an apparent resistivity of 1858 Ωm. The third layer is fractured biotite schist with a mean of 10.9 m and an apparent resistivity of 95 c2m. The basement rock has a mean apparent resistivity of 4606 Ωm. The depth drilled was 49.8 m and the yield was 0.6 m3/h.
At Framase, the resistivity profiling covered 190 m with apparent resistivity values ranging from 511.40 to 1566.92 Ωm. The sounding indicated three layered subsurface structures. The first layer presumably is Palecite with a mean thickness of 3.5 m and apparent resistivity of 517 Ωm. The second layer, which is very weathered granite, has its thickness varying between 3 to 12 m with a mean of 11.7 m and an apparent resistivity of 87 Ωm. The basement rock has a mean apparent resistivity of 2601 Ωm. The depth drilled was 39.0 m and the yield was 1.5 m3/h.
At Abotareye, the resistivity profiling covered 170 m with apparent resistivity values ranging from 292.22 to 842.95 Ωm. The sounding indicated four layered subsurface structures. The first layer presumably is laterite with a mean thickness of 1.4 m and apparent resistivity of 40 2m. The second layer, which is completely weathered material, has its thickness varying between 21 to 44 m with a mean of 8.9 m and an apparent resistivity of 717 Ωm. The third layer is granite with biotite with a mean of 12.1 m and an apparent resistivity of 1411 Ωm. The basement rock has a mean apparent resistivity of 216 Ωm. The depth drilled was 64.24 m and the yield was 4.0 m3/h.
At Agave, the resisitivity profiling covered 180 m with apparent resistivity values ranging from 451.63 to 1471.23 Ωm. The sounding indicated three layered subsurface structures. The first layer presumably is laterite with a mean thickness of 3.1 m and apparent resistivity of 30 Ωm. The second layer has its thickness varying between 3 to 7 m with a mean of 50 m and an apparent resistivity of 938 Ωm. The basement rock has a mean apparent resistivity of 3802 Ωm. The depth drilled was 69.5 m and the yield was 0.94 m3/h.
Description
A thesis submitted to the College of Science in partial fulfilment of the requirements for the degree of Master of Science in Geophysics, 2004