The ensuing geophysical investigation report is the result of field vertical electrical sounding(VES) carried out on 16 th March, 2018 for the sinking of a succesful borehole at Hon. Edeh Emmanuel Compound Off-Convent Road, Abakaliki L.G.A. Ebonyi State
Pre-drillin geophysica surveys happen to be the scientific method of determining the existence orotherwise ofunderground water. This therefore, necessitated our hydrogeologists to carry out this survey. The vertical electrical soundings were therefore carried out at the proposed site onthe7Th February, 2018
This reportinvolves studies ofthe physiography, local geology and hydrogeology as well as the conduct of vertical electrical sounding of the subsurface vertical stratigraphy.
Geophysica survey were employed to “lookintotheground” and find the detailed subsurface stratigraphy beneath the area. The method is employed in ground water investigations. This method makes use of variations in the electrical properties of rocks and minerals.It is generally used in studying geological formations of interest in relation to their depth, particularly with regard to their permeability.
It is well known that resistivity of a formation largely depends on the moisture content and on the physical and chemical properties of the saturating water.The equipment used for the fieldwork was the versatile GEOMATIVE GD-10 RESISTIVITY METRES ;however, Interpex 1D sounding inversion software was used in the interpretation. The apparent resistivity, which is the normal quantity determined from the field measurement, is not a physical constant, but reflects the distribution of the true resistivities in the subsurface, and depends on the spatial configuration of the measuringsy stems. Of the possible electrode configuration usedin measuring resistivity,the Schlumberger arrangement was the one employed in the present study. Apparent resistivity were obtained from the field resistance values using the equation
Where:
a =Apparent Resistivity in Ohm-metre
L =Distance between current Electrodes
b =Distance between potential Electrodes
R=Resistance of the ground
The obtained apparent resistivities were then plotted against corresponding electrode spacing (AB/2) to obtain a curve which was interpreted quantitatively by curve matching using auxiliary the point method and compared with direct interpretation to obtain the table below.
The Vertical Electric Sounding (VES) was conducted outside the compound where the proposed water scheme is to belocated but directed on the identified fracture system passing through the area of interest, and where a possible good spread was obtained.
The Vertical Electric Sounding (VES)r esultshows 3 and 4 layers at point 1 and 2 respectively. The first layers had an apparent resistivity 55.26 Ohm- mand 52.26 with thicknesses of 2.2 mand 2.1, while the second layers had apparent resistivity of 27.01 Ohm-m and 43.34 with thicknesses of 18.1 and 1.8 metres respectively . Detailed lithologic descriptions are shownin table 1 and 2.
Based on the existing hydrogeological considerations borehole depth of about 35 metres (120 feets) is recommended to be drilled at point two (2) as described with supervision of a qualified geologist or water engineer.
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