INTRODUCTION
The advent of technology has made the quest for water for all purpose
in life to drift from ordinary search for water to prospecting for steady
and reliable subsurface or ground water from boreholes. In Nigeria, presently,
borehole has rescued the citizenry from acute shortage of water. The geoelectric
method has been found to be very reliable for ground water studies over
the years. Asokhia et al. (2000), proposed a simple computer iteration
technique for the interpretation of vertical Electrical Sounding. Pulawski
and Kurht (1997), Zohdy et al. (1974) and Ujuanbi (2000) used this
method to map clay deposit in a dual geological environment. This method
was also used in the assessment of the ground water resources potentials
within the Obudu basement, Okwueze (1996). EtuEfeotor et al. (1998)
carried out an assessment of the near surface underground water resources
potential within the eastern Niger Delta. Olorunfemi et al. (1995)
carried out a predrilling geophysical investigation for ground water
development in the proterozoic basement of the northern rural part of
Kaduna State. The total well depth of borehole installed at the Jehovah
Witness Watchtower, Igieduma, is 680 ft (206.1 m).
This research therefore is centered on the used of vertical electrical
sounding for underground water studies in a sedimentary environment.
THEORY
Ohms law provides the relationship between electric field and current
density and it states that
Where:
σ = Conductivity (a constant)
For an isotropic medium, the conductivity will be a scalar quantity so
that J and E will be in the same direction. For anisotropic medium, the
conductivity is a tensor of second rank σ_{ij} so that
The subscript i and j maybe any of the X, Y, or Z spatial directions.
The basis of all resistivity prospecting with direct current is given
by:
In the isotropic case Eq. 3 reduces to laplaces equation
For a horizontal earth model the solution to Eq. 4 according
to Stefanesco et al. (1930) becomes
Where, J_{0} = is
the zero order Bessel function of the first kind and θ_{n},
is called the kernel function which is a function of the thickness and
reflection coefficient for an assumed earth model.
By differentiating Eq. 5, the Schlumbeger apparent resistivity
over an n layered earth becomes
Where, J_{1} is the first order Bessel function of the first
kind.
The evaluation of this integral of Eq. 6 has been done
in a number of ways. In this study we have adopted, Ghosh (1971) in which
it is possible to determine a linear digital filter, which converts resistivity
transform samples into apparent resistivity values for theoretical models.
MATERIALS AND METHODS
Vertical electrical sounding was carried out using schlumberger
electrode configuration. The ABEM SAS 300B Tetrameter was used for the
investigation in the two locations. The maximum current electrode spacing
(AB/2) was 400 and 500 m for VES 1 and VES 2, respectively. The resulting
sounding curves was interpreted using partial curve matching as well as
computer iteration techniques (Res1Dinverse or Schlumberger automatic
analysis). This study was carried out in Edo state (Igieduma and Ehor)
towards the end of June, 2006.
RESULTS AND DISCUSSION
The acquired data were processed and interpreted qualitatively and
quantitatively by using curve matching techniques to generate the initial
model for the computer interactive method of interpretation (Res1Dinverse).
From the results as shown in the Fig. 1 and 2
with respect to Table 1a, b and 2a,
b, seven geoelectric layers were encountered with resistivities as
shown in the model parameters 1 and 2.The VES 1 curve is KH curve type
with ρ_{1}<ρ_{2}>ρ_{3}<ρ_{4}
Asokhia (1995).
The VES 2 curve is also a KH curve type with ρ_{1}<ρ_{2}>ρ_{3}<ρ_{4}.
The sixth layer for both model 1 and 2 with thickness 66.6 and 105 m,
resistivities 8200 and 7350.00 Ωm, corresponds to a total depth of
241.48 m (796.90 ft) and 229.13 m (756.13 ft), respectively. In correlation
with the lithologic log of an existing borehole data from a nearby borehole,
total depth value of 206.1 m (680 ft). The VES results of both locations
presents a high correlation of the presented values with an existing functional
borehole. This study showed a clear support or confirmatory proof of the
depth to aquifer in a sedimentary environment, in Edo state.

Fig. 1: 
Resistivity curve of VES 1 

Fig. 2: 
Resistivity curve for VES 2 
Table 1a: 
Model parameter 1with RMS Error (%): 2.43 

Table 1b: 
Observed (field) and computed (theoretical) data 

Table 2a: 
Model parameter 2 with RMS Error (%): 2.47 

Table 2b: 
Observed (field) and computed (theoretical) data 

CONCLUSION
This research has shown that in a sedimentary environment, Vertical
Electrical Sounding (VES) have proved to be very reliable for underground
water studies and therefore the method can excellently be used for shallow
and deep underground water geophysical investigations.