EXPERIMENTALS                     

Electrical Resistivity method was applied using ABEM 300B (Self Averaging System) terrameter with current electrode spread up 928m apart so that ^AB/₂ is 464m.The terrameter was capable of measuring current and potential difference and displaying their ratio as the measured resistance. The Schlumberger array of Vertical Electrical Sounding (VES) was utilized using the table below for which ^AB/₂ was increased to  after each sounding from ^AB/₂ =1.00m. The electrode potential spacing MN varied from 1.00 – 60m as the need arose so that ^MN/₂ is (0.-30m)

(Fig. 2).  Nineteen VES soundings were utilized. The interpretations and results of the data were correlated with existing borehole data and some available well log data from Delta State Utilities board.

The theory and Data Evaluation: The theory is based on the fact that resistivities of rocks and soils are influenced by groundwater which acts as electrolyte in soil (Milsom, 1992).

Electrical resistivity technique utilizes the contrast in resistivity of ore bodies and their host rocks in determining presence of good conducting minerals.  In addition, the good electrical conductivity of groundwater makes the resistivity of a sedimentary rock much less (lower) when it is water logged than in dry condition (Lowrie, 1997). Generally, the arrangement consists of a pair of current electrodes and a pair of potential electrodes, which are driven into the ground, in a particular site of interest, to make a good contact in straight line. One of the current electrodes acts as the source while the other is the sink (Reinhard, 1974).   Current is passed into the earth through the terrameter to produce electric field within the subsurface with electric potentials. The potential at electrode C due to the source electrode A, in (fig 2) is

while the potential at C due to potential electrode D due to the sink electrode B is

 The resultant potential at electrode C due to the two current electrodes is therefore,

Similarly, the resultant potential at electrode D due to the two current electrodes is

where r_1, r₂, R₁ and R₂ are distances defined in fig.2

Thus, the potential difference (V_c – V_D) between the two inner electrodes measured by the voltmeter connected between C and D is

In the study of the earth interior or subsurface, resistivity measured is not the exert resistivity because the measurements are not made directly on the body especially where there is inhomogeneity (Mbipom and Archibong,1989).  Hence, it is called apparent resistivity (Okwueze and Ezeanyim, 1988).  In a homogeneous layer the resistivity is constant   it is independent of both electrode spacing and surface location (Egbai and Asokhia, 1998).  However, some terrains show inhomogeneous subsurface and anomalies in apparent resistivity values (Kearey and Brooks 1991).  In such studies the wider the electrode spacing, the deeper the penetration.  Okwueze et al, (1994) showed that current penetration to a depth say Z is achieved with a total current spread L, given by

L = 3Z so that

Z = ^L/₃ ………………….7.

For very large current electrode separation, the potential electrode separation is increased to maintain measurable potential difference.  In this array, the current and potential pairs of electrodes have a common midpoint but the distance between adjacent electrodes differs (Mamah and Ekine 1989).

Where “a” is the distance between the current electrode, station midpoint,

“b” is the distance between potential electrodes and “2a” is the current electrode separation.

Applying the theory above, the potential at electrode P₁ from C₁ in Fig 3 will be

The potential difference “dV” between the two potential electrodes is therefore, given by

where a>>b as in  Schlumberger array

Thus, with a = 1 and b = 0.2, K_s = 7.54.To maintain the relation (C₁C₂ > 5P₁P₂), the values of ^AB/₂ are increased to  each time while the potential electrodes separations are guided accordingly (Asokhia et al, 2000).

INTERPRETATION AND RESULTS

The field data were used to compute the apparent resistivity values and the smoothed values were plotted against half the current electrode separation on bi-log graphs. Qualitative and quantitative analyses of the curves were made by partial curve marching of field curves with relevant Schlumberger developed master and auxiliary curves. The resistivity values and corresponding thicknesses were fed into computer soft ware to obtain iterated curves. (i)  Qualitative Interpretation: The sites in the study area are predominantly characterized by A-type curves as in Ibusa, Ogwashi-uku, and Ubulu-uku. However, Asaba shows evidence of AK curve type (Zhody et al, 1974). (ii) Quantitative interpretation: Quantitative treatment and analyses of the data delineate three to five distinct layers in the study area. Using the results obtained from computer modeling and iteration within the limit of equivalence (Okwueze et al 1988), the geo-electric sections for the area were developed (Fig. 4).

DISCUSSION

The geo-electric sections for the sites show that the there are three to five distinct layers to depth of penetration. The resistivities of top soil formations are generally between 110Ωm and 130Ωm at Ogwashi-uku, Nsukwa and Ubulu-uku while it is as low as 73Ωm and 50Ωm at Asaba and Ibusa respectively with an average layer thickness of 1m. The topsoil is thickest at Ubulu-uku where it is up to 2 metres. This top layer is red loose Laterite and is underlain by a thick red lateritic burden whose thickness ranges from 6metres at Ogwashi-uku to 25 metres at Ubulu-uku. At Asaba however, the top layer is mainly clayey Sand of about 2.5 metres thick. This may be attributed to nearness to the bank of the River Niger. The resistivity of the second layer varies from 85Ωm at Ibusa to 230Ωm at Ubulu-uku. At Asaba, the third layer is essentially sand clay and fine to medium sand as one goes deeper with resistivity values within 320Ωm (Table 1). Ubulu-uku has third layer resistivity of 500Ωm while at Ibusa a layer, which consists of weathered rock and shale of 1100Ωm is encountered before getting into medium grain sand. Springs and streams are also common occurrence because of the undulating topography and rock faults on the Lee side of Ibusa.

The geo-electric section shows that the layer with a resistivity of 320Ωm at Asaba is water bearing. Thus, the aquifer at Asaba is within 35m from the surface (Fig. 5). At Ibusa, the third layer with resistivity value of 250Ωm consists of fine to medium grain sand. Water bearing medium may be encountered at 60m to 70m but the first aquifer may be struck at about 100 to130m. At Ogwashi-uku and Ubulu-Uku, viable aquifer is struck at depths above 140m to 180m. These are in agreement with available borehole records (Fig 5).

Conclusion: While groundwater at Asaba is estimated to be 35m, Ibusa, Ogwashi-Uku and Ubulu-uku have their peculiarities. The study shows that they have deep aquifers. At Ubulu-uku aquifer depths are found to be at about 180m.Ubulu-Uku. The town is remarkable for its thick red lateritic formation to very far depth. The aquifer in Ibusa is within 100m to 110m after piercing through hard-stone screen. Perched aquifer exists at 60m – 75m. Water may also be intercepted at 40m to 50m in some points where false aquifers exist. Between Ibusa and Okpanam is a very wide expanse of undulating formations that have given rise to many rock and aquifer outcrops like the Okpuzu spring and many long - time flowing streams (Oboshi and Atakpo streams) which empty into the river Niger. These have been the traditional source of water for the residence in these communities. Deep aquifer may be struck at 130m down but it is not advisable as there is possibility of getting into thin layer of lignite within Ibusa. At Ogwashi-Uku, viable aquifers may be sited at far depth on the south end of the town. Generally, aquifers in the territory are unconfined as there is no layer of thick clay.

REFERENCES