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The geoelectrical investigation of the proposed Aba River dam at Amapu-Ideobia, Akanu Ngwa Southeastern Nigeria has been carried out. The objective of this study is to determine the downward and lateral trends of the rock layers or units along and near the proposed dam axis and deduce the possible structures that may enhance workable design of the dam. ABEM Terrameter SAS 4000 model was used and the symmetrical Schlumberger configuration was adopted. Twelve (12) Vertical Electrical Sounding stations were located and fully occupied along the dam axis. Preliminary input data from the field were fed into Zohdy software to generate real resistivities and depths to geoelectric layers. Five geoelectric layers were interpreted as Loamy Top soil, Alluvial matter, Pebble bed, Sandy lateritic and Gravely sand. Layer 1 (the top loamy soil) was encountered in VES 1, 2, 3, 4, 5, 9, 10, 11 and 12 locations with maximum thickness of 1.5 m in VES 3 and 4. Resistivity values ranged from 216 to 519 Ohm-m. The second layer (lateritic matter) had a maximum lower depth of 5 m in VES 3 and 4 points. This was not encountered in VES 6 point being replaced by alluvium. Resistivity values ranged from 101 to 6190 Ohm-m. Layer 3 was interpreted as a restricted pebble bed which occurred only at VES 6, 7 and 8 locations flanking the river course with thickness of about 3.5 m and resistivity values range of 182 415 Ohm-m. The fourth layer was modeled as the alluvial matter and restricted to the river course (VES 6, 7, 8) locations with base at between 12.5 m in VES 8 and 8 m in VES 6. The last modeled layer (Layer 5) was composed of gravely sandstone that underlined the whole study area apart from the restricted pebble bed at the NE crestal portion (VES 12). No structures like fractures, lineaments and faults that would be of deleterious effect were observed in all the VES points down to about 40 m. However, it was observed that the axial length had overriding sandy matter with high porosity and potentially rife for great infiltration; a condition that could facilitate seepage around the reservoir portion of the dam.

There are various approaches available to source for information about the subsurface, and the best is undoubtedly the direct observation of earth materials. However, this approach is of course rarely possible to the extent that people would like [

Amapu-Ideobia is located at about 20 km southeast of Aba while the proposed dam axis is located at about 7 km east of Ugwunagbo Local Government Headquarters (

The southwest end of the dam axis is located on latitude 5˚1.127'N and on longitude 7˚24.090'E, the north eastern end of dam is located on latitude 5˚1.393'N and on longitude 7˚25.145'E. Although the project area is located in two communities in two different Local Governments, accessibility is only through Amapu-Ideobia in Akanu Ngwa. The same topographic set-up that exists around Ogbor Hill area in Aba metropolis seems to exist at Amapu-Ideobia in Akanu Ngwa, where the general

level terrain in Aba area is truncated by the Aba river valley. The only low land area is at the river course. However the southwest end of the proposed axis is lower in heights above sea level than the northeast end as shown in

While there seems to be steep fall from the south west crest to the lower high water point near the course, the river seems to stretch out toward the northeast end (A and B points on

The study involved the use of Electrical method. The Vertical Electrical Sounding (VES) array of Schlumberger was employed. The ABEM Terrameter model SAS

4000 (

Data from resistivity surveys are customarily presented and interpreted in the form of values of apparent resistivity ρ_{a}. Apparent resistivity is defined as the resistivity of an electrically homogeneous and isotropic half-space that would yield the measured relationship between the applied current and the potential difference for a particular arrangement and spacing of electrodes. An equation giving the apparent resistivity in terms of applied current, distribution of potential, and arrangement of electrodes can be arrived at through an examination of the potential distribution due to a single current electrode. The effect of an electrode pair (or any other combination) can be found by superposition. Consider a single point electrode, located on the boundary of a semi-infinite, electrically homogeneous medium, which represents a fictitious homogeneous earth. If the electrode carries a current I, measured in amperes (a), the potential at any point in the medium or on the boundary is given by:

where

U = potential, in Vρ = resistivity of the mediumr = distance from the electrode.

The mathematical demonstration for the derivation of the equation may be found in textbooks on geophysics, such as [

where r_{A} and r_{B} = distances from the point to electrodes A and B

In addition to current electrodes A and B,

where U_{M} and U_{N} = potentials at M and N respectively.

AM = distance between electrodes A and M, etc.

These distances are always the actual distances between the respective electrodes, whether or not they lie on a line. The quantity inside the brackets is a function only of the various electrode spacings. The quantity is denoted 1/K, which allows rewriting the equation as:

where K = array geometric factor.

Equation (4) can be solved for ρ to obtain:

The resistivity of the medium can be found from measured values of V, I, and K, the geometric factor. K is a function only of the geometry of the electrode arrangement.

Wherever these measurements are made over a real heterogeneous earth, as distinguished from the fictitious homogeneous half-space, the symbol ρ is replaced by ρ_{a} for apparent resistivity. The resistivity surveying problem is, reduced to its essence, the use of apparent resistivity values from field observations at various locations and with various electrode configurations to estimate the true resistivities of the several earth materials present at a site and to locate their boundaries spatially below the surface of the site.

An electrode array with constant spacing is used to investigate lateral changes in apparent resistivity reflecting lateral geologic variability or localized anomalous features. To investigate changes in resistivity with depth, the size of the electrode array is varied. The apparent resistivity is affected by material at increasingly greater depths (hence larger volume) as the electrode spacing is increased. Because of this effect, a plot of apparent resistivity against electrode spacing can be used to indicate vertical variations in resistivity. The types of electrode arrays that are most commonly used (Schlumberger, Wenner, and dipole-dipole) are illustrated in

For this array (

In usual field operations, the inner (potential) electrodes remain fixed, while the outer (current) electrodes are adjusted to vary the distance s. The spacing a is adjusted when it is needed because of decreasing sensitivity of measurement. The spacing a must never be larger than 0.4s or the potential gradient assumption is no longer valid. Also, the a spacing may sometimes be adjusted with s held constant in order to detect the presence of local inhomogeneities or lateral changes in the neighbourhood of the potential electrodes.

AB/2 values ranged from 1.5 m to 55 m, ensuring down to 37 m of depth probe. It is envisaged that at this depth structures and bed rock that characterize the dam site would have been probed. Equation (6) was used in data reduction. Plots of ρ_{a} values against various AB/2 values on log-log graph gave the characteristic curves from which preliminary input models were made.

Since curve shapes, and not necessarily the ρ_{a} values, are employed in resistivity sounding data interpretation, the log-log plots were employed for the determination of number of geoelectric layers prior to modeling. In all, the A-type curve was shown where there is an increasing resistivity trend with depth. Between 4 and 5 geoelectric layers were proposed in the study area.

Preliminary input data from the field were fed into Zohdy software [

Based on these model interpretations, the geoelectric section along the proposed dam axis is shown in

This is the top loamy soil encountered in VES 1, 2, 3, 4, 5, 9, 10, 11 and 12 locations with maximum thickness of 1.5 m in VES 3 and 4. Resistivity values range from 216 to 519 Ohm-m.

This is modeled as the lateritic matter with maximum lower depth at 5 m in VES 3 and 4 points. This was not encountered in VES 6 point being replaced by alluvium. Resistivity values range from 101 to 6190 Ohm-m).

This is modeled as a restricted pebble bed only at VES 6, 7 and 8 locations flanking the river course. Thickness is about 3.5 m in these locations (6, 7, 8). Resistivity values ranged from 182 - 415 Ohm-m).

This is modeled as the alluvial matter again restricted to the river course 6, 7, 8 locations with base at between 12.5 m in VES 8 and 8 m in VES 6.

This is the last modeled layer composed of gravely sandstone that underlines the whole study area apart from a restricted pebble bed at the NE crestal portion (VES 12). This model is shown in

The top sandy agricultural soil and lateritic layers are restricted to the south west and northeast flanks (

We thus conclude that the Dam axis is underlain by predominant sandy mater with gravel bed flanking it. Lateritic matter of 3.9 m thickness exists on both axial ends. The southwest end has a more subdued topography than the northeast end. Gullying is a common feature in the area possibly due to the overriding unconsolidated sediment underlying the Dam axis. This must be taken into account as it would introduce arenaceous and detrital matter from the uplands into the reservoir end of the dam thereby causing pronounced siltation of the impounded water. The differential topographic highs may deserve attention. Lastly, the river is very vibrant and must be taken into consideration during construction.