Abstract
This study highlights the application of geophysical and remote sensing/GIS techniques in the study of the proposed Otobi dam ax- is/foundation in southeastern Nigeria in order to establish: optimal depth to bed rock, the rock type which make up the foundation, the geological struc- tures of the foundation, overburden thickness, concealed basement morphology, fractures /seepage channels in the subsurface thus, enabling the eval- uation of the suitability of the proposed dam foundation. In this study, lineaments have been extracted directly from digital TM Land sat image after a series of enhancements and bands ratioing.The Arc GIS version 10.0 and ILWIS 3.1 was used to generate thematic drainage maps, lineament maps and rose diagram of structures. The interpretation of the satellite image of the area showed that the proposed dam foundation is dominated by NE-SW trend- ing fractures and this is in tandem with most of the fractures observed on outcrops of Otobi sandstone in the area. This was followed by an electrical resistivity profiling along the proposed dam axis using the Wenner array. The 2D resistivity imaging reveals that the upper layer of the study area to be 7- 10metres thick and consists of alluvial and residual soils with resistivity values ranging from 8-100 ohm-m. The second layer is over 10metres thick and has a resistivity value of 5-50ohm-m and made up of moderately weathered shales and mudstones with parallel permeable groundwater rich zones which are indicative of fractures. The results of the study indicate that the proposed Otobi dam foundation is underlain by numerous fractures which are detrimental to the dam and could serve as seepage channels through the dam.

Index Terms— Dam foundation, Electrical resistivity, Geographic information system, Lineament, Nigeria, Otobi, and Remote sensing, 


1. Introduction

The study are is Otobi village  which is about 12km southwest of Otukpo twon and about 1km northwest of Otobi community headquarters. It is in the Otukpo Local Government Area of Benue State and lies between longitude 8° 03' 06'' and 8° 06' 17'' East and Latitude 7° 05' 58'' and 7° 08' 08'' North (Fig.1 and 2). The proposed dam is to be constructed along the northwest southeast trending Okpokwu river.

Dams are among the largest and most important projects in civil engineering (Coduto, 1999). For geologic, hydrologic and topographic reasons, there are limited numbers of ideal sites for dams.

It is therefore very important to carry out geotechnical and geophysical investigations on any proposed dam site (Oladapo, 2011). The information provided by the study is expected to aid dam site investigation. Dams intended for wa- ter supplies require a low tolerance of seepage losses. Besides, the design of dam structures must be adapted to the existing site conditions (Ajayi et al., 2005) to minimize the losses. Fail- ure to do any of these may invariably result in unplanned seepage and/or total collapse of the structure (Olorunfemi et al., 2000). Biswas and Charttergee (1971) examined causes of dam failures worldwide and discovered that 25% of the fail- ures were due to geotechnical problems associated with seep- age, inadequate seepage cut-off, faults, settlements and land- slides. 

The proposed dam site location and dam orientation(Figure 2) was selected on the basis of existing hydrologic and hydraulic studies together with preliminary engineering geology investigations and testing conducted by the federal ministry of water resources in Nigeria.

The present study is an assessment of the dam axis/foundation using an integration relatively recent reomote sensing/gis approach and the conventional 2D resistivity technique to further investigate site geology, nature of over- burden materials, the geological structures and drainage char- acteristics of the proposed dam axis and reservoir areas. This paper present the result of an integrated geotechnical geo- physical survey and GIS studies of Otobi dam ax- is/foundation. 

Fig. 1. Map of Nigeria showing the study area. (Modified after Obaje 2009) 

2 GEOLOGIC SETTING OF STUDY AREA

The study area is specifically underlain by the rocks of the Anambra Basin (Nkporo Shales and Otobi Sandstones), SE Nigeria. The Anambra Basin is a Cretaceous sedimentary ba- sin, located in the southeast portion of Nigeria and bounded by the Abakaliki anticlinorium of the southern Benue Trough to the east and by the Basement Complex of the southwestern Nigeria to the west. 

Fig. 2. Geological map of the study area showing the different sedimen-tary units. 

Geologically, the origin of the Anambra Basin(Fig 3) is intimately related to the tectonic and sedimentary cycles responsible for the origin of the adjoining southern Benue Trough during the separation of African from the South American plate in the Mesozoic era. Studies of Nwachukwu hoque and ezepue and ladipo among others, have shown that, subsequent to the uplift of the Benue- Abakaliki fold belt during the Santonian, the geological histo- ry of the Anambra Basin is linked to the post-Santonian sub- sidence of the then Anambra platform. This was followed by a series of trans-regression cycles leading to sedimentation of about 6 km thick of Cretaceous and Tertiary sediments (Obaje et al. 1999) within the basin to the west of the Abakaliki uplift during the Campanian–– Paleocene. 

Fig. 3. A cross-sectional view of the sedimentary units within the Anam- bra Basin (After Aghamelu OP and Okogbue CO (2011) 

A summary of the main sedimentary units within the basin is briefly described below: 

(a) Enugu-Nkporo Shales Group: These early Campanian units underlie the eastern plain of the Udi-Enugu escarpment and consist of dark grey fissile, soft shale and mudstone with maximum thickness of about 1,000 m and characterized by interbedded sandy units and sulfur coated marl. A shallow marine environment was predicted due to the presence of foraminifera and ammonites (Reyment 1965; Agagu et al. 1985).In general the Nkporo group and their lat- eral/stratigraphic equivalents (i.e. Afikpo Sandstone, Enugu formation, Otobi Sandstone, Lafia Sandstone, Owelli Sand- stones etc) overlies an angular unconformity. The group com- prise the lowest stratigraphic units of the Anambra Basin. The Nkporo group consists of dark grey to black fissile, friable and soft shales and mudstones with occasional thin beds of sandy shale, fine sandstone and marl with coatings of Sulphur.

(b) Mamu Formation: This is a Late Campanian sedimentary unit, also known as the ‘‘Lower Coal Measures’’. It consists of mainly dark blue to grey shales/mudstones units with alter- nating sandy units and coal seam horizons to form a character- istically stripped rock unit. The Mamu Formation was depos- ited as shallow water of the paralic facies of a deltaic complex 

(c) Ajali Formation: This is a Maastrichtian sandy unit overlying the Mamu Formation and consists of white, thick friable, poorly sorted cross-bedded sands with thin beds of white musstone near the base. It is characterized by large scale cross bedding with dip angle as high as 20. Studies have suggested that the Ajali Formation is a continental/fluvio-deltaic sequence, characterized by a regressive phase of a short-lived Maastrichtian trans-regression with sediments derived from westerly areas of the Abakaliki anticlinorium and the granitic basement units of Adamawa-Oban massifs to the eastern side of the basin (Benkhelil 1986; Amajor 1987; Ladipo 1986, 1988; Adediran 1991). 

(d) Nsukka Formation: This is a Late Maastrichtian unit, lying conformably on the Ajali Formation and consists of alternating succession of sandstone, dark shales and sandy shales with thin coal seams at various horizons, hence termed the ‘‘Upper Coal Measures’’. The sedimentary successions indicate a paralic environment.

Surface geologic mapping revealed that the area is mainly un- derlain by the Nkporo shale formation and the Otobi sand- stone which are obscured by weathered shale and mudstones forming dark overburden in some parts of the area and red- dish to brownish top soils in other parts (Fig 2).

Exposures of the Nkporo shales were observed along the Ok- pokwu stream and consist essentially of clays, shales and mudstones. The Nkporo shales are highly weathered in most parts of the study area and are seen as residual soils (clays, silty clays and sandy clays).At the Southeastern part of the study area there are sandstones exposures which represents the Otobi sandstones. The sandstones displayed a high degree of jointing which suggests that the area has been affected by a tectonic episode in the past. Along the Okpokwu River, there are alluvial deposits mainly composed of silty clay, clayey silt and sandy and gravelly clay of Quaternary age.

3. MATERIALS AND METHODS

The study commenced with a reconnaissance survey which was accompanied by a geological mapping, a remote sensing and geographic information system studies and geophysical investigation. The geological investigation in the study area was done to ascertain the information provided by the existing geological maps of the area. Based on the observed field geol- ogy a geological map of the area was produced (Fig 2).

For the study, existing hydrogeological and relevant data on soils, geological/lithological units, and structural features, geomorphologic and climatic conditions of the study area were collated. The overall study concept involved integration of relevant thematic layers of conventional geology, drainage and lineament maps, using ArcGIS 10.0 and ILWIS 3.1 GIS software. All of the map themes were presented in UTM Projec- tion Zone 31, Datum WGS84. 
 

3.1 Preparation of thematic layers 
The anglogue drainage, lineament maps of the study area abtained from Nigeria Geological Survey Agency(NGSA) with scale 1:100,000 were georeferenced and digitized in ArcGis 10.0 software platform (Fig 4, Fig 5 and Fig 6 respectively). These were also complimented with data from Landsat images obtained from the National Centra for Remote Sensing in Nigeria (NCRS). Lineaments are manifestation of linear features that can be identified directly on the rock units or from remote sensing data while lineaments and their intersections play a significant role in the occurrence and movement of groundwater resources in rocks (Rao 2006; Prasad et al. 2008). The presence of lineaments may act as a conduit for ground- water movement which results in increased secondary porosi- ty and, therefore, can serve as groundwater potential zone or otherwise (Obi Reddy et al. 2000).

Drainage pattern reflects the characteristic of surface as well as subsurface formation. The drainage pattern of the basin also provides quantitative description of basin geometry. Drainage map (Fig 5) was obtained from DIVAGIS 

Subsequent to the preparation of all the different thematic maps (drainage density, geology and lineament thematic maps) with varied attributes, the maps were converted into raster format.

The geophysical studies was done in collaboration with Han- som Engineering Consultant Limited. A multi-electrode resis- tivity surveying system was used, which is composed of WDA-1 Super Digital DC Resistivity/IP Meter and WDZJ-3 Multi-electrode Converter. Three boxes of 90V batteries were used to provide working power. A pocket PC with blue tooth was used for data receiving, displaying and memorizing measured results. The BT-WYS electric Processing Specific Program was used to make the interpretation more convenient and visual. 

A total of 3.8km geophysical surveying profiles was accom- plished during the work.
The DC High Density Electrical Resistivity Survey was em- ployed in this study using the Wenner configurations. In the Wenner array configuration, potential electrodes are nested within the current electrodes with a common lateral distance between adjacent electrodes called the electrode a-spacing. For sounding measurements, the electrodes in a Wenner array are expanded about a center point by equally incrementing the a- spacing. The current therefore progressively passes into deeper layers, with the nominal depth of investigation being equal to the a-spacing. This procedure provides apparent re- sistivity values that are dependent upon vertical conductivity variations of the subsurface.This configuration was selected because it generally provides for high signal-to-noise ratios, good resolution of horizontal layers, and good depth sensitivi- ty(Ward,1990).

The unit electrode spacing of 10m was used for the study and the profile surveyed for one time is 590m long with 60 elec- trodes (each side of the station 30 electrodes) are arranged along the proposed dam axis in a straight line. The investiga- tion depth is almost 25m. 

4. RESULTS AND DISCUSSION
4.1 Geological mapping
Surface geologic mapping revealed that the proposed dam site is mainly underlain by the Nkporo shale formation and the Otobi sandstore(fig.2) which are obscured by the weathered shale and mudstones forming dark overburden in some parts of the are and reddish to brownish top soils in other parts

4.2 Remote sensing/Geographic information system

Figures 4 and 5 are drainage maps generated from satellite image and topographic map respectively. It would be ob- served that the drainage map in Figure 4 shows that some of the rivers have gone extinct as compared with the drainage from the topographic map (Fig. 5) and based on the dam axis initial plan it has to be adjusted to be able to meet up with the three main rivers tributaries (Figure 4).This would significant- ly increase the overall discharge into the dam. 

Fig. 3. Drainage map derived from Topographic map.