HYDROGEOPHYSICAL INVESTIGATION OF THE AREA AROUND ANGWARE IN JOS EAST PLATEAU STATE , NORTH CENTRAL NIGERIA

Hydrogeophysical investigation of the area around Angware and its environs was carried out. It is about 25kmand lies between latitudes 1000’N and 958’N and longitudes 905’E and 908’E. Schlumberger resistivity configuration method was used and interpreted with IP2Win 2.1 program. Geologically is underlain by Precambrian Basement complex(600±150Ma) and Mesozoic Younger Granites(130-150Ma).Computer aided results showed that the area is characterized by seven curve-types (A,H,Q,HA,QH,KA and QK) and 3,4 and 5 geo-electric layers with resistivity range and thickness of 100-692Ωm,1.4-51m; 15.7-1300Ωm,0.9-84m; and 72-504Ωm,0.9-36m respectively. The aquiferous zone revealed by the results is weathered/fractured Basement at a depth between 10-35m and resistivity range from 18-107Ωm, drilling depth of 20-45m is recommended for maximum yields.


INTRODUCTION
The nature of groundwater occurrence and distribution in typical Basement terrains is more complex due to the discontinuous pattern of crystalline aquifers.Successful groundwater development in such areas requires a quantitative knowledge of the hydrogeophysical parameters of the hydro-geologic units as well as the superficial materials overlying the crystalline rocks and the bedrock structures/relief, (Omosuyi et al, 2003;Olorunfemi et al, 1999 andJeff, 2006).Ground water development in areas of crystalline rocks depends on the climatic factors such as temperature, rainfall, humidity, relief and vegetation (Offodile, 2002).
In this study a detailed hydrogeophysical investigation of Angware and environs was carried out with a view of determining the geo-electric parameters (layer resistivity, thickness and aquifer potentials) of the superficial/overburden materials overlying the bedrock, the subsurface structural disposition of the bedrock and their hydrogeologic characteristics.The nature of the regolith materials overlying the bed rock in the study area is very influential in defining the groundwater potentials of the area.Vertical electrical sounding conducted in about thirty points was interpreted to establish the control of hydrogeophysics on the occurrence and distribution of groundwater in Angware area.

Geology of the Study area
The study area is located in Jos East Local Government Area (L.G.A) of Plateau State.It lies between latitudes 10 0 00'N and 9 0 58'N and longitudes 9 0 05'E and 9 0 08'E.The study area is part of maijuju sheet.Geologically the study area is underlain by Precambrian Basement Complex (600±150ma) and later intrusion of Mesozoic Younger Granites (130-150ma) as given by various workers including McLeod et al (1971), McCurry (1989), andTurner (1989).The study area is situated between the Shere Hills and Jarawa hills constituted by Undifferentiated Migmatites, Hornblende Biotite, Granite Porphyry, Older Biotite Granite, Early Rhyolite and Aplitic Pegmatite.(Fig. 1)

Materials and methods
The study was conducted in two phases; the geologic mapping preceded the hydrogeophysical investigation and was designed to collect and analyze rock samples with the aim of delineating areas with potentials of high yielding water wells.With the geology clearly established hydrogeophysical soundings were carried out at points well distributed to capture all the geologic Formations underlying the area.The main reason behind this approach was to bring out the geologic control on the hydrogeophysical parameters of significance in the groundwater occurrence and distribution.A total of twenty five probe points were investigated using campus allied ohmega resistivity, model number: 034CE to generate data on geoelectric layers, their resistivity, depths and thicknesses.(

RESULTS
The twenty five vertical electric sounding (VES) data acquired were processed using the partial curve matching method and computer interpreted curves.The result is presented on table1.The geo-electric layers range from three to five and the characteristic VES curves are shown in figures 3.0-3.2

Qualitative Analysis
The area constitutes a non uniform distribution of subsurface lithology in various locations, which is due to discontinuous weathering which is a common characteristic of crystalline rock typical of H-type curve (Offodile, 2002).
The HA-type curve, with resistivity p1>p2p<3<p4, constitutes 4% of the total curves and was obtained at VES3.The A-type curve with resistivity p1<p2<p3 and constitutes also 4% 0f the total curves and was obtained at VES2.

DISCUSSION
The study area is characterized by three -five layers geoelectric sections.The layers have been identified to portray different resistivities resulting from the variations in the lithologic units.The first layer is the top soil with resistivity and thickness range between 90-668Ωm and 0.9-2.35, the layer is zone of aeration, thus serve as medium for recharge.
The second layer is laterite, with a resistivity and thickness range between 668-2198Ωm and 0.58-9m.this is also part of the zone of aeration, but serves as shallow aquifers for hand dug wells.The third and fourth layers are weathered/fractured Basement with a resistivity and thickness range of 18.1-107Ωm and 10-35m, these are the expected aquiferous zones.
The fifth layer is slightly weathered-fresh unit, with a resistivity and thickness range from 139-745Ωmer and 4.08-100m, it is impervious, if fractured and well connected can serve as aquifer.

CONCLUSION
The results of the hydrogeophysical investigation revealed that 56% of the probe points are aquiferous.These correspond to weathered/fractured unit with thickness between 10-35m.The study area therefore is capable of producing ground water from borehole when drilled to depths of 20-45m.There is also need for proper well completion to check any likely contact of surface contamination of groundwater going by the shallow depth of its occurrence.
Department of Geology, Gombe State University, Gombe, Nigeria A. Olasehinde, Department of Geology, Gombe State University, Gombe, Nigeria I. S. Bute, Department of Geology, Gombe State University, Gombe, Nigeria

Figure 1 .
Figure 1.The Geological map of the study area Fig 2) Plots of such data produced different type curves ranging from Q-type curve, K-type curve, H-type curve 142 E. Y. MBIIMBE, A. OLASEHINDE AND I. S. BUTEA-type curve and combination of some of the type curves.The IPI2Win 2.1 version computer program was used to expedite the interpretation after manual partial curve matching.These data were interpreted to separate geologic units into aquiferous and non aquiferous.

Figure 2 .
Figure 2. The map of the study area showing VES points

Fig. 3 .
Fig.3.0.A three layers computer interpreted curve from the field data Logpigrand Geotechnics, three functional boreholes were drilled in 2008 by Fatigen Drillers, with mean yield of 2L/sec(Fatigen, 2009) and aquiferous zone corresponds to the weathered/fractured bed rock.The common drilled depth ranges from 25-35m depending on the location.The findings from the hydro-geophysical investigation are summarized on table2

Table 1 :
Interpreted Results of Field Data E. Y. MBIIMBE, A. OLASEHINDE AND I. S. BUTE

Table 2 .
Summary of the result showing groundwater potentials of the area