The Sekyere South District is underlain by the Basin Granitoids (Figure 1). These granitic rocks are related to the later stages of the Eburnean orogeny at or after the end of the Birimian deposition. They are often associated with schists and gneisses and intrude the Birimian meta-sedimentary rocks [2] [6] . The Basin Granitoids underlying the area are normally well foliated, often migmatic, potash rich Granitoids which take the form of muscovite-biotite granite, granodiorite, porphyroblastic biotite gneiss, aplites and pegmatites. The Granitoids are

characterised by the presence of many enclaves of schists and gneisses [6] .

One characteristic of these rocks is that it is not inherently permeable, but secondary permeability and porosity have developed as a result of fracturing and weathering. The hydraulic potential depends on the degree of fracturing and the potential recharge of the aquifer, which is directly related to the annual rainfall and water streaming.

Borehole information shows that, the depth to bedrock is shallow, ranging between 13 - 20 m with an average of 17 m. Fractures that aid in groundwater accumulation are not well developed and this therefore results in low groundwater potential in the area [7] .

The Sekyere South District located on a gentle hill is bounded to the north and south by two river valleys and is drained mainly by the Offin River to the west. Other streams which are tributaries to the Offin River such as Awyon and Bentiko are located in the northern and southern parts of Agona. The Kunkum stream flow from the east towards the west. There are other smaller streams that drain the area which flow from the north towards the south.

The Sekyere South District depends on boreholes (both mechanized and hand pump) installed for their water supply. The mechanized systems are located in the small towns including Agona and Jamasi and individual homes. The mechanized wells have an average yield of about 10 m³/hr [7] .

Aquifers can be obtained in the weathered and fractured granitic rocks which underlie the area. Boreholes obtaining its yields from both the weathered and the fractured zones are preferred for sustainable groundwater abstraction [7] . Recharge to the aquifers is from direct rainfall and the ephemeral streams in the area. The annual rainfall ranges between (1250 - 2000) mm. This suggests that there is a good groundwater recharge from precipitation.

Geophysical field measurements were carried out on pre-defined profiles using the electrical resistivity imaging (ERI) technique. These profiles were selected based on reconnaissance survey that was carried out. The profiles were selected to intercept the probable lineaments from air photo interpretation of the area. Measurements were carried out on eight profiles. Standard 400 m length profile measurements were carried out on each profile using the ABEM Terrameter SAS 4000. Measurement on a profile comprised the following:

・ Profile with 64 electrodes and 5 m spacing in the inner (100 to 300 m) cable loop and 10 m spacing at the outer (0 to 100 m and 300 to 400 m) cable lengths [4] .

・ Measurement configuration was the Wenner Long (WL) and the Wenner short (WS).

The ABEM Terrameter SAS 4000, which employs the static Lund automatic resistivity imaging systems suitable for high-resolution 2-D resistivity surveys and provides detailed information both laterally and vertically along the profile to investigate complex geological structures [9] [10] was employed. The resistivity data from the profile were inverted using the RES2DINV software to obtain the true 2-D resistivity model section [11] . Blue colors are generally used to represent areas of low resistivity and red colors identify relatively higher resistive areas [4] .

The 2-D model used for the inversion process also obeys the classical law of the resistivity theory and so the proposed model is one of the solutions and not the ultimate unique situation [12] . The geo-electrical model obtained by the inversion process is generally not a geological section and so interpretation must be carried out based on the considerations of the prevailing field geological conditions [12] .

Pumping test analysis was carried out on five (5) existing boreholes in the area. They were redeveloped by airlifting for the purpose of cleaning and establishing airlift yields since there were no available records.

Figures 2-9 give results of the ERI on profiles A to H in the area and the information on these profiles are summarized in Table 1.

It is observed from the pseudo-sections that the area is underlain by relatively high resistive rocks. The areas with very low resistivity distribution in such geology are therefore regarded as potential targets for groundwater accumulation. The profile A is carried out to calibrate one of the existing boreholes (station 200 m) in the area (Figure 2) to serve as a guide in the analysis.

The resistivity distribution of the area varies between (50 - 1200) Ω-m. The borehole (BH) located at station 200 m is obtaining its yield from the low resistivity distribution less than 500 Ω-m to a depth of 20 m. However, a possible borehole location at station 140 m which has resistivity values less than 150 Ω-m at depth (12 - 20) m could have resulted in a higher yield. Stations (0 - 160) m depicts low resistivity zones between the depths of (5 - 30) m which reduces in depth along the profile to station 400 m. The depths range has been able to produce successful boreholes in such granite geology.

The resistivity distribution of profile B varies between (80 - 2000) Ω-m (Figure 3). The profile shows three distinct resistivity layers. The first resistivity layer occurs to a depth of 9 m below the surface with resistivity values ranging from (300 - 1000) Ω-m. The second layer has low resistivity values (80 - 250) Ω-m with a thickness of 25 m. High resistivity of rocks above 1000 Ω-m is observed after 45 m.

Along profile C, it is observed that high resistivity values (>1000 Ω-m) occurs close to the surface (Figure 4). This is an indication that the Granitoids outcrop close to the surface in the area. After a depth of 9 m on the modeled section, a low resistivity layer between (100 - 300) Ω-m is located at stations (0 - 120) m and (240 - 300) m. These could indicate water bearing zones. Resistivity values between (500 - 800) Ω-m are observed after a depth of 30 m.

Profile D has a resistivity distribution of (20 - 1500) Ω-m. Along the profile, three distinct resistivity regions are observed (Figure 5). Regions with very low resistivity between (20 - 300) Ω-m are observed between stations 100 - 275 m, and are located between depths of (8 - 30) m. Regions with moderate resistivity (400 - 800) Ω-m surrounds the low resistivity areas. Resistivity values greater than 1000 Ω-m are observed between stations (235 - 275) m and with a thickness of 33 m.

Three distinct horizontally layered resistivities are observed beneath profile E (Figure 6). The first resistivity layer has values between (100 - 400) Ω-m, which occurs between depths of (8 - 30) m. The second resistivity layer occurs between depths of (30 - 42) m and has resistivity values ranging between (500 - 900) Ω-m. Resistivity values greater than 1000 Ω-m are observed below a depth of 45 m and also outcrops close to the surface.

Three significant low resistivity anomalous zones were observed beneath profile F at stations 85 m, 205 m and 310 m (Figure 7). These anomalies represent weak zones which may be suitable for the drilling of a productive borehole. The depths at which they occur are in the range (10 - 40) m which is similar to anomalies observed on profiles B and E. The resistivity distribution of the area varies between (200 - 3800) Ω-m.

Along profiles G and H, low resistivity zones (100 - 300) Ω-m are observed at depths (8 - 30) m. These zones are suspected to be zones of weakness and therefore suggest water bearing zone. Profiles G and H have resistivity distribution of values between (100 - 2000) Ω-m (Figure 8 and Figure 9).

Generally, the resistivity distribution of the area ranges between 90 - 4000 Ω-m with the low resistivity ranges (90 - 400) Ω-m occurring at relative depths of between 8 - 20 m. The modeled sections revealed that areas where low resistivity values forms bowl-shape which are underlain by high resistivity distribution yield water [4] . The high resistivity value which underlies the bowl-shape serve as an impermeable unit which retains the water. From the profiles, it can be deduced that groundwater in the area is controlled by the overburden thickness and fractures which have very low resistivity distribution. Information on resistivity distribution related to groundwater occurrence is summarized in Table 2. The groundwater potential has been categorized into very good, good and poor potential according to the resistivity distribution of the Granitoids in the area. In the weathered rock, resistivity of (50 - 300) Ω-m are likely to yield water, (100 - 600) Ω-m in the fractured rocks has a very high probability for groundwater occurrence and resistivity >1500 Ω-m are likely to result in dry wells, thus low groundwater potential.