GEOPHYSICAL INVESTIGATION FOR GROUND WATER EVALUATION AND DEPTH TO AQUIFER DETERMINATION

DEPARTMENT OF GEOLOGY AND GEOPHYSICS, FEDERAL UNIVERSITY,

 INTRODUCTION

The report highlights the result of geophysical survey made at Bishop Court in Onwe Road, Abakaliki, Ebonyi State. The survey aimed to located aquiferous unit for a groundwater borehole.

GEOLOGY

The area lies within the Asu River Group and comprises shale interbedded with siltstone, thin beds of

hard splintery metamorphose mudstone and granite that are highly jointed, fractured and faulted, clays and overburden `~overlie the area.

TABLE 1:  SCHLUMBERGER ELECTRICAL DATA SOUNDING SHEET

AB/2(m)	MN/2 (m)	Apparent Resistively (ohm-m)
2		603.1462
3		447.5779
6	1	490.8668
9		400.8721
9		335.1996
15		217.168
25	4	81.70575
40		52.01427
50		40.9571
75		42.70032
75		38.46008
100	20	30.51497
150		42.33405
200		42.24827

A total of 14 station data points were acquired as shown above.  The apparent resistivity at each station was generated by multiplying the resistance with the   geometric factor (K-) for Schlumberger electrode array. The data was curve matched and interpertated manually and automated using computer aided techniques and combination of software – Winglink and IP12 Win.

The investigation area is located in a hydrogeological complex terrain underlain basically by the Asu River Group sediments. These comprise mainly the Abakaliki Shale Formation with the associated siltstone, limestone and silicic sandstone.  Groundwater exploration in this area is thus difficult due to the low permeability of this rock unit. This low permeability makes this unit an aquitard. This situation is similar to that in basement terrain.

RESULT AND DISCUSSION

From the geo-electric sounding data (Fig 2), apparent resistivity curves were made from which a geo-electrical distribution  (Table 1) and geo – electrical section figure 3 were prepared. Depth to water   table was met at 40m while depth to ground water head was interpreted to be 52m. 

RECOMMENDATION

Following the interpretation of the VES curve, depth to aquifer was met at 40m. We therefore recommend a borehole depth of 52m.