Abstract
The low borehole yielding potential and the high drilling failure rate of the Voltaian sedimentary rocks of Northern Ghana have been of concern to many local hydrogeologists and international donors. Consequently, several donor-supported projects have been instituted within the last few years with the view to study the hydrogeological characteristics of this ‘difficult’ rock system. One such project is the geophysical borehole logging of 13 boreholes drilled into the Voltaian sedimentary rocks of Northern Ghana to enhance detailed hydrogeological assessment. Natural gamma detectors embedded in the five exploratory logging tools employed for the study ensured depth control by comparing their individual gamma log signatures. The combined gamma and formation resistivity/conductivity response logs provided more detailed lithological information than were shown in the driller’s/geologist’s logs. Significant discrepancies between the logging results and the reported drilled depths, construction depths, and screen settings were observed in seven of the thirteen investigated boreholes. Thus, the reliability of driller’s borehole records seems questionable, which will hamper hydrogeological studies and the mapping of groundwater resources. Further, it may be supposed that the productivity of most wells in Ghana is compromised by poor depth control of screen placement.
Highlights
Many hydrogeologists and groundwater experts have raised serious concerns about the low drilling success rate and the poor borehole yielding potential of the Paleozoic sedimentary rock system of Northern Ghana (Martin and van de Giesen 2005)
The low borehole yielding potential and the high drilling failure rate of the Voltaian sedimentary rocks of Northern Ghana have been of concern to many local hydrogeologists and international donors
The analyses and interpretation of the logging data in terms of drilled depths, construction details and profiles of soils/ rocks penetrated were compared with records obtained from the supervising geologist/driller to check for similarities and discrepancies
Summary
Many hydrogeologists and groundwater experts have raised serious concerns about the low drilling success rate and the poor borehole yielding potential of the Paleozoic sedimentary rock system of Northern Ghana (Martin and van de Giesen 2005). The Pendjari Oti comprises well-consolidated and closely compacted and impermeable rocks, whilst the Obosum group largely comprises variegated mudstone, clays, and siltstone interbedded with micaceous sandstone and interbedded conglomerates (Bates 1966). Many hydrogeologists such as Gills (1969), Frempong and Kortatsi (1994), MacDonald et al (1996), DapaahSiakwan and Gyau-Boakye (2000), Acheampong and Hess (2000) and Agyekum and Dapaah-Siakwan (2007) have reported about the generally poor yielding potential of the Voltaian rocks, though varying widely from 0.3 to 72 m3/hr (5–1,200 lpm) with the higher yields recorded in areas underlain by quartzitic sandstone rocks. X swl static water level a X means logging probe used on borehole b –means probe not used on borehole
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