Thomas Armah

Geophysical borehole logging for control of driller’s records: hydrogeological case study from Voltaian sedimentary rocks in northern Ghana

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.

Application of geochemical and stable isotopic tracers to investigate groundwater salinity in the Ochi-Narkwa Basin, Ghana

ABSTRACT Rainwater, groundwater and soil-water samples were analysed to assess groundwater geochemistry and the origin of salinity in the Ochi-Narkwa basin of the Central Region of Ghana. The samples were measured for major ions and stable isotopes (δ18O, δ2H and δ13C). The Cl− content in rainwater decreased with distance from the coast. The major hydrochemical facies were Na-Cl for the shallow groundwaters and Ca-Mg-HCO3, Na-Cl and Ca-Mg-Cl-SO4 for the deep groundwaters. Groundwater salinization is caused largely by halite dissolution and to a minor extent by silicate weathering and seawater intrusion. Stable isotope composition of the groundwaters followed a slope of 3.44, suggesting a mixing line. Chloride profiles in the soil zone revealed the existence of salt crusts, which support halite dissolution in the study area. A conceptual flow model developed to explain the mechanism of salinization showed principal groundwater flow in the NW–SE direction. EDITOR D. Koutsoyiannis ASSOCIATE EDITOR K. Heal

Integrating gamma log and conventional electrical logs to improve identification of fracture zones in hard rocks for hydrofracturing: a case study from Ghana

Hydrofracturing of low-yielding boreholes in hard rocks is a widely used technique in Africa for improvement of yield, thus making them qualified for installation of a hand-pump for domestic water supply. However, the success rate of the hydrofracturing campaigns seems not to be that high as generally claimed by contractors. One reason amongst others might be that the selection of zones for hydrofracturing in the individual borehole is based on pre-hydrofracturing investigation using conventional electrical logs only. Thereby, the zones selected are the occurring resistivity minima interpreted as weak zones with some fracturing. However, resistivity minima can also be caused solely by lithological reasons, which then in most cases could have been seen on a gamma log as corresponding increased gamma radiation. The advantages of using gamma logging in combination with conventional electrical logging technique for prediction of fractured zones in basement rocks is illustrated by investigations of three low-yielding boreholes located in different geological environments in crystalline basement rocks in Ghana.

Comment on “Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana” ()

We draw the reader’s attention to a statement made in the abstract and introduction that this study “is the first of its kind in Ghana” Boadu et al., 2005. The authors did good work in their case study, but we wish to state that this survey is not the first of its kind in Ghana, as they claim. In 1996, we conducted a similar survey and published the results in a local journal, as follows: “Determination of fracture trends in groundwater exploration in the Dahomeyan terrain, Ghana, using azimuthal resistivity survey,” 1997,Ghana Mining Journal,3, no. 1 and 2, 43–48. Groundwater exploration in Ghana is inundated with numerous challenges. Among these is the obvious lack of exposure of favorable structural targets for drilling because of the levels of weathering in most areas. This situation renders the application of aerial photographs a bit difficult, particularly in identifying trends of structural features required for setting up oriented geophysical field surveys. As a way around this problem, we adopted the azimuthal resistivity field techniques described by Taylor and Fleming 1988 and Skjernaa and Jorgensen1994. In our work, we showed how surface azimuthal resistivity, which involves measurements of the anisotropy resistivity characteristics of subsurface rocks, was used to investigate trends of fractures and other conductive zones in basement terrains of the Dahomeyan System in southeastern Ghana. The resulting anisotropy figuresRAFs representing the subsurface resistivity distributions were matched against subsurface-conductivity contour maps generated from very low-frequency electromagnetic VLF-EM surveys. These were found to complement each other in defining the conducting trends in the area, which were confirmed by follow-up drilling. Our surveys were carried out in areas underlain by the middle Precambrian Dahomeyan System consisting of alternate belts of felsic and mafic gneisses, which are characterized by crosscutting pegmatitic veins. These pegmatitic veins often weather to form ingresses for groundwater percolation and accumulation. The various areas were surveyed initially using VLF-EM techniques on a series of parallel traverses oriented orthogonally to the trends of observed surface features in the areas. The VLF-EM data then were processed into contour maps to delineate the spatial trends of inferred subsurface conductors. Sections of the inferred VLF-EM anomalies were investigated further with the azimuthal resistivity method using the Schlumberger electrode array with three different current electrode separations. The current electrode separations were used to determine the continuity, trends, or other aspects of the inferred resistivities with depth. The azimuthal data were processed to generate RAFs, which then were compared to the VLF-EM contours at the various points of measurement. From a total of seven sites at different locations in southeastern Ghana, the RAFs showed multiple axes with the maximum trending in the same direction as the VLF-EM inferred anomalies. It was concluded therefore that the azimuthal resistivity method can be used for reconnaissance surveys to establish the trends of subsurface conductors, which then could be used to set a grid system for other detailed surveys, particularly where satellite and/or aerial photographic interpretations are difficult or unavailable.

Comment on "Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana" "F. K. Boadu, J. Gyamfi, and E. Owusu, 2005, GEOPHYSICS, 70, no. 5, B35-B42…

We draw the reader’s attention to a statement made in the abstract and introduction that this study “is the first of its kind in Ghana” Boadu et al., 2005. The authors did good work in their case study, but we wish to state that this survey is not the first of its kind in Ghana, as they claim. In 1996, we conducted a similar survey and published the results in a local journal, as follows: “Determination of fracture trends in groundwater exploration in the Dahomeyan terrain, Ghana, using azimuthal resistivity survey,” 1997,Ghana Mining Journal,3, no. 1 and 2, 43–48. Groundwater exploration in Ghana is inundated with numerous challenges. Among these is the obvious lack of exposure of favorable structural targets for drilling because of the levels of weathering in most areas. This situation renders the application of aerial photographs a bit difficult, particularly in identifying trends of structural features required for setting up oriented geophysical field surveys. As a way around this problem, we adopted the azimuthal resistivity field techniques described by Taylor and Fleming 1988 and Skjernaa and Jorgensen1994. In our work, we showed how surface azimuthal resistivity, which involves measurements of the anisotropy resistivity characteristics of subsurface rocks, was used to investigate trends of fractures and other conductive zones in basement terrains of the Dahomeyan System in southeastern Ghana. The resulting anisotropy figuresRAFs representing the subsurface resistivity distributions were matched against subsurface-conductivity contour maps generated from very low-frequency electromagnetic VLF-EM surveys. These were found to complement each other in defining the conducting trends in the area, which were confirmed by follow-up drilling. Our surveys were carried out in areas underlain by the middle Precambrian Dahomeyan System consisting of alternate belts of felsic and mafic gneisses, which are characterized by crosscutting pegmatitic veins. These pegmatitic veins often weather to form ingresses for groundwater percolation and accumulation. The various areas were surveyed initially using VLF-EM techniques on a series of parallel traverses oriented orthogonally to the trends of observed surface features in the areas. The VLF-EM data then were processed into contour maps to delineate the spatial trends of inferred subsurface conductors. Sections of the inferred VLF-EM anomalies were investigated further with the azimuthal resistivity method using the Schlumberger electrode array with three different current electrode separations. The current electrode separations were used to determine the continuity, trends, or other aspects of the inferred resistivities with depth. The azimuthal data were processed to generate RAFs, which then were compared to the VLF-EM contours at the various points of measurement. From a total of seven sites at different locations in southeastern Ghana, the RAFs showed multiple axes with the maximum trending in the same direction as the VLF-EM inferred anomalies. It was concluded therefore that the azimuthal resistivity method can be used for reconnaissance surveys to establish the trends of subsurface conductors, which then could be used to set a grid system for other detailed surveys, particularly where satellite and/or aerial photographic interpretations are difficult or unavailable.

Comment on “Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana” (F. K. Boadu, J. Gyamfi, and E. Owusu, 2005, GEOPHYSICS, 74, no. 3, B35–B42)Azimuthal resistivity surveys in Ghana

We draw the reader’s attention to a statement made in the abstract and introduction that this study “is the first of its kind in Ghana” Boadu et al., 2005. The authors did good work in their case study, but we wish to state that this survey is not the first of its kind in Ghana, as they claim. In 1996, we conducted a similar survey and published the results in a local journal, as follows: “Determination of fracture trends in groundwater exploration in the Dahomeyan terrain, Ghana, using azimuthal resistivity survey,” 1997,Ghana Mining Journal,3, no. 1 and 2, 43–48. Groundwater exploration in Ghana is inundated with numerous challenges. Among these is the obvious lack of exposure of favorable structural targets for drilling because of the levels of weathering in most areas. This situation renders the application of aerial photographs a bit difficult, particularly in identifying trends of structural features required for setting up oriented geophysical field surveys. As a way around this problem, we adopted the azimuthal resistivity field techniques described by Taylor and Fleming 1988 and Skjernaa and Jorgensen1994. In our work, we showed how surface azimuthal resistivity, which involves measurements of the anisotropy resistivity characteristics of subsurface rocks, was used to investigate trends of fractures and other conductive zones in basement terrains of the Dahomeyan System in southeastern Ghana. The resulting anisotropy figuresRAFs representing the subsurface resistivity distributions were matched against subsurface-conductivity contour maps generated from very low-frequency electromagnetic VLF-EM surveys. These were found to complement each other in defining the conducting trends in the area, which were confirmed by follow-up drilling. Our surveys were carried out in areas underlain by the middle Precambrian Dahomeyan System consisting of alternate belts of felsic and mafic gneisses, which are characterized by crosscutting pegmatitic veins. These pegmatitic veins often weather to form ingresses for groundwater percolation and accumulation. The various areas were surveyed initially using VLF-EM techniques on a series of parallel traverses oriented orthogonally to the trends of observed surface features in the areas. The VLF-EM data then were processed into contour maps to delineate the spatial trends of inferred subsurface conductors. Sections of the inferred VLF-EM anomalies were investigated further with the azimuthal resistivity method using the Schlumberger electrode array with three different current electrode separations. The current electrode separations were used to determine the continuity, trends, or other aspects of the inferred resistivities with depth. The azimuthal data were processed to generate RAFs, which then were compared to the VLF-EM contours at the various points of measurement. From a total of seven sites at different locations in southeastern Ghana, the RAFs showed multiple axes with the maximum trending in the same direction as the VLF-EM inferred anomalies. It was concluded therefore that the azimuthal resistivity method can be used for reconnaissance surveys to establish the trends of subsurface conductors, which then could be used to set a grid system for other detailed surveys, particularly where satellite and/or aerial photographic interpretations are difficult or unavailable.