Hello Mr K. Fitzgibbon,

In hard rock environments, water yield generally depends on the tectonic history, which in turn, controls the extent of deformation of the rocks. The results of the tectonic disturbances are that the rocks will fracture thereby acquiring secondary porosities necessary for the transmission of fluids in the processes.

From my personal experience in the Oban Massif and Obudu Plateau and adjoining areas in Nigeria, where various types of hard rocks exist, fresh rocks do not usually have reasonably fractures so as to accommodate and transmit reasonable quantities of water. However, if they intrude into impervious sediments like clays and shales, they cause them to become baked, domed and fractured thereby acquiring secondary necessary for fluid transmission.

An experienced geophysicist can use inexpensive technique like VES in mapping these fractures. But I will strongly advise you against using available existing VES data because of the high levels of deceit and instrument sensitivity problems. I will rather appeal that you acquire all data by the team and then sit together to look at them critically. Borehole failures in such environments are usually very high and this, on its own, can be very frustrating. Without good geological information and good quality geophysical data, please be mindful of how you put money into the project.

If your project location was to be in or close to my country, I could have loved to join your project team.

I agree with Antony. Without natural fractures and fracture zones, there is no hope to have water flow from a well in granite. Fracture zones would be vertical and can be located say 50 meter apart. Between fracture zones there is unfractured solid granite. You need the combined help of an experienced geologist able to recognise open fractures and fracture zone trends, and an experienced geophysicist able to design a suitable survey and interpret the results.

You mention "underlying" granite. Does it means that there is non-granite rock above the granite? Is such rock (or sediment) permeable? As an alternative, try to identify zones with thicker sediments, possibly a buried paleo river bed, or thicker weathered granite.

Dear Kevin,

I agree with Anthony and Roberto I just want to add: If you can identify major fault zones than their surrounding damage zones might be a good target. Additionally I saw in my studies, that single fractures in contact metamorphic aureoles are hydraulically more active in any rock (maybe besides other intrusions). However, open fractures are usually spaced far away from each other.

Dear Fitzgibbon,

I agree absolutely with all the contributors so far. Just to add a few suggestions. I strongly believe that the granitic rock within the basement terrain under consideration has gone through a series of tectonic episodes, i.e., must have been intruded by other rock materials which probably will result in baked and deformed portions within the host rock. Intense deformation creates both macro- and micro-fractures which can be mapped using well articulated electrical resistivity techniques involving VES and ERT. Possible areas of multiple fracturing can be identified which could provide adequate transmissivity. Additionally, since you are considering depths of ~130m, the EM methods can also be employed as a means of validating the results of the former, in the absence of existing nearby productive water boreholes, hence, reducing the ambiguity problems associated with stand-alone method.

Just a comment that depending on the exposure/uplift history,  close to the top of the granite you may get a weathered zone and or sub-horizontal exfoliation  joints which could provide permeability and a little storage. The question would then be whether these are connected to any recharge or greater storage.  But what's really needed is local knowledge of the geology.

I agree fully with all the above suggestions. But I would suggest to take help of remote sensing data or study a satellite imagery. Before that you can collect the data of the existing bore wells in the surrounding and their drilling history. These data will be very useful in deciding for VES as well as weather to drill or not. It may be noted as well that the deeper we drill the fractures tend to close.

To All who provided answers and insight to my question above: thank you. Your suggestions are entirely sensible and valid, and clearly come from good knowledge of relevant disciplines, and from experience. I apologise for my delay in replying. Our concern at the time was the seemingly random recommendations and 'geophysical survey' results we had received. But then we discovered a company who had previously drilled several boreholes in the area without success. Even though I suspect they did not use any advanced techniques to identify the locations, this did not bode well for our project, which had enough for a single drilling effort only. But the village in question does have access to a surface waterhole in a shallow ground depression, so I think this was the most obvious way to address their water security needs - even though it needs treatment. Many thanks again - I just thought I would give you feedback on how it turned out. Kind regards.