The use of ground electrical survey methods for siting water-supply boreholes in shallow crystalline basement terrains

Abstract

Resistivity methods are widely used as an aid to siting water-supply boreholes in basement terrain. Adverse conditions due to shallow bedrock can be identified readily but specific targets such as narrow conductive zones may be missed; quantitative interpretation may be less reliable than is often assumed. Electromagnetic (EM) profiling will detect localized zones of deeper weathering and the effects of fracturing at shallow depths, and it is used for the precise location of anomalous ground indicated by lineations on aerial photography. Routine EM surveys give better lateral resolution than resistivity for the conductive targets of interest. The 2-D and 3-D EM modelling techniques needed for interpreting the conductor geometry realistically are still under development. Geophysical methods must be applied with due regard for the targets being sought and the local geological setting: they will not always be appropriate and resources should be targeted on districts where problems can be solved. Qualitative interpretations are adequate in some situations but more rigorous field procedures and analytical techniques are needed to ensure that useful hydrogeological information is obtained.

Anomaly effects of orthogonal paired-arrays for 3D geoelectrical resistivity imaging

“…The thickness of the weathered zone is assumed to vary between a minimum of 5.75 m (depth 8.25 m) at the centre of the model structure where the least weathering occurs to a maximum of 13.50 m (depth 16.0 m) at the edges of the model considered to be most weathered. The weathered zone in crystalline basement complex is a product of chemical weathering which is usually a low resistive saprolite overlying a more resistive basement rocks (Carruthers and Smith 1992;Hazell et al 1992). In addition, this zone is commonly aquiferous; thus, low resistivity model values varying between 150 and 100 X m were assigned to this layer.…”

Evaluation of groundwater resource potential in rural part of northcentral Nigeria using vertical electrical resistivity method

“…A common exploration strategy for groundwater in a crystalline basement terrains like this study area is to site water supply borehole where the regolith is thickest, the expectation being that it is under such circumstances that the saturated thickness is greater and the frequency of bedrock fissures also greater (Beeson and Jones, 1988;Carruthers and Smith, 1992;Chilton and Foster, 1995). However, in this aspect of research, there was a consideration on other parameters for deciding on the optimal and reliable borehole site.…”

Geophysical Investigation of Shallow Subsurface Fracture Distribution in Parts of the Eastern Basement Complex of Nigeria

“…Resistivity surveys are also commonly used to map fracture zones in hard rock terrain [1] and [2] because high resistivity contrasts usually occur between solid rocks and saturated fracture zones. It has also been revealed that the success of borehole siting in this kind of environment depends on the borehole intersecting some fracture zone in the bedrock.…”