|
British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
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.
