Geophysics in site investigation

(Contributed by Dr P. W. McDowell, University of Portsmouth, and Dr M. C. Matthews, University of Surrey.)

Field geophysical surveys provide a means of mapping variations in ground conditions and properties. Geophysical data can be acquired rapidly and at relatively small cost, compared with the more conventional direct investigation methods. This can be very useful for large sites, for interpolation between, and extrapolation from, boreholes, and also for small sites where considerable lateral variability or small-scale anomalous features are anticipated. Also, unlike trial pits, geophysical surveys do not disrupt the ground conditions or damage services.

However, the interpretation of the data may not always lead to accurate results, particularly if the method used is inappropriate for the ground conditions and the survey is poorly designed to achieve the investigation objectives. It is only when the survey objectives have been properly considered in relation to the ground conditions and the targets of the investigation, the most appropriate geophysical techniques are employed and sufficient sub-surface information is available in order to calibrate the survey data that geophysics will provide reliable information. Even taking these precautions, geophysics cannot be regarded as a solution to all subsurface investigation problems, and there will be some cases where the use of geophysics will be totally inappropriate.