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Technical paper: The use of geophysical methods in engineering geology - part 2 - Electrical resistivity, magnetic and gravity methods

By I E Higginbottom, Wimpey Laboratories

This paper was first published in GE’s March 1976 issue.

Part 1 in this series of papers was written by A N Burton on the subject of seismic techniques and was published in GE’s January 1976 issue.

Introduction

These methods are used on a smaller scale in civil engineering than those described in part 1 in this series of papers. On the whole, they are less suited than seismic techniques for determining the depths of interfaces in layered systems, such as the profiling of —rockhead below weaker superficial deposits probably the most important geophysical problem in civil engineerng but usually more readily solved by seismic methods. Moreover, electrical resistivity and magnetic methods are especially sensitive to shallow variations in the properties measured and the quality of the data obtained falls off rapidly with depth. They are therefore very liable to interpretational ambiguities caused by near-surface features and artificial disturbances, which may be irrelevant to the problem under consideration. Gravity methods are sensitive in practice only to large features, and in an engineering context they would normally be considered only for the location of important faults. The methods to be described tend to be used more for the detection of suspected features whose exact position is unknown, than for the investigation of layered systems, although in favourable circumstances some electrical methods can give useful vertical information to a limited depth.

Unlike seismic methods, those described here depend on physical parameters which are much less closely related to mechanical properties than is the velocity of sound, and they are therefore less widely useful for assessing engineering behaviour, as in problems of rock excavation or ‘groutability’, where a relationship with seismic velocity has been shown.

Fig. 1 shows the relative importance of various geophysical methods in site investigation, in terms of the number of surveys carried out by each method over the past ten years in a firm of site investigation specialists. It includes only work carried out for civil engineering purposes; surveys made for the mining or oil industry, which include most gravity surveys, are omitted. The diagram may not exactly reflect the division of work in the entire industry, since individual firms tend to specialise in certain methods, but it is believed to be accurate enough for illustrative purposes.

The comparatively subordinate role of the non-seismic methods is clearly shown, and it should also be remembered that since they are often employed in the search for features whose approximate whereabouts is already known, surveys tend to be small in scale compared with most seismic investigations, in which it is usual to seek continuity of information throughout the site area.

Because of space limitations, electrical and radiometric borehole logging techniques are not considered here. They have an established role in providing supplementary stratigraphic information where continuous core drilling is not possible but. although they have engineering applications in special circumstances, they are much more closely associated with mineral and petroleum exploration.

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