Engineering geophysics ? a geologist's view

Abstract

Thirty years ago some engineering organizations expected geophysics, applied virtually in isolation, to provide cheap answers to engineering questions. After some expensive lessons it is recognized today that geophysical methods are fully effective only when used with sufficient geological understanding and calibration. Common applications most useful in engineering are in two categories. First, is the location of features with anomalous physical properties. Such features include dykes, deeply weathered zones, major fault zones, buried channels, cavities and landslipped masses, all of which are potential hazards during the construction of tunnels and dams. Basic dykes have been located by airborne magnetometer, and where forming ridges on the sea bed, by side-scan sonar. During the planning of Sugarloaf Inlet Tunnel a deeply weathered unit was located by airphoto interpretation and refraction seismic traverses. Early recognition of this unit enabled the tunnel to be located so as to pass beneath the weathered zone. Seismic refraction and magnetic methods have also been successful in locating faults and buried channels. Concealed cavities have proven more difficult to locate, despite the use of micro-gravity, seismic, resistivity, shallow EM, Sirotem and ground-radar techniques. Landslipped masses can usually be recognized by their distinctive geomorphological features, but where such features are absent, a landslipped mass might be detected by its anomalously low seismic wave velocity. The second application category is the delineation of boundaries between features with contrasting physical properties, for example between 'overburden' and 'bedrock'. Where the overburden is geologically young transported material overlying unweathered rock, the boundary is readily determined by the seismic refraction method. Where the overburden comprises weathered rock in-situ, its boundary with the fresh rock is often highly irregular and gradational, and weathered profile reversals can occur. Caution is suggested in the interpretation of travel-time plots obtained from such variable masses. The author questions the value of geophysical methods for assessing the strength and compressibility of rock masses, and the effectiveness of grouting. He proposes seven other civil and mining engineering applications as more worthy of research, viz: location of hazardous features offshore, location of thin weak seams in rock masses, delineation of fresh rock 'boulders' within residual soils, location of cavities, development of remote-reading instruments for monitoring rock subsidence during and after mining, assessment of thicknesses of mechanically loosened rock, and detection of leakage areas in reservoir floors.