Sinkholes occurring over covered karst and land subsidence pose major risks to engineering structures including highways. Geotechnical risks associated with these hazards are greatly increased if potentially unstable, weak or cavernous regions are not identified and defined. Geophysical technologies implemented from the surface, boreholes or from underground conduits can significantly reduce such risks by non-destructively mapping and imaging significant karst features. As many karst sites occur in built-up areas, site screening of covered karst areas can pose problems for surface geophysical methods due to interference from external sources of noise. For example, power lines and surface or buried metals can severely restrict the application of electrical, induction EM and Ground Probing Radar (GPR) methods. Ground vibrations from traffic can cause inaccurate seismic and gravity measurements which reduces the effectiveness of these methods. In the light of these problems there is still a requirement for relatively low cost, rapid geophysical screening methods which are sensitive to a range of shallow covered karst features and relatively insensitive to interference. EM methods are attractive because they generally do not require ground contact.


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