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Feb1-08, 03:15 PM
P: 749
Ground Penetrating Radar (GPR) is a possibility, although you would need to calibrate it carefully and the method could still fail if the geological conditions were unfavourable. Magnetometetry would be another possibility, provided that the limestone had some magnetic susceptibility to differentiate it from the cavity (i.e. the absence of rock). Seismics would also work but you would need to deal with high frequencies and would therefore need quite specialist geophones to detect a small cavity. Perhaps even some kind of neutron or gamma ray detector could work, although I somehow doubt unless you were presented with very accomodating geological conditions. Gravitometry is the most tried and tested method here, and by far the most reliable - why would you want to account for density variations in the rock? Surely that is exactly what you are trying to detect - or to put it more precisely - you are trying to detect the density variation in the subsurface, a cavity would stand out like a sore thumb. There are lots of other corrections you might want to apply however, eg: instrument drift, tidal, free-air, Bouguer, latitude, etc...