Book excerpt: "The first phase of geophysical exploration is carried out on the subsurface.The subsurface in this phase is inaccessible to direct observation. D.W. Strangway presents an interesting and effective method: audiofrequency magnetotelluric (AMT) sounding. In the second phase where observations can be made, D.J Buchanan writes of fault detection in coal seams, and A. Hussain writes about of gravity surveys.Other contributions include, improvement of geophysical logs by filtering and a troublesome problem which can appear in either phase of exploration-discovery and exploration of caves and abandoned mine workings.'