AN elementary textbook is much concerned with the fundamentals of a subject and these do not change with time. In this respect the second edition of our book •has been little modified. In every other it is much altered for the subject has advanced greatly since we gathered together the material for the original version. The changes have been brought about mainly by developments in electronics and in the science of comput• ing. The former have led to the appearance of much lighter, more sensitive and altogether much improved instruments, making it possible to collect better data far more quickly. The latter have made possible the rapid reduction of the very large amounts of data that can now be collected during a survey but even more important they make possible the use. of far .more sophisticated interpretation techniques than formerly. It is no exaggeration, therefore, to say that applied geophysics has moved into a new era in the interim since this book was first conceived.
Thus, though the sections in the first edition dealing with basic ideas remain little modified, extensive rewriting has been necessary and certain alterations have had to be made to the general plan to bring about the required changes of emphasis. •
As the preface to the first edition states, the book was originally written.
with the needs of the civil engineer in mind, though the interests of others were not forgotten. The book has, however, proved to be popular with students of geology and the alterations to the balance of the material that have been made have taken this into account. For example, more attention has been paid to applications of geophysics in the search for ore minerals, this being reflected in considerable strengthening of the sections dealing with electromagnetic and induced polarization techniques

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