This little book is different. It is written, primarily, for geologists, reservoir engineers, and log analysts. Why? Because today's seismic method is more than a tool for reconnaissance exploration, for finding structures; it has become a tool for studying the discovered reservoir-its extent, its barriers, its variations of thickness, and its trends of porosity. Today, the geophysicist, the geolog- ist, the engineer, and the log analyst can do great things together. Because the book is not written primarily for geophysi- cists, it can skip much of the "mechanics" of the seismic method. The reader who reaches the last page (bless him!) will not be able to practise the seismic method, but he will understand how the seismic method can help to solve his problems. It may also be of value to those practitioners of the seismic method-in the field or in the processing centre-who already know the mechanics of the method, but would like to take a broader view. 1 1 Seismics and Structure Of all the geophysical methods working from the surface, there is only one which is widely accepted for studies of individual petroleum reservoirs-the seismic reflection method. We would dearly love to have something better, but alas ... Reduced to its essentials, the method is this: we make a bang, and we listen for echoes.