Introduction

Seismic investigations utilize the fact that elastic waves (also called seismic waves) travel with different velocities in different rocks. By generating seismic waves at a point and observing the times of arrival of these waves at a number of other points on the surface of the earth, it is possible to determine the velocity distribution and locate subsurface interfaces where the waves are reflected or refracted.

Traditionally, seismic methods are classified into two major divisions, depending on the energy source of the seismic waves: (1) Earthquake seismology, and (2) Explosion seismology. The main contributions of earthquake seismology are information on the physical properties and structure of the earth's interior. In a geotectonics and civil engineering context, important applications include studies of earthquake magnitude and frequency relationship, evaluation of seismic hazards, and prediction of earthquakes and mitigation of damages. These topics will not be discussed because earthquake seismology is not included in the scope of the book.

Methods of explosion seismology (the reflection and refraction survey methods) are widely applied to studies of crustal structure, geological correlation of layered sequences and mapping of structures in the uppermost crust, and to hydrocarbon exploration. In the same way, but on a smaller scale, these methods are applied to environmental and geotechnical problems such as location of water table, waterbearing fracture zones, cavities and sinkholes, delineation of faults and fractures, and site investigations of foundation conditions including determination of depth to bedrock.