SEISMIC SURVEY

method of investigating subterranean structure, particularly as related to exploration for oil, gas, and ore deposits. The technique is based on determinations of the time interval that elapses between the initiation of a sound wave and the arrival of reflected or refracted impulses at one or more seismic detectors. Though dynamite exploded underground is most commonly used to initiate such sound waves, an electric vibrator or falling weight (thumper) may also be employed at sites where an underground explosion might cause damage; e.g., where caverns are present. Upon arrival at the detectors, the amplitude and timing of waves are recorded to give a seismogram (record of Earth vibrations). The illustration depicts seismic survey methods in simplified form. Generally, the density of rocks near the surface of the Earth increases with depth. Thus, the second layer in the diagram is denser than the first. Sound waves initiated at the shot point may reach the receiving point by reflection, refraction, or both. When the shot point is close to the receiving point, reflected waves usually reach the receiving point first. At greater distances, however, the pulse travels faster by the refraction path because its velocity is greater along the top of the second layer than it is through the first; in this case, the refracted wave arrives first. Interpretation of the depths and media reached by seismic waves thus depends on the distance between shot points and receiving points and the densities of the strata. The results of a seismic survey may be presented in the form of a cross-sectional drawing of the subsurface structures as if cut by a plane through the shot point, the detector, and the centre of the Earth. Such drawings are called seismic profiles.