Instruments \ Seismic \ 
Seismic Items

Subsurface formations are mapped by measuring the times required for a seismic wave (or pulse), generated in the earth by a near-surface explosion of dynamite, mechanical impact, or vibration, to return to the surface after reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion. They are laid along the ground at distances from the shot point which are generally small compared with the depth of the reflector.

Variations in the reflection times from place to place on the surface usually indicate structural features in the strata below. Depths to reflecting interfaces can be determined from the times using velocity information that can be be obtained either from the reflected signals themselves or from surveys in wells. Reflections from depths as great as 20,000 ft can normally be observed from a single explosion, so that in most areas geologic structure can be determined throughout the sedimentary section.