next up previous

1 Introduction to Wave Propagation     continued...

The acoustic assumption is also in contrast to the elastic assumption. A fluid medium supports the propagation of a pressure wave. An elastic medium supports both shear and pressure waves. Marine seismic data are collected using water borne receivers or receivers which rest on the ocean bottom. Propagation of shear energy is restricted to solid media, and this makes land seismic measurement the principal generator of elastic seismic data. However, it is possible to find mode converted elastic information within an acoustic marine seismic dataset. In an elastic medium when waves impinge on a reflector, both shear and pressure waves are created, i.e. there is a mode conversion.

Seismic pressure waves are recorded by using a geophone, a microphone on a spike. The geophone has a small weight which is spring mounted with a magnet and a coil of wire. The pressure wave from the vibrating earth generates a vertical displacement moving the coil while the weight tries to resist this motion. The magnetic field generates a voltage proportional to the earth acceleration. Elastic waves have three components of displacement. Three geophones are aligned in orthogonal directions to measure the pressure wave and the two shear waves.

The seismic wave is man-made and requires a significant amount of energy to propagate any distance in the earth. Dynamite charges are used to generate the primary pressure wave (P-wave) and some shear wave energy (S-wave).