The Seismic Method
Seismic reflection utilizes a process of imparting sound energy to the sea floor and underlying substrate. Energy is reflected from those surfaces and is received by hydrophones at, or near, the sea surface.
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| Diagram of the elements of marine seismic reflection. A research vessel, equipped with navigation devices (GPS, Differential, RTK systems) tow a seismic source from the stern (shown in yellow) and a receiving hydrophone (shown in red). The outgoing and returning (reflected) energy is depicted by three rays, shown in red. |
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The mechanism of imparting energy into the substrate is quite complex in the marine realm, as opposed to land surveys, due to a highly-conductive water column and the separation of the source (usually located close to the sea surface) and the sea floor.
Early on, dynamite or other explosives were used as a source since they generate a short, intense pulse of energy. But, due to the danger, detriment to marine life, low firing repetition rates and the costly need for two ships (one to 'shoot' and one to receive) these explosives are no longer used. In the 1950's the boomer, electrical sparker, and a gas gun (which detonated a propane and oxygen mixture) were developed (after Sengbush, 1982). Presently compressed air or water alone is used in air and water guns respectively, singly or as part of a potentially tune-able array that can be configured to best define the geology of an area and enable signal optimization (noise and bubble energy minimization). The bubble pulse – a non-desirable secondary pulse generated after initial discharge from an air gun – obscures the waveform. The bubble pulse problem was addressed by the development of the water gun, which injects water into the surrounding water column rather than compressed air.
Another source used in seismic reflection is chirp, or 'swept' frequency. Energy sources can provide a limited range of frequencies over a finite period of time as described above for impulse sources, or alternately, can 'sweep' through frequencies over a relatively extended time period. Here, a long, oscillatory signal is transmitted whose frequency changes with time, either linearly or non-linearly; thus the 'swept' frequency terminology. It is based on post- World War II chirp radar developed by Bell Laboratories (Klauder et al., 1960). Rather than intensify the signal, the outgoing energy – usually less at any one instance than an explosive derived signal – is transmitted over the short time period and a range of frequencies. The received signal is then compressed to the desired 'pulse' by cross-correlating it with the input signature (Sengbush, 1982).
Higher–frequency, for example 3.5 and 12 kHz, transducer-type sources were developed for use as both bathymetric and shallow-penetration subbottom systems. Their history is mentioned in the bathymetry history section, accessible from the tree to the right.
When the outgoing energy pulse is discharged by a system, the signal attenuates as it travels away from the source due to several factors; it is reflected at acoustic interfaces (i.e. horizons that are defined by a contrast of impedance), absorption of energy through conversion into heat (after McQuillin et al., 1979) and spherical spreading resulting in a decrease in amplitude with time (i.e. distance).
Higher frequencies attenuate more rapidly, but generally produce a high resolution signal with limited penetration. Lower frequencies generate a signal that can penetrate quite deeply, but in return yield lower resolution.
This brief history is not intended to be a complete one. It is an overview from which readers may further explore the topic in libraries or on the web, using the topics presented above as starting points.References
Klauder, J.R., Price, A.S., Darlington, S., and Albersheim, W.J., 1960, The theory and design of chirp radars, Bell systems Tech. Jour., v. 39, p.745.
McQuillin, R., Bacon, M., and Barclay, W., 1979, An introduction to seismic interpretation: Graham and Trotman, Limited, London, 199 p.
Sengbush, R.L., 1983, Seismic exploration methods: IHRDC, Boston, 296 p.
Urick, Robert J., 1983, Principles of underwater sound: New York, McGraw-Hill Book Company.
