deep seismic reflection profiling

deep seismic reflection profiling For over 50 years, images of the subsurface produced by recording seismic energy reflected from velocity discontinuities in the Earth's upper crust have been the primary tool in the exploration work of petroleum companies (see seismic exploration methods). The expenditure (about a billion pounds a year) on acquiring seismic reflection data has fuelled technological advances that have made this technique an important driving force in the development of computer hardware and software. In employing this technique, the goal is to produce an image that is so clear that its geological interpretation will be readily evident. There are also subtleties in the data, such as amplitude variations with distance from the source, which can reveal details about the physical properties of a region of the image. Unlike other active source methods, the reflection technique is configured to record seismic waves (usually P waves; see seismic body waves) at receivers whose distances from the source are small in relation to the reflecting target. Many repetitive sources are used and many hundreds of groups of receivers are deployed. This approach results in a great deal of multiplicity, which is used to suppress noise and enhance the usually weak reflected signals and to infer variations in velocity. A knowledge of velocities is needed to form the image, and velocity variations can help to identify the types of rock present and their physical properties, such as their porosity. An individual seismogram on a typical seismic reflection record section is in fact the sum of many thousands of individual seismic pulses that have travelled through the Earth.

The technology involved in the reflection technique comes at a high price. Each kilometre of data costs thousands of pounds for data recorded on land. A marine vessel equipped for conducting reflection surveys is a technological wonder capable of determining its position with great precision as well as recording huge volumes of data. Once deployed, such a vessel tows the seismic sources and a streamer containing the receivers, and it is thus very efficient. The unit cost of acquiring data is consequently reduced.

Once the data have been recorded, the task of organizing and manipulating the data to produce an image is still considerable. The data-processing and analysis entailed are greater than for other seismic methods. The computations required are, in fact, some of the most intensive in any scientific endeavour. Once a satisfactory image has been obtained, the task of modelling and interpreting the data is also very computer-intensive.

Geophysicists interested in deep Earth structure and processes recognized the value of seismic reflection data early in the evolution of the method, but academic researchers were frustrated for many years because they could not publish proprietary data from the petroleum industry and could not afford to acquire their own data. From the 1970s, however, deep seismic reflection data began to be gathered by academic groups on a regular basis. This advance was led by the Consortium for Deep Continental Reflection Profiling (COCORP), which was funded by the National Science Foundation in the USA. By the 1980s, many other countries were funding the collection of deep seismic reflection data for academic research.

The images produced by these efforts have provided many insights into the structure and evolution of the Earth's crust. On occasion, the uppermost mantle has also been imaged. These contributions are too numerous to discuss in detail. However, one example is that the effects of compression in the lithosphere have been shown to take many forms. In the southern Appalachian Mountains of the eastern USA, reflection data indicate that crystalline basement rocks have been transported hundreds of kilometers inward from the edge of the continent as result of the collision of Africa and North America about 300 million years ago. In the Alps, the convergence of Africa and Europe caused virtually the entire crusts of the two plates to be stacked on top of each other. Elsewhere, relatively simple crustal-scale faults have accommodated compression by uplifting large blocks of the crystalline basement.

Another example of the use of deep seismic reflection data is in rift zones where extension has greatly modified the crust. Reflection profiles have shown that most rift basins are asymmetrical, indicating that one bounding fault is dominant. They have also shown that the modification of the crust by magmatism is extremely variable, ranging from nil to the replacement of virtually the entire pre-existing crust.

G. R. Keller

Bibliography

Blundell, D.,, Freeman, R.,, and and Mueller, St. (1992) A continent revealed: the European Geotraverse. Cambridge University Press. [With maps and database on CD-Rom.]
Meissner, R. (1986) The continental crust: a geophysical approach. Academic Press, San Diego.
Meissner, R.,, Brown, L.,, Durbaum, H.- J.,, Franke, W.,, Fuchs, K.,, and and Seifert, F. (1991) Continental lithosphere: deep seismic reflections. American Geophysical Union, Geodynamics Series, Vol. 22.