controlled-source seismology

controlled-source seismology Seismological methods for determining Earth structure are often classified as being either active or passive in nature. Passive methods involve waiting for an earthquake to occur and provide the seismic source for recording. In contrast, controlled-source seismology refers to active methods where the experimenter provides the source by an explosion or a mechanical device such as a hammer, a weight that is dropped, or a vibrator. Active methods can, in turn, be divided into two basic classifications. The first is seismic reflection profiling, which is a clearly defined approach in which the goal is to produce an image of the subsurface in which structures can be seen directly, in the way that an X-ray image reveals features inside an object. This technique is discussed in more detail elsewhere (see deep seismic reflection profiling and seismic exploration methods). Other active seismic methods infer seismic velocities and the presence of discontinuities in velocity and structure (such as faults) using a variety of approaches that analyse the arrival times and sometimes the shape of seismic waves travelling along different paths through the Earth. As a consequence of advances in seismic instrumentation and national programmes increasing the number of instruments available, there have been many recent developments in these techniques that do not directly image the Earth. A convenient aspect of the theoretical basis for virtually all active-source techniques is that they are largely independent of scale. Thus, detailed studies to address environmental problems and regional studies to determine deep Earth structure employ the same basic techniques of analysis.

Controlled-source seismology is not a precisely defined term and, as discussed above, could be considered to encompass all active-source techniques. The common practice is, however, to associate this term with large-scale investigations to determine deep Earth structure and large offsets (of at least tens of kilometres) between the sources and receivers. I exclude tomographic approaches which may have these characteristics but which tend to form a distinct line of investigation on their own (see seismic tomography).

Most introductory texts use the term ‘refraction techniques’ to describe what is often thought of as controlled-source seismology. If one assumes an Earth composed of constant-velocity layers separated by planar interfaces, simple equations can be derived that can be solved for the velocities of each layer and the geometry of the boundaries between each layer (see seismic waves, principles). This approach is used in many small-scale applications, such as the search for the top of the water table. As practised in large-scale studies, the refraction technique classically focused on determining the thickness of the crust and velocity variations in the crust and upper mantle. Early refraction studies employed a few dozen seismographs so that spacing between recordings was large and resolution was low. However, in modern studies, the distinction between refraction and reflection techniques is gradually disappearing because it is becoming possible to record the entire wave field generated by a seismic source.

Modern studies of lithospheric structure usually employ hundreds of portable seismographs of portable seismographs, tens of seismic sources, intervals between recording stations of less than 1 km, digital data processing, and sophisticated computer modelling schemes capable of handling very complex Earth structure. Formal inversion techniques and synthetic seismogram modelling (see synthetic seismograms) to match observed waveforms are also employed regularly. Although a modern experiment may be referred to as a refraction study, the actual seismic waves recorded and interpreted are often mostly wide-angle reflections. An example of data and an initial interpretation of Earth structure from a recent seismic experiment are shown in Fig. 1. The non-proliferation experiment (NPE) was a large chemical explosion which was detonated in Nevada in 1993. Its purpose was to provide data for efforts to discriminate between nuclear blasts and mine blasts. About 600 seismographs were deployed eastward from this explosion along a line which crossed Death Valley, the Sierra Nevada range, and California's Great Valley. The data show strong Pn arrivals from the upper mantle and strong reflections from the Moho (Fig. 2).

Controlled-source seismology has made a major contribution to our understanding of the Earth's crust and upper mantle. This approach generally produces our best constraints on the thickness of the crust (depth to the Moho) and the velocity of the upper mantle (Pn velocity). These two values have become measurable quantities, understood and used by a large segment of the geoscience community because of the insight they provide in terms of plate tectonic processes. Areas of extension (see grabens and rift valleys) are associated with crustal thinning, and the amount of thinning reflects the amount of extension. On the other hand, large compressional mountain belts, such as the Alps and Himalayas, have thick crust as a result of crustal-scale thrust faulting. Upper mantle velocities are an indication of the tectonic and heat flow regime of an area, and a linear relationship between heat flow and Pn velocity has been shown to exist. Tectonically active areas tend to have slow Pn velocities and high heat flow. In addition, global and regional compilations of controlled-source seismology results are showing indications of crustal structure variations that reveal differences that might even correlate with crustal age.

The lower crust has been the focus of much interest in recent studies. The lower crust has often been found to be highly reflective. This has been particularly true in extended areas and may indicate a fabric imparted by ductile flow. In some areas, the lower crust has been found to have unusually high velocities, suggesting that it may have been modified by magmatism.

Still other long-range results are discovering interfaces in the upper mantle which may be related to flow, as suggested by the presence of anisotropy (see seismic anistropy). A major development in controlled-source seismology has been the release of ultra-long seismic profiles from the former Soviet Union which employed peaceful nuclear explosions to probe several hundreds of kilometres into the mantle. These results will probably never be duplicated. The deeper we look into the Earth, the more heterogeneity we see. We can thus look forward to many discoveries from controlled-source seismology.

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.