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Their objective was to build an instrument that could be used both for recording earthquakes and for seismic refraction. For 'passive' earthquake operation, incoming seismic waves triggered an analog tape recorder which stored the signal as well as pre-trigger data from a continuously operating buffer, allowing the instrument to record information arriving before the trigger. For 'active' seismic refraction measurements, the instrument was programmed to record during preset time windows; explosions or airguns set off on a fixed schedule provided the seismic energy. The instrumental package itself was in a hollow waterproof sphere constructed of thick glass that allowed it to operate at the high pressures found on the ocean floor. For seafloor deployment, the sphere was attached to a disposable anchor; at a preset time, the instrument released the anchor and floated to the surface.
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During this time, they have undergone numerous design modifications, including changing from analog to digital recording. One early project that utilized the OBS both as earthquake and refraction instruments investigated the New Hebrides. This project and many subsequent OBS projects have involved collaboration with French scientists from ORSTOM, the French government's overseas scientific research organization. Between 1978 and 1982 there were also several earthquake OBS projects offshore of Alaska. These were all designed to investigate seismic activity associated with the Aleutian subduction zone, where extraordinarily large earthquakes can occur, but where there is also commercially valuable oil. One of the Alaskan OBS earthquake experiments included the deployment and successful recovery, with data, of an OBS from the Aleutian trench at a depth of 7112 meters.
Elsewhere, these instruments have also been used extensively in conjunction with multichannel seismic profiling in a series of programs to map the structure of the Gulf of Mexico margins. The resulting crustal structure cross-sections revealed huge thicknesses of sedimentary rocks around the Gulf of Mexico margins and led to the modern appreciation of the importance of sediment loading in the depression of the crust and mantle along all continental margins.
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Cross-section of the Chicxulub impact crater |
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A recent OBS project that has received considerable attention is a survey of the Chicxulub impact crater, which lies buried beneath about 1000 meters of sediment under the Yucatan Peninsula and the Gulf of Mexico. Scientists from UTIG and elsewhere collected seismic reflection, refraction, gravity and magnetic data in order to determine the actual size of the crater and to characterize its internal structure. The Chicxulub seismic survey produced the first high-resolution image of a well-preserved large crater and provided the first direct evidence that the crater possessed the multi-ring basin morphology that is typical of the largest impact features on the Moon and Venus. The results led the scientists to conclude that the Chicxulub crater is about 195 kilometers in diameter, and that the impact ejected about 50,000 cubic kilometers of material into the atmosphere, excavating a 12 kilometers-deep cavity at the Earth's surface. Prior to this research, the size and morphology of the Chicxulub crater had been in dispute, with estimates of its diameter ranging between 180 and 300 kilometers. Such a large discrepancy in size corresponds to a factor of ten difference in impact energy, and considerably different consequences for the Earth's environment. Some have suggested that the Chicxulub impact's effect on the Earth's atmosphere was responsible for the extinction of the dinosaurs.
At present, UTIG operates 19 OBS instruments. The current OBS design utilizes digital microprocessors to control data acquisition and essential functions, and commercially available hard disks to store the data. Just as in 1977, the OBS instrument and control electronics are still housed in glass spheres. A surface ship deploys the instrument, and it free-falls to the seafloor where it detects and records seismic signals, produced either by earthquakes or by airgun sources. After the data acquisition is completed, the system receives a sound signal from a surface ship, which triggers the OBS to release from its anchor and float to the surface for recovery. On board the ship, the data are downloaded to standard computer workstations for analysis.
Recently, UTIG scientists have undertaken a program to develop an entirely new 'broad-band' OBS instrument, i.e., an OBS which faithfully detects ground motions with periods ranging from less than 0.1 sec to 60 sec or more. Since many ocean bottom sites are relatively quiet, such an instrument could record earthquakes with magnitudes of 5.5 or greater occurring over the entire Earth, just as the better land-based seismic stations do. If these OBSs were relatively inexpensive, easy-to-deploy, and could operate for a year or more without maintenance, it would become feasible to extend the present land-based global station network to the oceans, which cover 70% of the Earth's surface.
