The mathematical methods of geophysics, now being developed on the basis of computer technologies, provide for rigorous standards with respect to the precision and fidelity of observation data. That is why the Institute of Computer Mathematics and Mathematical Geophysics (Siberian Branch of the Russian Academy of Sciences) set up a new research subdivision - the De-part-ment of Geophysical Computer Science - as far back as 1978. Doctor B. Glinsky supplies the details.
This department is involved with an innovative technology and techniques used for the registration of geophysical fields, seismic fields in the first place as being most informative. Mobile field complexes, equipped with the most up-to-date computer technology (quite on a par with the best seismological observatories) have been developed for a program of in-depth sounding by means of vibration signals rather than by the conventional, orthodox pulse techniques (explosions, quakes). Now vibration signals, if sustained for some period of time, may be equal in their cumulative effect to a powerful explosion; however, they do not destroy or modiiy the ground. Besides, they make it possible to emit targeted pulses with preassigned parameters again and again - something that is impossible with the explosive sounding techniques.
The registration of such signals (and obtaining thereby data on the geological medium structure) has made it necessary to synthesize special algorithms because at a significant distance from the source these signals are attenuated to an infinitesimal fraction of even microseismic noises. The high-precision systems of computer-aided vibrators designed at the Department of Geophysical Computer Science allow to pick up 100 KW signals of force 100 tons at a distance of 300 km and get substantive seismo-grams besides. So there is some promise of setting up global systems of active seismic sounding for a detailed study of the internal structure of the earth and geodynamic processes.
The first models of powerful vibrators made use of centrifugal forces generated by massive rotary shafts with a displaced center of gravity. However, this mode did not allow to increase the power ofvibroeffect infinitely. Therefore the research staff has suggested (and patented) a novel principle of vibrator design based on the hydraulic resonance effect and built pilot models of hydraulic (hydro) resonance vibrators of 50 and 200 tons. Here a water-filled reservoir (tank, well, mine, pit) with an "air cushion" is used. A water mass and air pressure combine into a resonance system which is activated by a pneumatic drive. The impact acts upon the ground surface, and the force of this
impact can be enhanced open-end-ediy in practical terms, to thousands of tons, i.e. what is necessary for a global system of active seismic sounding. The parsimony of a like plant is obvious, for its "force elements" are water and air, something you can actually get gratis.
The high-precision level of sounding thus achieved has made it possible to undertake a study of geological medium variability in real time. It has been found to be caused by an infrared wave that is generated by oscillations of the ground surface (amplitude, 10 mm) around the vibrator. A close study of this phenomenon has helped to detect the phenomenon of atmos-pheric-hthospheric wave induction and illustrate it with direct measurements. It is most intensive where the velocities of waves in the ground surface and of sound in the atmosphere are close in value.
Further research led to the discovery of anomalies of seismic waves propagation as a result of lithospher-ic deformation under the effect of lunar-solar tides. In fact, the gravitational pull of these two celestial bodies is viewed as a natural standard of deformation forces that is amenable to a precise mathematical description and that can be predicted both in time and in space; this standard is of global scope. The high-precision systems of depth seismosounding in geodynamics studies ought to
respond to tidal effects. Once, in the course of a special experiment, Siberian researchers found the time of waves propagation to be changing with a period of 12 and 24 hours, which correlates with tidal acceleration characteristics, while relative changes in the rate of waves propagation are equal to 10-6. A system of sounding with such metrological parameters may be useful for monitoring variations in the strain of the medium in the foci of potential earthquakes.
As to the queries of some experts abroad about the fidelity of vibroac-tion results obtained by the Siberian Branch of RAS, it would be appropriate to recall that in 1995 our scientists, in cooperation with their colleagues from the Tokyo Institute of Earthquake Studies, carried out a joint experiment for studying the structure of a seismic field generated by powerful vibrators installed on a testing range at Bystrovka in the Novosibirsk Region (Western Siberia). The Russian research team
was led by Academician Anatoly Alexeyev, and his Japanese counterpart was Juno Kasahara. On the Russian side, this experiment involved such organizations as the Geophysics Institute (RAS Siberian Branch), the Novosibirsk Field Expedition engaged in vibration research, and the GEON Center (involved with geophysical and geoe-conomic studies, the RF Ministry of Natural Resources). All the field parties used instruments of their own, and so vibrator-emitted signals could be registered simultaneously at different distances and directions. The data thus obtained were collated and evaluated. All sets of instruments had registered intensive signals. This experiment demonstrated that the resonance properties of the ground surface layer are playing a substantial role in the emission of vibration waves. At the Bystrovka testing-range this layer exhibits enhanced elasticity and is thus resonance capable. Ground masses about the vibrators "get into the swing" and impart a
magnified mechanical impact to bedrock over a large area, with the emission of waves "through resonance" being tenfold more intensive than if they were "sent" into bedrock directly, i.e. with the use of pulse techniques. This means that powerful hydroresonance vibrators would be particularly effective if installed in places devoid of resonating ground (for such vibrators possess a resonance system of their own).
So, the use of unorthodox techniques involving computer methods and related hardware and instruments allows to combine technical sounding of the geological medium with its mathematical modeling within a single information process of research.
Nauka v Sibiri (Science in Siberia), 1999
Prepared by Emma SOLOMATINA
Опубликовано 08 сентября 2018 года
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