1887
Volume 14 Number 2
  • E-ISSN: 1365-2478

Abstract

Abstract

The screening effect of thin, relatively shallow high‐velocity layers often presents considerable problems in seismic exploration. Such layers prevent the greater part of the seismic energy from travelling to greater depths and introduce additional refraction arrivals, confusing the seismogram still further.

In order to investigate both the screening and refractive properties of high‐velocity layers, scale‐model experiments have been made over a wide range of layer‐thickness/ wavelength ratios (0.05 < < 2) for suitably chosen material contrasts. The results may be summarised as follows.

Refraction arrivals from thin layers in the field may be recognised by their relatively rapid amplitude decay. Furthermore, the “echeloning”‐effect observed for refraction first arrivals may be due to the presence of a (thin) layered structure. Since the apparent refraction velocity varies with when < 1, differences between vertical well‐log velocities and velocities observed along the surface may be expected, making time/depth conversion using surface velocity data inaccurate.

Transmission of elastic energy may be expected, if anywhere, only near the shotpoint, at small geophone offset, and for relatively thin screens ( < 0.1). The transmitted signal shape is then independent of the layer thickness. This transmitted energy may be registered either in a reflection set‐up with geophones near the shotpoint, or in long‐distance refraction work.

Three possibilities are offered for overcoming the screening effect of thin high‐velocity layers: Use longer‐wavelength signals

Apply short‐spread reflection shooting

Apply long‐distance refraction shooting

The experimental results obtained in scale‐model arrangements of such set‐ups confirm the potentialities of these methods.

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2006-04-27
2020-04-01
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References

  1. Arbogast, J., 1965, Private communication.
  2. Berzon, I. S. and Epinat'eva, A. M., 1950, Seismic screening. Bull. Akad. Sci. U.S.S.R., Ser. Geogr. and Geoph., v. 10, No. 6.
    [Google Scholar]
  3. Carabelli, E. and Folicaldi, R., 1957, Seismic model experiments on thin layers. Geophys. Prosp., v. 5, pp. 317–327.
    [Google Scholar]
  4. Davydova, N. I., 1958, The dependence of the dynamic characteristics of longitudinal head waves for thin layers on the velocity contrast. Izv. Akad. Nauk. S.S.S.R., Geophys. Ser., pp. 1181–1191.
  5. Davydova, N. I., 1959, The dependence of the amplitude of longitudinal head waves for thin layers on the velocity contrast. Izv. Akad. Nauk. S.S.S.R., Geophys. Ser., pp. 658–668.
  6. Davydova, N. I., 1962, Model studies on the dependence of the dynamic characteristics of longitudinal head waves on the layer thickness. Izv. Akad. Nauk. S.S.S.R., Geophys. Ser., pp. 11–22.
  7. Donato, R. J., 1965, Measurements on the arrival refracted from a thin high‐speed layer. Geophys. Prosp., v. 13, pp. 387–404.
    [Google Scholar]
  8. Dunkin, J. W., 1963, A study of two‐dimensional head waves in fluid and solid systems. Geophysics, v. 28, pp. 563–581.
    [Google Scholar]
  9. Heelan, P. A., 1953, On the theory of head waves. Geophysics, v. 18, pp. 871–893.
    [Google Scholar]
  10. Lavergne, M., 1961, Étude sur modèle ultrasonique du problème des couches minces en sismique refraction. Geoph. Prosp., v. 9, pp. 60–73.
    [Google Scholar]
  11. Levin, F. K. and Hibbard, H. C., 1955, Three‐dimensional seismic model studies. Geophysics, v. 20, pp. 19–32.
    [Google Scholar]
  12. Levin, F. K. and Ingram, J. D., 1962, Head waves from a bed of finite thickness. Geophysics, v. 27, pp. 753–765.
    [Google Scholar]
  13. O'Brien, P. N. S., 1955, Model seismology, the critical refraction of elastic waves. Geophysics, v. 20, pp. 227–242.
    [Google Scholar]
  14. Oliver, J., Press, F. and Ewing, M., 1954, Two‐dimensional model seismology. Geophysics, v. 19, pp. 202–219.
    [Google Scholar]
  15. Ott, H., 1942, Reflexion und Brechung von Kugelwellen. Effekte 2. Ordnung, Ann. d. Phys., v. 41, pp. 443–466.
    [Google Scholar]
  16. Parkhomenko, I. S., 1958, The intensity of a head wave during its passage through a high‐velocity layer, studied on models. Izv. Akad. Nauk. S.S.S.R., Geophys. Ser., pp. 449–457.
  17. Poley, J. Ph., 1964, The critical‐angle effect in seismic exploration. Geophys. Prosp., v. 12, pp. 397–421.
    [Google Scholar]
  18. Press, F., Oliver, J. and Ewing, M., 1954, Seismic model study of refractions from a layer of finite thickness. Geophysics, v. 19, pp. 388–401.
    [Google Scholar]
  19. Riznichenko, Yu. V. and Shamina, O. G., 1957, Elastic waves in a solid layered medium, as investigated on two‐dimensional models. Izv. Akad. Nauk. S.S.S.R.. Geophys. Ser., No. 7, pp. 17–37.
    [Google Scholar]
  20. Rosenbaum, J. H., 1961, Refraction arrivals along a thin elastic plate, surrounded by a fluid medium. J. Geophys. Res., v. 66, pp. 3899–3906.
    [Google Scholar]
  21. Rosenbaum, J. H., 1965, Refraction arrivals through thin high‐velocity layers. Geophysics, v. 30, pp. 204–212.
    [Google Scholar]
  22. Shamina, O. G., 1965, Attenuation of head waves from thin beds for rigid and sliding contact. Izv. A.N. Phys. Solid Earth, pp. 148–153.
  23. Toksöz, M. N. and Schwab, F., 1964, Bonding of layers in two‐dimensional seismic modeling. Geophysics, v. 29, pp. 405–413.
    [Google Scholar]
  24. Vassil'ev, Yu. I., Kovalev, O. I. and Parkhomenko, I. S., 1958, Study of the crystalline basement by the refracted wave method under conditions of partial masking. Izv. Akad. Nauk. S.S.S.R., Geoph. Ser., pp. 317–329.
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