Relationship between head wave amplitudes and seismic refraction velocities to detect lateral variation in the refractor

نویسنده

Assistant Professor, Geology Group, Urmia University, Iran

چکیده

Refractor ambiguities are big problem in seismic refraction method especially in seismic engineering. There can be hidden subsurface geological phenomena such as hidden faults and shear zones which are not simply predicted by the travel-time graph or some geophysical methods. Head wave amplitudes are used to show the resolution of refractor ambiguities and the existence of anisotropy in complex geological area. Wave amplitude is proportional to the square root of energy density; it decays as 1/r. In practice, velocity usually increases with depth, and causes further divergence of the wave front and a more rapid decay in amplitudes with distance. Amplitudes measured from first peak to first trough and corrected for geometric spreading, can be demonstrated some subsurface information such as anisotropy. Meanwhile, amplitudes are not commonly study by researchers in seismic refraction studies, because of being the very large geometric spreading components due to variations related to wave speeds in the undulated refractor. The variations in amplitudes are described with the transmission coefficient of the Zoeppritz equations. This variation in velocity and density produces head wave amplitude and head coefficient changes in refractor, even with refractors exhibiting large variations in depth and wave speeds. The head coefficient can be approximately calculated by the ratio of the specific acoustic impedance in the overburden layer in the refractor. This study shows that there is a relationship between the amplitude and the seismic velocity which the lower the contrast in seismic velocity and/or density, the higher the amplitude and vice versa.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Relationship between head wave amplitudes and seismic refraction velocities to detect lateral variation in the refractor

نویسنده [English]

  • Ramin Nikrouz
Assistant Professor, Geology Group, Urmia University, Iran
چکیده [English]

Refractor ambiguities are big problem in seismic refraction method especially in seismic engineering. There can be hidden subsurface geological phenomena such as hidden faults and shear zones which are not simply predicted by the travel-time graph or some geophysical methods. Head wave amplitudes are used to show the resolution of refractor ambiguities and the existence of anisotropy in complex geological area. Wave amplitude is proportional to the square root of energy density; it decays as 1/r. In practice, velocity usually increases with depth, and causes further divergence of the wave front and a more rapid decay in amplitudes with distance. Amplitudes measured from first peak to first trough and corrected for geometric spreading, can be demonstrated some subsurface information such as anisotropy. Meanwhile, amplitudes are not commonly study by researchers in seismic refraction studies, because of being the very large geometric spreading components due to variations related to wave speeds in the undulated refractor. The variations in amplitudes are described with the transmission coefficient of the Zoeppritz equations. This variation in velocity and density produces head wave amplitude and head coefficient changes in refractor, even with refractors exhibiting large variations in depth and wave speeds. The head coefficient can be approximately calculated by the ratio of the specific acoustic impedance in the overburden layer in the refractor. This study shows that there is a relationship between the amplitude and the seismic velocity which the lower the contrast in seismic velocity and/or density, the higher the amplitude and vice versa.

کلیدواژه‌ها [English]

  • Head wave amplitudes
  • Seismic velocity
  • Acoustic impedance
  • seismic refraction

مراجع

Brown, A. R., 1987, The value of seismic amplitude, The Leading Edge, October 1987, 30-33.
Brown, A. R., 1996, Interpretation of three-dimensional seismic data, AAPG Memoir, 42.
Cerveny, V. and Ravindra, R., 1971, Theory of seismic head waves, University of Toronto Press.
Gebrande, H. and Miller, H., 1985, Refraction seismology, In: Bender, F. (Ed), Applied Geosciences, methods of applied geophysics and mathematical procedures in geosciences, F. Enke publishing house, Stuttgart.
Heelan, P. A., 1953, On the theory of head waves, Geophysics, 18, 871-893.
Nikrouz, R., 2005, Three-dimensional (3D) three-component (3D) shallow seismic refraction surveys across a shear zone associated with dryland salinity at the spicers creek catchment, PhD thesis, School of Geology, NSW, Sydney, Australia.
Nikrouz, R., 2006, Near-surface corrections with the GRM, 17th International Geophysical Congress & Exhibition by CGET, 14-17 Nov., Ankara-Istanbul.
Nikrouz, R. and Palmer, D., 2004, 3D 3C seismic refraction imaging of shear zone sources of dryland salinity, 17th Geophysical Conference and Exhibition, 15-19 August, Sydney, Australia.
O’Brien, P. N. S., 1967, The use of amplitudes in seismic refraction survey, in: Musgrave, A. W. (Ed), Seismic refraction prospecting, Society of Exploration Geophysicists, Tulsa.
Palmer, D., 1986, Refraction seismics, the lateral resolution of structure and seismic velocity, Geophysical Press.
Palmer, D., 2001a, Resolving refractor ambiguities with amplitudes, Geophysics, 66, 1590-1593.
Palmer, D., 2001b, Digital processing of shallow seismic refraction data with the refraction convolution section, PhD thesis, School of geology, NSW, Sydney, Australia.
Palmer, D., 2001c, Imaging refractors with the convolution section, Geophysics, 66, 1582-1589.
Palmer, D., 2003a, Application of amplitudes in shallow seismic refraction inversion, ASEG 19th Geophysical Conferences and Exhibition, Adelide.
Palmer, D., 2003b, Processing and interpreting shallow seismic refraction data with the GRM and the CRM, unpublished: university of New South Wales, Sydney, Australia.
Palmer, D., 2006, Refraction travel time and amplitude corrections for very near-surface in homogeneities, Geophysical Prospecting, 54, 589-604.
Palmer, D., Nikrouz, R. and Spyrou, A., 2005, Statics corrections for shallow seismic refraction data, Exploration Geophysics, 36, 7-17.
Werth, G. A., 1967, Method for calculating the amplitude of the refraction arrival, in: Musgrave, A. W. (Ed), seismic refraction prospecting, Society of Exploration Geophysics, 119-137.

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