Resolving a velocity inversion at the geotechnical scale using the microtremor (passive seismic) survey method

  • Roberts James C. (Monash University) ;
  • Asten Michael W. (Monash University)
  • Published : 2004.02.01

Abstract

High levels of ambient noise and safety factors often limit the use of 'active-source' seismic methods for geotechnical investigations in urban environments. As an alternative, shear-wave velocity-depth profiles can be obtained by treating the background microtremor wave field as a stochastic process, rather than adopting the traditional approach of calculating velocity based on ray path geometry from a known source. A recent field test in Melbourne demonstrates the ability of the microtremor method, using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However, this layer of sediments is resolved by the microtremor method, and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records. Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrate that estimates of shear velocities and layer thicknesses are accurate to within approximately $10\%\;to\;20\%$ using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity-thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.

References

  1. Aki, K., 1957, Space and time spectra of stationary stochastic waves, with special reference to microtremors: Buttetm of the Earthquake Kesearch Institute, 35, 415-456
  2. Asten, M.W., (1976), The use of microseism.s in geophysical exploration: PhD Thesis, Macquahe University, Australia
  3. Asten, M.W., 2003, Lessons from alternative array design used for high-frequency microtremor array studies: in Wilson, J.L., Lam, N.K., Gibson, G., and Butler, B., (eds.), Earthquake Risk Mitigation: Proceedings of a Conference of the Australian Earthquake Engineering Soc., Melboume, Paper 14
  4. Asten, M.W., Dhu, T., Jones, A., and Jones, T, 2003, Comparison of shear-velocities measured from microtremor array studies and SCPT data acquired for earthquake site hazard classification in the northern suburbs of Perth W.A.: in Wilson, J.L., Lam, N.K., Gibson, G., and Butler, B., (eds.) Earthquake Risk Mitigation: Proceedings of a Conference of the Austratian Earthquake Engineering Soc., Melboume, Paper 12
  5. Asten, M.W., 2004, Method for site hazard zonation, Santa Clara valley: Thickness and shear-velocity mapping of Hotocene-Pleistocene sediments by array studies of microtremors: Proceedings of First Annual Northern California Earthquake Hazards Workshop, USGS, Menlo Park
  6. Bloch, S., Hales, A.L., and Landisman, M., 1969, Velocities in the crust and upper-mantle of southern Africa, from multimode surface-wave dispersion: Bulletin of the Seismological Society of America, 59, 1599-1629
  7. Herrmann, R.B., 2001, Computer programs in seismology - an overview of synthetic seismoeram computation Version 3.1'. Department of Earth and Planetary Sciences, St Louis University
  8. Koopmans, L.H., 1974, Spectral analysis of time series'. Academic Press
  9. Mooney, H.M., and Bolt, B.A., 1966, Dispersive characteristics of the first three Rayleigh modes for a single surface layer: Buttelin of the Seismological Societyof America, 56, 43-67
  10. Nakamura, Y., 1989, A method for dynamic characteristics estimation of subsurface using microtremors on the ground surface: Quarterty Reports of the Raitway Technical Research Institute, Tokyo, 30, 25-33
  11. Neilson, J.L., 1970, Geological report on the western portion of the proposed additional span for the railway bridge over the Yarra River between Hawlhom and Bumley: Unpublished report. Department of Mines, Victoria, Australia
  12. Okada, H., 2003, The Microseismic Survey Method: Society of Exploration Geophysicists of Japan. Translated by Koya Suto, Geophysical Monograph Series No. 12, Society of Exploration Geophysicists
  13. Tokimatsu, K., 1997, Geotechnical site characterization using surface waves: in Ishihara (ed..), Earthquake Geotechnical Engineering: Balkema
  14. Toksoz, M.N. and Lacoss, R.T., 1968, Microseisms: mode structures and sources: Science, 159, 872-873
  15. Whiteley, R.J., and Greenhalgh, S.A., 1979, Velocity inversion and the shallow seismic refraction method: Geoexploration, 17, 125-141.
  16. Whiteley, R.J., 1994, Seismic re&acdon testing - a tutorial: in Woods, R.C. (ed.), Geophysical Characterisation of Sites: Balkema, 45-47
  17. Xia, J., Miller, R.D., and Park, C.B., 1999, Estimation of shear-wave velocity by inversion of Rayleigh waves: Geophysics, 64, 691-700