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Dispersion Analysis Of Rayleigh Surface Waves And Applications

Posted on:2010-12-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:D M PanFull Text:PDF
GTID:1100360308990023Subject:Earth Exploration and Information Technology
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As long ago as 1885, the English mathematician and physicist Lord Rayleigh theoretically proved that there was a kind of elastic waves which propagate along the free surface with an anti-clockwise elliptic particle trajectory and its amplitude with an exponential decay with depth. The wave was named as Rayleigh wave or R-wave later. Subsequently, Rayleigh waves have been widely applied to studying Earth's internal structure by seismologists from the analysis of earthquake records, and near surface soil structures through the use of Spectral Analysis of Surface Waves (SASW) by engineers. To overcome the weakness of SASW method, such as, low precision of dispersion curves,multimode data mixture,body wave energy contamination and the difficulty to calculate the high-mode dispersion curves, the Multi-channel Analysis of Surface Waves (MASW) has been proposed. Velocity dispersion curves are extracted from multiple trace records by wave field analysis. The fundamental mode of Rayleigh waves,however, are easily contaminated by the high-modes or body wave energy in the real data.By studying the characteristics of the Rayleigh-wave propagation and dispersion, in this paper the modified propagation matrix method was adopted to calculate the dispersion curves of Rayleigh-wave disperse in multi-layered media. Common Rayleigh-wave velocity dispersion curves are extracted through the use ofτ?p transform or Radon transform. Additionally, the author has studied the Rayleigh-wave synthetic method based on velocity dispersion curve and adopted reflectivity method to analyze synthetic records and velocity dispersion of whole seismic wave wavefield. I have also analyzed comprehensively the influence of acquisition parameters on Rayleigh-wave velocity dispersion. The results have showed that array characteristics, source pattern and target depth, site condition, and type of the geophone play an important role on the Rayleigh-wave frequency composition. Among these, the source energy played a dominant role in exciting the low frequency Rayleigh-wave signals.The computation of velocity dispersion method in this paper does not use any digital process such as 1-D filtering, f-k filtering, and it avoids the impacts of digital processing just like Gipps effect. However it discriminates the direct waves and reflected waves using the frequency analysis and the geological condition of the sites to choose frequency and velocity range. It is further verified that the velocity dispersion method is valid and credible through the analysis of different sites, sources, and different pattern of geophones. So it gives rise to better results.Receiver array length, offset distribution range, performance of geophone and seismograph, time sampling rate and length of the record have no effects on Rayleigh-waves in coal exploration. Although some measures has been taken to avoid the Rayleigh-waves such as using the deep hole to excite and the high-frequency geophone to receive, the energy of the Rayleigh-waves still appears because geophone grouping is rarely applied in coal exploration. It provides advantage to carry out Rayleigh-wave exploration in reflected waves. From the velocity dispersion analysis results, extracting the S-wave velocity distribution of near-surface using the Rayleigh-wave analysis from reflected waves can provide reliable data for converted wave statics.
Keywords/Search Tags:Rayleigh wave(Rayleigh-wave), velocity dispersion analysis, Rayleigh-wave synthesize, Rayleigh-wave acquisition
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