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Long duration noisy-looking waveforms such as those obtained in randomly multiple scattering and reverberant media are complex; they resist direct interpretation. Nevertheless, such waveforms are sensitive to small changes in the source of the waves or in the medium in which they propagate. Monitoring such waveforms, whether obtained directly or obtained indirectly by noise correlation, is emerging as a technique for detecting changes in media. Interpretation of changes is in principle problematic; it is not always clear whether a change is due to sources or to the medium. Of particular interest is the detection of small changes in propagation speeds. An expression is derived here for the apparent, but illusory, waveform dilation due to a change of source. The expression permits changes in waveforms due to changes in wave speed to be distinguished with high precision from changes due to other reasons. The theory is successfully compared with analysis of a laboratory ultrasonic data set and a seismic data set from Parkfield California. [ABSTRACT FROM AUTHOR]

This paper explores the applicability of wavelet transform for the analysis of strong and weak ground motions recorded on rock at Yerba Buena Island and on dredged sandfill at nearby Treasure Island about 2 km away during the 1989 Loma Prieta earthquake in California. An interpretation of the results from this analysis is presented and discussed. The major important feature of this data set is that liquefaction occurred at the Treasure Island site during the strong shaking. The Daubechies 20-coefficient wavelet transform is utilized in this study. Results of the application of the wavelet transform can be summarized as follows: most of the energy of the earthquake at the Treasure Island site is in the frequency range of 0.58-1.77 Hz and arrives in the time interval between 11 and 15 s. For the Yerba Buena Island site, most of the energy lies within the frequency range of 1.15-3.53 Hz and arrives between 8 and 13 s. The analysis indicates that there are some large peaks at the highest resolution in the wavelet transforms at the Treasure Island site, which can be attributed to a progressive change in the stiffness characteristics of the soil. [ABSTRACT FROM AUTHOR]