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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Splitting functions retrieved from spectra of the free oscillations are sensitive to the lateral variations in P velocity (α), S velocity (β), and density (ρ) simultaneously. In this study they are used to constrain the values of the ratios d ln α/d ln β and d ln ρ/d ln α for the lower mantle. Assuming that the upper mantle structure is obtainable from model M84A (this is not a crucial assumption as experiments indicate), the optimal value of d ln α/d ln β inferred from the modal data is 0.44 and d ln α/d ln β lies in the interval (0.39, 0.60) with 75 per cent confidence, strongly discriminating against the value (0.8) often used. The constraints on density structure of the current data are insufficient to yield new definitive results. The analysis demonstrates, however, that the value of d ln ρ/d ln α could be estimated from a larger set of modal data.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2019
    Description: 〈span〉〈div〉ABSTRACT〈/div〉Ground penetrating radar (GPR) is one of the most widely used geophysical survey methods to locate cavities under roads due to its speedy exploration and high-resolution imaging. To locate underground cavities using GPR, we need to distinguish between cavity-induced reflections and other reflections, which can be achieved by examining the polarity change in reflections compared to the polarity of the transmitted signal. The polarity change can be measured from the phase shift between the target and first reflections. To estimate the phase shift in reflections, the method of computing the power spectrum difference between the original trace and background signal was proposed, but the method has a limitation for shallow reflectors. As an alternative method to avoid this limitation, we propose using only one component of the power spectrum difference, the cross-correlation between the target reflection and background signal. The cross-correlation has its maximum peak at a time lag between the target and first reflection (from the air-ground interface). Additionally, the phase at that time lag represents a phase shift between the two reflections. We compare our cross-correlation-based method with the conventional method of computing the whole power spectrum difference and investigate the feasibility of our method for distinguishing cavity-induced reflections using a 2D field data set acquired in a testbed in Sudeoksa, Korea.〈/span〉
    Print ISSN: 1083-1363
    Electronic ISSN: 1943-2658
    Topics: Geosciences
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