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Numerical and Experimental Analysis of the Effects of Underground Cavities on the Rayleigh Seismic Waves in the Frequency Domain - Ellipticity and Diffraction PatternNormal access

Authors: C. Filippi, D. Leparoux, G. Grandjean and A. Bitri
Event name: 79th EAGE Conference and Exhibition 2017
Session: Near Surface - Seismic, Mining
Publication date: 12 June 2017
DOI: 10.3997/2214-4609.201701414
Organisations: EAGE
Language: English
Info: Extended abstract, PDF ( 3.09Mb )
Price: € 20

Summary:
In the aim to contribute to identify the capacity of Rayleigh waves for underground cavities detection, we propose a combined approach based on numerical and experimental data obtained in a reduced-scale laboratory. First, we propose to calculate the H/V ratio of the direct R-wave for each frequency of the spectral content. The presence of the void is revealed by a peak which is higher amplitude and higher frequency than the H/V anomaly due to a high velocity cylinder. Therefore, the ellipticity seems to be a good criterion to detect a cavity and distinguish a void from a rigid object. In a second step, the diffraction pattern of a shallow cavity illuminated by the elastic wave-field is analysed in the frequency domain through numerical tests in an homogeneous medium. We show that 1) the amplitude of the horizontal components are strongly disturbed by the cavity and 2) the forward scattering part is predominant compared to the backscattering one if the cavity is large compared to the propagated wavelength. Thus, it becomes necessary to consider the direct wave (which is a combination of the forward scattering part and the incident part) on both components in order to efficiently locate the cavities.


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