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Shear waves by an explosive point-source: the Earth surface as a generator of converted P-S wavesNormal access

Author: J. Fertig
Journal name: Geophysical Prospecting
Issue: Vol 32, No 1, February 1984 pp. 1 - 17
DOI: 10.1111/j.1365-2478.1984.tb00713.x
Organisations: Wiley
Language: English
Info: Article, PDF ( 963.6Kb )

Summary:
The most common source of seismic energy is an explosion at some depth in a borehole. The radiated waves are reflected not only at the subsurface layers but also at the free surface. The earth's surface acts as a generator of both P- and S-waves.

If the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force. Theoretical seismograms were computed with different methods to look for the relevance of the surface-reflected waves. The numerical experiments show reflected shear waves even for small shotpoint—receiver distances. Due to their polarization these waves can be detected most easily on in-line horizontal geophones. The existence of these waves was examined during a conventional survey in Northern Germany. Conventional data analysis shows a large variability in the νps ratio. The method used here produced a shear-wave section with a rather good signal-to-noise ratio down to 4 s S-wave reflection time.


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