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Near‐field seismic effects in a homogeneous medium and their removal in vertical seismic profile attenuation estimates
Author(s) -
Haase Arnim B.,
Stewart Robert R.
Publication year - 2010
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.2010.00883.x
Subject(s) - attenuation , geology , isotropy , anelastic attenuation factor , logarithm , magnitude (astronomy) , field (mathematics) , point source , seismic wave , near and far field , rotation (mathematics) , seismology , computational physics , geometry , physics , optics , mathematical analysis , mathematics , astronomy , pure mathematics
ABSTRACT To better understand (and correct for) the factors affecting the estimation of attenuation ( Q ), we simulate subsurface wave propagation with the Weyl/Sommerfeld integral. The complete spherical wavefield emanating from a P‐wave point source surrounded by a homogeneous, isotropic and attenuative medium is thus computed. In a resulting synthetic vertical seismic profile, we observe near‐field and far‐field responses and a 90° phase rotation between them. Depth dependence of the magnitude spectra in these two depth regions is distinctly different. The logarithm of the magnitude spectra shows a linear dependence on frequency in the far‐field but not in those depth regions where the near‐field becomes significant. Near‐field effects are one possible explanation for large positive and even negative Q ‐factors in the shallow section that may be estimated from real vertical seismic profile data when applying the spectral ratio method. We outline a near‐field compensation technique that can reduce errors in the resultant Q estimates.