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The competing effects of stress and water saturation on in situ Q for shallow (< 1 m), unconsolidated sand, evaluated with a modified spectral ratio method
Author(s) -
Crane James M.,
Lorenzo Juan M.,
Shen Jie,
White Chris D.
Publication year - 2018
Publication title -
near surface geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.639
H-Index - 39
eISSN - 1873-0604
pISSN - 1569-4445
DOI - 10.3997/1873-0604.2017048
Subject(s) - attenuation , saturation (graph theory) , geology , sorting , soil science , effective stress , mineralogy , range (aeronautics) , geotechnical engineering , mathematics , materials science , optics , composite material , physics , algorithm , combinatorics
A publicly available seismic dataset from a lab experiment shows the dependence of quality factor( Q )simultaneously on water saturation and stress in unconsolidated sand. Large Q gradients (e.g., > 10 m −1 ) necessitate a spectral ratio method modified to assume that Q changes with each ray path, thereby eliminating false Q values (e.g., < 0). Interval Q values( Q i n t)increase the most with depth ( d Q / dz = 43 m − 1 ) and stress( d Q / da = 0.0025 / Pa )in dry sand and the least in partially saturated sand ( d Q / dz = 10 m − 1and d Q / da = 0.0013 / Pa ) where attenuation created by local fluid flow reaches a maximum. ExpectedQ i n tvalues can be extrapolated from d Q / da and are bounded byQ i n tof the dry( Q d r y)and partially saturated( Q w e t)media (e.g.,Q d r y ≥ Q i n t ≥ Q w e t).Q i n tdeviations outside this range may be explained by changes in effective stress, attenuation mechanism, or sediment composition. Field estimation of seismic attenuation in natural settings may be helped by these constraints, although attenuation remains subject to careful consideration of other factors, e.g., grain size, sorting, and shape.