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

Abstract

ABSTRACTA publicly available seismic dataset from a lab experiment shows the dependence of quality factor simultaneously on water saturation and stress in unconsolidated sand. Large gradients (e.g., > 10 m−1) necessitate a spectral ratio method modified to assume that Q changes with each ray path, thereby eliminating false values (e.g., < 0). Interval values increase the most with depth and stress in dry sand and the least in partially saturated sand ( and ) where attenuation created by local fluid flow reaches a maximum. Expected values can be extrapolated from and are bounded by of the dry and partially saturated media (e.g., ). deviations 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.