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
A publicly available seismic dataset from a lab experiment shows the simultaneous dependence of quality factor (Q) 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 (Qint) increase the most with depth (dQ/dz = 43 m−1) and stress (dQ/dσ = 0.0025/Pa) in dry sand and the least in partially saturated sand (dQ/dz = 10 m−1 and dQ/dσ = 0.0013/Pa) where attenuation created by local fluid flow reaches a maximum. Expected Qint values can be extrapolated from dQ/dσ and are bounded by Qint of the dry (Qdry) and partially saturated (Qwet) media (e.g., Qdry ≥ Qint ≥ Qwet). Qint deviations outside this range may be explained by changes in effective stress, attenuation mechanism, or sediment composition. Field values 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.