Q_s estimation for unconsolidated sediments using first-arrival SH wave critical refractions

On the basis of the assumptions of multilayered and homogenous media with constant velocities and Q values, and plane seismic waves, the seismic pulse-broadening equation, τ=τ₀+Ct/Q (C=0.5), is used to estimate the shear wave quality factor (Q_s) of unconsolidated sediments from SH wave critical refractions in the upper Mississippi embayment. The principal advantage of using the SH wave pulse-broadening technique instead of other methods for deriving Q_s is that only one-fourth to one-half wavelength of signal is needed. Results from the study show (1) Q_s varies from 10 to 60 in the unconsolidated sediments studied, (2) Q_s is independent of frequency in the range of 10 to 70 Hz, and equivalent to the constant(Q₀) of power law attenuation (Q = Q₀f^y, 0<γ<1.0), and (3) Q_s, to the first approximation, is related to the shear wave velocities (V_s) of the unconsolidated sediments by the relationship Q_s = 0.08 V_s + 6.99 ± 12.10.