JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B7, PAGES 13,543-13,551, JULY 10, 1994 estimation for unconsolidated sediments using first-arrival SH wave critical refractions Z. Wang, R. Street, • and E. Woolery Departmentof Geological Sciences, University of Kentucky, Lexington J. Harris Kentucky GeologicalSurvey,University of Kentucky,Lexington Abstract. On the basis of the assumptions of multilayered and homogenous media with constant velocities and Q values, andplane seismic waves,the seismic pulse-broadening equation, x--Xo+Ct/Q (C=0.5), is usedto estimate the shear wave qualityfactor (Q•) of unconsolidated sediments from SH wave criticalrefractions in the upperMississippi embayment. Theprincipal advantage of using the SH wave pulse-broadening technique instead of othermethods for derivingQ• is that only one-fourth to one-halfwavelength of signal is needed. Results from the studyshow(1) Qsvaries from 10 to 60 in the uncon- solidated sediments studied, (2) Qsis independent of frequency in the rangeof 10 to 70 Hz, andequivalem to the constant(Qo) of powerlaw attenuation (Q = Q•, 0<¾<1.0), and (3) Q•, to the first approximation, is relatedto the shear wave velocities (Vs) of the unconsolidated sediments by the relationship Q• = 0.08 V• + 6.99 +_ 12.10. Introduction The amplification of seismic waves by low-velocity, loose, andpoorlyconsolidated sediments is knownto be a major contributor to seismic hazard [Aki, 1988]. Some factors that influence ground motions at the surface include the nature of the incident wave, topographic irregularities on the bedrock and at the observation point of the ground motions,the component of motion being observed, the number andshape of the layersmaking up the soils, along with impedance contrasts between bedrock and soil as well as within the soil column [Dravinski and Mossessian, 1987]. In order to accurately predict siteresponse, it is important to acquire a detailed knowledge of ihe in sireshear wavevelocities, densities, and damping ratios of the lithologies within the soil column. Siteeffects in the upper Mississippi embayment (Figure 1) areexpected to be of particular concern because of (1) the depth to bedrock,which is of the order of severaltens of meters near WIKY to several hundreds of meters at LATN [Crone and Brockman, 1982] and(2) the largenear-surface impedance contrasts due to the widespread prevalence of thick, saturated, low-velocity, loose, and poorly consolidated sediments. As partof a series of ongoing studies in western Kentucky having the objective of quantifying linearsiteeffects in the area,it is necessary to estimate damping ratiosof the near- •Also at Kentucky Geological Survey, University of Kentucky,Lexington. Copyright 1994 by the American Geophysical Union. Paper number 94JB00499. 0148-0227/94/94IB-00499 $05.00 surface, low-velocity sediments. Damping ratios (D) are commonly relatedto Q, the specific quality factor, by the relationship (1) for strains less than 10'3% [Moket al., 1988]. In this study, we weremostconcerned with ground motions due to shear waves,so Q, is the parameter of interest. In sireQ, values have been derived in various ways. For example, the spectral ratio technique has been used extensively for estimating, in situ, the shear wave quality factor(Q,). Studies by McDonaMet al. [1981],Kudoand Shima [1981], Boatwright et al. [1986], Redpath and Lee [1986], Hauksson et al. [1987], Blakeslee and Malin [1991], Brockman andBollinger[1992], and Fukushima et al. [1992] usethe spectral ratio technique for estimating Q,. But, as pointed out by McDonaldet al. [1981], a difficultyin using the spectral ratio technique for estimating Q, is the requirement of using a certain length of signal with a high signal-to-noise ratio. This limitation prevents general useof the technique with shallow seismicdata because of the multitude of noises from cultural sources and interference with the signal by reflections, diffractions, and convened waves. Pulse-BroadeningTechnique Ricker [1953] was the first to studythe propagation of seismic wavelets and concluded that the rise time of a wavelet,x, is proportional to the square root of the travel time. Later, Knopoff and McDonald [1960], Futterman [1962], Azimi et al. [1968], Strick [1981], and Kjartansson [1979] discussed the characteristics of a propagating pulse in terms of different attenuation models. 13,543