Estimating quality factor and mean grain size of sediments from high-resolution marine seismic data

Abstract

Seismic quality factor has the potential to characterize sediment properties but seldom is used by the industry for offshore site investigations because of practical difficulties with reflection seismology (e.g., restricted bandwidth) and because of uncertainties in rock-physics models. A spectral-ratio analysis of high-resolution marine seismic data can determine a quality factor to within a 95% confidence of [Formula: see text] within the uppermost [Formula: see text] of unconsolidated marine sediments. Our spectral-ratio technique does not require assumptions on how attenuation scales with frequency. Emphasis is placed on interpretation of spectral signatures before applying an iteratively reweighted robust least-squares regression to subdue the effects of noise and local heterogeneities when determining the quality factor of a sediment package. We combined data from boomer and chirp sources toexamine attenuation over four octaves of frequency [Formula: see text] and to demonstrate that expanding the frequency range improves the precision and accuracy of quality-factor fits. We obtain frequency-independent quality factors with 95% confidence intervals of 135 [Formula: see text] and 107 [Formula: see text] for silty clays with mean grain sizes of 7.7 and [Formula: see text], respectively, and 63 [Formula: see text] for a modern sand deposit with mean grain size [Formula: see text], from the Solent (U. K.). Sediments with higher quality factors require more independent observations to achieve a desirable 95% confidence. We required only 45 traces over sands and 1250 traces over the lowest attenuating silty clays. By constructing an empirical model of quality factor against mean grain size from published sediment studies, the mean grain sizes of our Solent sediments can be located, and we find that quality factor can be used to distinguish between coarse grain-dominated and clay-dominated sediments.