Sediment shear Q from airgun OBS data

Abstract

SUMMARY Direct measurement of the sediment shear-wave quality factor, Q,j, has been hindered by the lack of an effective shear-wave source. We show that if a satisfactory horizontal component ocean bottom seismometer (OBS) is available, then sediment Q, can be determined directly by using spectral ratios of converted shear-wave reflections. Spectral ratios are formed with the PS reflection from the sediment/basement interface and the PSSS multibounce sediment shear-wave reflection. As a check, we also computed Qfi from the peak amplitudes of PS and PSSS. We applied the spectral ratio method to airgun OBS data collected over 356 m of primarily high-porosity biosiliceous clay in 5467 m of water in the northwest Pacific at 4355.44'N, 159'47.84'E (DSDP Site 581). An average sediment shear-wave velocity of about 0.2 km s-' was obtained from the PS traveltime. Effective Q,< for the sediment column was found to be 97 f 11 (a = 0.281 f 0.032 dB A-') in the frequency band 3-18 Hz. We tested the methods by applying them to reflectivity synthetic seismograms computed for various velocity profiles with both frequency-dependent Q and frequency-independent Q. The Q, estimate obtained from synthetic seismograms was within 15 per cent of the true QB for each velocity profile. (2, estimates within 25 per cent of the true Q were obtained with the addition of up to 6.5 per cent signal-generated noise, whereas the addition of only 3 per cent signal-generated noise energy makes estimates of the frequency dependence of Q unreliable using spectral ratios. We conclude that the two-octave band of the data is not wide enough to determine the frequency dependence of Q,. Tests on synthetic seismograms, computed from models containing alternating layers of high impedance contrast with realistic velocities, indicated that apparent attenuation due to intrabed multiples does not significantly affect the spectral ratio Q, estimates, although a shift in spectral content to higher frequencies for PS and PSSS phases and a delay in the apparent arrival time of PSSS were observed. However, the alternative peak amplitude ratio method gave Q, estimates more than 25 per cent lower than the true Q for multilayer sediment models. We also tested the methods on synthetic data subjected to hard and soft clipping. Spectral ratio estimates of Q,, from synthetic data with PS clipped up to 50 per cent, were within 25 per cent of the true Qs.