Sediment shear Q from horizontal component airgun OBS data

Abstract

In this dissertation, I estimate the effective sediment shear-wave quality factor, Qβ, from PS and PSSS basement converted shear-wave reflections. I use airgun horizontal component ocean bottom seismometer (OBS) data collected over 356 m of primarily high-porosity biosiliceous clay in .5467 m of water in the northwest Pacific at 4:3°55.44'N, 159°47.84'E (DSDP Site 581) (Duennebier et al., 1987) to estimate Qβ. Direct measurement of the sediment shear-wave quality factor, Qβ, has been hindered by the lack of an effective shear-wave source. I show that if a satisfactory horizontal component OBS is available, then sediment Qβ can be determined directly by using spectral ratios of converted shearwave reflections. Spectral ratios are formed with the PS reflection from the sediment/basement interface and the PSSS multi-bounce sediment shearwave reflection. As a check, I also computed Qβ from the peak amplitudes of PS and PSSS. To evaluate the reliability of the Qβ estimate, I tested the spectral ratio method on synthetic seismograms that model the OBS data. Core logs from a nearby borehole were used to constrain the sediment thickness and density, while the OBS data constrains the sediment and basement velocity structure for the models used to generate the synthetic seismograms. Tests show that the spectral ratio method is reliable in the presence of moderate amounts of noise, signal clipping, and scattering from cyclic layering. Effective Qβ for the sediment column was found to be 97 ± 11 (α = 0.281 ± 0.032 dB/λ) in the frequency band 3-18 Hz. To increase confidence in a Q estimate, more than one method should be applied to the same arrivals. A new method to estimate Q from two arrivals of the same signal, called the Q-gram method, is presented. In this application of the Q-gram method, I use a new definition of pulse width applicable to oscillatory arrivals to measure the propagation loss. The propagation loss is defined as the change in the pulse width divided by the difference in traveltime between the arrivals. An average of either the instantaneous frequency or instantaneous pulse width over the duration of the arrival is used as the definition of pulse width. The PS arrival serves as the reference wavelet. The Q-gram method is based on propagating the reference wavelet with a plane-wave Q-propagator for various values of Q^-1. The Q-propagator includes a dispersion relation and the measured difference in traveltime between the data arrivals. The plot of synthetic propagation loss between the reference and propagated wavelets, versus Q^-1, is called a Q-gram. The Q-gram, together with the measured propagation loss of the data, gives the Q of the data. The Q-gram method is found to be robust in the presence of moderate amounts of noise and signal clipping by tests on synthetic seismograms that model the OBS data. The OBS data indicate that effective sediment Qβ is 75 ± 15. Application of the spectral ratio method using shortened windows that exclude interfering arrivals identified by means of the instantaneous frequency gives a similar estimate of sediment Qβ: