Derivation of Acoustic and Physical Properties from High-Resolution Seismic Reflection Data

Abstract

Acoustic measurements in sediments can remotely provide information about their physical properties. This thesis develops methods using high-resolution marine seismic reflection data to generate accurate, precise, and repeatable estimations of the in situ seismic quality factor (Q) and velocity, within the uppermost 100 m of unconsolidated marine sediments. This was achieved using a single channel Chirp sonar (1.5-13.0 kHz) and a Boomer sonar (0.2-4.0 kHz) towed with a multi-channel streamer that provides offsets up to 80 m. These methods were applied in the Solent (Southampton, U.K.) to test empirical models that predict sediment physical properties from these acoustic properties, and in lake Windermere (Cumbria, U.K.) where the new quantitative information helps distinguish between, and identify, previously unknown glacial and lacustrine deposits. The spectral-ratio technique was used to reliably estimate Q from seismic reflection data, without assumptions on how attenuation varies with frequency. Based on empirical observations, it is demonstrated here that Q can characterise the dominant fabric of the sediment that is supporting wave-propagation - whether it is clay dominated and has a Q over 75, or coarse-grain dominated and has a Q less than 75. The Chirp source is the most effective single sonar to estimate Q, however, improved results were obtained by combining Boomer and Chirp data to examine attenuation over four octaves (0.5-8.0 kHz). Frequency-independent Q (with 95% confidence intervals) of 135 (+12; -10) was obtained for silty-clay, 63 (+10; -7) for ne-sand, and 33 (+6; -4) for gyttja. Boomer multi-channel seismic data were acquired in the Solent over a 30 m core and a sampled sand deposit, and in lake Windermere where over 150 km of survey lines were required to cover most of the lake. An optimal processing methodology was developed for high-resolution seismic data using iterative Kirchhoff prestack depth migration, Burg deconvolution and F-K filtering; velocities were derived using migration velocity analysis. Additionally, a method was developed to obtain the streamer depth profile from the surface ghost. The Solent study proved that measurements of velocity and Q together can identify unconsolidated coarse-grain sediment and distinguish between gravel, sand and silt, and differentiate between unconsolidated and over-consolidated clay sediments. In Windermere, five seismic stratigraphic sequences were identified from the depth migrated data: I - till (c. 2000-3000 m/s); II - glacio-uvial (c. 1750 m/s); III - glacio-lacustrine/lacustrine (1500 m/s + 6 s-1); IV - disturbed/slumped (c. 1500 m/s); V - lacustrine (c. 1490 m/s). A new deglacial history for lake Windermere is proposed, which has important implications on the overall character of the ice-sheet retreat in Britain after the last glacial maximum.

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