3D seismic imaging of buried Younger Dryas mass movement flows: Lake Windermere, UK

Windermere is a glacially overdeepened lake located in the southeastern Lake District, UK. Using the threedimensional(3D) Chirp subbottom profiler, we image mass movement deposits related to the Younger Dryas(YD) within a decimetre-resolution 3D seismic volume, documenting their internal structure and interactionwith preexisting deposits in unprecedented detail. Three distinct flow events are identified and mappedthroughout the 3D survey area. Package structures and seismic attributes classify them as: a small (totalvolume of c. 1500 m3) debris flow containing deformed translated blocks; a large (inferred total volume ofc. 500,000 m3), homogeneous fine-grained mass flow deposit; and a debris flow (inferred total volume ofc. 60,000 m3) containing small (c. 8.0×2.0 m) deformed translated blocks. Geomorphological mapping oftheir distribution and interaction with preexisting sediments permit the reconstruction of a depositionalhistory for the stratigraphic units identified in the seismic volume.<br/

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

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

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

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 ±10 within the uppermost 30 m 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 (0.5–8.0kHz) 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 (+12; ?10) and 107 (+6; ?5) for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 (+10; ?7) for a modern sand deposit with mean grain size 2.5 phi, 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

Deglacial history of glacial lake Windermere, UK; implications for the central British and Irish Ice Sheet

In the UK, a combination of outcrop mapping, satellite Digital Elevation Models, high-resolution marine geophysical data, and a range of dating techniques have constrained the maximum limit and overall retreat behaviour of the British and Irish Ice Sheet (BIIS). The changing styles of deglaciation have been most extensively studied in the west and north-western sectors of the BIIS, primarily using offshore geophysical surveys. The surviving record in the southern, terrestrial sector is fragmentary, permitting only large-scale (tens of kilometres) and longer timescale (c. 1 ka) reconstructions of ice-margin movement, with limited information on deglacial processes. Here we present a high-resolution study of the retreat behaviour for a section of the southern ice-margin from Windermere in the Lake District, using high-resolution 2D multi-channel seismic (MCS) data, processed using prestack depth migration. By combining the seismic stratigraphy with landform morphologies, extant cores, and seismic velocity measurements, we are able to distinguish between: over-consolidated till; recessional moraines; De Geer moraines; flowed till/ice-front fan; supra-/en-glacial melt-out till; and subsequent glaciolacustrine/lacustrine sedimentation. The results reveal a complex and active valley glacier withdrawal from Windermere that changed character between basins and produced two small, localised areas of ice-stagnation and downwasting. This study indicates that similar active ice-margin retreats probably took place in other valleys of the Lake District during the Late Devensian deglaciation rather than the previously held view of rapid ice-stagnation and downwasting. When combined with the regional terrestrial record, this supports a model of early ice loss in terrestrial England compared to other parts of the United Kingdom