Summary Demonstrate the successful application of multiple decimetre-resolution 3D seismic volumes from an exposed, challenging survey location a significant distance offshore. Image subsurface structure in 3D to depths of at least 10 m below seafloor, along with point targets such as boulders or potential UXO. Provide decimetre-accuracy positions for target features in X,Y, and Z. First application of decimetre-resolution 3D seismic imaging for site investigation on an offshore wind site. Despite exposed site, coarse grain sub-surface, and large distance (18 km) from base station, data demonstrate true 3D imaging using a Chirp (1.5 – 13.0 kHz source). Target volumes are small, focused areas (c. 30 × 30 m) or narrow (c. 5 m) corridors primarily for UXO identification around monopoles and along cable routes. Almost 100 % (>95 %) coverage of target areas was obtained in 3 – 4 hours and processed in near real-time. Subsurface stratigraphy is successfully imaged to > 10 m depth below seafloor and multiple point targets are identified. Illustrates the potential of ultra-high-resolutions 3D seismic systems for offshore site survey applications.

Marine Unexploded Ordnance (UXO) identification and clearance is a massive growth industry, driven by the expansion into deeper water resources and offshore renewables. The standard survey too of choice is a magnetic gradiometry system, which benefits from being cheap, easy to use, and able to rapidly cover spatially extensive areas. In theory, using the 3D Analytical Signal, such systems are also capable of characterizing the magnetic susceptibility of the UXO targets. Given the huge areas to cover and often large number of identified targets, this kind of material properties information is invaluable for distinguishing between UXO that need removing and those that can remain in situ. However, inversion of the Analytical Signal is a multi-paramter, non-unique process, difficult to perform reliably based on magnetometry data alone. Here we demonstrate the combined use of decimetre-resolution 3D seismic and gradiometer data for UXO characterization. The 3D seismic data are used to constrain the location, size, and burial depth of UXO targets prior to inversion of the Analytical Signal. Together, these techniques are used in combination to characterize 89 UXO targets that are verified using post-survey dredging.

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. 1ka) 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 two-dimensional multi-channel seismic 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, localized 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 with other parts of the UK. Copyright (c) 2012 John Wiley & Sons, Ltd.

The stratigraphy and sedimentological processes operating over the last 15,000 years within glacial lake Windermere (UK), at the mouth of Cunsey Beck, were imaged by a decimetre-resolution seismic reflection survey. A complex of fifteen mass movement deposits was identified as contemporaneous with the Younger Dryas, and two within the overlying Holocene drape. The high vertical resolution and dense grid of profiles allowed pseudo three-dimensional mapping of individual events, along with the determination of their relative temporal relationships. The size of the mass wasting deposits has been estimated to range between 2100 and > 100,000 m3. The geometry, structure and relationship to the existing stratigraphy suggest a rapid emplacement of the Younger Dryas mass movement deposits, facilitated by climatic changes making subaqueous slopes unstable, with possible triggering by seismic activity. Morphometric parameters, such as volume and planar surface area, indicate a greater mobility of the Younger Dryas mass movement deposits compared to the Holocene events. The sediments of all imaged mass movement deposits are believed to originate from the slope deposits of the lake. The age of two Holocene mass movement deposits, triggered by flooding or terrestrial debris flows, is estimated to be 2400 and 4400 years BP.

Windermere is a glacially overdeepened lake located in the southeastern Lake District, UK. Using the three-dimensional (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 interaction with preexisting deposits in unprecedented detail. Three distinct flow events are identified and mapped throughout the 3D survey area. Package structures and seismic attributes classify them as: a small (total volume of c. 1500 m3) debris flow containing deformed translated blocks; a large (inferred total volume of c. 500,000 m3), homogeneous fine-grained mass flow deposit; and a debris flow (inferred total volume of c. 60,000 m3) containing small (c. 8.0 × 2.0 m) deformed translated blocks. Geomorphological mapping of their distribution and interaction with preexisting sediments permit the reconstruction of a depositional history for the stratigraphic units identified in the seismic volume.

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.

We present estimates of in-situ compressional-wave attenuation and velocity within the uppermost 30 m of unconsolidated lacustrine sediments within Lake Windermere (U.K.), using high-resolution seismic data acquired with Boomer and Chirp sources. The wide frequency bandwidth of the Chirp source allows attenuation to be examined over a frequency range of approximately 2-9 kHz, and by using a spectral ratio technique incorporating robust re-weighted least squares regression, the apparent quality factor of sediments can be accurately and precisely determined. A 60 m multi-channel streamer used with the Boomer source allows interval velocities between target reflectors to be obtained. Models relating quality factor to mean grain-size distinguish between coarse grain-dominated and clay-dominated sediments. The interval velocities improve the classification of the sediment sequences to clays, laminated silts & sands, and coarse sand and gravel deposits. The results are evaluated against geological and core data.