Acoustic attenuation measurements in gassy intertidal sediments in Dibden Bay, Southampton Water (UK) show significant and systematic changes over a tidal cycle.Modelling of the attenuation-frequency response curves, based on extant theory with modifications for hydrostatic pressure/gas bubble size relations and bubble size distributions, reproduces the observations over the tidal cycle.However, more work is needed to constrain the model input parameters to verify the theory more completely, particularly bubble size distribution and morphology.

In response to the requirement for new techniques to undertake non-destructive surveys ofsubmerged archaeological sites, adaptation of conventional acoustic characterisation techniques has been proposed.Identi cation of archaeological material (particularly wood) using such methods requires knowledge of their acoustic properties.Approaches for the calculation ofp-wave velocity and density for waterlogged wooden artefacts are presented Preliminary results suggest acoustic methods can be used to identify wooden artefacts and may be su iciently sensitive to determine their degradation state.Such techniques could have a signi cant impact on the management and conservation of our submerged cultural heritage.

The application of seismic inversion techniques to the foundation and drilling top hole zones has garnered significant interest in recent years. The shift towards more geologically complex and deeper water sites, combined with the global economic climate, has driven a requirement for more cost-effective site characterisation. More often used by the exploration industry, seismic inversion has been touted as a potentially valuable tool for quantifying the spatial and depth variability in sediment properties. In doing so, this approach can reduce the risk of encountering unforeseen ground conditions and the need for excessive over-design. Despite its potential, the inversion of high-resolution seismic data has yet to see widespread use, leaving unanswered questions regarding how and where this tool can best fit into the site characterization work flow. We test the potential usefulness of seismic inversion using a range of existing site investigation data sets. We apply several different inversion methods, including acoustic impedance and seismic quality factor inversion, as well as artificial neural network multi-attribute regression, to tackle end-member potential uses. First, explore early-phase potential uses, showing how seismic quality factor and acoustic impedance inversion can be used to capture the spatial variability in facies architecture and bulk sediment properties that could be used in appraisal and pre-FEED studies to optimize borehole and penetrometer (CPT) depths/locations and to ensure effective site-wide characterization. Second, we apply a combined acoustic impedance and artificial neural network workflow to link seismic properties with CPT profiles. These results demonstrate the potential late-phase use of seismic inversion for short-range interpolation/extrapolation of more complex geotechnical properties through the generation of synthetic CPT profiles useful for infrastructure design and micro-siting late in the development cycle. While not a comprehensive list of applications, together these examples illustrate how seismic inversion can be utilized throughout the development cycle. If the required objectives are clearly defined and an appropriate inversion workflow developed, seismic inversion can help to reduce uncertainty in site-wide characterization and drive efficiencies in layout and design studies throughout a project lifetime.

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.

Twin inverted pulse sonar (TWIPS) is here deployed in the wake of a moored rigid inflatable boat (RIB) with propeller turning, and then in the wake of a moving tanker of 4580 dry weight tonnage (the Whitchallenger). This is done first to test its ability to distinguish between scatter from the wake and scatter from the seabed, and second to test its ability to improve detectability of the seabed through the wake, compared to conventional sonar processing techniques. TWIPS does this by distinguishing between linear and nonlinear scatterers and has the further property of distinguishing those nonlinear targets which scatter energy at the even-powered harmonics from those which scatter in the odd-powered harmonics. TWIPS can also, in some manifestations, require no range correction (and therefore does not require the a priori environment knowledge necessary for most remote detection technologies).

This paper describes the detection and classification of targets against clutter by distinguishing between linear and nonlinear scatterers and, further, by distinguishing those nonlinear targets that scatter energy at the even-powered harmonics from those that scatter in the odd-powered harmonics. This is done using twin inverted pulse sonar (TWIPS), which can also, in some manifestations, require no range correction (and therefore does not require the a priori knowledge of the environment needed for most remote detection technologies). The method applies, in principle, to a range of sensor technologies, including the use of radar to distinguish between circuitry, metal and soil; Light Detection and Ranging (LIDAR) to detect combustion products; and Magnetic Resonance Imaging (MRI). A sonar application is demonstrated, detecting objects in bubbly water (including in the wake of a ship of 3953 gross register tonnage). A manmade sonar that can operate in bubbly water is relevant: Cold War sonar is not optimized for the shallow coastal waters that typify many current operations. The US Navy use dolphins in such waters. TWIPS arose as a demonstration that echolocation was possible in bubbly water in response to a video showing dolphins generating bubble nets when hunting: if echolocation were impossible in these nets, then during this hunt, the dolphins would have blinded their sonar.

The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000±50,000m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment.

The presence of free gas can dramatically alter the acoustic properties of marine sediments. The effect of different shapes and sizes of gas pockets is of particular interest. Results from one acoustic transmission and two-frequency acoustic scattering experiments at intertidal gassy mud sites on the south coast of England provide evidence for the presence of both spherical and non-spherical gas voids. The characteristics of the bubble population can be estimated from new models of nonlinear bubble dynamics.

GeoChirp 3D: High Resolution 3D Sub-Bottom Profiling Martin Gutowski (GeoAcoustics Ltd) Jon M. Bull (National Oceanography Centre) Justin K. Dix (National Oceanography Centre) Timothy J. Henstock (National Oceanography Centre) Peter I. Hogarth (GeoAcoustics Ltd) and Thomas M. Hiller (GeoAcoustics Ltd) SUMMARY____________________________________________________________ This article describes the concept of high resolution 3D sub-bottom profiling outlines the design and application of the GeoChirp 3D system and demonstrates its capabilities using a dataset imaging a buried cofferdam in the Port of Southampton (UK). EAGE 69 th Conference & Exhibition — London UK 11 - 14 June 2007 Introduction The hydrocarbon exploration industry has routinely

To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured group velocities at specific locations for the silty sites. For both silts and sands predicted dispersions are comparable to the intrinsic errors in group velocity and hence undetectable. The attenuation coefficients predicted by the Grain Shearing model adequately describe the measured attenuation coefficients, within the observed variability.

The continental margin of the George V Land represents the seaward termination of one of the largest sub-glacial basins (the Wilkes Basin) of the East Antarctic lee Sheet (EAIS) and hence is a potentially useful site for the investigation of the Cenozoic glacial history of Antarctica. Because the seafloor morphology relates strictly to recent glacial marine sedimentary processes, we have compiled all available echo-soundings data collected until the year 2001 and integrated the data set with satellite altimetry data. As a result, we have produced a new bathymetric map of the margin, covering an area of more than 80,000 km(2) with a spatial resolution of about 1 km. The bathymetric data have been integrated with sub-bottom profiler data with the purpose of defining sedimentary processes and their variations during the Quaternary.The continental shelf of the Wilkes Land margin is characterised by alternating banks and glacial troughs connected to sub-marine canyons that cut into the continental slope. Our study focussed on the continental rise, where asymmetrical ridges alternate with large deep-sea channels. The ridges have a long gentle eastern side and short steep western side, with axis elongated approximately in north-south direction, perpendicular to the margin. The channels represent the main sediment drainage pattern feeding the ridge depositional system found along the continental rise. The sediment is supplied to the continental shelf edge by ice sheets, and sediment gravity flows are considered the main process for sediment supply to the rise.The modern sedimentary environment of the deep margin is affected by turbiditic down-slope sediment transfer with a minor contribution from along-slope contour currents. The WEGA channel is currently affected by transport and settling of sediment through High-Salinity Shelf Water (HSSW), originating on the continental shelf. We infer that thermohaline circulation has contributed to sediment transport and deposition since the mid-Pleistocene. (c) 2006 Elsevier Ltd. All rights reserved.

The spatial and temporal accumulation of slip from multiple earthquake cycles on active faults is poorly understood. Here, we describe a methodology that can determine the time period of observation necessary to reliably constrain fault behaviour, using a high-resolution long-timescale (the last 17kyr) fault displacement dataset over the Rangitaiki Fault (Whakatane Graben, New Zealand). The fault linked at ca. 300ka BP and analysis of time periods within the last 17kyr gives insight into steady-state behaviour for time intervals as short as ca. 2kyr. The maximum displacement rate observed on the Rangitaiki Fault is 3.6±1.1mmyr−1 measured over 17kyr. Displacement profiles of the last 9ka of fault movement are similar to profiles showing the last 300ka of fault movement. In contrast, profiles determined for short time intervals (2–3kyr) are highly irregular and show points of zero displacement on the larger segments. This indicates temporal and spatial variability in incremental displacement associated with surface-rupturing slip events. There is spatial variability in slip rates along fault segments, with minima at locations of fault interaction or where fault linkage has occurred in the past. This evidence suggests that some earthquakes appear to have been confined to specific segments, whereas larger composite ruptures have involved the entire fault. The short-term variability in fault behaviour suggests that fault activity rates inferred from geodetic surveys or surface ruptures from a single earthquake may not adequately represent the longer-term activity nor reflect its future behaviour. Different magnitude events may occur along the same fault segment, with asperities preventing whole segment rupture for smaller magnitude events.

We present high-resolution 'Chirp' sub-bottom profiler data from Thingvallavatn, a lake in Iceland's western rift zone. These data are combined with stratigraphic constraints from sediment cores to show that movement on normal faults since 9 ka are temporally correlated with magmatic events, indicating that movements were controlled by episodic dyke intrusion. Sediment depo-centres and the focus of subsidence migrated westwards over 3-4 kyr towards the locus of subsequent brittle failure. We interpret this subsidence as related to dyke intrusion a few km along strike, originating from the Hengill volcanic system, which occurred prior to major dyking, faulting and subsidence within the lake at 1.9 ka.