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.

Methane hydrates are ice-like compounds that can exist only under restricted thermobaric conditions, at low temperatures or under high ambient pressure. They are important because of their potential contributions as a future source of energy, to global warming, and as a possible trigger for long run-out submarine slope instability. This paper describes laboratory experiments to synthesise disseminated methane hydrates and to characterise them under small-strain dynamic loading in the resonant column apparatus. The effects of depositing varying quantities of methane hydrate within a sand are investigated by reference to their shear and bulk modulus, and damping, over a range of isotropic effective stress. Results are compared with those obtained on the same sand without hydrate bonding and after dissociation.

Remote seismic methods, which measure the compressional wave (P wave) velocity (Vp) and shear wave (S wave) velocity (Vs), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate‐bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate‐bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both Vp and Vs made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both Vp and Vs. This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.

Compressional wave (P wave) and shear wave (S wave) velocities (V-p and V-s, respectively) from remote seismic methods have been used to infer the distribution and volume of gas hydrate within marine sediments. Recent advances in seismic methods now allow compressional and shear wave attenuations (Q(p)(-1) and Q(s)(-1), respectively) to be measured. However, the interpretation of these data is problematic due to our limited understanding of the effects of gas hydrate on physical properties. Therefore, a laboratory gas hydrate resonant column was developed to simulate pressure and temperature conditions suitable for methane gas hydrate formation in sand specimens and the subsequent measurement of both Q(p)(-1) and Q(s)(-1) at frequencies and strains relevant to marine seismic surveys. 13 dry (gas saturated) sand specimens were investigated with different amounts of methane gas hydrate evenly dispersed throughout each specimen. The results show that for these dry specimens both Q(p)(-1) and Q(s)(-1) are highly sensitive to hydrate saturation with unexpected peaks observed between 3 and 5 per cent hydrate saturation. It is thought that viscous squirt flow of absorbed water or free gas within the pore space is enhanced by hydrate cement at grain contacts and by the nanoporosity of the hydrate itself. These results show for the first time the dramatic effect methane gas hydrate can have on seismic wave attenuation in sand, and provide insight into wave propagation mechanisms. These results will aid the interpretation of elastic wave attenuation data obtained using marine seismic prospecting methods.

Remote prediction of gassy marine sediment properties is important for geohazard assessment. Gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity‐frequency and attenuation‐frequency relationships that depend on gas bubble size, gas content, and sediment elastic properties. An acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at Dibden Bay, Southampton, United Kingdom. A vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed. A miniboomer in the seabed above the array was used to generate broadband (600 Hz to 3000 Hz) acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m (subaerial exposure) at low tide and 2.35 m at high tide. The calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum (over 200 dB/m) that shifts in frequency from 1050 Hz at low tide to 1250 Hz at high tide, thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments. Modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum, supported by X‐ray computed tomography scan observations on a pressure core (which also indicate that bubble shape depends on sediment type). Modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum, implying that gas diffusion and nonspherical bubbles are significant.

The use of bender elements has become increasingly popular for the assessment of the small strain shear stiffness G(max). This may be attributed to the simplicity of the test (i.e. both test equipment and procedure) and the fact that it is non-destructive. The test yields results that have often been reported to compare well, qualitatively, with laboratory measurements using other test methods, although doubts over the accuracy of absolute values have remained, and continuing efforts have been made to improve both the bender element technique and the interpretation of its results. This paper provides a preliminary report of further developments. A continuous sine wave signal and low-frequency spectrum analyser has been used to obtain the phase velocity between applied and measured voltage of a conventional bender element configuration, over a wide range of frequency. Bender elements have also been installed on the side of the specimen, so that both group and phase velocities could be measured between sensors remote from both the source and from the rigid specimen ends. The results suggest that repeatable and consistent determinations of G(max) may be achieved by measuring the group velocity of a single-pulse sine wave between side-mounted bender elements.

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.

While ultrasound is an established technique for the non-destructive evaluation of both cut timber and standing trees, it has not been applied to water-saturated wood. Hence, laboratory experiments were performed to discover the acoustic propagation mechanism in wood. If valid, the Biot model for acoustic waves in porous media should allow the prediction of laboratory measurements (taken at 300-1200 kHz). The results show that, for the best obtainable input parameters, the Biot theory models the observed anisotropy between the longitudinal, radial and tangential directions in wood. The model also predicts velocity dispersion of 1.4% between 300-1000 kHz in the longitudinal direction while that observed was 4.2%.

Compressional wave velocity and attenuation were measured at frequencies of 200–1500 Hz on seafloor sediments at Lough Hyne, Ireland, using a mini-boomer source and hydrophone array. Velocity and attenuation were also measured in the laboratory at 200–800 kHz on a 1 m long sediment core taken from the site. The in situ results indicate an average sediment phase velocity of about 1600 m/s and sediment quality factor of 10–20. The laboratory core measurements give an average phase velocity of 1793±26 m/s and quality factor of 16±5. The poorly sorted, Lough Hyne sediments are highly attenuating and highly dispersive when compared to values published in the literature for well-sorted, marine sediments such as clean sands and marine clays. The results are consistent with the few published data for poorly sorted sediments, and indicate that intrinsic attenuation is highest when the mass ratio of mud (clay + silt) to sand grade particles is close to unity. It is proposed that compliance heterogeneities are most abundant when mud and sand grade particles are present in roughly equal proportions, and that the observations support local viscous fluid flow as the most likely loss mechanism.

Recent marine forensic investigations have largely unravelled the sequence of events concerning the sinking of the R.M.S. Titanic and its descent through nearly 3800 m of water to the seafloor on the morning of 15 April 1912. In particular, the velocity and attitude of the Titanic's bow section (at present lying upright, reasonably intact, and embedded by similar to 12 m at the prow) as it hit the bottom are of general interest to marine accident investigators. During the 1998 Titanic Science Expedition, a single sediment sample was retrieved from the seafloor (depth 20-30 cm) near the wreck by the deep water submersible, Nautile. Published geological studies suggest the seafloor in this area has remained largely undisturbed since 1912. Geotechnical analysis of the sediment sample reveals that the impact was probably a substantially undrained event and that the characteristic undrained shear strength of the sediment is similar to 25kPa within 10-16 m below the seafloor. A simple analytical model was used to calculate the embedment of a cuboid with dimensions and mass of the water-filled bow as a function of impact velocity, impact angle, and the undrained shear strength of the sediment. The results indicate the impossibility of a steep angle of impact and fast velocity. The most likely scenario is an impact velocity of 5-10 m/s at a fairly shallow angle (<40), which corroborates the results of hydrodynamic investigations.

Accurate density and P-wave velocity measurements on marine sediment cores are needed for quantitative acoustic impedance studies. An increasing number of international marine laboratories routinely measure P-wave velocity and bulk density, the product of which gives the acoustic impedance, on marine sediment cores using automated core loggers (MSCLs). Despite the progressive standardisation of MSCLs in recent years, no standard calibration technique has been decided that will enable datasets from different marine laboratories to be validated and compared with confidence. This problem is particularly acute for bulk density measurements because systematic MSCL errors are typically much greater than 10%. One solution is to use a calibration tube (comprising a stepped aluminium insert inside a water-filled core liner) that mimics marine sediment porosities and velocities, and that can be used during normal core logging. The calibration tube enables a reliable gamma ray reference intensity to be measured, and an apparent Compton mass attenuation coefficient to be derived that is suitable for most marine sediments and that simplifies data processing. The calibration tube results show that the MSCL belonging to Southampton Oceanography Centre (SOC), used as part of the standard analysis procedure of the British Ocean Sediment Core Repository (BOSCOR) gives measurement accuracies of better than ±0.07 g/cm3 for bulk density and ±0.3% for P-wave velocity.

The SAPPA (Sediment Acoustic and Physical Properties Apparatus) is a new instrument designed for the rapid acquisition of seafloor geophysical and geotechnical data. The present system can measure P-wave velocity and attenuation down to Im sub-bottom depth in sands and gravels, and the velocities of horizontally and vertically polarised shear waves at the surface. Preliminary tests show that sufficient energy is produced to propagate P-waves at frequencies up to 10 kHz in water and S-waves at about 120 Hz through at least 1 m of wet sand.

Laboratory ultrasonic measurements of compressional wave velocity and attenuation were made as a function of effective pressure on samples of limestone, sandstone and siltstone taken from a shallow borehole test site. The results indicate that the sandstones are pervaded by grain contact microcracks which dramatically affect their compressional wave attenuations. Clean sandstone shows a compressional wave quality factor (Q(p)) of 24 +/- 2 at 5 MPa effective pressure (close to the estimated in situ burial pressure) and a Q(p) of 83 +/- 29 at 60 MPa. The Q(p) of limestones and siltstones at the site show negligible and small increases with pressure in the laboratory, respectively. The strong pressure dependence of Q(p) in clean sandstone was used to infer the presence of in situ microcracks. Sediment velocities measured in the laboratory at about 1 MHz were compared with those from the full waveform sonic log at about 10 kHz. Significant velocity dispersion was observed in clean sandstones, but none in limestones and siltstones. The fact that clean sandstones are highly attenuating at 1 MHz implies that they must also be highly attenuating over a significant part of the frequency range 10 kHz to 1 MHz, to account for the magnitude of the observed velocity dispersion. Assuming the laboratory Q(p) values measured at 5 MPa remain constant down to 10 kHz predicts the observed dispersion quite well. Furthermore, the sonic log velocities of sandstones, limestones and siltstones (after normalizing each lithology for porosity and clay content) were found to reflect the same pressure (depth) trends observed in the laboratory. The results provide evidence for the existence of in situ microcracks in near-surface sediments.