Acoustic reflectors in the surface layers of deep sea sediments

Using ultrasonic scanning it is possible to display the position of an acoustic discontinuity within a sediment core and the amplitude of the reflected signal which results from it. From these results it has been possible to examine the effect of variations in the geotechnical properties on the reflection of compressional waves at normal incidence within the core at a frequency of 100 kHz. The occurrence of acoustic reflectors in the surface layers of North Atlantic sediments has also been examined.Two ultrasonic scanning systems for the non-destructive testing of marine sediment cores have been developed. The performance of the two systems has been evaluated using the results from measurements made on a suite of marine sediment cores from the North Atlantic in conjunction with X-ray scanning and geological analysis of the same cores.     

Chlorinated hydrocarbons in seawater: Analytical method and levels in the northeastern Pacific

A method is described for analyzing nanogram quantities of chlorinated hydrocarbons from 1-1 samples of seawater. Seawater samples are pumped through a copper column containing a mixture by weight of 5% activated carbon powder, 10% MgO and 85% refined diatomaceous earth. The chlorinated hydrocarbons in the seawater are adsorbed or trapped on the column and subsequently eluted with 30% benzene in acetone (v/v) for analysis by gas-liquid chromatography.This procedure was used to analyze chlorinated hydrocarbon levels in samples collected off the southern California coast. We suggest that anthropogenic chlorinated hydrocarbons can be used for the investigation of large-scale ocean currents and mixing processes.     

Alboran Sea late cenozoic tectonic and stratigraphic evolution

Analysis of airgun seismic profiles from the Alboran Sea reveals a complex morphostructure with margins, basins, and structural highs. North of the Alboran Ridge, south-facing margins have a passive style of evolution, with thick progradational sequences of post-Messinian deposits, whereas north-facing margins are tectonized along structural highs with reduced sediment cover. Basins are extensional features developed since the Early Miocene by mechanisms of tectonic escape and pull-apart, under generalized northwest-southeast to north-south compression. Depositional sequences in this semi-land-locked sea were controlled by the local tectonism and influenced by global sea-level oscillations.     

Wave forces on a single and on a group of cylinders in response to two wave spectra of same significant wave height

Spectral analysis is used to determine the wave force characteristics on structures exposed to random waves. Considerable work has been carried out to determine the magnitude of random wave forces acting on a single cylinder, but little information is available in the case of a group of cylinders in random waves. Such situations arise when structures comprise multiple tubular members which are in close proximity, and wave forces cannot be calculated precisely by analytical methods due to complicated flow conditions past the group. Experimental studies are also required for proper understanding and analysis. An experimental scheme was carried out to study the wave force characteristics on a single cylinder and on a group of cylinders in response to two different wave spectra with the same significant wave height, and the results are compared.     

Comparative tolerance of two estuarine annelids to fluoranthene under normoxic and moderately hypoxic conditions

The tolerance of the oligochaete Monopylephorus rubroniveus and the polychaete Streblospio benedicti to sediment-associated fluoranthene was characterized under normoxic (>80% dissolved oxygen saturation) and moderately hypoxic ( approximately 50% dissolved oxygen saturation) conditions. Under both conditions, M. rubroniveus was highly tolerant of fluoranthene. Streblospio benedicti was considerably less tolerant of fluoranthene compared with M. rubroniveus. In addition, S. benedicti was less tolerant to fluoranthene under moderately hypoxic conditions, although no differences in sensitivity between the two oxygen conditions were observed based upon median lethal tissue residues. Bioaccumulation factors were higher for S. benedicti exposed to moderate hypoxia, suggesting that behavioral adaptations to compensate for the lower dissolved oxygen increased its bioaccumulation of fluoranthene. The results of the present laboratory study demonstrate that (1). changes in annelid tolerance to fluoranthene under varying oxygen conditions is a species-dependent phenomenon and (2). the differential tolerance of these two annelids to these combined stressors is consistent with their relative abundances in the field.     

Physical and anthropogenic effects on observed long-term nutrient changes in the Irish Sea

The trend in Irish Sea nutrient concentrations over the last four decades has been considered to reflect changes in anthropogenic loading. Comparison of a long-term database for the Menai Strait, North Wales, with an established historic data set for the Cypris station, Isle of Man, indicates that climate also has a significant influence on observations of nutrient concentrations. Data are presented detailing long-term shifts in nitrate, phosphate and silicate measurements since the 1960s at these two fixed sampling sites in the Irish Sea. Broad systematic changes observed in all three nutrients over the decades show a rise from the 1960s through to the 1980s, followed generally by an overall decline in the 1990s. Decadal-scale salinity changes occur in the opposite sense to nutrient changes. Anthropogenic inputs from freshwater cannot fully account for observed nutrient trends, neither is there evidence for shifts in nutrient concentrations in oceanic waters over the past four decades. Climatically forced movement in the geographical position of the freshwater/seawater mixing zone over a decadal time scale could, however, give rise to the observed shifts in nutrient concentration and salinity. This cannot alter nutrient concentration and salinity per se, but causes the measurements taken at fixed sampling sites to fluctuate inversely over this time scale. It is concluded that there is complex interplay between anthropogenic loading and climate affecting the distribution of nutrients in the Irish Sea.     

Dynamics and control of a towed underwater vehicle system, part I: model development

Autonomous vehicles are being developed to replace the conventional, manned surface vehicles that tow mine hunting towed platforms. While a wide body of work exists that describes numerical models of towed systems, they usually include relatively simple models of the towed bodies and neglect the dynamics of the towing vehicle. For systems in which the mass of the towing vehicle is comparable to that of the towed vehicle, it becomes important to consider the dynamics of both vehicles. In this work, we describe the development of a numerical model that accurately captures the dynamics of these new mine hunting systems. We use a lumped mass approximation for the towcable and couple this model to non-linear numerical models of an autonomous surface vehicle and an actively controlled towfish. Within the dynamics models of the two vehicles, we include non-linear controllers to allow accurate maneuvering of the towed system.