Improved in situ gamma-ray transmission densitometer for marine sediments

The first-generation University of Illinois gamma-ray transmission densitometer, designed for the in situ measurement of sediment bulk density, was modified by incorporating in the detector probe (1) an Americium-241 alpha particle pulser and an anti-walk gain stabilization control to maintain better temperature stability and (2) a small power supply and a IC preamplifier to eliminate the need for a high-voltage coaxial cable between the detector and external signal conditioning electronics package. This second-generation Lehigh University system has been successfully deployed since 1971 in routine use from ships and submersibles in the Atlantic and Pacific Oceans and the Gulf of Mexico. Results are presented of system operations to (1) measure bulk density over the range of 1.2–1.8 Mg/m³ in the Hudson Canyon, (2) penetrate 1.9 m into the seafloor in the San Diego trough and, (3) be lowered to a water depth of 3.6 km in the Gulf of Mexico.     

Relationship between Shot-Induced Noise and Shot Interval in Seismic Refraction Experiments at Sea

Seismic noise induced by the seismic source during continuous profiling reduces the signal-to-noise ratio and hence the data quality. This noise is largely dependent on the shot interval. In this paper, the noise amplitude of refraction seismic records from a special experiment is analysed as a function of the shot interval. An empirical exponential relationship between mean peak noise amplitude and shot interval is deduced. By increasing the shot interval, the induced noise can be minimized on all successive records. This results in an improvement of the data, predominantly the signal-to-noise ratio. Because the seismic signal and the shot-induced noise have nearly the same spectra, the chance of improving the signal-to-noise ratio by stacking is significantly reduced.     

Experimental investigation of the combustive sound source

In this paper, we describe a unique low frequency underwater sound source called the combustive sound source (CSS). In this device, a combustible gas mixture is captured in a combustion chamber and ignited with a spark. The ensuing combustion produces expanding gases which in turn produce high intensity, low frequency acoustic pulses. With high-speed motion pictures of the CSS event, we relate the motion of the bubble to the acoustic waveform. We also compare the measured first bubble period in the CSS pressure signature with the predictions of the Rayleigh-Willis equation, including the dependence of the radiated acoustic waveform on the volume and depth of the bubble. Measurements of the first bubble period agree with Rayleigh-Willis theory in trend, but not in absolute value. In addition, we discuss the variation of the acoustic output with the fuel/oxygen mixture. Finally, several other factors that affect the acoustic output of CSS are discussed. These include the shape of the CSS combustion chamber, the type of oxidizer and fuel, and the ignition source     

Submersible observations of an iceberg pit and scour on the Grand Banks of Newfoundland

An elongated sea bed depression (pit), 80 by 125 m and 10 m deep located on the Grand Banks of Newfoundland was identified on high resolution surficial geophysical survey lines and investigated using a manned submersible. The pit occurs at the termination of a long (greater than 3 km) iceberg scour, 1.5 m deep, and 80 m wide in 91 m of water. It is similar to other depressions in the region that indent the sea bed up to seven times deeper than the surrounding deepest iceberg furrows. The pit is interpreted to be the sea bed response to impact and loading by a rolling and grounding iceberg. The iceberg scour and pit appear to be of recent origin.     

Dynamic response of dolos armor units to drop test impact loads

Dolos concrete units have been used extensively throughout the world for the protection of shorelines and rubble structures. A three-dimensional finite element model is used to determine the states of dynamic stress in dolosse with varying dimensions and concrete properties. An analytical procedure is developed which accurately predicts the tensile stress in the shank and horizontal fluke of dolosse subject to drop test loading conditions Numerical examples are presented which illustrate the application of prediction and iso-stress equations.     

Bioavailability: the organism as sensor

The use of a system of continuous culture of phytoplankton at constant population density, the ‘cage-culture’ turbidostat, as a monitor of the bioavailability of pollutant materials, is illustrated. In the case of proposed dredge spoils from two polluted areas, it is shown that bulk analyses for scheduled materials cannot be equated with bioavailability. Monitoring for the effect of the whole pollutant mixture on a properly chosen test population could obviate the necessity for chemical analyses for each component.     

Interactions of cadmium with the reproductive cycle of Mytilus edulis L.

Sublethal effects of cadmium on reproduction can have detrimental consequences for the long-term survival, even of species with high LC₅₀ values. The sea mussel, Mytilus edulis, is well-known for its use in biomonitoring work due to its relatively low sensitivity towards many toxicants, including cadmium.¹ Because little is known on sublethal effects of this metal in invertebrates,^2–6 we have studied these effects, with emphasis on reproduction, by exposing mussels to 100 ppb cadmium in sea water. This concentration significantly inhibits follicle development in both male and female gonads, but has subsequently a stimulating effect on the spawning frequency. Due to these opposite effects the consequences of cadmium stress for gamete production are expected to be small, as the average number of gametes released by spawning animals was not significantly changed.     

Sea-level, sediment supply and coastal changes: Examples from the coast of Ireland

Sediment supply and pre-existing shoreline morphology are crucial factors in controlling coastal changes due to sea-level rise. Using examples from both southeast and northeast Ireland, it can be shown that sea-level change may trigger a sequence of events which leads to both static and dynamic shoreline equilibrium. Cliff erosion and longshore sediment movement in east Co. Wexford has led to injection of sediment onto the shelf, and the growth, under both wave and tide regimes, of linear offshore shoals. These shoals now control the pattern of shoreline erosion and provide a template for possible stepwise evolution of the coast under any future sea-level rise. In contrast, the nearby coast of south Co. Wexford comprises a series of coarse clastic barriers moving monotonously onshore, via overwash processes. Here the behavior of the barrier is conditioned by the antecedent morphology of both the beach face and stream outlet bedforms. Finally, the rock platform coast of Co. Antrim presents a far more resistant shoreline to incident marine processes, yet even here there is strong evidence of present process control over so-called ‘raised’ platforms and embayments. It is concluded that coastal sediment supply and dynamics, together with coastal morphology and its interaction with waves, present a far more complex variety of sea-level indicators than is normally acknowledged.