A direct recording ocean bottom seismometer

We have designed a simple, cheap and reliable ocean-bottom seismometer. Signals from three-component geophones are recorded directly on magnetic tape running continuously at a speed of 1 mm s¹. Time reference is derived from a temperature-compensated quartz crystal oscillator and encoded on a fourth channel as an amplitude modulation of a 20 Hz carrier. A bipolar square-root signal-compression scheme doubles the tape dynamic range to 80 db, and the available bandwidth is 2 to 100 Hz. Tape and batteries are capable of 500-hr operation, and the unique magnetic release comes close to being a fail-safe system. A heavy, high-drag concrete anchor shaped like a flower-pot provides easy launching, fast stable descent and good coupling to the ocean floor. We have had numerous successful field emplacements which have yielded good earthquake and shot-refraction data.     

Exposure to 2,4-decadienal negatively impacts upon marine invertebrate larval fitness

Diatoms liberate volatile, biologically active unsaturated aldehydes following cell damage, which negatively impact upon invertebrate reproductive processes such as fertilization, embryogenesis and larval survival. 2,4-Decadienal is frequently identified among the aldehydes produced and is one of the more biologically active. The majority of studies which have examined the toxic effects of diatom aldehydes to invertebrate reproduction have scored egg production and/or hatching success as indicators of biological impacts. There are very few studies which have dealt specifically with the impacts of diatom-derived aldehydes on larval fitness. Larval stages of the polychaetes Arenicola marina and Nereis virens and the echinoderms Asterias rubens and Psammechinus miliaris exposed to 2,4-decadienal at sub 1 microg ml(-1) concentrations suffered reduced survival over the incubation period (day 1-8 post fertilization) with detectable differences for the polychates at a concentration of 0.005 and 0.01-0.1 microg ml(-1) for the echinoderms. Susceptibility of larval N. virens was investigated using stage specific 24 h exposures at 2,4-decadienal concentrations up to 1.5 microg ml(-1). A clear stage specific effect was found, with earlier larval stages most vulnerable. Nectochaete larvae (9-10 d) showed no reduction in survival at the concentrations assayed. Fluctuating asymmetry (FA), defined as random deviations from perfect bilateral symmetry, was used to analyse fitness of larval P. miliaris exposed to 2,4-decadienal at concentrations of 0.1, 0.5 and 1 microg ml(-1). The degree and frequency of asymmetrical development increased with increasing 2,4-decadienal concentration. Equally, as FA increased larval survival decreased. These results provide further support for the teratogenic nature of 2,4-decadienal and its negative impact on invertebrate larval fitness.     

Seasonal abundance, distribution and recruitment of mud crabs (Scylla spp.) in replanted mangroves

The abundance and distribution of mud crabs were studied in a replanted mangrove forest in Buswang, Aklan, Philippines. Two fishing gears, lift nets and bamboo traps, were used to monitor relative abundance of Scylla spp. populations from March 2002 to December 2003 inside the mangrove forest. A third gear, a stakenet set across a creek, was used to monitor crabs migrating out of the mangroves during the ebb tide. Scylla olivacea formed 99.3% and 70.3% of the catch in the mangrove and the stakenet, respectively. The percentage of Scylla tranquebarica increased from <1% in the mangrove catches to 29% in the stakenet. Scylla serrata was present at very low levels in both catches. The lack of modal progression in the size–frequency plots and the year-round catch rate of gravid females suggested that recruitment was constant throughout the year. Even though relative abundance decreased over the study period indicating that the stock is being over-exploited, mud crab production is more than equivalent to that of most natural mangroves.     

Numerical simulation of secondary consolidation of soil: Finite element application

A rheological model has been developed for simulating the secondary consolidation of soils. Numerical procedures have been incorporated into a coupled consolidation program using results from a representative oedometer test. A solution of a two-dimensional problem has also been performed. The technique is numerically stable and has provided satisfactory predictions for the consolidation settlements and the dissipation of pore water pressure within soils.     

Finite element modelling for simulating the surface subsidence above a compacting hydrocarbon reservoir

The surface subsidence above a compacting saturated oil reservoir is the main topic of this paper. From a literature review, it is obvious that extensive efforts have been conducted for investigating this phenomenon in various situations. Herein, a numerical model, based on the finite element method, was used for simulating three-dimensional three-phase fluid flow in a deforming saturated oil reservoir. The mathematical formulation describes a fully coupled governing equation system which consists of the equilibrium and continuity equations for three immiscible fluids flowing in a porous media. An elastoplastic soil model, based on a Mohr Coulomb yield surface, was utilized. The finite element method was applied to obtain simultaneous solutions to the governing equations where the displacements and the fluid pressures are the primary unknowns. The final discretized equations are solved by a direct solver using fully implicit procedures. A linear analysis was used to study the stability conditions of the present model. Finally, a series of simulations were conducted to indicate the validity and the utility of the developed model.     

Potential conflict in the South China sea and environs

There are many territorial disputes in Asia — both on continental Asia and in the surrounding seas. These disputes are often extensions of wider ethnic, historical and ideological rivalries. A relatively new development in disputes over island territories and EEZ boundaries is the economic dimension that has been added to old disputes and rivalries: the disputing nations, aware of the benefits that offshore oil and minerals, as well as fishing, can provide to their economies, have a clear financial incentive to pursue their claims as strongly as they dare. National pride and wider political and ideological conflicts with their rival claimants further reduce the prospects for compromise. Tensions are increased by the possibility that a military conflict in pursuit of a claim could lead to escalation, drawing the USA and the USSR into the dispute. This article outlines some of the major tension points in the area of the South China Sea.