Detection of free gas and gas hydrate based on 3D seismic data and cone penetration testing: An example from the Nigerian Continental Slope

We present a new method to characterize free gas, gas hydrates and carbonate concretions occurrence which are considered as high-risk factors for sub-sea developments in the Niger delta. This method is based on the combination of 3D seismic data to the geotechnical site characterizations using piezocone CPTU tests (Cone Penetration Test with additional measurement of the pore water pressure). A special processing of the 3D seismic data has enabled the determination of the interval compressional velocity. Using the effective-medium theory, velocity anomalies (negative and positive) within the first 15 m were translated in gas hydrate and free gas distribution. The calibration of the P wave velocity anomalies was done thanks to in-situ geotechnical testing carried out during two oceanographic surveys (2003 and 2004). Comparison between in-situ testing, recovered cores and the prediction of the gas and the gas hydrate distribution based on the compressional wave velocity have shown that 3D seismic data is a valuable tool to identify heterogeneous areas but the use of the piezocone was essential to discriminate between gas hydrate occurrences and carbonate concretions' presence. Furthermore, in-situ compressional wave velocity (V_p) measurements have clearly demonstrated what it was suspected from the 3D seismic data, the co-existence in the study area between gas hydrate and free gas.     

Plankton distribution and vertical flux of biogenic matter during high summer stratification in the Krka estuary (Eastern Adriatic)

The aim of this study was to investigate phytoplankton abundance, composition and vertical export in the highly stratified Krka estuary, Croatia. The estuary is stratified throughout the year, and an interface between fresh- and brackish water plays an important role in production and degradation of biogenic matter. Vertical export of particulate organic carbon (POC), phytoplankton carbon (PPC) and faecal pellet carbon (FPC) was studied by deploying sediment traps in the middle and lower reach of the estuary and in the adjacent coastal zone. Zooplankton faecal pellet (FP) production experiments were conducted to provide additional information on the potential contribution of FP to the total carbon flux. High suspended concentrations of POC, chlorophyll a and phytoplankton was found in the lower reaches of the Krka estuary, adjacent to a source of anthropogenic eutrophication. The fraction of organic detritus to the total POC flux was 61–69% inside the estuary but only 7% at the marine station. This indicates that the primary producers in the surface layer of the Krka estuary are decomposed in and below the interface and then settle as detritus to the bottom. Low sedimentation rates in the coastal zone outside the estuary revealed that the eutrophication does not spread out of the estuary. Mesozooplankton played a modest role in vertical flux regulation, due to their low abundance and dominance of smaller forms as well as low faecal pellet production rates. It is concluded that processes taking place at the freshwater-seawater interface are of major importance for the vertical carbon flux in the investigated area.     

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s^-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171.     

Topography and a magnetic analysis of an area south-east of the Azores (36°N, 23°W)

A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m^-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity.     

Sand ridges on the inner Atlantic shelf of North America: Morphometric comparisons with Huthnance stability model

Sand ridge fields on the inner shelf of the Middle Atlantic Bight are generally thought to have formed in response to northeasterly storm flows as the shoreface underwent erosional retreat with postglacial sea-level rise. However, the hydrodynamic mechanism is poorly unerstood. Coastal boundary models see the ridges as responses of the seafloor to distortions in the flow induced by the coastal boundary. Stability models propose that an irregular initial topography will evolve toward an ordered array of bedforms in response to repeated flow events. The two classes of models are not mutually exclusive, nor are members within each class mutually exclusive. Results of measurements of ridge spacing on the inner Atlantic shelf of North America agree with the predictions of stability models.     

The use of controlled experimental ecosystems: A review

During the past 20 years, the controlled experimental ecosystem has found wide-spread use in marine sciences. Uses have included the study of natural ecology, environmental pollution and computer model analysis. In all these roles, the experimental ecosystem serves as a model of nature and results can be used to obtain greater insight into the oceanography of the sea. Four different types of experimental apparatus are described, and experimental procedures and results are reviewed and discussed.     

Radium and lead-210 in marine barite

Measurements of ²²⁶Ra and ²¹⁰Pb in barite samples separated from different depths of a piston core from the eastern equatorial Pacific are reported. ²²⁶Ra is enriched in the barite as would be expected from the chemistries of Ra and Ba; the ${}^{226}\text{Ra}\text{Ba}$ ratio centers around 1.5 · 10¹⁴ atoms/mole. The results suggest that barite is concentrating Ra at a rate comparable to its decay rate, viz. 5.78 · 10^?3 yr^?1. The gross ²¹⁰Pb depletion with respect to ²²⁶Ra is indicative of ²²²Rn loss due to the smallness of the barite crystals.     

Water colour index of the Guinean sector of the Atlantic Ocean

The distribution of the colour index is considered in the region bounded by 8–11°N and 13°30–18°30W based on the results of measurements made on board vessels of the Marine Hydrophysical Institute of the Ukrainian SSR Academy of Sciences (MHI) in 1977–1985. Mean values and statistical characteristics are calculated for the colour index variability over one-degree squares. A map of its multi-yearly average distribution is plotted.Translated by M. M. Trufanov.     

The NDRE-AUV flight control system

The flight control system of an autonomous underwater vehicle (AUV) developed at the Norwegian Defence Research Establishment (NDRE) is presented. A mathematical model of the vehicle is derived and discussed. The system is separated into lightly interacting subsystems, and three autopilots are designed for steering, diving, and speed control. The design of the separate controllers is based on PID techniques. Results from extensive sea testing show robust performance and stability for the autopilot     

PCDD/F and PCB in harbour seals (Phoca vitulina) from British Columbia: response to exposure to pulp mill effluents

Blubber of harbour seals (Phoca vitulina) sampled from the Strait of Georgia, BC, in 1991 and 1992 contained higher concentrations of polychlorinated dibenzo-p-dioxins and furans (PCDD/F) than did samples from Quatsino Sound on western Vancouver Island. The higher proportions of 1,2,3,6,7,8-hexachlorodibenzodioxin and 2,3,7,8-tetrachlorodibenzofuran in the Strait of Georgia samples probably reflect contamination arising from the discharge of effluents from bleached kraft mills. Higher concentrations of non-ortho- and mono-ortho substituted polychlorinated biphenyls were also found in the Strait of Georgia samples, probably as a result of general industrial activity in the region. Principal components analysis showed that residues were distributed differently between males and females. Within males, concentrations of PCDD, PCDF and PCBs appeared to increase with age. Toxic equivalents (TEQ) to 2,3,7,8-tetrachlorodibenzodioxin in the Quatsino Sd. samples were close to the working guidelines for edible fish used by Health Canada; TEQ calculated for the Strait of Georgia samples exceeded these guidelines by up to a factor of five.