A program for computation of the gravity and magnetic anomalies associated with two-dimensional structures has been adapted for use in an interactive mode using a storage-screen CRT linked to a remote computer via standard telephone lines. This new program aids the user in developing a structural model that satisfies both magnetic and gravity observations. Examples are included to demonstrate the increased efficiency of interpretation using this low-cost graphics approach and to illustrate the detail of the model resulting from the compromise necessary to satisfy both potential-field data sets. 相似文献
Inversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple `Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple `flood' geometry. Complete burial, with a typical sediment density of 2300 kg m–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m. 相似文献
Chlorophyll and carotenoid pigments were determined from the gut sediments of five species of bathyal holothurian in the NE Atlantic, sampled shortly after the spring/summer phytoplankton bloom in 2001 and prior to the spring bloom in 2002. Three species, Laetmogone violacea, Paroriza pallens and Bathyplotes natans, sampled within a similar depth range (900–1100 m) in the summer of 2001 showed significant differences in their chlorophyll and carotenoid pigment concentrations. This suggests they may select for slightly different components from the available food resource. Four species sampled in early spring 2002, Laetmogone violacea, Paroriza pallens, Benthogone rosea and Benthothuria funebris, also had significant differences in their pigment concentrations. These species were sampled over a wider depth range (1000–3100 m) showing a bathymetric trend in pigment concentrations. There was a distinct seasonal change in the composition and concentration of the pigments, linked to a reduction in the availability of fresh organic material during autumn and winter periods.Ovarian tissue was also examined. The carotenoid pigments found in the ovary also occurred in the OM ingested by the holothurians. The dominant gonadal carotenoid pigments were β-carotene, echinenone and zeaxanthin. The potential for using these carotenoids to gain a competitive advantage through selectivity of chlorophyll and carotenoid pigment biomarkers are discussed in relation to competition for food resources by deposit-feeders. The results were also compared with selectivity in abyssal species. 相似文献
Surveys of imposex in dogwhelk (Nucella lapillus) populations around oil terminals in Sullom Voe (Shetland) and Scapa Flow (Orkney) have confirmed that the effects of tributyltin (TBT) released from large vessels in coastal waters can be recognised. The impact of large vessels can be separated from the effects of other inputs of TBT in the areas studied. The intensity of the effects was greater in Shetland than in Orkney, reflecting the higher exposure to TBT resulting from the greater volume of traffic using the Shetland terminal, and the more restricted water exchange in that area. A large proportion of the female dogwhelks in Sullom Voe were unable to reproduce. Females in Scapa Flow were still able to reproduce, except for a small proportion close to two fish farm sites where TBT net-treatments were used prior to 1987. 相似文献
Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane.
Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf.
Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction.
Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble.
The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km2. This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged. 相似文献