The springtime stratospheric ozone (O3) layer over the Antarctic is thinning by as much as 50 percent, resulting in increased midultraviolet (UVB) radiation reaching the surface of the Southern Ocean. There is concern that phytoplankton communities confined to near-surface waters of the marginal ice zone will be harmed by increased UVB irradiance penetrating the ocean surface, thereby altering the dynamics of Antarctic marine ecosystems. Results from a 6-week cruise (Icecolors) in the marginal ice zone of the Bellingshausen Sea in austral spring of 1990 indicated that as the O3 layer thinned: (i) sea surface- and depth-dependent ratios of UVB irradiance (280 to 320 nanometers) to total irradiance (280 to 700 nanometers) increased and (ii) UVB inhibition of photosynthesis increased. These and other Icecolors findings suggest that O3-dependent shifts of in-water spectral irradiances alter the balance of spectrally dependent phytoplankton processes, including photoinhibition, photoreactivation, photoprotection, and photosynthesis. A minimum 6 to 12 percent reduction in primary production associated with O3 depletion was estimated for the duration of the cruise.
Three independent ocean color sampling methodologies are compared to assess the potential impact of instrumental characteristics and environmental variability on shipboard remote-sensing reflectance observations from the Santa Barbara Channel, California. Results indicate that under typical field conditions, simultaneous determinations of incident irradiance can vary by 9-18%, upwelling radiance just above the sea surface by 8-18%, and remote-sensing reflectance by 12-24%. Variations in radiometric determinations can be attributed to a variety of environmental factors such as Sun angle, cloud cover, wind speed, and viewing geometry; however, wind speed is isolated as the major source of uncertainty. The above-water approach to estimating water-leaving radiance and remote-sensing reflectance is highly influenced by environmental factors. A model of the role of wind on the reflected sky radiance measured by an above-water sensor illustrates that, for clear-sky conditions and wind speeds greater than 5 m/s, determinations of water-leaving radiance at 490 nm are undercorrected by as much as 60%. A data merging procedure is presented to provide sky radiance correction parameters for above-water remote-sensing reflectance estimates. The merging results are consistent with statistical and model findings and highlight the importance of multiple field measurements in developing quality coastal oceanographic data sets for satellite ocean color algorithm development and validation.
This paper compares four methods that predict jackup rig spudcan penetration with measured load-penetration records at thirteen Gulf of Mexico locations. Predicted load-penetration curves, for the four methods, are determined using site-specfic geotechnical information for normally to slightly over-consolidated clay soil profiles. The four methods include two based on SNAME (2002) recommendations, using bearing capacity formulations as proposed by Skempton (1951) and Hansen (1970), a method recommended by Houlsby & Martin (2003), and a method recommended by Hossain et al. (2006). Detailed spudcan geometries and dimensions from the seven different types of jackup rigs installed at the thirteen locations are also provided. Spudcan diameters of the seven jackup rigs vary from 11.0 to 20.1 meters and spudcan bearing pressures range from 162.8 to 507.4 kPa. Interpreted soil parameters are presented for all thirteen sites. Boring logs, illustrating the interpretation of parameters from the measured soil properties are presented for two of these locations. The logs present the results of standard laboratory tests typically performed for Gulf of Mexico site investigations: undisturbed undrained shear strength measurements from Torvane, miniature vane, pocket penetrometer and unconsolidated-undrained (UU) triaxial tests; remolded shear strength from miniature vane and UU triaxial tests; submerged unit weights (derived from measured bulk wet unit weight); water content; and Atterberg limits. Theoretical submerged unit weight values, used as an aid in parameter selection, are also plotted on the boring logs. The results of this study are presented in a series of figures that compare the predicted load-penetration curves based on each of the four methods to the measured load-penetration data recorded during the installation of the jackup rig at each of the thirteen locations. The comparisons indicate that (1) the Houlsby & Martin method provides good lower bound loadpenetration predictions, generally predicting a deeper penetration under a given load than measured; (2) the Hossain et al., method provides an upper bound load-penetration prediction, usually predicting a shallower penetration under a given load than that measured, and (3) the modified SNAME methods, with Skempton and Hansen bearing capacity factors, provide reasonable predictions of the average penetration under a given load. Two factors affecting the predictions are also discussed: spudcan geometry and spudcan cavity depth. The results demonstrate the difficulties all four methods may experience providing accurate load-penetration predictions for jackup rigs with spudcan geometries that do not lend themselves to easy modeling using standard bearing capacity factors and equivalent projected circular bearing areas. At four of the sites, with three different types of jackup rigs, all of the four methods provide more acceptable predictions after application of empirical rig-specific correction factors. The depths of spudcan cavities are examined using the methods ...
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