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81.
82.
An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented 相似文献
83.
Carey W.M. Evans R.B. Davis J.A. Botseas G. 《Oceanic Engineering, IEEE Journal of》1990,15(4):324-334
The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter 相似文献
84.
85.
Current plate motions 总被引:57,自引:0,他引:57
86.
Solar Physics - What are the observational effects if type I bursts are deflected by lower-hybrid waves? The deflection creates an elliptical radio mirage, with the direct source at one end of the... 相似文献
87.
A.G.G.M. Tielens L.B.F.M. Waters F.J. Molster K. Justtanont 《Astrophysics and Space Science》1997,255(1-2):415-426
This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal
the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known
10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source
to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are
dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting
grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed
to amorphous silicate grains.
These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence
of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation
sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that
the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates
in these outflows, resulting in amorphous grains with little characterizing spectral detail.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
88.
89.
R. Wieler Th. Graf P. Signer S. Vogt G. F. Herzog C. Tuniz D. Fink L. K. Fifield J. Klein R. Middleton A. J. T. Jull P. Pellas J. Masarik G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ~20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ~65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. 相似文献
90.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献