Tests of models for high-frequency seafloor backscatter   总被引：3，自引：0，他引：3  

Jackson  D.R. Briggs  K.B. Williams  K.L. Richardson  M.D. 《Oceanic Engineering, IEEE Journal of》1996,21(4):458-470

The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles  相似文献   

Can changes in the fisheries yield in the Kattegat (1950-1992) be linked to changes in primary production?     

Nielsen  E.; Richardson  K. 《ICES Journal of Marine Science》1996,53(6):988-994

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A comparison of ozone and chlorine toxicity to three life stages of the American oyster Crassostrea virginica     

Leonard B Richardson  Dennis T Burton  Ann M Stavola 《Marine environmental research》1982,6(2):99-113

The effects of ozone-produced oxidants (OPO) on three developmental stages of the American oyster, Crassostrea virginica, were evaluated. Both straight-hinge (48-h post fertilisation) and setting pediveliger larvae were exposed to a series of OPO concentrations ranging from 0·05 to 0·30 mg/litre OPO (as total residual chlorine) at 25°C for 96 h. Mortality was used as the response parameter and results were quantitatively compared with the effects of chlorine-produced oxidants (CPO) obtained from the literature on these same developmental stages. Adult oysters (2- to 4-year class) were exposed to OPO concentrations ranging from 0·01 to 0·50 mg/litre at 15°C for 5 days and to the same concentration range at 25°C for two consecutive 6-day periods. Shell deposition and faecal matter accumulation were used as the response parameters for this stage.Straight-hinge larvae were significantly more sensitive to OPO than setting pediveliger larvae. Both larval stages were significantly more sensitive to CPO than to OPO. Shell deposition was significantly inhibited at all OPO concentrations for both 15°C and 25°C acclimated adult oysters. No significant differences were found between the shell deposition response of oysters at the two acclimation temperatures. Faecal matter accumulation was significantly reduced at OPO concentrations ≥ 0·05 mg/litre at both 15°C and 25°C. It was significantly (p < 0·05) greater for 25°C acclimated oysters than for 15°C acclimated oysters at OPO concentrations < 0·10 mg/litre but not at higher concentrations.  相似文献   

The age determination and growth rate of the European flat oyster, Ostrea edulis, in British waters determined from acetate peels of umbo growth lines     

Richardson  C. A.; Collis  S. A.; Ekaratne  K.; Dare  P.; Key  D. 《ICES Journal of Marine Science》1993,50(4):493-500

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High-resolution atmospheric observations by the Mars Odyssey Thermal Emission Imaging System     

Ai Inada  Mark I. Richardson  Timothy H. McConnochie  Melissa J. Strausberg  Huiqun Wang  James F. Bell III 《Icarus》2007,192(2):378-395

High-resolution observations of atmospheric phenomena by the Mars Odyssey Thermal Emission Imaging System (THEMIS) during its first mapping year are presented. An atmospheric campaign was implemented on the basis of previous spacecraft imaging. This campaign, however, proved of limited success. This appears to be due to the late local time of the Odyssey orbit (the locations of activity at 4–6 p.m. appear to be different from those at 2 p.m.). Ironically, images targeting the surface were more useful for study of the atmosphere than those images specifically targeting atmospheric features. While many previously recognized features were found, novel THEMIS observations included persistent clouds in the southern polar layered deposits, dust or condensate plumes on the northern polar layered deposits, dust plumes as constituent parts of local dust storms, and mesospheric clouds. The former two features tend to be aligned parallel and normal to polar troughs, respectively, suggesting a wind system directed normal to troughs and radially outward from the center of the polar deposits. This is consistent with katabatic drainage of air off the polar deposits, analogous to flow off Antarctica. The observation of dust lifting plumes at unprecedented resolution associated with local dust storms not only demonstrates the importance of mean wind stresses (as opposed to dust devils) in initiation of dust storms, but is also seen to be morphologically identical to dust lifting in terrestrial dust storms. As Odyssey moves to earlier local times, we suggest that the atmospheric campaign from the first mapping year be repeated.  相似文献   

Scour and Burial Mechanics of Objects in the Nearshore     

Jenkins  S.A. Inman  D.L. Richardson  M.D. Wever  T.F. Wasyl  J. 《Oceanic Engineering, IEEE Journal of》2007,32(1):78-90

A process-based, numerical, hydrodynamic vortex lattice mine scour/burial model (VORTEX) is presented that simulates scour and burial of objects of arbitrary shape resting on a granular bed in the nearshore. There are two domains in the model formulation: a far-field where burial and exposure occur due to changes in the elevation of the seabed and a near-field involving scour and transport of sediment by the vortices shed from the object. The far-field burial mechanisms are based on changes in the equilibrium bottom profiles in response to seasonal changes in wave climate and accretion/erosion waves spawned by fluxes of sediment into the littoral cell. The near-field domain consists of one grid cell extracted from the far-field that is subdivided into a rectangular lattice of panels having sufficient resolution to define the shape of the object. The vortex field induced by the object is constructed from an assemblage of horseshoe vortices excited by local pressure gradients and shear over the lattice panels. The horseshoe vortices of each lattice panel release a pair of vortex filaments into the neighboring flow. The induced velocity of these trailing vortex filaments causes scour of the neighboring seabed and induces hydrodynamic forces on the object. Scour around the object and its subsequent movement into the scour depression contribute to burial, while far-field changes in local sand level may increase burial depth or expose the object. Scour and burial predictions of mines and mine-like objects were tested in field experiments conducted in the nearshore waters off the Pacific coast of California at Scripps Pier, the Gulf Coast of Florida at Indian Rocks, and off the Atlantic coast of Massachusetts at Martha's Vineyard. Model predictions of mine scour and burial are in reasonable agreement with field measurements and underwater photographs.  相似文献   

Numerical simulations of asteroids modelled as gravitational aggregates with cohesion     

D.C. Richardson  P. Michel  K.W. Flynn 《Planetary and Space Science》2009,57(2):183-192

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Laboratory simulations of prebiotic molecule stability in the jarosite mineral group; end member evaluation of detection and decomposition behavior related to Mars sample return     

J. Michelle Kotler  C. Doc Richardson  Jill R. Scott 《Planetary and Space Science》2009,57(12):1381-1388

Recently, the prebiotic amino acid glycine has been found associated with natural jarosite samples from locations around the world. Since the discovery of jarosite on Mars, extensive research focuses on linking this mineral group with possible detection of biosignatures in the geologic record on Earth and Mars. Multiple analytical methods, including extraction and mass spectrometry techniques, have identified glycine and other biomolecules in jarosite samples. The jarosite end members jarosite (sensu stricto-potassium jarosite), natrojarosite (sodium jarosite), and ammoniojarosite (ammonium jarosite) have different thermodynamic stabilities, decompose at different rates, and have potentially different susceptibilities to substitution. The relationship between the thermodynamic stability of the jarosite end members and the effect that glycine has on the thermal decomposition behavior of each end member was investigated using thermal gravimetric analysis. Introducing glycine into the synthesis procedure (75 ppm) of the potassium, sodium, and ammonium jarosite end member has elucidated the effects that glycine has on the thermal stability of the mineral group. Potassium jarosite appears to be the least susceptible to the effects of glycine, with the sodium and ammonium end members showing marked changes in thermal decomposition behavior and decomposition rates. These results suggest that the sodium and ammonium jarosites are more suitable targets for identifying signs of prebiotic or biotic activity on Mars and Earth than the potassium jarosites. These results have implications for current in situ investigations of the martian surface and future sample return missions.  相似文献   

Power spectrum for the small-scale Universe     

Lawrence M. Widrow  Pascal J. Elahi  Robert J. Thacker  Mark Richardson  Evan Scannapieco 《Monthly notices of the Royal Astronomical Society》2009,397(3):1275-1285

The first objects to arise in a cold dark matter (CDM) universe present a daunting challenge for models of structure formation. In the ultra small-scale limit, CDM structures form nearly simultaneously across a wide range of scales. Hierarchical clustering no longer provides a guiding principle for theoretical analyses and the computation time required to carry out credible simulations becomes prohibitively high. To gain insight into this problem, we perform high-resolution  ( N = 720³–1584³)  simulations of an Einstein–de Sitter cosmology where the initial power spectrum is   P ( k ) ∝ k ⁿ ,  with  −2.5 ≤ n ≤− 1  . Self-similar scaling is established for   n =−1  and −2 more convincingly than in previous, lower resolution simulations and for the first time, self-similar scaling is established for an   n =−2.25  simulation. However, finite box-size effects induce departures from self-similar scaling in our   n =−2.5  simulation. We compare our results with the predictions for the power spectrum from (one-loop) perturbation theory and demonstrate that the renormalization group approach suggested by McDonald improves perturbation theory's ability to predict the power spectrum in the quasi-linear regime. In the non-linear regime, our power spectra differ significantly from the widely used fitting formulae of Peacock & Dodds and Smith et al. and a new fitting formula is presented. Implications of our results for the stable clustering hypothesis versus halo model debate are discussed. Our power spectra are inconsistent with predictions of the stable clustering hypothesis in the high- k limit and lend credence to the halo model. Nevertheless, the fitting formula advocated in this paper is purely empirical and not derived from a specific formulation of the halo model.  相似文献   

Fragment properties at the catastrophic disruption threshold: The effect of the parent body’s internal structure   总被引：1，自引：0，他引：1  

Martin Jutzi  Patrick Michel  Derek C. Richardson 《Icarus》2010,207(1):54-346

Numerical simulations of asteroid breakups, including both the fragmentation of the parent body and the gravitational interactions between the fragments, have allowed us to reproduce successfully the main properties of asteroid families formed in different regimes of impact energy, starting from a non-porous parent body. In this paper, using the same approach, we concentrate on a single regime of impact energy, the so-called catastrophic threshold usually designated by , which results in the escape of half of the target’s mass. Thanks to our recent implementation of a model of fragmentation of porous materials, we can characterize for both porous and non-porous targets with a wide range of diameters. We can then analyze the potential influence of porosity on the value of , and by computing the gravitational phase of the collision in the gravity regime, we can characterize the collisional outcome in terms of the fragment size and ejection speed distributions, which are the main outcome properties used by collisional models to study the evolutions of the different populations of small bodies. We also check the dependency of on the impact speed of the projectile.In the strength regime, which corresponds to target sizes below a few hundreds of meters, we find that porous targets are more difficult to disrupt than non-porous ones. In the gravity regime, the outcome is controlled purely by gravity and porosity in the case of porous targets. In the case of non-porous targets, the outcome also depends on strength. Indeed, decreasing the strength of non-porous targets make them easier to disrupt in this regime, while increasing the strength of porous targets has much less influence on the value of . Therefore, one cannot say that non-porous targets are systematically easier or more difficult to disrupt than porous ones, as the outcome highly depends on the assumed strength values. In the gravity regime, we also confirm that the process of gravitational reaccumulation is at the origin of the largest remnant’s mass in both cases. We then propose some power-law relationships between and both target’s size and impact speed that can be used in collisional evolution models. The resulting fragment size distributions can also be reasonably fitted by a power-law whose exponent ranges between −2.2 and −2.7 for all target diameters in both cases and independently on the impact velocity (at least in the small range investigated between 3 and 5 km/s). Then, although ejection velocities in the gravity regime tend to be higher from porous targets, they remain on the same order as the ones from non-porous targets.  相似文献