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Goodman R.R. Caruthers J.W. Stanic S.J. Wilson M.A. 《Oceanic Engineering, IEEE Journal of》2000,25(4):501-506
An experiment was performed off the shore of Panama City, FL, to measure the spatial and temporal coherence of high-frequency signals that were transmitted between fixed towers. Transmission was along paths at mid depths in about 10 m of water. During the time of the experiment, there were two stormy days with breaking waves and nearby high surf. It was observed that pulse-to-pulse variations (over seconds) in travel times, over a range of frequencies, increased dramatically from those observed on quiet days. Average travel times also increased by about 3%. Dispersion was also observed. The distance between the source and the receiver towers was approximately 60 m. By assuming that bubbles were either generated by breaking waves and advected down ward and/or generated by surf and advected outward, these results are explained. Estimates of the average bubble density and bubble-density variations are made 相似文献
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Pavle TAN?I? Radovan DIMITRIJEVI? Maja POZNANOVI? Aleksandar PA?EVSKI Slobodanka SUDAR 《《地质学报》英文版》2012,86(6):1524-1538
The crystal structure of ludwigite from Vranovac ore deposit (Boranja Mt., Serbia) was refined using the X-ray powder diffraction (XRPD) Rietveld method in the space group Pbam to a final RB=7.45% and RF=5.26%. It has the unit cell dimensions of: a=9.2515(2) ?; b=12.3109(2) ?; c=3.03712(7) ?; and V=345.91(1) ?3. The calculated distances and angles are mostly in good agreement with the Mg2+-Fe2+ substitutions across the M(1) and M(3) sites, as well as with the Fe3+-Al3+ replacement in the M(4) site. However, the mean observed M(2)-O distance is considerably shorter than prescribed, due to a slight increase of the Fe3+ content in the M(2) site. Such replacement was compensated by slight increase of the Fe2+ content in the M(4) site, resulting in the (Mg1.48Fe2+0.46Fe3+0.05Mn0.02)2.01(Fe3+0.94Fe2+0.04Al0.02)1.00B1.00O5 composition. The formation temperature was estimated to be about 500–600°C. The influences of the various chemical compositions to the crystallographic parameters, M-O distances, M(3) and M(4) sites shift, distortion parameters and estimated valences, were also studied and compared with other reference samples. 相似文献
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A formal theory exists for determining sound attenuation from a known distribution of bubble sizes in the ocean; however, an integral equation must be inverted if attenuation is given and the distribution of bubbles is not. An approximate distribution can be determined based on the resonant bubble approximation (RBA). An iterative approach, for which the RBA represents the zeroth iteration, was proposed and carried out to the first iteration in a previous paper . It was suggested that additional iterations would improve the bubble-distribution results. Here we formulate the procedure to carry the results to a higher order and demonstrate, based on a theoretical distribution of a multiple power law form, the improvements in successive approximations of the bubble distribution to the fourth iteration level. A recursion relation is developed that allows one to carry the iteration out to an arbitrary order. It is shown that regions of the distribution that change in the power-law exponent are places where the higher order corrections improve the results the most. 相似文献
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PA Franklin 《新西兰海洋与淡水研究杂志》2013,47(1):112-126
Maintenance of suitable conditions in lowland rivers for both fish passage and resident species is crucial to ensuring the long-term sustainability of fish populations. The dissolved oxygen concentration of water is a key factor controlling habitat quality for fish and a critical measure of stream health. Continued land use intensification and greater exploitation of water resources has contributed towards increasing the frequency and duration of low dissolved oxygen events in lowland rivers and the associated risk of adverse effects on fish communities. Revised guidelines are therefore proposed to support setting of biologically relevant dissolved oxygen limits for the protection of New Zealand freshwater fish communities. These guidelines account for both event magnitude and duration, identify different protection levels based on the risk of negative impacts and are based on current scientific knowledge on the tolerances of New Zealand fish species. 相似文献
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了解气候变化对冬小麦生育期的影响,可为农业应对气候变化、调整农作物种植结构提供科学依据。本文利用1996—2022年近26 a拉萨冬小麦生育期以及逐日平均气温(Tm)、最高气温(Tmax)、最低气温(Tmin)、气温日较差(DTR)、相对湿度(RH)、降水量(Pr)、日照时数(S)和≥0 ℃积温(∑T0)等资料,采用线性倾向估计、Pearson相关系数和优势分析方法等,分析了拉萨河谷冬小麦生育期变化特征以及影响的主导气象因子。结果表明:(1)近26 a拉萨河谷冬小麦营养生长期(Vegetative Growth Period,VGP)、生殖生长期(Reproductive Growth Period,RGP)和全生育期(Whole Growth Period,WGP)的Tm均呈上升趋势,Tmin升温率大于Tmax的升温率,DTR、RH呈减小趋势,∑T0、Pr和S趋于增加。(2)冬小麦拔节前各生育期提早,提早率为0.47~6.75 d/10 a;拔节后各生育期推迟,推迟率为3.17~7.03 d/10 a;VGP、WGP延长,RGP缩短。(3)拉萨河谷10月、1月Tmin和2月Tm的升高,是冬小麦出苗、分蘖和返青期提前的主要因素,导致冬小麦拔节后生育期推迟的重要因素则是4—5月S增加及6月RH减少。 相似文献
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An upgrade to bistatic scattering strength modelling that is based on the authors' current understanding of bottom topographic scattering with an emphasis on modeling the `forward lobe' where Lambert's law fails quite significantly is reported. Low-frequency bottom scatter modeling is reviewed with particular emphasis on the issues of the forward scattered lobe. A specific model (a modified version of BISSM) is proposed, and the model's advantages and limitations are discussed. The requirement for certain high-resolution geomorphic data needed to support the model is discussed. Like the original BISSM, the version does not modify the accepted form for diffuse scattering, but it does modify the form of the forward lobe 相似文献
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Rouseff D. Henyey F.S. Caruthers J.W. Stanic S.J. 《Oceanic Engineering, IEEE Journal of》2001,26(1):131-140
In March of 1997, a shallow water experiment was conducted near the Scripps Pier in La Jolla, CA, USA. The goal was to determine the dynamics, distribution, and acoustic effects of bubbles just offshore from active surf. A major component of the experiment was the “Delta Frame”, an apparatus that supported two acoustic sources and eight receivers. Acoustic intensity was measured at frequencies between 39 and 244 kHz over the resulting 16 horizontal ray paths. Paths ranged in length from 2.5 to 8.6 m. In the present paper, a tomography algorithm is developed and implemented using Delta Frame data. Measurements are combined to produce quantitative cross-sectional images of the attenuation associated with the bubble cloud. Numerical simulations predict that the Frame ran resolve details of the field down to about 2 m. Images constructed at different acoustic frequencies are scaled and compared. A 5-min sequence of images is studied in detail. Swell waves are shown to cause rapid fluctuations in the images 相似文献
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Scattering from the ocean bottom is often assumed to be controlled by two spatial scales: the larger scale associated with reflections from plane facets, and the smaller one associated with diffuse scattering from height variations. Choosing the wavenumber for this partitioning has proven to be important but troublesome. For this work, scattering data are simulated using Helmholtz-Kirchhoff or physical optics theory and selected input geomorphology. These data are inverted to provide rms slope of facets and rms heights of small-scale roughness using a simple two-scale roughness model introduced previously (J. W. Caruthers and J. C. Novarini, IEEE J. Oceanic Eng., vol. 18, pp. 100-106, 1993). Bottom relief is described by power spectra of the power law form, and the bottom is assumed to be impenetrable. The work introduces a new criterion for effecting this partition based on setting a roughness parameter equal to unity. The criterion is shown to be valid for the cases analyzed based on the ability of the inversion model to recover the input geomorphology 相似文献