Interference fringes on GLORIA side-scan sonar images from the Bering Sea and their implications     

Q. J. Huggett  A. K. Cooper  M. L. Somers  A. R. Stubbs 《Marine Geophysical Researches》1992,14(1):47-63

GLORIA side-scan sonographs from the Bering Sea Basin show a complex pattern of interference fringes sub-parallel to the ship's track. Surveys along the same trackline made in 1986 and 1987 show nearly identical patterns. It is concluded from this that the interference patterns are caused by features in the shallow subsurface rather than in the water column. The fringes are interpreted as a thin-layer interference effect that occurs when some of the sound reaching the seafloor passes through it and is reflected off a subsurface layer. The backscattered sound interferes (constructively or desctructively) with the reflected sound. Constructive/destructive interference occurs when the difference in the length of the two soundpaths is a whole/half multiple of GLORIA's 25 cm wavelength. Thus as range from the ship increases, sound moves in and out of phase causing bands of greater and lesser intensity on the GLORIA sonograph. Fluctuations (or wiggles) of the fringes on the GLORIA sonographs relate to changes in layer thickness. In principle, a simple three dimensional image of the subsurface layer may be obtained using GLORIA and bathymetric data from adjacent (parallel) ship's tracks. These patterns have also been identified in images from two other systems; SeaMARC II (12 kHz) long-range sonar, and TOBI (30 kHz) deep-towed sonar. In these, and other cases world-wide, the fringes do not appear with the same persistence as those seen in the Bering Sea.  相似文献   

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling     

Karin E. Gustafsson  Jrgen Bendtsen 《Estuarine, Coastal and Shelf Science》2007,74(4):641

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.  相似文献   

加固含水平软弱层高陡边坡预应力锚索倾角的取值范围     

宋书志孔纪名  杨宗佶刘正梁 《山地学报》2006,24(B10):217-221

用直线和曲线滑面两种模型，模拟预应力锚索加固含有水平软弱层的高陡边坡的潜在滑动面，通过对锚索加固的力学机理分析来阐述预应力在滑动面主滑方向上产生分力的原因。从锚索受力角度上对锚索倾角的取值范围进行探讨，结合实际造孔过程有不同程度的孔斜或弯曲以及获取较好的锚固灌浆效果，就仰角锚固和俯角锚固两种锚固条件分别得出锚索倾角的取值范围，并以此来判定锚索的预应力是否会在滑动面走向方向上产生不利于坡体稳定的下滑力。  相似文献   

干旱半干旱地区农田土壤NO3-N深层积累及其影响因素   总被引：7，自引：0，他引：7  

郭胜利  张文菊  党廷辉  吴金水  郝明德 《地球科学进展》2003,18(4):584-591

以长期试验资料为基础,着重分析了干旱半干旱地区农田系统中施肥、作物、降水、耕作措施以及土壤类型和特性对产生土壤NO3-N深层积累的影响.分析发现,氮肥的过量施用和400～800 mm降水量偏低是导致干旱半干旱地区土壤NO3-N积累在100～300 cm土层的主要因素.随着氮肥用量的增加,NO3-N深层积累显著增加;氮磷配施有助于降低其积累量.不同作物对氮素的吸收利用效率也是影响NO3-N深层积累的因素,作物之间的轮作方式会有效降低NO3-N深层积累;休闲期种植合理植物可有效降低NO3-N深层积累.NO3-N深层积累主要产生在质地较重的土壤上,带正电荷粘土矿物对NO3-N吸附是导致热带土壤中NO3-N积累的主要因素.深入研究深层积累NO3-N的生物有效性、迁移变化机理、与作物根系之间的关系以及对土壤性状和环境的影响具有重要意义.  相似文献   

An EFTEM/HRTEM high-resolution study of the near surface of labradorite feldspar altered at acid pH: evidence for interfacial dissolution-reprecipitation   总被引：1，自引：0，他引：1  

R. Hellmann  J.-M. Penisson  R. L. Hervig  J.-H. Thomassin  M.-F. Abrioux 《Physics and Chemistry of Minerals》2003,30(4):192-197

Using an approach combining high-resolution and energy-filtered transmission electron microscopy (HRTEM and EFTEM), we have studied with Å to nm-spatial resolution the interfacial region that delimits the near-surface altered zone and non-altered labradorite feldspar after dissolution under acid pH conditions. The interface is characterized by extremely sharp and spatially coincident changes in structure and chemistry. The 500-nm-thick altered zone is depleted in interstitial cations (Ca, Na, K) and Al, a framework element, whereas it is enriched in H, O, and Si. Modeling H⁺-alkali interdiffusion within a 500-nm-thick altered zone shows that volume interdiffusion cannot reproduce the sharp chemical interfaces measured by EFTEM. Based on these new data, we propose that the near-surface altered zone is a result of interfacial dissolution-reprecipitation, and not of preferential leaching of cations and interdiffusion with H⁺. This implies an intrinsic dissolution process that is stoichiometric, where the breaking of bonds and release of interstitial cations and framework elements (Al, Si, and O) to solution occur contemporaneously at equal relative rates from the original fluid–mineral interface.  相似文献   

The mean and turbulence structure simulation of the monsoon trough boundary layer using a one-dimensional model withe-l ande-ε closures     

Kusuma G. Rao  V. N. Lykossov  A. Prabhu  S. Sridhar  E. Tonkacheyev 《Journal of Earth System Science》1996,105(3):227-260

An attempt has been made here to study the sensitivity of the mean and the turbulence structure of the monsoon trough boundary layer to the choice of the constants in the dissipation equation for two stations Delhi and Calcutta, using one-dimensional atmospheric boundary layer model withe-ε turbulence closure. An analytical discussion of the problems associated with the constants of the dissipation equation is presented. It is shown here that the choice of the constants in the dissipation equation is quite crucial and the turbulence structure is very sensitive to these constants. The modification of the dissipation equation adopted by earlier studies, that is, approximating the Tke generation (due to shear and buoyancy production) in theε-equation by max (shear production, shear + buoyancy production), can be avoided by a suitable choice of the constants suggested here. The observed turbulence structure is better simulated with these constants. The turbulence structure simulation with the constants recommended by Aupoixet al (1989) (which are interactive in time) for the monsoon region is shown to be qualitatively similar to the simulation obtained with the constants suggested here, thus implying that no universal constants exist to regulate dissipation rate. Simulations of the mean structure show little sensitivity to the type of the closure parameterization betweene-l ande-ε closures. However the turbulence structure simulation withe-ε. closure is far better compared to thee-l model simulations. The model simulations of temperature profiles compare quite well with the observations whenever the boundary layer is well mixed (neutral) or unstable. However the models are not able to simulate the nocturnal boundary layer (stable) temperature profiles. Moisture profiles are simulated reasonably better. With one-dimensional models, capturing observed wind variations is not up to the mark.  相似文献   

高密度电法勘察岩土工程实例   总被引：6，自引：1，他引：6  

陈泉霖  陈新奇 《中国煤田地质》2004,16(6):52-54

利用岩土层间的导电性差异特性，使用由WDJD-1型主机与WDZJ-1多路电极转换器组成的高密度电法测量系统，按照不同的工程要求，采取不同的排列方式进行勘探，并用RES2DINV(演示版)高密度电阻率数据二维反演软件对所采集的数据处理解释，可实现对不同岩土层界面的划分。与常规电法相比，其方法手段更为先进，定性定量解释可靠，影像图件直观清晰。  相似文献   

准噶尔盆地表层结构基本类型及特征   总被引：1，自引：0，他引：1  

赵峰  陈志刚  郑鸿明  杨晓海  王泉 《新疆地质》2004,22(2):191-195

地震勘探区内表层及近地表结构的地球物理特征，往往取决于相应的地质体结构的变化．研究不同表层及近地表地质体的地质特征和地球物理特征，对地震勘探中静校正方法的确定以及静校正量的求取至关重要．准噶尔盆地就其表层地质结构可分为山前冲积扇、戈壁、农田、沙漠等类型，不同类型的地质结构体具有各自独特的地球物理特征。  相似文献   

单管高压旋喷注浆技术在砂层加固、止水工程中的应用   总被引：1，自引：0，他引：1  

李文华  张瑞琼 《江苏地质》2004,28(2):103-106

就其他工艺在砂层的加固、止水工程中难以解决的问题，提出了单管高压旋喷注浆技术，并通过一些工程实例证实此工艺的可行性。  相似文献   

某滑坡软弱夹层抗剪强度取值方法的研究   总被引：3，自引：0，他引：3  

王义军  李天斌  王宝国 《中国地质灾害与防治学报》2004,15(4):25-29

文章主要针对软弱夹层抗剪强度取值所存在的争议，通过对贵州新街子滑坡的软弱夹层分别选择比例极限、屈服极限及峰值作为剪应力值，采用最小二乘法获得比例抗剪强度、屈服抗剪强度和峰值抗剪强度。以参数选取中应用较为成熟的反分析方法推求力学性质参数c、φ值作为判据对以上3种参数进行比较。计算结果表明：本滑坡软弱夹层中由于含泥量较大，而且粘塑性较强，屈服抗剪强度和反分析得到的抗剪强度偏差最小，比例抗剪强度偏差最大，峰值抗剪强度居中。并以此3种抗剪强度进行天然状态下滑坡稳定性验算，结果表明选用比例极限抗剪强度与当前滑坡的地质现象不符，选用峰值极限抗剪强度安全储备较低，而选取屈服极限抗剪强度最为合理。用此参数进行设计，滑坡已经得到了良好的治理。在取值研究中认为采用试验与反分析相结合的方法确定滑坡稳定性计算参数是比较合理的。以上结论为滑坡中软弱夹层抗剪强度取值的选择提供了重要的依据。  相似文献