Environmental significance of bedrock jointing in Southern Ontario,Canada     

N. Eyles  A. E. Scheidegger 《Environmental Geology》1995,26(4):269-277

The bedrock surface of many glaciated areas is obscured by thick drift deposits. In southern Ontario, Canada, the buried bedrock surface is dissected by channels, infilled with glacial deposits as much as 150 m thick, that are part of a wider mid-continent preglacial fluvial system that predates formation of the modern Great Lake basins. The infills of bedrock channels form major groundwater aquifers, influence regional groundwater flows and contaminant migration to Lake Ontario, and may localize the release of thermogenic methane and radon within heavily urbanized surface environments. A quantitative comparison of the regional pattern of bedrock joints and the orientation pattern of buried bedrock channels and modern river valleys shows that all these orientation patterns are virtually coincident. Buried bedrock channels in south-central Ontario are not part of a simple antecedent drainage system but were likely predesigned by bedrock joint patterns that have subsequently been propagated upward into overlying Pleistocene sediments. Joints in sediments are of considerable environmental significance (for example, subsurface contaminant and gas migration in fine-grained clayey sediments) and of many origins (stress release, desiccation, etc.) but are widely assumed to be a predominantly surface-related phenomena; the existence of deeper joints has been noted by some authors but their origin is obscure. Data presented herein from south-central Ontario confirm that, in addition to surface-related joints, a second population of bedrock-related joints, reflecting the upward propagation of bedrock fractures, is present in Pleistocene sediments of south-central Ontario.  相似文献   

风云二号可见光通道噪声分析     

谷松岩 《气象》1998,24(8):26-28

通过空间结构函数分析法,分析了风云二号扫描辐射计可见光通道的噪声。计算结果表明,风云二号扫描辐射计可见光４个通道的噪声水平不完全一样,其中通道１的等效噪声计数值最大,为０．５１,其它３个通道的等效噪声计数值为０．４２￣０．４５。  相似文献   

An Initial Guess for the Levenberg–Marquardt Algorithm for Conditioning a Stochastic Channel to Pressure Data     

Fengjun Zhang  Albert C. Reynolds  Dean S. Oliver 《Mathematical Geology》2003,35(1):67-88

A standard procedure for conditioning a stochastic channel to well-test pressure data requires the minimization of an objective function. The Levenberg–Marquardt algorithm is a natural choice for minimization, but may suffer from slow convergence or converge to a local minimum which gives an unacceptable match of observed pressure data if a poor initial guess is used. In this work, we present a procedure to generate a good initial guess when the Levenberg–Marquardt algorithm is used to condition a stochastic channel to pressure data and well observations of channel facies, channel thickness, and channel top depth. This technique yields improved computational efficiency when the Levenberg–Marquardt method is used as the optimization procedure for generating realizations of the model by the randomized maximum likelihood method.  相似文献   

Tailings fans and valley‐spur cutoffs created by hydraulic mining     

L. Allan James 《地球表面变化过程与地形》2004,29(7):869-882

Sand and gravel tailings from nineteenth century open‐pit hydraulic gold mines formed large alluvial fans at tributary con?uences in the northwestern Sierra Nevada, California. In the Bear River watershed, several of these fans were so large that they blocked main channels for decades. Some channels not only aggraded deeply, but also moved laterally and cut across the inner bends of valley spurs. Now locked in bedrock channels, these valley‐spur cutoffs impose local controls on geomorphic, hydraulic, and sedimentary processes. One cutoff has incised 25 m into bedrock over the past century (25 cm a^?1) with rapid initial incision rates of up to 50 cm a^?1 (1884–1890). Recognition of spur cutoffs in the geological record may help to identify large landslides and provide an analogue for a type of natural earth?ll dam spillway not prone to catastrophic failures. Tailing fans, valley‐spur cutoffs, and the sediment they trap are described from contemporary accounts and recent ?eld conditions in the Bear River watershed. These anthropogenic changes represent a major shift in the watershed from supply‐limited to transport‐limited sediment budgets and a change in geomorphic processes away from long‐term drainage evolution dominated by ingrown meanders. The large volumes of mining sediment stored in these landforms will be slowly released over the next millennium and could be signi?cant to contemporary ecological and public health issues due to recent ?ndings of high mercury loadings associated with hydraulic mines. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

An experimental study of rip channel flow   总被引：2，自引：0，他引：2  

Nils Drnen  Harshinie Karunarathna  Jrgen Fredse  B. Mutlu Sumer  Rolf Deigaard 《Coastal Engineering》2002,45(3-4)

A laboratory study of the flow over a bar with a single rip channel has been performed. First, the well-known pattern of a bar circulation cell with a strong offshore-directed current out through the rip channel and a weaker onshore-directed return flow over the bar is documented. Then measurements of the three-dimensional structure of the flow in the area where the rip channel, the bar and the trough meet and well inside the rip channel are presented. These measurements reveal that 3D effects play an important role, and that a depth-integrated viewpoint may not always be sufficient for predicting the flow in the near bed region. Particle-tracking experiments illustrate the near bed flow pattern over the entire area. These demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest: For some conditions, the rip current is fed from the trough and for other conditions it is fed directly from the bar. Both the 3D measurements and the trajectory tests show the existence of a weaker onshore-directed near-bed drift in the area where the rip current ceases. Finally, in a series of sensitivity tests, measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height (both normalized).  相似文献   

Effect of channel bifurcation on residual estuarine circulation: Winyah Bay, South Carolina     

Yong H. Kim  George Voulgaris 《Estuarine, Coastal and Shelf Science》2005,65(4):671-686

The residual circulation pattern of Winyah Bay, the fourth largest estuary on the eastern coast of the US, is examined using stationary and shipborne current measurements during periods of low freshwater discharge. The estuary has a complex morphology with a single channel and narrow banks at the river entrance and the bay mouth, and a bifurcated channel system (main and western channels, respectively) in the middle part that appears to affect the residual circulation.Overall, the upper (single channel morphology) and middle (dual-channel morphology) parts of the estuary exhibit a baroclinic residual circulation. The presence of bifurcated channels in the middle part of the estuary modifies the typical gravitational circulation. The near-bed landward-directed residual flow is stronger in the deeper main channel than the shallower western channel. This is the result of the fact that the magnitude of residual flow scales with the water depth of the channel and it is also influenced by the opposing patterns of channel alignment in the northern and southern junctions. Analytical modeling confirms that the observed residual currents in the upper and middle estuary are density-induced. In the lower estuary, residual flow is directed seaward throughout the water column of the channel while in the adjacent shoals the residual flow is directed landward, suggesting that in contrast to the upper and middle estuary, the residual flow near the mouth is barotropic, controlled by the tides and the channel-bank morphology.  相似文献   

Automated drainage extraction in mapping the Monterey submarine drainage system,California margin     

Lincoln F. Pratson  William B. F. Ryan 《Marine Geophysical Researches》1996,18(6):757-777

Drainage-extraction algorithms traditionally used for extracting river networks and watersheds from gridded land topography are applied to gridded multibeam bathymetry of the mid-California margin. The algorithms are used to automatically map two regional tributary networks of submarine canyons and deepsea channels draining Monterey Bay, the principal conduits of which are Acension and Monterey Canyons. The algorithms reliably map subaqueous drainage areas, but are prone to error in mapping the extent of submarine canyon and channel thalwegs due to operator subjectivity and algorithm limitations. A geomorphic comparison of the Acension and Monterey Canyon networks, with 12 river networks in the continental U.S., illustrates both the potential and weaknesses of using drainage extraction algorithms to analyze sediment pathways in gridded bathymetry.  相似文献   

Seismic facies analysis of Westphalian sequences of the southern North Sea     

David J. Evans  Alan Meneilly  Gordon Brown 《Marine and Petroleum Geology》1992,9(6)

Differing seismic facies are observed from within the Westphalian sequences of the southern North Sea. A review of published synthetic seismograms and seismic data from known coal-bearing basins establishes seismic facies types and their relationships. Similar reflection character is recognized from Westphalian sequences in the southern North Sea and lithologies confirmed by borehole information. Probable sand/silt or clastic-prone Upper Coal Measures (Westphalian C/D) stages and coal-prone Lower and Middle Coal Measures (Westphalian A, B and part C) stages can be defined and mapped, providing information on the regional distribution of source and reservoir rocks and stratigraphic plays. It is suggested that sand-prone channel complexes or clastic dominated areas within the mainly A/B stages can be recognized and mapped, providing information on migration pathways and stratigraphic plays.  相似文献   

Quasi-three-dimensional modelling of the morphology of longshore bars     

N. Drønen  R. Deigaard 《Coastal Engineering》2007

A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.  相似文献   

双偏振天气雷达水平和垂直通道回波对比     

何田勇  胡明宝  艾未华  陈楠 《气象科学》2019,39(3):368-376

本文针对南京溧水S波段双偏振多普勒天气雷达获取的回波I/Q信号,采用FFT算法处理水平和垂直通道I/Q数据,分别得到水平和垂直通道回波的强度、速度值,定性分析了两通道回波强度、速度数据差异,引入平均差及均方根误差参数做定量分析。对比结果表明:两通道I/Q数据处理得到的回波层次分布基本一致;两通道速度数据差异较小,强度数据差异较大。以水平通道回波数据为基准,当回波强度小于16 dBz时,水平通道强度偏小,这可能是由云系边缘较小的针状冰晶所致;当回波强度大于32 dBz时,水平通道强度值偏大,且随回波强度的增加两通道强度差值增大,这反映了强降水中大粒子较多,而粒子越大,降落过程中扁平程度越大;两通道回波强度的差值与回波强度大小呈现一定相关性,这可能反映了单偏振天气雷达在定量测量降水中精度不高,通过双偏振雷达联合利用两通道数据有望改善雷达定量测量降水精度,提升雷达定量估测降水能力。  相似文献