共查询到20条相似文献，搜索用时 703 毫秒
1.
Michael F. Dacey 《Mathematical Geosciences》1979,11(6):655-668
The model for bed formation by Kolmogorov consists of an unending sequence of alternating periods of deposition and erosion of sediments, with the amounts of deposition and erosion being independent random variables. This paper examines this model in relation to recent mathematical studies that are relevant to, and simplify, the analysis of the thickness of the bed that remains after the sequence is operated for a long time. This thickness obeys the exponential probability law when the amounts of deposition and erosion also obey the exponential law distributed. For the discrete version formulated by Schwarzacher, the thickness obeys the geometric probability law when the amounts of deposition and erosion obey the geometric law. 相似文献
2.
A stochastic model of bed thickness distribution is given. The following scheme of sedimentation and bed formation is assumed: (1)sedimentation consists of an unending sequence of alternating periods of deposition and erosion; (2)succession of sedimentary environments is described in terms of a transition probability matrix; (3)each depositional period accumulates a bed of sediments; (4)net accumulation exceeds erosion; (5)amounts of deposition and erosion are independent random variables. The equations for transformation of bed thickness distribution from sedimentary environment to geologic record are obtained. The sedimentological significance of these equations is discussed. 相似文献
3.
Noritaka Endo 《Mathematical Geosciences》2010,42(8):955-968
Bed thickness is an important factor when interpreting geologic records to determine the past environment; it is related to
the sediment transport and debris production rates. Because of the inherent uncertainty of these phenomena, a probabilistic
model is useful for dealing with the problem. Many probabilistic models are variations of the Kolmogorov model, which is a
type of random-walk model. The Kolmogorov model is a simple mathematical model that has a wide range of applications. However,
when estimating paleo-environments from geologic records, the inverse problem is more practical than the forward problem,
but the former has not been well discussed. Previous applications have estimated the probability density function (PDF) for
stochastic steady states (including virtual cumulative erosion that is not physically observable) but not for independent
events, and the difference between the results for these two kinds of PDFs has not been analyzed. This study considers the
inverse problem of the Kolmogorov model and the properties of its solution. This study found that (1) the inverse problem
can be solved analytically in a general form; (2) the difference between the above two PDFs can exceed 10% in some cases;
and (3) different PDFs for the deposition and erosion magnitudes of independent events can reproduce the same bed thickness
distribution of preserved layers. 相似文献
4.
海底浊流在坡道转换处的流动及沉积的数值模拟 总被引：2，自引：1，他引：1
根据一经多项试验数据验证的基于三维不可压缩流体Navier-Stokes方程和湍流 k-ε 模型的重力流数值计算的数学模型,模拟并分析了单粒径沉积物的海底浊流沿不同斜坡流至近似平坦坡的流动及沉积特征。模拟结果显示了有关海底浊流的一些重要特征:连续入流的浊流在斜坡上的流速随着斜坡的增大而增大,同时浊流厚度由于对环境水体的夹带而渐渐增厚,坡度越大,增厚越快;流至近水平坡时,流速均有明显的降低,但大斜坡入流依然保持相对较高的流速。在沉积方面,初步的模拟结果显示对给定的沉积物来说存在一相对应的临界坡度:当坡度小时,坡上沉积多,坡下少,这样整体的坡度有逐渐增大之势;当坡度大时,坡上沉积少或为侵蚀,而坡下沉积相对较多,坡度有整体减小之势。了解了不同坡度转换的浊流沉积的上述特点,对于我们根据实测的浊流沉积的剖面特征推测其形成的环境,进而推测相关油气储层的分布状况会有一定的参考作用。 相似文献
5.
Tetsuji Muto 《地学学报》1995,7(4):417-423
Kolmogorov's model of the distribution of bed thicknesses is assessed by numerical simulations of a sedimentation process, assumed to be a random time-series of alternating depositional and erosional episodes conformable with a stationary Markov process in a state of equilibrium. The study supports the validity of the main point of the model. The random time-series process generates a succession of beds with 'positive' (preserved) and 'negative' (eroded) thicknesses, the frequency distribution of which, f(x), spans the range of positive to negative x -values. The beds with negative thicknesses are absent in the stratigraphic record, whereby the measured bed thicknesses show a frequency distribution, f*( x | x > 0), that is left-side truncated, cut off at the zero thickness value. The numerical simulations further indicate that f( x ) is a 'composite' geometrical distribution, whose actual form changes progressively with p d , the probability of sediment deposition relative to erosion. The distribution f( x ) invariably has a maximum at x ≤0, such that the truncated distribution f*( x ) for p d ≥0.5 is a simple geometrical distribution regardless of p d value. The f*( x ) distribution will appear to be a negative exponential distribution when based on the bed-thickness data measured in a conventional metric scale. Data sets from four different turbidite successions in the Cenozoic of Japan, each comprising a few thousand beds, show this type of distribution. However, the sandstone-capping shales in one of the turbidite successions show a truncated Gaussian distribution, attributed to a significant component of non-turbiditic mud. No universal form of bed-thickness distribution can be assumed for the Kolmogorov model. The form of bed-thickness distribution may vary with the type of the depositional process and the character of the sedimentary environment. 相似文献
6.
On the frequency distribution of turbidite thickness 总被引：1，自引：0，他引：1
Peter J. Talling 《Sedimentology》2001,48(6):1297-1329
The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present. 相似文献
7.
Fluvial bedrock erosion rates due to impacting sediment particles are thought to be proportional to the energy delivered to the bedrock. When sediment particles cover the bed, they reduce the energy transmitted to the bed by an impacting particle. We measured the decline of energy transferred through sediment cover of increasing thickness in laboratory experiments. The energy arriving at the bed is a function both of the cover thickness and the grain size of the covering sediment. Using a simple stochastic model of cover distribution, the experimental results were upscaled to the reach scale. Although cover thickness influences energy delivery heavily at a given point, when averaging over the whole bed, cover-free areas dominate total energy delivery, making partial energy transfer through the cover negligible when a small or intermediate fraction of the bed is covered by sediment. Partial energy delivery through the bed cover is not negligible when a large fraction or the complete bed is already covered, but in this situation, an erosion threshold may become important. On grounds of the presented data, we expect that the areal distribution of sediment in a bedrock channel dominates total energy delivery and that partial energy delivery to the bed through a sediment layer can be neglected for most modelling purposes. 相似文献
8.
ZOLTÁN SYLVESTER 《Sedimentology》2007,54(4):847-870
Turbidite bed thickness distributions are often interpreted in terms of power laws, even when there are significant departures from a single straight line on a log–log exceedence probability plot. Alternatively, these distributions have been described by a lognormal mixture model. Statistical methods used to analyse and distinguish the two models (power law and lognormal mixture) are presented here. In addition, the shortcomings of some frequently applied techniques are discussed, using a new data set from the Tarcău Sandstone of the East Carpathians, Romania, and published data from the Marnoso‐Arenacea Formation of Italy. Log–log exceedence plots and least squares fitting by themselves are inappropriate tools for the analysis of bed thickness distributions; they must be accompanied by the assessment of other types of diagrams (cumulative probability, histogram of log‐transformed values, q–q plots) and the use of a measure of goodness‐of‐fit other than R2, such as the chi‐square or the Kolmogorov–Smirnov statistics. When interpreting data that do not follow a single straight line on a log–log exceedence plot, it is important to take into account that ‘segmented’ power laws are not simple mixtures of power law populations with arbitrary parameters. Although a simple model of flow confinement does result in segmented plots at the centre of a basin, the segmented shape of the exceedence curve breaks down as the sampling location moves away from the basin centre. The lognormal mixture model is a sedimentologically intuitive alternative to the power law distribution. The expectation–maximization algorithm can be used to estimate the parameters and thus to model lognormal bed thickness mixtures. Taking into account these observations, the bed thickness data from the Tarcău Sandstone are best described by a lognormal mixture model with two components. Compared with the Marnoso‐Arenacea Formation, in which bed thicknesses of thin beds have a larger variability than thicknesses of the thicker beds, the thinner‐bedded population of the Tarcău Sandstone has a lower variability than the thicker‐bedded population. Such differences might reflect contrasting depositional settings, such as the difference between channel levées and basin plains. 相似文献
9.
分汊河流的江心洲洲头是分汊河段分水分沙和泥沙冲淤的动态区域,其冲淤过程受来水来沙和地形条件变化呈现冲淤交替。通过几何概化洲头的浅滩逆坡和侧向顺坡,考虑逆坡促淤和顺坡分沙作用,建立洲头浅滩的推移质冲淤的概化模型,推导洲头由淤积转向冲刷的临界流量表达式。上游来流量增加,洲头淤积速率先增加到最大值后逐渐减少,直至达到临界流量,再转向冲刷。洲头淤积速率与上游水面纵比降和床沙粒径成正比,与浅滩逆坡、侧向水面比降与顺坡成反比。若其他条件相同,洲头浅滩形状越不对称和上游来沙量越小,越有利于冲刷。模型预测武汉河段天兴洲的临界流量范围与水文观测值较为接近,对其出现超过某个大临界流量仍出现淤积的特殊性,给出的新解释是天兴洲洲头的整治工程使得在大流量条件下洲头抵抗冲刷和阻挡作用促进泥沙淤积。 相似文献
10.
NATHAN HAWLEY 《Sedimentology》1981,28(5):699-712
A series of flume runs using a 12·5 h tidal cycle was conducted in order to determine whether or not flaser beds could be formed in nearshore subtidal areas as a result of purely tidal activity. The effects of clay composition, sand size, bed thickness, and maximum current velocity were investigated. The resistance to erosion of mud beds 10 mm thick or less is low; shear stresses approximately equal to those needed to initiate sand movement resulted in rapid erosion of the mud beds. In the light of the low resistance to erosion of the beds, and the necessity for increased thicknesses and very long time periods for their resistance to be increased substantially, it is concluded that the formation of these beds is unlikely to result from purely tidal action unless very large amounts of mud are available. A mechanism which more readily permits the deposition of thicker beds and longer consolidation times, such as storm action, is preferred. 相似文献
11.
Shih-Nan Chen Lawrence P. Sanford Evamaria W. Koch Fengyan Shi Elizabeth W. North 《Estuaries and Coasts》2007,30(2):296-310
The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified
Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment
transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function
of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction
of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation
qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased
as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital
velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in
higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted
lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion
rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification.
Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates
alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed. 相似文献
12.
13.
应用高分辨率层序地层学与沉积学相关理论,综合运用岩芯、测井资料,建立喇萨杏地区PI 组薄层叠覆、前积及削蚀减薄地层发育模式。其中,PI1、PI21 小层为薄层叠覆模式,PI4 小层为前积模式,PI3 与PI4 之间为消蚀减薄模式。薄层叠覆模式显示近源沉积厚度薄,单元叠置紧密,可分性差; 向远源方向沉积厚度增大,旋回发育完整,单元易于划分。消蚀减薄模式显示近源早期沉积遭受强烈侵蚀,向远源方向侵蚀程度减弱、消失。前积作用显示向深湖方向沉积厚度增大,层数增多。3 种模式显示的共同作用,造成了北部近源至南部远源地层厚度增加、层数增多。地层发育模式的确立,对明确各开发区块的小层对应关系、小层间的接触关系及平面分布特征,建立地层对比方案和进行沉积演化规律研究具有指导意义。 相似文献
14.
A process called ‘piping’, which often occurs in the soil at dams, levees, and dykes, involves the formation and development of a continuous tunnel between upstream and downstream ends. The hole erosion test is commonly used to quantify the critical stress and the rate of piping erosion progression. The aim of this study is to draw up a model for interpreting the results of this test. A characteristic internal erosion time is defined and expressed as a function of the initial hydraulic gradient and the coefficient of surface erosion. It is established here that the product of the coefficient of erosion and the flow velocity is a significant dimensionless number: when this number is small, the kinetics of erosion are low, and the particle concentration does not have any effect on the flow. This finding applies to most of the available test results. Theoretical and experimental evidence is presented showing that the evolution of the pipe radius during erosion with a constant pressure drop obeys a scaling exponential law. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
15.
应用雷诺平均纳维尔—斯托克斯模型模拟探讨了等量突然释放型浊流在流经不同坡折渠道的流动及沉积特性,取得如下主要结论:随着底坡增加,浊流流速增长率因卷吸作用的增强而减缓;浊流深度平均速度和浓度形态相似,头部大且向尾部呈线性下降;在小坡折处产生沉积,沉积最厚处离坡折处不远且上下游平均粒径相差不大,往下游厚度呈线性减小;而在大坡折处产生侵蚀,随着坡度的增大侵蚀增加,沉积最厚处离坡折处较远且上下游平均粒径相差大,形态上呈上拱状。这些认识对于根据浊流沉积特征推测其形成环境具有一定的参考作用。 相似文献
16.
Erosion by turbidity currents changes the morphology of the sea floor. The relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition; this could, in turn, affect flow and deposition patterns in subsequent events. This study investigates shallow, centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the Oligocene Macigno Formation of North‐west Italy, where erosion and deposition are variably coupled at the bed scale in a net‐aggradational setting. The research focus was on: (i) the recognition of scour edges and erosive surfaces; (ii) quantification of spatial differences in the amount of erosion; and (iii) an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces. Where they can be observed, scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded. A consequence of this de‐laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges, giving the appearance of normal conformable bed bases. Despite their cryptic nature, such bedding‐parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs (here, 16 bed‐parallel scour surfaces were identified in a succession comprising 95 beds). Different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief. It is found that differential erosion is on average under‐compensated by differential sedimentation. In some cases, the overlying deposits anti‐compensate, being thinner at the location where more erosion has occurred. Unequal spatial distribution of differential erosion in the study area combines with sedimentary under‐compensation to result in a trend of accumulating section thickness differences over multiple beds. In one ca 25 m thick package, the maximum cumulative change in lateral gradient during some 20 events reached 0·17°, before being reset by a single event. This process can be interpreted either as a lobe compensation effect, or as a scour enhancement effect, depending on the orientation of the palaeohorizontal datum. If allowed to proceed, the latter process could force the system past a channellization threshold, prompting a change from sheet to channelled architecture. This type of shallow substrate scouring and differential deposition is likely to be an important process in the build‐up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea‐floor gradients. 相似文献
17.
Data from Ordovician and Tertiary turbidites and from the Hekla ash fall show good correlation between bed thickness and grain size. Most of the thicker beds are coarser grained. The relation between bed thickness and grain size in graded beds can be explained by a theory which uses a sedimentation formula based on the decay law of turbulence in the deposition zone of a turbidity current. 相似文献
18.
地层模型中的半马尔可夫过程分析同时考察岩性状态的转移和状态的延续时间--层厚分布,更全面地反映沉积过程,但在求解半马尔可夫过程转移概率时需要进行拉普拉斯逆变换,造成实际应用的困难.本文从另一思路导出了直接计算半马尔可夫过程转移概率的方法,并以某受潮汐影响的三角洲体系的详细沉积断面为例介绍半马尔可夫过程分析在定量地研究沉积作用过程及聚煤规律中的应用。 相似文献
19.
针对长江中游瓦口子至马家咀河段(弯曲分汊河段)的水沙运动特点,给出了二维水沙数学模型尤其是推移质不平衡输沙计算的模式,主要包括推移质不平衡输沙方程、床沙级配方程、河床变形方程;对模型中的几个关键问题提出了处理方法,如非均匀沙起动及输移规律、床面混合层厚度等.利用大量的水流及河床变形资料,率定了模型的一些参数,进行了水面线、流速分布及河床变形的详细验证.在此基础上,根据设计部门提供的进出口水沙边界条件,预测了三峡工程蓄水初期该河段的冲淤过程与分布及航道条件的变化. 相似文献
20.
在弯道水槽中展开系列试验,研究水力冲刷过程中非粘性岸坡冲刷崩塌与河床冲淤交互作用过程及其影响因素,进一步分析塌岸淤床泥沙贡献率。试验成果表明,水流冲刷过程中岸坡破坏是水流淘刷岸坡坡脚、岸坡崩塌及崩塌体淤积坡脚并在河床上输移掺混的交互作用反复循环过程。塌岸淤床模式及掺混程度与近岸流速、主流贴岸程度、水位及河床边界条件等关系密切。近岸流速越大、水位越高,岸坡总冲刷坍塌量、河床总淤积量以及河床累计淤积率也越大,稳定后的岸坡越趋平缓;河床可动程度越大,岸坡总冲刷坍塌量及其在河床上的总淤积量也越大,但河床累计淤积率却越小;水位越高,在弯道段等横向输沙强度较大的地方,岸坡冲刷崩塌体与河床发生掺混的程度也越大。 相似文献