Does the permeability of gravel river beds affect near‐bed hydrodynamics?          下载免费PDF全文

James R. Cooper  Annie Ockleford  Stephen P. Rice  D. Mark Powell 《地球表面变化过程与地形》2018,43(5):943-955

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

Joint facies and rock properties Bayesian amplitude‐versus‐offset inversion using Markov random fields          下载免费PDF全文

James Gunning  Mark Sams 《Geophysical Prospecting》2018,66(5):904-919

Seismic reflection pre‐stack angle gathers can be simultaneously inverted within a joint facies and elastic inversion framework using a hierarchical Bayesian model of elastic properties and categorical classes of rock and fluid properties. The Bayesian prior implicitly supplies low frequency information via a set of multivariate compaction trends for each rock and fluid type, combined with a Markov random field model of lithotypes, which carries abundance and continuity preferences. For the likelihood, we use a simultaneous, multi‐angle, convolutional model, which quantifies the data misfit probability using wavelets and noise levels inferred from well ties. Under Gaussian likelihood and facies‐conditional prior models, the posterior has simple analytic form, and the maximum a‐posteriori inversion problem boils down to a joint categorical/continuous non‐convex optimisation problem. To solve this, a set of alternative, increasingly comprehensive optimisation strategies is described: (i) an expectation–maximisation algorithm using belief propagation, (ii) a globalisation of method (i) using homotopy, and (iii) a discrete space approach using simulated annealing. We find that good‐quality inversion results depend on both sensible, elastically separable facies definitions, modest resolution ambitions, reasonably firm abundance and continuity parameters in the Markov random field, and suitable choice of algorithm. We suggest usually two to three, perhaps four, unknown facies per sample, and usage of the more expensive methods (homotopy or annealing) when the rock types are not strongly distinguished in acoustic impedance. Demonstrations of the technique on pre‐stack depth‐migrated field data from the Exmouth basin show promising agreements with lithological well data, including prediction accuracy improvements of 24% in and twofold in density, in comparison to a standard simultaneous inversion. Much clearer and extensive recovery of the thin Pyxis gas field was evident using stronger coupling in the Markov random field model and use of the homotopy or annealing algorithms.  相似文献   

Identification and Explanation of a Change in the Groundwater Regime using Time Series Analysis     

Christophe Obergfell  Mark Bakker  Kees Maas 《Ground water》2019,57(6):886-894

Time series analysis is applied to identify and analyze a transition in the groundwater regime in the aquifer below the sand ridge of Salland in the Netherlands, where groundwater regime refers to the range of head variations throughout the seasons. Standard time series analysis revealed a discrepancy between modeled and observed heads in several piezometers indicating a possible change in the groundwater regime. A new time series modeling approach is developed to simulate the transition from the initial regime to the altered regime. The transition is modeled as a weighted sum of two responses, one representing the initial state of the system, the other representing the altered state. The inferred timing and magnitude of the change provided strong evidence that the transition was the result of significant dredging works that increased the river bed conductance of the main river draining the aquifer. The plausibility of this explanation is corroborated by an analytical model. This case study and the developed approach to identify a change in the groundwater regime are meant to stimulate a more systematic application of time series analysis to detect and understand changes in groundwater systems which may easily go unnoticed in groundwater flow modeling.  相似文献   

Heli-borne gravity gradiometry in rugged terrain     

Mark H. Dransfield  Tianyou Chen 《Geophysical Prospecting》2019,67(6):1626-1636

For airborne gravity gradiometry in rugged terrain, helicopters offer a significant advantage over fixed-wing aircraft: their ability to maintain much lower ground clearances. Crucially, this provides both better signal-to-noise and better spatial resolution than is possible with a fixed-wing survey in the same terrain. Comparing surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan, demonstrates that there is some loss of spatial resolution in the more rugged terrain. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components is done by a Fourier method over gentle terrain and an equivalent source method for rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth model inversion, and the source distribution must be designed to suit the survey design.  相似文献   

Mega-blowouts in Qinghai–Tibet Plateau: Morphology,distribution and initiation     

Wanyin Luo  Zhongyuan Wang  Junfeng Lu  Linghai Yang  Guangqiang Qian  Zhibao Dong  Mark D. Bateman 《地球表面变化过程与地形》2019,44(2):449-458

Blowouts are wind-eroded landforms that are widely distributed in the north-eastern part in Qinghai–Tibet Plateau (QTP), China. These blowouts are thought to form in response to climate change and/or human activity but little is known about their morphodynamics. Using field surveys, remote sensing and geographic information system (GIS) spatial analysis, the distribution and morphology of blowouts are analysed and their initiation considered. Results show the QTP mega-blowouts are some of the largest in the world. The orientations of the trough shaped blowouts are parallel with the prevailing wind, but the saucer and bowl-shaped blowouts are influenced by bi-directional transport. Whilst regional patterns of blowout shape and size were observed to reflect the extent of aeolian sediments and wind regimes, the relationship between the different morphological parameters showed consistency. During initial stages of development, the length to width ratios of blowouts increase rapidly with area but after they reach a mega size this relationship stabilizes as blowouts widen. Initial luminescence dating shows that blowouts appear to have initiated ~100 to 500 years ago, coinciding with the Little Ice Age (LIA) climate event when northwest winds are known to have intensified. Further work is required to confirm this initiation period and establish the significance of mega blowouts for landscape degradation and human activities. © 2018 John Wiley & Sons, Ltd.  相似文献   

A study case of bioluminescence potential dynamics in the Delaware Bay with observations and modeling     

Igor Shulman  Mark A. Moline  Stephanie Anderson  Peter Sakalaukus  Clark Rowley  Sherwin Ladner 《Ocean Dynamics》2017,67(3-4):383-396

Stormy geomorphology: geomorphic contributions in an age of climate extremes          下载免费PDF全文

Larissa A. Naylor  Tom Spencer  Stuart N. Lane  Stephen E. Darby  Francis J. Magilligan  Mark G. Macklin  Iris Möller 《地球表面变化过程与地形》2017,42(1):166-190

The increasing frequency and/or severity of extreme climate events are becoming increasingly apparent over multi‐decadal timescales at the global scale, albeit with relatively low scientific confidence. At the regional scale, scientific confidence in the future trends of extreme event likelihood is stronger, although the trends are spatially variable. Confidence in these extreme climate risks is muddied by the confounding effects of internal landscape system dynamics and external forcing factors such as changes in land use and river and coastal engineering. Geomorphology is a critical discipline in disentangling climate change impacts from other controlling factors, thereby contributing to debates over societal adaptation to extreme events. We review four main geomorphic contributions to flood and storm science. First, we show how palaeogeomorphological and current process studies can extend the historical flood record while also unraveling the complex interactions between internal geomorphic dynamics, human impacts and changes in climate regimes. A key outcome will be improved quantification of flood probabilities and the hazard dimension of flood risk. Second, we present evidence showing how antecedent geomorphological and climate parameters can alter the risk and magnitude of landscape change caused by extreme events. Third, we show that geomorphic processes can both mediate and increase the geomorphological impacts of extreme events, influencing societal risk. Fourthly, we show the potential of managing flood and storm risk through the geomorphic system, both near‐term (next 50 years) and longer‐term. We recommend that key methods of managing flooding and erosion will be more effective if risk assessments include palaeodata, if geomorphological science is used to underpin nature‐based management approaches, and if land‐use management addresses changes in geomorphic process regimes that extreme events can trigger. We argue that adopting geomorphologically‐grounded adaptation strategies will enable society to develop more resilient, less vulnerable socio‐geomorphological systems fit for an age of climate extremes. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

3‐D uncertainty‐based topographic change detection with structure‐from‐motion photogrammetry: precision maps for ground control and directly georeferenced surveys          下载免费PDF全文

Mike R. James  Stuart Robson  Mark W. Smith 《地球表面变化过程与地形》2017,42(12):1769-1788

Structure‐from‐motion (SfM) photogrammetry is revolutionising the collection of detailed topographic data, but insight into geomorphological processes is currently restricted by our limited understanding of SfM survey uncertainties. Here, we present an approach that, for the first time, specifically accounts for the spatially variable precision inherent to photo‐based surveys, and enables confidence‐bounded quantification of 3D topographic change. The method uses novel 3D precision maps that describe the 3D photogrammetric and georeferencing uncertainty, and determines change through an adapted state‐of‐the‐art fully 3D point‐cloud comparison (M3C2), which is particularly valuable for complex topography. We introduce this method by: (1) using simulated UAV surveys, processed in photogrammetric software, to illustrate the spatial variability of precision and the relative influences of photogrammetric (e.g. image network geometry, tie point quality) and georeferencing (e.g. control measurement) considerations; (2) we then present a new Monte Carlo procedure for deriving this information using standard SfM software and integrate it into confidence‐bounded change detection; before (3) demonstrating geomorphological application in which we use benchmark TLS data for validation and then estimate sediment budgets through differencing annual SfM surveys of an eroding badland. We show how 3D precision maps enable more probable erosion patterns to be identified than existing analyses, and how a similar overall survey precision could have been achieved with direct survey georeferencing for camera position data with precision half as good as the GCPs'. Where precision is limited by weak georeferencing (e.g. camera positions with multi‐metre precision, such as from a consumer UAV), then overall survey precision can scale as n^‐½ of the control precision (n = number of images). Our method also provides variance–covariance information for all parameters. Thus, we now open the door for SfM practitioners to use the comprehensive analyses that have underpinned rigorous photogrammetric approaches over the last half‐century. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

New insights on the relative contributions of coastal processes and tectonics to shore platform development following the Kaikōura earthquake          下载免费PDF全文

Wayne J. Stephenson  Mark E. Dickson  Paul H. Denys 《地球表面变化过程与地形》2017,42(13):2214-2220

We describe the immediate impact of the 14 November 2016 Kaikōura magnitude 7.8 (Mw) earthquake on shore platforms and cliffs around Kaikōura Peninsula. The earthquake caused an instantaneous uplift of ~1.01 m of the peninsula. We resurveyed seven profiles previously used for erosion monitoring and observed changes in the configuration of the shoreline. The coseismic uplift has fundamentally changed the process regime operating on the platforms and altered the future trajectory of shore platform and cliff development. Our observations highlight the interplay of waves, weathering, biology and tectonics. At this location tectonism strongly modulates the process regime, driving instantaneous changes in morphology and altering rates and patterns of erosion. Finally, the uplift of the Kaikōura coast has implications for changing resilience to climate change and sea level rise. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Estimating gas saturation in a thin layer by using frequency‐dependent amplitude versus offset modelling          下载免费PDF全文

Zhaoyu Jin  Mark Chapman  Xiaoyang Wu  Giorgos Papageorgiou 《Geophysical Prospecting》2017,65(3):747-765

Various models have been proposed to link partial gas saturation to seismic attenuation and dispersion, suggesting that the reflection coefficient should be frequency‐dependent in many cases of practical importance. Previous approaches to studying this phenomenon typically have been limited to single‐interface models. Here, we propose a modelling technique that allows us to incorporate frequency‐dependent reflectivity into convolutional modelling. With this modelling framework, seismic data can be synthesised from well logs of velocity, density, porosity, and water saturation. This forward modelling could act as a basis for inversion schemes aimed at recovering gas saturation variations with depth. We present a Bayesian inversion scheme for a simple thin‐layer case and a particular rock physics model and show that, although the method is very sensitive to prior information and constraints, both gas saturation and layer thickness theoretically can be estimated in the case of interfering reflections.  相似文献