The initial stages of cave formation: Beyond the one-dimensional paradigm

The solutional origin of limestone caves was recognized over a century ago, but the short penetration length of an undersaturated solution made it seem impossible for long conduits to develop. This is contradicted by field observations, where extended conduits, sometimes several kilometers long, are found in karst environments. However, a sharp drop in the dissolution rate of CaCO₃ near saturation provides a mechanism for much deeper penetration of reactant. The notion of a “kinetic trigger” – a sudden change in rate constant over a narrow concentration range – has become a widely accepted paradigm in speleogenesis modeling. However, it is based on one-dimensional models for the fluid and solute transport inside the fracture, assuming that the dissolution front is planar in the direction perpendicular to the flow. Here we show that this assumption is incorrect; a planar dissolution front in an entirely uniform fracture is unstable to infinitesimal perturbations and inevitably breaks up into highly localized regions of dissolution. This provides an alternative mechanism for cave formation, even in the absence of a kinetic trigger. Our results suggest that there is an inherent wavelength to the erosion pattern in dissolving fractures, which depends on the reaction rate and flow rate, but is independent of the initial roughness. In contrast to one-dimensional models, two-dimensional simulations indicate that there is only a weak dependence of the breakthrough time on kinetic order; localization of the flow tends to keep the undersaturation in the dissolution front above the threshold for non-linear kinetics.     

Assessing Canada’s disaster baselines and projections under the Sendai Framework for Disaster Risk Reduction: a modeling tool to track progress

Natural Hazards - Under the United Nations (UN) Sendai Framework for Disaster Risk Reduction (2015–2030), 187 countries including Canada committed to increasing disaster resilience and...     

HSR mechanisms and effects on the spatial structure of regional tourism in China

Chinese railway has entered the “HSR era”, while the structure of “four vertical and four horizontal” railways for transit passengers is almost completed. Taking the Beijing- Shanghai High-Speed Rail (hereinafter referred to as HSR) as an example, this paper first explores HSR’s effects on the spatial structure of regional tourist flows using the social network analysis. Next, it notes changes in the accessibility of regional transportation. After analyzing the factors including initial endowment of regional tourism resources, hospitality facilities, the density of the regional tourism transportation network, and locations, the paper discusses the mechanisms through which HSR affects regional tourist flows. The study shows the following: (1) HSR’s effects on the spatial structure of regional tourist flows are manifested through the Matthew effect, the filtering effect, the diffusion effect and the overlying effect, and (2) the Matthew effect of HSR is manifested under an obvious interaction of the location, the initial endowment of tourism resources, hospitality capacity, tourist transportation network density and “time-space compression”. The filtering effect of HSR is manifested for those tourism nodes without favorable location conditions, endowment of tourism resources, hospitality capacity, or tourist transportation network density and without obvious benefits from “time-space compression”. Those tourism nodes that boast advantages in terms of location condition, endowment of tourism resources, hospitality capacity, tourist transportation network density and obvious “time-space compression” will become sources for the diffusion effect. HSR will strengthen the aggregation effects of tourist flow in these diffusion sources, which will thereafter diffuse to peripheral tourist areas, manifesting “aggregation-diffusion”. HSR has overlapped tourists’ spatial traveling range over large-scale spaces. However, the overlying effect is only generated in those tourism nodes with a favorable location condition, an endowment of tourism resources, hospitality capacity, tourist transportation network density, and obvious “time-space compression”.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – III. Multimolecular line mapping

We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H  ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO⁺ and N₂H⁺ are anticorrelated.     

Optimizing early detection of non-indigenous species: Estimating the scale of dispersal of a nascent population of the invasive tunicate Ciona intestinalis (L.)

Knowledge of dispersal and establishment during the early stages of invasion is essential for allocating monitoring effort, detecting nascent populations and predicting spread. The scarcity of these data, however, provides little guidance for monitoring programs. Here we present data on the adult distribution and the subsequent pattern of larval recruitment from a nascent population of the invasive tunicate Ciona intestinalis in Prince Edward Island, Canada. Existing niche models indicate the entire study site is suitable for recruitment, suggesting an equal probability of detection throughout the site. In contrast, we found a heterogeneous pattern of larval recruitment, including areas of zero recruitment. By fitting a dispersal kernel, we show Ciona is not capable of naturally dispersing between bays, restricting further spread, and provide guidance for future monitoring. Our results also highlight how large-scale models, although important, lack the small-scale patterns essential for monitoring and early detection of invasive species.     

Free-Phase Gas Detection in Groundwater Wells via Water Pressure and Continuous Field Parameters

Detection of free-phase gas (FPG) in groundwater wells is critical for accurate assessment of dissolved gas concentrations and the occurrence of FPG in the subsurface, with consequent implications for understanding groundwater contamination and greenhouse gas emissions. However, identifying FPG is challenging during routine groundwater monitoring and there is poor agreement on the best approach to detect the occurrence of FPG in groundwater. In this study, laboratory experiments in a water column were designed to mimic nonflowing and flowing conditions in a groundwater well to evaluate how the presence of FPG affects water pressure and commonly used continuous field parameters. The laboratory results were extrapolated to interpret field data at an abandoned exploration well with episodic release of free-gas CO₂. The FPG effect on water pressure varied between flowing and nonflowing wells, and depending on whether the FPG was above or below the sensor. Electrical conductivity values were decreased and/or behaved erratically when FPG was present in the water column. Findings from this study have shown that the combined measurement of water pressure, electrical conductivity, and total dissolved gas pressure can provide information about the occurrence of FPG in groundwater wells. Measurement of these parameters at different depths can also provide information about relative depths and amounts of FPG within the well water column. This approach can be used for long-term monitoring of groundwater gases, managing gas-locking in production wells with gassy groundwater, and measuring fugitive greenhouse gas emissions from groundwater wells.     

GPS+GLONASS组合测量

最近人们开始讨论把ＧＰＳ和ＧＬＯＮＡＳＳ组合起来以扩大测量应用的可行性问题,主要是因为对这两种卫星系统的数据组合起来时会遇到技术上的挑战。本文将向您表明ＧＰＳ＋ＧＬＯＮＡＳＳ技术能够为测量带来显著优势,这些优势包括大大增加卫星可用性和定位可靠性,而且提高作业效率又不降低精度,并能获得与当前ＧＰＳ系统相同的精度。Ｗｉｌｌｉａｍ Ｍａｒｔｉｎ等用该设备采集了数据并给出了实际结果。说明单频ＧＰＳ＋ＧＬＯ     

An algorithm for the simulation of curvilinear plane-strain and axisymmetric hydraulic fractures with lag using the universal meshes

We present an algorithm to simulate curvilinear hydraulic fractures in plane strain and axisymmetry. We restrict our attention to sharp fractures propagating in an isotropic, linear elastic medium and driven by the injection of a laminar, Newtonian fluid governed by lubrication theory, and we require the existence of a finite lag region between the fluid front and the crack tip. The key novelty of our approach is in how we discretize the evolving crack and fluid domains: we utilize universal meshes (UMs), a technique to create conforming triangulations of a problem domain by only perturbing nodes of a universal background mesh in the vicinity of the boundary. In this way, we construct meshes, which conform to the crack and to the fluid front. This allows us to build standard piecewise linear finite element spaces and to monolithically solve the quasistatic hydraulic fracture problem for the displacement field in the rock and the pressure in the fluid. We demonstrate the performance of our algorithms through three examples: a convergence study in plane strain, a comparison with experiments in axisymmetry, and a novel case of a fracture in a narrow pay zone.