MINI-MACtm—A new generation dual—Band surveyor

Summary Litton Divisions presently produce both high accuracy GPS surveyors and low—cost GPS navigation sets. Aero Service'sMACROMETER ^R Interferometric Surveyors, have become the standard against which GPS surveying equipment is measured. Litton Aero Products has developed a highly digitized, low costL ₁,C/A code GPS card set. The integration of these technologies had led to the development of a low-cost, high-precision, GPS survey system which can be configured with or without a codelessL ₂ capability. TheMINI-MAC surveying system is the first member of the new generation of GPS survey systems resulting from this joint development. The system design is described in this paper, and initial survey test results using a prototypeMINI-MAC surveying system are presented.     

The effects of floods on the temperature of riparian groundwater

The transition area between rivers and their adjacent riparian aquifers, which may comprise the hyporheic zone, hosts important biochemical reactions, which control water quality. The rates of these reactions and metabolic processes are temperature dependent. Yet the thermal dynamics of riparian aquifers, especially during flooding and dynamic groundwater flow conditions, has seldom been studied. Thus, we investigated heat transport in riparian aquifers during 3 flood events of different magnitudes at 2 sites along the same river. River and riparian aquifer temperature and water‐level data along the Lower Colorado River in Central Texas, USA, were monitored across 2‐dimensional vertical sections perpendicular to the bank. At the downstream site, preflood temperature penetration distance into the bank suggested that advective heat transport from lateral hyporheic exchange of river water into the riparian aquifer was occurring during relatively steady low‐flow river conditions. Although a small (20‐cm stage increase) dam‐controlled flood pulse had no observable influence on groundwater temperature, larger floods (40‐cm and >3‐m stage increases) caused lateral movement of distinct heat plumes away from the river during flood stage, which then retreated back towards the river after flood recession. These plumes result from advective heat transport caused by flood waters being forced into the riparian aquifer. These flood‐induced temperature responses were controlled by the size of the flood, river water temperature during the flood, and local factors at the study sites, such as topography and local ambient water table configuration. For the intermediate and large floods, the thermal disturbance in the riparian aquifer lasted days after flood waters receded. Large floods therefore have impacts on the temperature regime of riparian aquifers lasting long beyond the flood's timescale. These persistent thermal disturbances may have a significant impact on biochemical reaction rates, nutrient cycling, and ecological niches in the river corridor.     

Tree‐ring records unveil long‐term influence of the Pacific Decadal Oscillation on snowpack dynamics in the Stikine River basin,northern British Columbia

Long hydroclimate records are essential elements for the assessment and management of changing freshwater resources. These records are especially important in transboundary watersheds where international cooperation is required in the joint planning and management process of shared basins. Dendrochronological techniques were used to develop a multicentury record of April 1 snow water equivalent (SWE) for the Stikine River basin in northern British Columbia, Canada, from moisture‐sensitive white spruce (Picea glauca) tree rings. Explaining 43% of the instrumental SWE variability, to our knowledge, this research represents the first attempt to develop long‐term snowpack reconstructions in northern British Columbia. The results indicated that 15 extreme low April 1 SWE events occurred from 1789 to the beginning of the instrumental record in 1974. The reconstruction record also shows that the occurrence of hydrological extremes in the Stikine River basin is characterized by persistent below‐average periods in SWE consistent with phase shifts of the Pacific Decadal Oscillation (PDO). Spectral analyses indicate a very distinct in‐phase (positive) relationship between the multidecadal frequencies of variability (~40 years) extracted from the SWE tree‐ring reconstruction and other reconstructed winter and spring PDO indices. Comparison of the reconstructed SWE record with other tree‐ring‐derived PDO proxy records shows coherence at multidecadal frequencies of variability. The research has significant implications for regional watershed management by highlighting the hydrological response of the Stikine River basin to prior climate changes.     

The Alaska Wrangell Arc: ~30 Ma of subduction‐related magmatism along a still active arc‐transform junction

The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for ~500 km across south‐central Alaska (USA) into Canada at a volcanic arc‐transform junction. Previously, geochemistry documented mantle wedge and slab‐edge melting in <12 Ma WVB volcanic rocks; new geochemistry shows that the same processes characterized ~18–30 Ma WVB magmatism in Alaska. New ⁴⁰Ar/³⁹Ar ages demonstrate that WVB magmatism in Alaska initiated at ~30 Ma due to flat‐slab subduction of the Yakutat microplate and that the dextral Totschunda fault was active at this time. Our results, together with prior studies, show that Alaskan WVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g. the Wrangell Arc). The WVB provides a long‐term geological record of subduction, strike‐slip and magmatism. Slab‐edge upwelling, flat‐slab defocused fluid‐flux and faults acting as magma conduits are likely responsible for the exceptionally large volcanoes and high eruption rates of the Wrangell Arc.     

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...     

Geology of possible Martian methane source regions

Recent observations of methane on Mars suggest that spatially localized source regions are present. Here we discuss the surface morphology and mineralogy of these regions, focusing on features that may provide insights into mechanisms of methane production and/or release. Preliminary trends among methane source regions include old age, deep fractures, past or present subsurface water, and the presence of hydrated minerals, sometimes including serpentine. As the spatial and temporal coverage of Martian methane is expanded, geological observations of proposed source regions will be a powerful tool for understanding the methane cycle on modern Mars.     

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