How hydrofractures become arrested     

Agust Gudmundsson  & Sonja L. Brenner 《地学学报》2001,13(6):456-462

Fluids in the earth's crust are commonly transported by hydrofractures, such as dykes and mineral veins, many of which become arrested at various crustal depths. Hydrofractures are commonly arrested – some showing blunt tips – at contacts between soft (low Young's modulus) and stiff (high Young's modulus) layers. For example, many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows, and vein tips at contacts between soft marl and stiff limestone. Theoretical models indicate that overpressured, buoyant hydrofractures in homogeneous, isotropic host rocks should normally reach the surface. In layered host rocks, however, abrupt changes in Young's moduli, horizontal discontinuities, and layers with unusually high fracture-perpendicular stresses encourage hydrofracture arrest. It is proposed that for layer-parallel loading, stiff layers favour hydrofracture arrest during active compression but soft layers during extension. It is concluded that for hydrofracture propagation to occur, the stress field along its potential pathway must be essentially homogenous.  相似文献   

The scientific basis of marine pollution control     

Sonja Boehmer-Christiansen 《Marine Policy》1982,6(1):2-10

The author examines the scientific basis of the work of the Joint Group of Experts on the Scientific Aspects of Marine Pollution (GESAMP). Problems of definition and methodology are discussed, and the work of GESAMP working groups on, among others, Marine Waste Disposal, Coastal Water Quality Criteria and the Impact of Oil on the Marine Environment are reported. The author concludes that marine pollution control at the international level has not been able to make much practical headway because political expectations have outstripped scientific and administrative possibilities.  相似文献   

A sediment record of trophic state change in an Arkansas (USA) reservoir     

Byron Winston  Sonja Hausmann  Jaime Escobar  William F. Kenney 《Journal of Paleolimnology》2014,51(3):393-403

Reservoir sediments are used cautiously in paleolimnological studies because of dating uncertainties, possible sediment disturbances and even concerns that indicators of trophic status may behave differently in reservoirs as opposed to natural lakes. We measured loss on ignition (LOI), carbon to nitrogen ratio (C:N), diatom abundance, total nitrogen (TN), total phosphorus (TP), TN:TP ratio, and carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) in an 83-cm sediment core to track recent trophic status changes in Beaver Reservoir, Northwest Arkansas, USA. Measurements showed that LOI, TN, TP and diatom abundance increased significantly from the bottom to the top of the core (p < 0.001). The C:N ratio and δ¹³C indicated a predominantly algal source for organic matter in the sediments. Increases in TN and TP were positively correlated with human population growth (p < 0.01) and the TN:TP ratio recorded a shift from phosphorus to nitrogen limitation around 1990. This shift may have encouraged cyanobacterial growth that caused episodes of taste and odor problems in the reservoir. This study suggests that despite concerns about sediment dating and disturbance, reservoir sediments can provide valuable information on past water quality changes.  相似文献   

On the conditions of sheet injections and eruptions in stratovolcanoes     

Agust Gudmundsson  Sonja L. Brenner 《Bulletin of Volcanology》2005,67(8):768-782

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell  相似文献   

Impact of marine pollution on society: edited by Virginia K. Tippie and Dana R. Kester Centre for Ocean Studies,Praeger, New York, 1982, 313 pp, $27.95     

Sonja Boehmer-Christiansen 《Marine Policy》1983,7(2):130-131

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Modelling and analysis of attenuation anisotropy in multi-azimuth VSP data from the Clair field     

Sonja Maultzsch  Mark Chapman  Enru Liu  Xiang-Yang Li 《Geophysical Prospecting》2007,55(5):627-642

Anisotropic variations in attenuation are of interest since they can give information on the fracture system and may be more amenable to measurement than absolute attenuation values. We examine methods for detecting changes in relative attenuation with azimuth from VSP data, and validate the techniques on synthetic data. Analysis of a multi‐azimuth walkaway VSP data set from a fractured hydrocarbon reservoir indicates that such azimuthal variations in P‐wave attenuation are observable. The effects are localized in the reservoir, and analysis allows the prediction of a fracture strike direction, which agrees with geological information. The observed effects can be modelled under reasonable assumptions, which suggests the validity of the link between the anisotropic attenuation and the fracturing.  相似文献   

The effect of fluid saturation in an anisotropic multi-scale equant porosity model   总被引：5，自引：0，他引：5  

Mark Chapman  Sonja Maultzsch  Enru Liu  Xiang-Yang Li 《Journal of Applied Geophysics》2003,54(3-4):191

It is recognised that exchange of fluid between fractures and the rock matrix can have a strong impact on a rock's anisotropic elastic properties. A recent theoretical advance considers the effect of the scale length of the fractures. We show that under certain circumstances, this model can be simplified. The simplified model matches laboratory data. A prediction of the model is that frequency-dependent effects are important for fluid substitution in the anisotropic case.  相似文献   

Propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields     

Agust Gudmundsson  Ingrid Fjeldskaar  Sonja L. Brenner 《Journal of Volcanology and Geothermal Research》2002,116(3-4)

Palaeofluid-transporting systems, observed as networks of mineral-filled veins in deeply eroded parts of extinct geothermal fields, indicate that hydrofractures commonly supply fluids to geothermal fields. Here we examine well-exposed vein networks that occur at crustal depths of around 1.5 km below the initial surface of the Tertiary lava pile in North Iceland. The veins are located in the damage zone of a major fault zone that dissects basaltic lava flows, the most common host rocks of geothermal fields in Iceland. The lava flows contain numerous weaknesses, particularly columnar (cooling) joints and contacts. For hydrofractures to supply fluids to geothermal fields, the fractures must be able to propagate, and transport fluids, to the surface. We explore hydrofracture pathway formation using boundary-element models of hydrofractures with fluid overpressure varying linearly from 10 MPa at the fracture centre to 0 MPa at the fracture tip (or the fluid front). The hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock-matrix Young’s modulus of 1×10¹⁰ Pa and a Poisson’s ratio of 0.25. The joints and contacts between layers are modelled as internal springs, each with a stiffness (‘strength’) of 6 MPa/m. The location and sizes of discontinuities, as well as the location of the hydrofracture tip, vary between the models. The results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip, and that these discontinuities eventually link up to form the hydrofracture pathway. Analytical models indicate that for a hot spring of a given yield associated with a fault, the dimensions of the fluid-transporting part of the fault are likely to be similar for a typical normal fault and a strike-slip fault. Also, a hot spring of yield 180 l/s (the maximum in the low-temperature fields of Iceland) can be supplied through a hydrofracture of aperture 3 mm and trace length 1.2 m. These dimensions are very similar to those of typical veins in the studied networks. Buoyancy, rather than excess pressure in the fluid source, appears to be the primary driving force of hydrofractures in the geothermal fields of Iceland.  相似文献   

Climate action for food security in South Asia? Analyzing the role of agriculture in nationally determined contributions to the Paris agreement     

T. S. Amjath-Babu  Pramod K. Aggarwal  Sonja Vermeulen 《Climate Policy》2019,19(3):283-298

The Nationally Determined Contributions (NDCs) submitted under the Paris Agreement propose a country’s contribution to global mitigation efforts and domestic adaptation initiatives. This paper provides a systematic analysis of NDCs submitted by South Asian nations, in order to assess how far their commitments might deliver meaningful contributions to the global 2°C target and to sustainable broad-based adaptation benefits. Though agriculture-related emissions are prominent in emission profiles of South Asian countries, their emission reduction commitments are less likely to include agriculture, partly because of a concern over food security. We find that income-enhancing mitigation technologies that do not jeopardize food security may significantly augment the region’s mitigation potential. In the case of adaptation, analysis shows that the greatest effort will be directed towards protecting the cornerstones of the ‘green revolution’ for ensuring food security. Development of efficient and climate-resilient agricultural value chains and integrated farming bodies will be important to ensuring adaptation investment. Potentially useful models of landscape level climate resilience actions and ecosystem-based adaptation are also presented, along with estimates of the aggregate costs of agricultural adaptation. Countries in the region propose different mixes of domestic and foreign, and public and private, adaptation finance to meet the substantial gaps.

Key policy insights

• Though substantial potential for mitigation of agricultural emissions exists in South Asia, governments in the region do not commit to agricultural emissions reductions in their NDCs.

• Large-scale adoption of income-enhancing technologies is the key to realizing agricultural mitigation potential in South Asia, whilst maintaining food security.

• Increasing resilience and profitability through structural changes, value chain interventions, and landscape-level actions may provide strong options to build adaptive capacity and enhance food security.

• Both private finance (autonomous adaptation) and international financial transfers will be required to close the substantial adaptation finance gap

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Internal flow structures in columnar jointed basalt from Hrepphólar, Iceland: II. Magnetic anisotropy and rock magnetic properties     

Bjarne S. G. Almqvist  Sonja A. Bosshard  Ann M. Hirt  Hannes B. Mattsson  Gy?rgy Hetényi 《Bulletin of Volcanology》2012,74(7):1667-1681

The anisotropy of magnetic susceptibility (AMS) and rock magnetic properties were measured on specimens from a basalt plate that was cut from a vertical section of a basalt column from Hrepphólar, Iceland. Macroscopic structures are clearly distinguishable in the plate, including banding inferred to represent viscous fingering parallel to the vertical axis of the column. Rock magnetic experiments indicate that the dominant ferromagnetic (sensu lato) mineral is titanomagnetite, Fe _3?x Ti _x O₄, with a Ti-composition of x?=?~0.6. Magnetic properties are related to the position within the plate and reveal a dominant volume fraction of single domain titanomagnetite in the center of the basalt column, with multidomain titanomagnetite away from the center. The AMS determined by low-field measurements shows an inconclusive relationship with the visual structures, which arises from variation of the grain size (i.e., single domain versus multidomain) across the column. In contrast, the AMS measured with a high-field torsion magnetometer avoids the complication of magnetic domain state, as is demonstrated in this contribution, and additionally allows for the separation of ferrimagnetic from paramagnetic sub-fabrics. Both sub-fabrics display a clear relationship with the macroscopic structures and support the hypothesis that vertical flow of melt took place during development of the Hrepphólar columnar basalt. Maximum susceptibility axes of the ferrimagnetic sub-fabric are grouped near the vertical axis of the column. The paramagnetic sub-fabric varies systematically across the column in coincidence with internal structure. The shape of the magnetic susceptibility ellipsoid varies across the basalt column, showing an increasingly prolate fabric toward its center.  相似文献