Showing papers on "Pore water pressure published in 1996"

Flow-induced uptake of particulate matter in permeable sediments

Abstract: We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z 2 x 10-l l m2) incubated in a laboratory flume. Increased pressure up- and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the core, carrying suspended particles (1 pm) to 2.2-cm sediment depth within 10 h. Simultaneously, decreased pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z 5 10 cm) to the surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation and layers of increased particle concentration. Ripple topography (0.8-2.8 cm high) enhanced interfacial particle (1 pm) flux by a factor 2 .3 when compared to a level control core. The pathways of the particle and solute tracers below a sediment ripple are explained with a source-sink model that describes the pore flow velocity field. Our results suggest that bedform-induced interfacial flows are important for the uptake of particulate organic matter into permeable shelf sediments.

How Permeability Depends on Stress and Pore Pressure in Coalbeds: A New Model

Abstract: In naturally fractured formations, such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., effective stress). This paper presents a new theoretical model for calculating pore volume compressibility and permeability in coals as a function of effective stress and matrix shrinkage, using a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). Pore volume compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. Pore volume compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values. We have also history matched rates from a {open_quotes}boomer{close_quotes} well in the fairway of the San Juan basin using various stress-dependent permeability functions. The best fit stress-permeability function is then compared with the new theory.

Postseismic Rebound in Fault Step-Overs Caused by Pore Fluid Flow

Abstract: Near-field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation. Synthetic aperture radar (SAR) interferometry revealed several centimeters of postseismic uplift in pull-apart structures and subsidence in a compressive jog along the Landers, California, 1992 earthquake surface rupture, with a relaxation time of 270 ± 45 days. Such a postseismic rebound may be explained by the transition of the Poisson's ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium.

Effect of rainfall on matric suctions in a residual soil slope

Abstract: A slope stability study involving shallow slip surfaces should include the effect of negative pore-water pressures in a slope. A field instrumentation program was carried out to monitor negative pore-water pressure (i.e., in situ matric suction) in a residual soil slope in Singapore. Variations in matric suction and the matric suction profiles under (1) a canvas-convered grassed surface, (2) a grassed surface, and (3) a bare ground surface, in response to rainfalls were investigated. Changes in matric suction due to changes in climatic conditions decrease rapidly with depth. The change was found to be most significant in the bare slope and least significant under the canvas-covered slope. The amount of decrease in matric suction after a rainstorm was observed to be a function of the initial matric suction just prior to the rainstorm. Positive pore-water pressures were observed above the groundwater table, suggesting the development of a perched water table within the slope. These observations are also typ...

Earthquakes as a coupled shear stress‐high pore pressure dynamical system

Abstract: The migration, coalescence and localization of slip, seismicity, and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic. Increased pore pressure in discrete cells creates zones of low effective stress, which induces slip that may propagate to surrounding cells depending on the local state of stress. At slip, stress is transferred using the solution for a rectangular dislocation in an elastic half-space, and pore pressures are redistributed by conserving fluid mass. Using simple assumptions about fault rheology and permeability, it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events, locked zones, and large variations in fault strength. The model evolves from a uniform shear stress state on a strong fault, to a heterogeneous shear stress state on a weak fault.

Theory for the propagation of tidally induced pore pressure variations in layered subseafloor formations

Abstract: Tidally induced pore pressure variations below the seafloor depend on the elastic moduli and transport properties of the pore fluid and formation. Hence observations of pore pressure variations, in conjunction with model predictions, can provide important constraints on these formation properties. In this paper, we study the propagation of tidally induced pore pressure variations in a layered poroelastic medium. We derive an analytic solution and use the solution to investigate the effects of various parameters, in particular, the bulk modulus of the formation, the bulk modulus of the pore fluid, and the formation permeability. Specific examples are considered that include the typical continuous depth variation of properties that occurs through normal sediment consolidation and imbedded layers of contrasting properties. Diffusive propagation of tidal pressure variations from the seafloor depends on the hydraulic diffusivity. The depth limit of diffusive propagation scales with the inverse square root of permeability and the period of the signal; for typical fine-grained marine sediments the depth scale at tidal frequencies is only a few meters. Any internal contrast in elastic properties, due to the presence of free gas for example, can give rise to large instantaneous pressure changes across a layer boundary, which in turn can induce diffusive propagation of signals above and below the interface. Long-term pressure records from sealed deep-ocean boreholes that included tidal signals are considered in light of the model results. In Ocean Drilling Project (ODP) Hole 857D on the Juan de Fuca Ridge, the observed attenuation of the seafloor tidal signal to 15% is consistent with the relatively low compressibility of the hydrothermally indurated section intersected by the open part of the borehole and with the high compressibility of the hot formation fluid. In ODP Hole 892B in the Cascadia accretionary prism, the attenuation to 55% and several degree phase lead of the formation tidal signal are probably the result of the open part of the hole being connected to an overlying interval bearing a few percent free gas via a high-permeability fault zone.

Elastic visco-plastic modelling of one-dimensional consolidation

Abstract: This paper incorporates an elastic visco-plastic (EVP) constitutive model into the consolidation equation to calculate settlements and excess pore water pressures in clays under multi-stage constant vertical loads in one-dimensional vertical straining. A finite difference method is used to solve the resulting non-linear differential consolidation equation. Results from the model are compared with test data and results from other models. The EVP model is also used to simulate consolidation in different thicknesses of clay to investigate the influence of the thickness on total settlements, dissipation of pore water pressures, strains and stresses. Consolidation analysis using the model can describe phenomena caused by the viscous nature of clays: excess pore water pressures can become larger than their initial values immediately after loading, effective stresses in the clay may become smaller than their initial values, and the relationship between strains and effective stresses at the end of primary consoli...

Preferential water flow in a frozen soil - A two-domain model approach

Abstract: Earlier modelling studies have shown the difficulty of accurately simulating snowmelt infiltration into frozen soil using the hydraulic model approach. Comparison of model outputs and field measurements have inferred the occurrence of rapid flow even during periods when the soil is still partly frozen. A one-dimensional, physically based soil water and heat model (SOIL) has been complemented with a new two-domain approach option to simulate preferential flow through frozen layers. The ice is assumed to be first formed at the largest water filled pore upon freezing. Infiltrating water may be conducted rapidly through previously air-filled pores which are not occupied by ice. A minor fraction of water is slowly transferred within the liquid water domain, which is absorbed by the solid particles. A model validation with field measurements at a location in the middle-east of Sweden indicated that the two-domain approach was suitable for improving the prediction of drainage during snowmelting. In particular, the correlation between simulated and observed onset of drainage in spring was improved. The validation also showed that the effect of the high flow domain was highly sensitive to the degree of saturation in the topsoil during freezing, as well as to the hydraulic properties at the lower frost boundary regulating the upward water flow to the frozen soil and ice formation.

Oxygen pore water profiles in continental shelf sediments of the North Sea: turbulent versus molecular diffusion

Abstract: Oxygen pore water profiles in North Sea sed~ment s were measured with microelectrodes during 2 contrasting seasons The measurements were conducted In a wide variety of sediments, including non-depositional areas on the southern shelf as well as depositional areas in the Skagerrak All measurements were performed within minutes on board at In sltll temperature The curvature of oxygen profiles in sandy sediments on the southern shelf Indicated the presence of a surface layer characterised by enhanced diffusion The occurrence of enhanced diffusion was related to sedimentological and seasonal differences Quantitative evaluat~on of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coeff~c~ents In a 02 to 16 mm subsurface layer were 15 to > l 0 0 times higher than the ~nolecular diffusion coefflcient Highest effective diffusion coefficients were reported for non-depositional sediments characterised by low diffusive oxygen fluxes Oxygen profiles in sediments of the depositional area of the Skagerrak indicated constant diffusivity throughout the sediment column Diffusive fluxes cdlculated from profiles ranged from 52 to 89 ~nlnol m-2 d-' in August 1991, and were between 08 and 62 mm01 nl ' d-' in February 1992 Seasonal differences in sediments locatt!d In the Skagerrak area were minor It is proposed that near-bottom tidal currents induce enhanced diffus~on transport processes in the upper millimetres of the sandy sediments of the southern North Sea, while less energetic hydrodynamical conditions in the depos~tional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only Biogeochemical implications of the enhanced diffusivity close to the sediment-water interface are discussed

Wave-induced soil response in a nearly saturated sea-bed of finite thickness

Abstract: Consolidation and storage equations are used to develop a closed-form solution for the wave-induced pore pressure, soil displacements and effective stresses in an elastic sea-bed subject to a system of two intersecting waves. A homogeneous soil matrix of finite thickness in isotropic and saturated conditions only is considered. The three-dimensional general solutions so developed are readily reducible to the conditions for soil of infinite thickness, and also for the limiting cases of two-dimensional progressive and standing waves for soil of finite thickness, for which no explicit solutions have previously been available. Verification of this solution with results of two-dimensional solutions available from a semi-analytical method, a boundary-layer approximation and a numerical model is carried out. Validation is performed by comparison with experimental results. The effects on wave-induced pore pressure of sea-bed thick- ness, shear modulus of soil and grain size are discussed. Les equations de stockag...

Interpretation of chemical and isotopic data from boreholes in the unsaturated zone at Yucca Mountain, Nevada

Abstract: Analyses of pore water from boreholes at Yucca Mountain indicate that unsaturated-zone pore water has significantly larger concentrations of chloride and dissolved solids than the saturated-zone water or perched-water bodies. Chemical compositions are of the calcium sulfate or calcium chloride types in the Paintbrush Group (Tiva Canyon, Yucca Mountain, Pah Canyon, and bedded tuffs), and sodium carbonate or bicarbonate type water in the Calico Hills Formation. Tritium profiles from boreholes at Yucca Mountain indicate tritium-concentration inversions (larger tritium concentrations are located below the smaller tritium concentration in a vertical profile) occur in many places. These inversions indicate preferential flow through fractures. Rock-gas compositions are similar to that of atmospheric air except that carbon dioxide concentrations are generally larger than those in the air. The delta carbon-13 values of gas are fairly constant from surface to 365.8 meters, indicating little interaction between the gas CO{sub 2} and caliche in the soil. Model calculations indicate that the gas transport in the unsaturated zone at Yucca Mountain agrees well with the gas-diffusion process. Tritium-modeling results indicate that the high tritium value of about 100 tritium units in the Calico Hills Formation of UZ-16 is within limits of a piston-flow model with a water residence time of 32 to 35 years. The large variations in tritium concentrations with narrow peaks imply piston flow or preferential fracture flow rather than matrix flow. In reality, the aqueous-phase flow in the unsaturated zone is between piston and well-mixed flows but is closer to a piston flow.

Calcite dissolution in sediments of the Ontong‐Java Plateau: In situ measurements of pore water O2 and pH

Abstract: We present in situ electrode measurements of sediment resistivity, pore water oxygen, and pore water pH from three stations between 2300 and 3000 m depth on the Ontong-Java Plateau in the western equatorial Pacific. One of these stations is also the site of a concurrent benthic chamber incubation experiment [Jahnke et al., 1994]. The pore water oxygen data and a steady state diffusion and reaction model constrain the depth-dependent rate of oxic respiration in the sediments and imply a diffusive flux of oxygen to the sediments of 10–21 μmol cm−2 yr−1. Given these respiration rates, the pore water pH data cannot be explained without calcite dissolution driven by metabolically produced CO2. The dissolution necessary to explain the observations, quantified by a statistical approach, is 3.5–6 μmol cm−2 yr−1, which corresponds to at least 20–40% of the calcite rain to these sediments. Over 65% of the total dissolution is driven by metabolic CO2. Oxygen fluxes and net calcite dissolution constrained by the electrode data are compatible with the benthic chamber measurements of Jahnke et al. [1994]. The dissolution flux, while a significant part of the early diagenesis of calcite in these sediments, is less than would be predicted by earlier models of dissolution, and Jahnke et al. [1994] probably could not distinguish it from zero with the benthic chamber technique. The dissolution rates found in this study are lower than previous estimates because the respiration reaction is concentrated near the sediment-water interface, and the calcite dissolution rate constants are very small. The statistical evaluation of the pore water pH data and model constrain the calcite dissolution rate constant to 0.005–0.16% d−1, following the general trend of lower values determined by in situ techniques rather than by laboratory methods.

Impact of boundary layer flow velocity on oxygen utilisation in coastal sediments

Abstract: Small pressure gradients generated by boundary flow-topography interactions cause advective pore water flows in permeable sediments. Advective pore water exchange enhances the flux of solutes between the sediment and the overlying water, thus generating conditions for an increased utilisation of oxygen. We compared a less permeable (k = 5 X 10-l2 m2) with a permeable sediment (k = 5 X 10-" m') typical for coastal and shelf sediments. Total oxygen utilisation (TOU) in incubated sediment cores was measured in 10 laboratory experiments using recirculating flow tanks (33 runs). TOU was a function of flow velocity in permeable sediment where advective pore water flow occurred. TOU increased with the increasing volume of sediment flushed with oxygenated water. We found that TOU increased by 91 2 23% in coarse sand when flow increased from 3 to 14 cm S-' (38 mounds m-', height 10 to 30 mm, flow measured 8 cm above the sediment). Additlon of fresh algal material caused a stronger stimulation of TOU in the coarse sand than in the fine sand (4 additional flume runs). After the addition, intensive oxygen consumption reduced the oxygen penetration depth in the advectively flushed zone of the coarse sediment. However, counteracting this process, advectlve flow maintained an oxic sediment volume still larger than that in the less permeable sediment. Flow-enhanced oxygen utilisation is potentially effective in permeable beds of coastal and shelf regions, in contrast to the situation in cohesive sediments limited by predominantly diffusive oxygen supply.

Troubled waters: Environmental applications of electrical and electromagnetic methods

Abstract: The relatively new subdiscipline of environmental geophysics has grown enormously in the last five years. The size and diversity of the field, and the associated literature, is such that it is extremely difficult to keep up with even a small portion of the field. Electrical and electromagnetic (E & EM) methods, including ground penetrating radar and time-domain reflectometry, play a central role in environmental geophysics. One reason for the utility of E & EM methods in groundwater studies is the similarity in the way that current flow and fluid flow depend on the connectivity and geometry of the pores in soils and rocks. Another reason is the influence of the pore water quality on the geophysical response. More than any other geophysical technique, E & EM methods are directly affected by the presence of conductive pore fluids in the subsurface, such as leachates from landfill sites and sea water invading a coastal groundwater supply that has been placed under stress because of population expansion. The chloride ion is one of the most electrically active of the naturally-occurring ions, and allows us to detect sea water incursion; leachates from landfill sites contain the by-products of organic decay, such as acetic acid, which are generally less conductive than chloride, but nonetheless enhance the pore water and formation electrical conductivities. Landfill leachate plumes are thus easily mapped. The shallow subsurface electrical and dielectric properties exhibit hysteresis due to seasonal changes in water content; the physical properties will be different for the same degree of saturation, depending on whether the water level is rising or falling. Topographic effects are also important; an empirical correction method works well to remove a background trend in the conductivity due to changes in elevation. Heterogeneity and anisotropy of the electric properties may be related to similar effects in the hydraulic properties. New technology and the adaptation of existing technology has lead to the development of fresh instruments, such as electrode arrays towed across the ground, resistivity logging while drilling, fast-rise time TEM, NMR combined with TEM, electric quadripole, et cetera. The applications of E & EM methods cover a wide range of geographic areas and groundwater problems, but have had particularly wide use for groundwater exploration in arid and semi-arid regions, for mapping and monitoring salt-water incursion in susceptible aquifers, and for mapping and monitoring contaminants.

Solute Exchange by Convection Within Estuarine Sediments

Abstract: The rates of material transport through sediments are central to determining the biochemical status of sediments. Transport within sediments can occur by molecular diffusion through porewaters and by the activities of burrowing biological organisms, but a number of physical processes can enhance transport within porewaters. Here, convection is investigated as a mechanism for solute transport within sediments. Convection will occur as the gravitational adjustment to the occurrence of porewater which is of lower salinity and density than the water at the sediment surface. This process could be important for exchange between sediments and the water column in regions subject to significant temporal variations in salinity such as estuaries.

Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

Abstract: High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments. From our modeling, we conclude that significant overpressures have been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated. We further predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the oil window remained at approximately 2.5±0.5 km below sea floor (bsf). Similarly, the depth to the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively constant position of the top of the oil window defines a maturation "front" that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsidence.

Seismic triggering by rectified diffusion in geothermal systems

Abstract: Widespread seismicity was triggered by the June 28, 1992, Landers California, earthquake at a rate which was maximum immediately after passage of the exciting seismic waves. Rectified diffusion of vapor from hydrothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant levels within the duration of the earthquake. In a hydrothermal system modeled as a two-component H_2O-CO_2 fluid in porous rock the pressure initially increases linearly with time. The rate of pressure buildup depends sensitively on the mean bubble radius and is large for small bubbles. The diffusion-induced pressure is relaxed by percolation and resorption of vapor into the liquid solution. The induced seismicity itself also relieves stress. Values of parameters used in the present calculations give results consistent with observations of triggered seismicity at Long Valley caldera after the Landers earthquake. For one representative condition, at 250°C and 5.6 km depth, oscillating strain acting on 10-μm-diameter bubbles increases pore pressure at the rate of 151 Pa/s resulting in a pressure increase of 12 kPa in the 80-s duration of the Landers earthquake. The elevated pressure induced by a single 26-m-diameter cloud of bubbles in saturated rock relaxes by percolation through soil of 0.2-mdarcy permeability in 53.6 hours. Observations of earthquake swarms at other locations suggest that self-induced buildup of pore pressure by rectified diffusion can provide a positive feedback mechanism for amplifying seismicity.

Geochemical Estimates of Paleorecharge in the Pasco Basin: Evaluation of the Chloride Mass Balance Technique

Abstract: The Pasco Basin in southeastern Washington State provides a unique hydrogeologic setting for evaluating the chloride mass balance technique for estimating recharge. This basin was affected by late Pleistocene catastrophic floods when glacial dams in western Montana and northern Idaho were breached. It is estimated that multiple Missoula floods occurred between ∼13,000 and 15,000 years B.P. and reached a high water elevation of ∼350 m. These floods removed accumulated chloride from the sediment profile, effectively resetting the chloride mass balance clock at the beginning of the Holocene. The rate of chloride accumulation qCl in the sediments was determined by two methods and compared. The first method measured qCl by dividing the calculated natural fallout of 36Cl by a measured ratio of 36Cl/Cl in the pore water, while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had accumulated since the last flood. Although the two methods are based on different approaches, they showed close agreement. In laboratory studies the sediment to water ratio for chloride extraction was sensitive to the grain size of the sediments; low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride concentration. Br/Cl ratios were useful for distinguishing nonatmospheric (e.g., rock) sources of chloride. Field studies showed little spatial variability in estimated recharge at a given site within the basin but showed significant topographic control on recharge rates in this semiarid environment. An extension of the conventional chloride mass balance model was used to evaluate chloride profiles under transient, time-varying annual precipitation conditions. This model was inverted to determine the paleorecharge history for a given soil chloride profile, and the parameters of the root extraction model required to estimate paleoprecipitation

Deformation of water-saturated sand: II. effect of pore water flow and shear banding

Abstract: Previous undrained biaxial experiments on water-saturated sand are analysed in relation to shear banding and linear stability under locally drained and globally undrained conditions. The regulariza...

Parameter estimation for determining hydraulic properties of a fine sand from transient flow measurements

Abstract: A cone penetrometer method for measuring hydraulic conductivity of unsaturated soils at depth is under development. Successful advancement of this method hinges on using parameter estimation to obtain hydraulic parameter values from pore water pressure and flow rate data. A finite element model is employed to predict flow responses, and objective functions describe differences between “true” and simulated responses. Contour plots in parameter space show the relative sensitivity of objective functions to field-saturated hydraulic conductivity, Kfs, field-saturated moisture content,θfs, and the van Genuchten hydraulic parameters, α and n. Principal curvatures and directions in parameter space describe the nature of objective functions near “true” parameter values. An objective function based on flow rate and pore water pressures does not provide better parameter sensitivity than one based on pore water pressures alone. It appears possible to obtain estimates of Kfs and α but unlikely that the other parameters will be identifiable.

Pore-Water Pressures in Freezing and Thawing Fine-Grained Soils

Abstract: Laboratory freezing tests were performed on laterally confined samples of lightly overconsolidated fine-grained soil exposed to one-dimensional freezing at a constant temperature gradient. Pore-wat...