Showing papers on "Pore water pressure published in 1992"

Radon transport from soil to air

Abstract: Radon generated within the upper few meters of the Earth's crust by the radioactive decay of radium can migrate during its brief lifetime from soil into the atmosphere. This phenomenon leads to a human health concern as inhalation of the short-lived decay products of radon causes irradiation of cells lining the respiratory tract. This paper reviews the factors that control the rate at which two radon isotopes, 222Rn and 220Rn, enter outdoor and indoor air from soil. The radium content of surface soils in the United States is usually in the range 10–100 Bq kg−1. The emanation coefficient, which refers to the fraction of radon generated in a material that enters the pore fluids, varies over a wide range with a typical value being 0.2. Radon in soil pores may be partitioned among three states: in the pore air, dissolved in the pore water, and sorbed to the soil grains. Except in the immediate vicinity of buildings, radon migrates through soil pores principally by molecular diffusion. Average reported flux densities from undisturbed soil into the atmosphere are 0.015–0.048 Bq m−2 s−1 for 222Rn and 1.6–1.7 Bq m−2 s−1 for 220Rn. Soil is the dominant source of radon in most buildings. Advective flow of soil gas across substructure penetrations is a key element in the transport process. The advective flow is driven by the weather (wind and indoor-outdoor temperature differences) and by the operation of building systems, such as heating and air conditioning equipment. A typical radon entry rate into a single-family dwelling of 10–15 kBq h−1 can be accounted for by weather-induced pressure-driven flow through moderately to highly permeable soils. The extent to which diffusion through soil pores contributes to radon entry into buildings is not known, but in buildings with elevated concentrations, diffusion is believed to be less important than advection.

Chapter 20 Fault Stress States, Pore Pressure Distributions, and the Weakness of the San Andreas Fault

Abstract: The San Andreas Fault (SAF) is weak in an absolute sense, in that it moves under shear stresses far smaller than implied by the most obvious reading of laboratory friction results (Byerlee law with hydrostatic pore pressure and friction coefficient F = 0.6-0.9). It is also weak in a relative sense, in that the adjoining crust seems to be mechanically stronger; this is implied by the stress state there having a horizontal maximum principal direction that makes a steep angle to the trace of the SAF, much larger than the 25-30° angle (i.e., 45° - 0.5 arctan F ) expected from standard frictional failure considerations, and in the range of 60° to nearly 90°. It is shown that a maturely deformed fault zone which is weak relative to its surroundings, owing to inherent material strength and/or pore pressure differences, develops stresses within it which are distinct from those of its surroundings. Because of those stress differences, it is found that pore pressure distributions which are high, and near to the fault-normal compressive stress, within the fault zone, but which decrease with distance into the adjacent crust, are consistent with both the absolute and relative weakness of the SAF; the pore pressure in such distributions is less than the least principal stress at every point, so there is no hydraulic fracturing, even though the pressure in the fault zone may be greater than the least principal stress in the nearby crust. Such pore pressure distributions are shown to result from the following assumptions: (1) there is a supply of fluids near the ductile roots of crustal fault zones, where pore pressure must be nearly lithostatic; (2) active fault zones are far more permeable than the adjoining rock of the middle crust; and (3) fault permeability is a rapidly diminishing function of effective normal stress. Evidence in support of these assumptions is discussed. The resulting pore pressure distributions adjust significantly from hydrostatic, such that the effective normal stress, and hence also the brittle frictional strength, becomes approximately independent of depth along the fault zone. These assumptions also predict the possibility of diffusive surges of pore pressure that propagate upward along a fault in a slow wavelike manner.

Stress field constraints on intraplate seismicity in eastern North America

Abstract: Focal mechanisms of 32 North American midplate earthquakes (mo = 3.8-6.5) were evaluated to determine if slip is compatible with a broad-scale regional stress field derived from plate-driving forces and, if so, under what conditions (stress regime, pore pressure, and frictional coefficient). Using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes (_ 10o), the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb = (S2 - S3)/(S - S3) from the fault/stress geometry. As long as the focal mechanism has a component of oblique slip (i.e., the B axis does not coincide with the intermediate principal stress direction), this calculation identifies which of the two nodal planes is a geometrically possible slip plane (Gephart, 1985). Slip in a majority of the earthquakes (25 of 32) was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad-scale uniform regional stress field. Slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity. Significantly, all five of these events occurred prior to 1970 (when many regional networks were installed), and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events. The frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory. The ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field. Regional stress magnitudes were determined from (1) S/S3 ratios based on the frictional strength of optimally oriented faults (the basis for the linear brittle portion of lithospheric strength profiles), (2) the computed relative stress magnitude (b) values, and (3) a vertical principal stress assumed equal to the lithostat. Two end-member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes. First, the frictional coefficient was held constant on all faults, hydrostatic pore pressure was assumed regionally, and the fault zone pore pressure was determined. Since pore pressure is a measurable quantity with real limits in the crust (P0 < S3), this end-member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes. Alternately, pore pressure was fixed at hydrostatic everywhere, and the required relative lowered frictional coefficient of the fault zone was computed. Slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate-driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults (assuming hydrostatic pore pressure regionally). The conditions for slip on these 23 relatively "well-oriented" earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure (between about 0.4-0.8 of lithostatic, hydrostatic is about 0.37 of lithostatic) or, alternately, moderate lowering (<50%) of the frictional coefficient on the faults which slipped. Superlithostatic pore pressures are not required. Focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field, however, did require pore pressures far exceeding the least principal stress (or extremely low coefficients of friction). These events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms. The analysis also confirmed a roughly north to south contrast in stress regime between the central eastern United States and southeastern Canada previously inferred from a contrast in focal mecha- nisms between the two areas: most central eastern United States earthquakes occur in response to a strike-slip stress regime, whereas the southeastern Canadian events require a thrust faulting stress regime. This contrast in stress regime, with a constant maximum horizontal stress orientation determined by far-field plate-driving forces, requires a systematic lateral variation in relative stress magnitudes. Superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation, but maximum computed rebound-related stress magnitude changes are quite small (about 10 MPa) and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths.

Chapter 3 Frictional Strength and the Effective Pressure Law of Montmorillonite and lllite Clays

Abstract: Low-strength clay minerals are a common constituent of fault gouges, and are often cited as a possible explanation for the low ambient shear stresses along the San Andreas fault inferred from heat flow constraints and in situ stress measurements. Montmorillonite, the weakest of the clay minerals, undergoes a gradual phase transition to illite with depth. In order to compare the shear stresses supported by these two minerals with those thought to exist along the San Andreas, we have measured the frictional sliding behavior of pure montmorillonite, mixed montmorillonite/illite and pure illite as a function of effective pressure, simulating burial to seismogenic depths. Strength measurements verify that the effective pressure law for friction holds for these minerals under all conditions. That is, the measured stresses were a function of the effective pressure, P c - P p , independent of the choice of confining and pore pressure. This relation, common for many other rock types, was previously untested for these clays under most conditions. Results show that dry samples were consistently stronger than saturated samples, and that strength increased with increasing illite content. In addition, the coefficient of friction increased as a function of pressure for the montmorillonite gouge, but was independent of pressure for the illite gouge. This behavior may be explained by the presence of loosely bonded interlayer water in the montmorillonite, which is squeezed out at higher pressures, changing the frictional characteristics of the clay. The nonexpanding illite was not affected in this way. For the montmorillonite-to-illite compositional profile, an average shear stress of 60 MPa was determined for crustal conditions to 15 km, assuming a normal hydrostatic gradient. If montmorillonite remains stable at depth, the resulting average shear stress is reduced to 30 MPa. In either case, these values are above the 10-20 MPa shear stress limit along the San Andreas inferred from heat flow constraints. Strength may be reduced to in-situ levels if fluid pressures become greater than hydrostatic within the gouge zone.

Pore pressure, stress increase, and fault weakening in low‐angle normal faulting

Abstract: Low-angle (dip < 30°) normal faults accommodate much extension of the continental crust They apparently move under low resolved shear stress and are anomalously weak, characteristics that they share with the San Andreas fault Structural, textural, and geochemical arguments suggest that low-angle normal faults are weak in both the ductile and brittle regimes, partly or totally due to elevated pore fluid pressure High pore pressure in detachment zones may be contained by upper-plate strata, mineral precipitation in their hanging walls, formation of low-permeability microbreccia layers, threshold pressure gradients, and low-permeability mylonites below chlorite breccia Mechanical analysis shows that fault weakening may preclude equality of the regional and fault-zone stress tensors, and predicts reorientation and increase of principal stresses in weak fault zones These changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks Fault weakening focuses extension in the upper crust onto low-angle normal ductile-brittle shear zones in the midcrust, promoting propagation of low-angle brittle normal faults into the upper crust

Mechanics of seismic instabilities induced by the recovery of hydrocarbons

Abstract: We review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models, poroelastic stress transfer and isostasy theory. These three mechanisms trigger or induce seismic instabilities at both local scale (D≤5 km) and at regional scale (D≥20 km). The modeled changes in stress are small (≤1 MPa), whatever the tectonic setting. Each mechanism corresponds to different production processes. (1) Local hydraulic fracturing due to fluid injection induces seismic-slip on cracks (M L≤3) within the injected reservoir through decreasing the effective stress. (2) Pure fluid withdrawal causes pore pressure to decrease within the reservoir. It triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion. Poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereM L≤5 seismic instabilities occur either above or below the reservoir. (3) Massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust. It can induce larger earthquakes (M L≥6) at greater distance from the hydrocarbon fields than the two other mechanisms. Due to the mechanical properties of the shallow rock matrices involved, seismic slip triggered either by mechanism (1) or (2), is a second-order process of the main elastoplastic deformation. for a minimum of 80% of commercially productive basins, most of the local deformation is reported as aseismic, i.e., there is no evidence forM L≥3 earthquakes. Nevertheless, the induced stresses vary as a function of time in a manner that depends on the hydraulic diffusivity (i.e., permeability) of the reservoir and surrounding rocks. Because small earthquakes (M L≤3) indicate changes in stress and pore pressure, monitoring of seismicity is a means of assessingin situ reservoir behavior. The less constrained seismic response to hydrocarbon recovery is the possible connection between local fluid manipulations, triggered earthquakes and major regional earthquakes. Positive feedback mechanisms suggest that the region of seismic hazard changes is much larger than the area where hydrocarbons are extracted. These observations and models testify that fluid movement and pore pressure changes (increase or decrease) play important roles in the mechanics of earthquakes and in the triggering of natural earthquakes.

Chapter 8 The Determination of Permeability and Storage Capacity: Pore Pressure Oscillation Method

Abstract: With the aim of exploring the possibility of monitoring the changes in the pore space as they take place during deformation of rocks at high pressure and temperature, we have implemented a method which utilizes the measurement of the attenuation and phase retardation of a sinusoidal pore pressure variation as it propagates through a sample under test. In this paper, the theory and some design considerations as well as detailed discussion of adequate data analysis are presented. The results of our measurement are presented in Chapter 9 of this volume.

Gravity-driven groundwater flow and slope failure potential. 1. Elastic effective-stress model

Abstract: Hilly or mountainous topography influences gravity-driven groundwater flow and the consequent distribution of effective stress in shallow subsurface environments. Effective stress, in turn, influences the potential for slope failure. To evaluate these influences, we formulate a two-dimensional, steady state, poroelastic model. The governing equations incorporate groundwater effects as body forces, and they demonstrate that spatially uniform pore pressure changes do not influence effective stresses. We implement the model using two finite element codes. As an illustrative case, we calculate the groundwater flow field, total body force field, and effective stress field in a straight, homogeneous hillslope. The total body force and effective stress fields show that groundwater flow can influence shear stresses as well as effective normal stresses. In most parts of the hillslope, groundwater flow significantly increases the Coulomb failure potential Φ, which we define as the ratio of maximum shear stress to mean effective normal stress. Groundwater flow also shifts the locus of greatest failure potential toward the slope toe. However, the effects of groundwater flow on failure potential are less pronounced than might be anticipated on the basis of a simpler, one-dimensional, limit equilibrium analysis. This is a consequence of continuity, compatibility, and boundary constraints on the two-dimensional flow and stress fields, and it points to important differences between our elastic continuum model and limit equilibrium models commonly used to assess slope stability.

Wave-induced advective transport below a rippled water-sediment interface

Abstract: The exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements. In this paper we examine quantitatively the effects of modulations in the profile of the water-sediment interface on this exchange and on the advective transport inside the sediment bed. The flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using Darcy's law. The forcing is due to progressive gravity waves in the water above. The results of two different models for the pressure variation imposed on the upper boundary are compared. The two pressure profiles are derived from potential flow theory and from a numerical solution to the Navier-Stokes equations for the oscillatory flow over a rippled bed. From an analytic solution to the two-dimensional model, the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed. The shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface. The total exchange across the water-sediment interface, averaged over one wave period, is significantly higher across a rippled interface than across a flat bed. This difference increases with increasing ripple slope and the strength of the wave motion, and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave. Since rippled bed forms are commonly found in coastal waters, the increase in the total exchange across a rippled water-sediment boundary can enhance the exchange of solutes due to “wave pumping.” Immediately below the water-sediment interface, circulation cells with net advective transport over a wave period are found. Such net advection patterns can lead to spatial (in the horizontal direction) inhomogeneities of the vertical concentration (or temperature) profiles if the overlying water column and/or the sediment bed act(s) as source or sink. This gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements.

The effects of fluid escape on accretionary wedges 2. Seepage force, slope failure, headless submarine canyons, and vents

Abstract: The high pore pressure gradients inherent to accretionary complexes affect the force balance of the wedge via seepage force, which acts in the direction of flow and is proportional to the pressure (head) gradient. If sufficiently large, this seepage force can offset gravity and friction and lead to failure. At the toe of the wedge sediments are weak, slopes are over-steepened by folding and faulting, and fluid pressure gradients can be high; these conditions are conducive to seepage-induced failure. For the 14–16° slope at the toe of the southern Cascadia wedge, the pore pressure gradient necessary to initiate failure is λ=0.74–0.86. The gradient necessary to cause failure is sensitive to surface slope and sediment strength, but is insensitive to porosity. Reasonable estimates of sediment strength for most accretionary wedges require pore pressure gradients ranging from 10 to 60% of lithostatic to cause failure. These values are within the range of modeled and measured pore pressures in accretionary complexes, suggesting that seepage-induced slope failure should be an expected feature in this environment. If these failure features are observed, then their presence can be used to constrain the pore pressure gradient within the wedge, independent of any assumptions regarding fluid discharge or permeability. If seepage failure repeats and is localized in the same region, then it can lead to channel, gully, and canyon formation. Two convergent margins, southern Cascadia and northern Hispaniola, show many regularly spaced headless canyons that cannot be attributed to downslope erosive flow. We suggest that these canyons are forming from internally driven seepage-induced failure. Both the Oregon and Hispaniola accretionary wedges also contain evidence for non-uniform fluid flow based on the observed and inferred presence of vents. Using Darcy's Law, the pore pressure constraint from the slope failure analysis and an estimate of the total fluid discharge, we examine the relationship between wedge and vent permeabilities, the areal extent of focused fluid venting, and the percent of the total fluid discharge that flows out of vents. Given reasonable estimates of the total fluid discharge out of the southern Cascadia wedge, we find that the wedge must be less permeable than 2 × 10−17 m2 in order for focused fluid venting to occur at all. If the permeability of the vents is much higher than the wedge permeability, then the vents will occur over a very small percentage of the wedge; these vents, however, could accommodate much of the fluid flowing out of the wedge. Using permeability measurements from samples collected at the toe of the Oregon margin [Horath, 1989], we estimate that vents at the toe of the southern Cascadia accretionary complex comprise less than 0.2% of the wedge area, but that these vents can accommodate up to 60% of the total fluid discharge.

Effect of restricted geometries on the structure and thermodynamic properties of ice

Abstract: The thermal properties and structure or water in porous Vycor glass and various silica gels with pore radii in the range 23-70 A were investigated using calorimetry and X-ray diffraction. Samples containing pore water only and those containing pore water in equilibrium with bulk water were studied. The amounts or bound and freezable water in the samples were determined by measuring the heat or melting as a function or pore water content and by application or thermoporometry. The melting point or ice was depressed by 9-20 K, depending on the pore size

Chapter 9 Measurement of Permeability and Storage Capacity in Rocks During Deformation at High Temperature and Pressure

Abstract: Both permeability and storage capacity have been determined in a marble, a limestone, and a sandstone during deformation at high pressure and temperature. The method used utilizes the attenuation and phase retardation of a sinusoidal pore pressure oscillation as it propagates through a sample under test. These parameters were measured at a confining pressure of 300 MPa, argon pore fluid pressures ranging from 20 to 270 MPa, temperatures to 873 K and strains of 0, 10% and 20%. The results for marble indicate that temperature elevated to moderate levels can enhance both the permeability and storage capacity owing to the anisotropy in thermal expansion of the constituent minerals, while the activation of plastic processes at higher temperatures causes the collapse of void spaces and reduction of permeability and storage capacity. It has been observed at all temperatures, however, that when the pore fluid pressure approaches the confining pressure, both measured parameters increase rapidly, and also that ongoing deformation can, when pore pressure remains relatively high, have a profound effect on the transport and storage properties of polycrystalline materials, pushing the measured permeability to up to two orders of magnitude above the level of virgin rock. A model analysis is presented from which information regarding the in-situ pore structure under the experimental conditions can be deduced (permeability, storage capacity, pore structure, crack aspect ratio).

Diminished pore pressure in low‐porosity crystalline rock under tensional failure: Apparent strengthening by dilatancy

Abstract: Rupture tests on internally pressurized, thin-walled hollow cylinders of Westerly granite with impermeable inner membranes suggest that the conventional, or Terzaghi, effective stress law does not describe tensile failure at high internal pressurization rates near 6 MPa/s. Unjacketed and saturated samples, with an initial pore pressure and for which the inner cavity pressure was increased rapidly with respect to the diffusivity, display substantially increased apparent tensile strengths and deformational moduli much higher than similarly configured but more slowly pressurized tests. Alternatively, the properties of completely dry test pieces with no pore pressure show little, if any, dependence on pressurization rate. Further, the behavior of the rapid unjacketed tests was similar to that for completely dry samples. These observations cannot be explained by the predicted undrained response, but they provide indirect evidence for diminished pore pressure effects reminiscent of dilatant hardening observed in compressive failure experiments. Calculated pore pressure diffusion rates support this suggestion as pore pressure perturbations cannot be damped out on the time scale of the rapidly pressurized tests. It is not clear if these effects are produced by elastic microcrack dilatancy, of which the nonlinear stress-strain curve of granites is symptomatic, or the irreversible production of new porosity as in compressive shear failure tests.

Forward modelling of porosity and pore pressure evolution in sedimentary basins

Abstract: The gravitational compaction of sediments is an important process in forward basin modelling. This paper presents a mathematical model for the one-dimensional compaction of an accreting layer of argillaceous sediments. Realistic constitutive laws for the clay compressibility and the clay permeability, based on soil mechanics tests, were incorporated into the model. The governing equations were put in dimensionless form and the extent of abnormal pore fluid pressure development was found to depend on the sedimentation parameter, a dimensionless group representing the ratio of the sediment hydraulic conductivity to the sediment accumulation rate. The effects of clay compressibility were studied and highly colloidal clays such as montmorillonite developed higher overpressures than less compressible materials. The results also showed that overpressuring developed in shales for cases in which the clay permeability did not go to zero in the limit of zero porosity. Linear models based on simplifying assumptions inappropriate for sedimentary basins were found to give significantly different estimates for the conditions leading to overpressuring. Using reasonable ,parameters, the model adequately reproduced porosity and pore pressure profiles measured in the sandshale sequences of the South Caspian Sea.

Geochemical cycling of heavy metals in a marine coastal area: benthic flux determination from pore water profiles and in situ measurements using benthic chambers

Abstract: Concentrations of the heavy metals Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn were measured in sea water, suspended matter, sediments and pore water samples collected in a coastal area of the middle Tyrrhenian Sea. Concentration factors between pore water (extracted from the first centimeter of the sediments) and the overlying sea water (taken 30 cm above the sea bed) were less than 1 for Cr, Cu and Pb, 1–10 for Cd and Ni, 10–100 for Fe and Co, 100–1000 for Mn, and 1–100 for Zn.

Characterization of saturated granular bases under repeated loads

Abstract: The behavior of typical granular materials with different gradation was investigated under saturated, undrained, repeated triaxial loading conditions. Of particular interest is the comparative behavior of open-graded and dense-graded base courses and the influence of fines content on the dynamic response. The effect of aggregate gradation on the cyclic stress-strain behavior, pore pressure, damping, resilient modulus, compressibility, and permeability is investigated. Results indicate that saturated granular materials will develop excess pore water pressure under undrained repeated triaxial loading. This could lead to a decrease in resilient modulus and a potential increase in volume compressibility. Open-graded aggregates are more resistant to pore pressure buildup than dense-graded aggregates and are therefore less likely to induce damage in pavements under saturated conditions. In this respect, the estimated damage per repetition could be as much as 70 to 100 times more for pavements with dense-graded bases.

Mobility and Degradation of Organic Contaminants in Subsurface Environments

Abstract: . Introduction. Contaminant Vapors as a Component of Soil Gas in the Unsaturated Zone. Liquid Contaminants Adhering to "Water-Dry" Soil Particles in the Unsaturated Zone. COntaminants Dissolved in the Water Film Surrounding Soil Particles in the Unsaturated Zone. Contaminants Sorbed to "Water-Wet" Soil Particles or Rock Surface (After Migrating Through the Water) in Either the Unsaturated or Saturated Zone. Liquid Contaminants in the Pore Spaces between Soil Particles in the Saturated Zone. Liquid Contaminants in the Pore Spaces between Soil Particles in the Unsaturated Zone. Liquid Contaminants Floating upon the Water Tables. Contaminants Dissolved in Groundwater. Contaminants Sorbed onto Colloidal Particles in Water in Either the Unsaturated or Saturated Zone. Contaminants That Have Diffused Into Mineral Grains or Rocks in Either the Unsaturated or Saturated Zone. Contaminants Sorbed onto or into Soil Microbiota in Either the Saturated or Unsaturated Zone. Contaminants Dissolved in the Mobile Pore Water of the Unsaturated Zone. Liquid Contaminants in Fractured Rock or Karstic Limestone in Either the Unsaturated or Saturated Zone. Glossary.

a Theoretical Treatment of the Effect of Microscopic Fluid Distribution on the Dielectric Properties of Partially Saturated ROCKS1

Abstract: Microscopic fluid distribution can have a significant effect on the dielectric properties of partially saturated rocks. Evidence of this effect is found in the laboratory data presented by Knight and Nur in which different methods for controlling saturation produced very different results for the dependence of the dielectric response on water saturation. In this study, previously derived models for the dielectric response of a heterogeneous medium are generalized and the case of a pore space occupied by multiple pore fluids is considered. By using various geometrical distributions of water and gas, it is observed that both the pore geometry in which saturation conditions are changing and the gas–water geometry within a given pore space are critical factors in determining the effective dielectric response of a partially saturated rock. As an example, data for a tight gas sandstone undergoing a cycle of imbibition and drying are analysed. Previous research has demonstrated that significantly different microscopic fluid distributions result from the application of these two techniques to control the level of water saturation. By approximating these microscopic fluid distributions using simple geometrical models, good agreement is found between experimental data and calculated dielectric properties.

Nutrient regeneration processes in bottom sediments in a Po delta lagoon (Italy) and the role of bioturbation in determining the fluxes at the sediment-water interface

Abstract: A study on nutrient regeneration processes and a measure of their fluxes at the sediment-water interface was carried out in two different stations of a shallow lagoon of the Po delta river (Italy). A few parameters on the solid fraction (grain-size, porosity, C, N) and pore water profiles of o-P, NH3, NO inf3 sup− , SiO2, Tot-CO2, SO inf4 sup2− , Fe, Mn, Ca, Mg, pH, Eh were determined. At both stations the results were typical for fine sediments rich in organic matter. The ratio of variations of sulphate (ΔSO inf4 sup2− ) to total carbonate demonstrates the main role sulphate reduction plays on the organic matter decay. The use of the ratios of variations of sulphate (ΔSO inf4 sup2− ) to ammonia (ΔNH3) and of sulphate (ΔSO inf4 sup2− ) to phosphate (Δo-P) in pore waters enabled us to calculate the C/N/P of the decomposing organic matter. Obtained C/N/P indicated an enrichment of N and P with regard to C/N/P ratios of the solid fraction, due to the selective stripping of N and P during organic matter mineralization. This phenomenon decreases with depth, where organic matter becomes more refractory. Calculations on saturation degrees have shown the possibility of authigenic calcite, apatite and rhodochrosite precipitation in sediments. Nutrient fluxes were estimated for SiO2, NH3 and o-P by means of benthic chambers and modelling the pore water profiles. The model used for the calculation of fluxes allowed us to account for the bioturbation-irrigation influence near the interface, by means of a cumulative diffusion coefficient. Directly measured fluxes proved to be always significantly greater than the theoretical ones. These differences seem to be due to surface processes which do not affect pore water concentrations (degradation of fresh materials at the interface; micro-bioturbation by small gasteropoda such as Hydrobia ventrosa) and/or to the different concept of the two methods in time and space. Number, size and biomass of macrobenthic species living in the sediment underneath the benthic chambers were determined. The comparison between data on macrobenthic populations and flux values showed a good relationship between the number of organisms and benthic fluxes within each station. However, flux variations between stations are to be attributed mainly to the different arrangement of the tubes of the polychaetes Polydora ciliata in the sediment.

Measurements of matric suction and volume changes during inundation of collapsible soil

Abstract: Reduction in soil volume due to inundation under a constant total stress is a phenomenon referred to as collapse. Collapse is exhibited by soils during a change of state from an unsaturated to a saturated condition. Several researchers have postulated various theories to explain collapse behavior. Recent published research has attempted to explain the collapse phenomenon using theories of unsaturated soil mechanics. However, the theoretical explanations require further verification by experimental data. A simple experimental procedure is suggested in this paper to measure changes in matric suction and soil volume during inundation. The suggested measurements provide experimental data that can be used to verify the applicability of the unsaturated soil mechanics theories to collapsible soil behavior. Typical test results from this experimental program indicate that collapsible soil behavior can be explained using unsaturated soil mechanics theories.