Solid–liquid–air coupling in multiphase porous media

This paper addresses various issues concerning the modelling of solid–liquid–air coupling in multiphase porous media with an application to unsaturated soils. General considerations based on thermodynamics permit the derivation and discussion of the general form of field equations; two cases are considered: a three phase porous material with solid, liquid and gas, and a two phase porous material with solid, liquid and empty space. Emphasis is placed on the presentation of differences in the formulation and on the role of the gas phase. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The two formulations are then analysed with respect to a documented drainage experiment carried out by the authors. The merits and shortcomings of the two approaches are shown. Copyright © 2003 John Wiley & Sons, Ltd.     

Rift-zone development around a plume-related magma centre on the Isle of Skye (Scotland): a model for stress inversions

ABSTRACT Using the example of the Isle of Skye in Scotland, we investigate the influence of pressure variations in upper-crustal magma reservoirs on the development of rift-type normal faulting around central volcanoes. The regional synmagmatic stress regime is of strike-slip type in Scotland during the Lower Tertiary. During a prolonged period of overall high pressure in the Skye magma reservoir (gabbro intrusion stage), crustal extension results from the injection of basaltic dykes parallel to the trend of the far-field maximum stress. During a subsequent period of pressure decrease in the reservoir (granites intrusion stage) normal faults trending parallel to the dykes are initiated. These faults tilt the upper-crustal blocks along with the former dyke swarm and associated lava pile. Finite-element modelling shows that a decrease of magma pressure in a circular cavity may lead, as in Skye, to a change from a regional strike-slip to a local rift-type normal stress regime.     

Backscatter calibration of high-frequency multibeam echosounder using a reference single-beam system,on natural seafloor

The calibration of multibeam echosounders for backscatter measurements can be conducted efficiently and accurately using data from surveys over a reference natural area, implying appropriate measurements of the local absolute values of backscatter. Such a shallow area (20-m mean depth) has been defined and qualified in the Bay of Brest (France), and chosen as a reference area for multibeam systems operating at 200 and 300 kHz. The absolute reflectivity over the area was measured using a calibrated single-beam fishery echosounder (Simrad EK60) tilted at incidence angles varying between 0° and 60° with a step of 3°. This reference backscatter level is then compared to the average backscatter values obtained by a multibeam echosounder (here a Kongsberg EM 2040-D) at a close frequency and measured as a function of angle; the difference gives the angular bias applicable to the multibeam system for recorded level calibration. The method is validated by checking the single- and multibeam data obtained on other areas with sediment types different from the reference area.     

Radionuclides as tracers of water fronts in the South Indian Ocean—ANTARES IV Results

Anthropogenic ⁹⁰Sr, ^239,240Pu and ²⁴¹Am were used as tracers of water mass circulation in the Crozet Basin of the South Indian Ocean, represented by three main water fronts—Agulhas (AF), Subtropical (STF) and Subantarctic (SAF). Higher ⁹⁰Sr concentrations observed north of 43°S were due to the influence of AF and STF, which are associated with the south branch of the Subtropical gyre, which acts as a reservoir of radionuclides transported from the North to the South Indian Ocean. On the other hand, the region south of 43°S has been influenced by SAF, bringing to the Crozet Basin Antarctic waters with lower radionuclide concentrations. The ²³⁸Pu/^239,240Pu activity ratios observed in water and zooplankton samples indicated that, even 35 years after the injection of ²³⁸Pu to the Indian Ocean from the burn-up of the SNAP-9A satellite, the increased levels of ²³⁸Pu in surface water and zooplankton are still well visible. The radionuclide concentrations in seawater and their availability to zooplankton are responsible for the observed ²¹⁰Po, ^239,240Pu and ²⁴¹Am levels in zooplankton.     

Huge landslide blocks in the growth of piton de la fournaise, La réunion, and Kilauea volcano, Hawaii

Piton de la Fournaise, on the island of La Réunion, and Kilauea volcano, on the island of Hawaii, are active, basaltic shield volcanoes growing on the flanks of much larger shield volcanoes in intraplate tectonic environments. Past studies have shown that the average rate of magma production and the chemistry of lavas are quite similar for both volcanoes. We propose a structural similarity — specifically, that periodic displacement of parts of the shields as huge landslide blocks is a common mode of growth. In each instance, the unstable blocks are within a rift-zone-bounded, unbuttressed flank of the shield. At Kilauea, well-documented landslide blocks form relatively surficial parts of a much larger rift-zone-bounded block; scarps of the Hilina fault system mark the headwalls of the active blocks. At Fournaise, Hilina-like slump blocks are also present along the unbuttressed east coast of the volcano. In addition, however, the existence of a set of faults nested around the present caldera and northeast and southeast rift zones suggests that past chapters in the history of Fournaise included the slumping of entire rift-zone-bounded blocks themselves. These nested faults become younger to the east southeast and apparently record one of the effects of a migration of the focus of volcanism in that direction. Repeated dilation along the present set of northeast and southeast rift zones, most recently exemplified by an eruption in 1977, suggests that the past history of rift-zone-bounded slumping will eventually be repeated. The record provided by the succession of slump blocks on Fournaise is apparently at a relatively detailed part of a migration of magmatic focus that has advanced at least 30 km to the east-southeast from neighboring Piton des Neiges, an extinct Pliocene to Pleistocene volcano.     

A novel experimental procedure to investigate the biodegradation of NAPL under unsaturated conditions

Soils need to be thoroughly investigated regarding their potential for the natural attenuation of non-aqueous phase liquids (NAPL). Laboratory investigations truly representative of degradation processes in field conditions are difficult to implement for porous media partially saturated with water, NAPL and air. We propose an innovative protocol to investigate degradation processes under steady-state vadose zone conditions. Experiments are carried out in glass columns filled with a sand and, as bacteria source, a soil from a diesel-fuel-polluted site. Water and NAPL (n-hexadecane diluted in heptamethylnonane (HMN)) are added to the porous medium in a two-step procedure using ceramic membranes placed at the bottom of the column. This procedure results, for appropriate experimental conditions, in a uniform distribution of the two fluids (water and NAPL) throughout the column. In a biodegradation experiment non-biodegradable HMN is used to provide NAPL mass, while keeping biodegradable n-hexadecane small enough to monitor its rapid degradation. Biodegradation is followed as a function of time by measuring oxygen consumption, using a respirometer. Degradative activity is controlled by diffusive transfers in the porous network, of oxygen from the gas phase to the water phase and of n-hexadecane from the NAPL phase to the water phase.     

Assessing Groundwater Depletion and Dynamics Using GRACE and InSAR: Potential and Limitations

In the last decade, remote sensing of the temporal variation of ground level and gravity has improved our understanding of groundwater dynamics and storage. Mass changes are measured by GRACE (Gravity Recovery and Climate Experiment) satellites, whereas ground deformation is measured by processing synthetic aperture radar satellites data using the InSAR (Interferometry of Synthetic Aperture Radar) techniques. Both methods are complementary and offer different sensitivities to aquifer system processes. GRACE is sensitive to mass changes over large spatial scales (more than 100,000 km²). As such, it fails in providing groundwater storage change estimates at local or regional scales relevant to most aquifer systems, and at which most groundwater management schemes are applied. However, InSAR measures ground displacement due to aquifer response to fluid‐pressure changes. InSAR applications to groundwater depletion assessments are limited to aquifer systems susceptible to measurable deformation. Furthermore, the inversion of InSAR‐derived displacement maps into volume of depleted groundwater storage (both reversible and largely irreversible) is confounded by vertical and horizontal variability of sediment compressibility. During the last decade, both techniques have shown increasing interest in the scientific community to complement available in situ observations where they are insufficient. In this review, we present the theoretical and conceptual bases of each method, and present idealized scenarios to highlight the potential benefits and challenges of combining these techniques to remotely assess groundwater storage changes and other aspects of the dynamics of aquifer systems.