The theory of poroelasticity-one of the theories addressing deformation-fluid flow coupling processes-which can be used to explain processes occurring in the geosphere, is described. Both undrained and drained conditions are explained in some detail to make it easier to understand both coupled and uncoupled processes under this theory. Then, the following three examples of applications of this theory to earth sciences are presented ; 1. redistribution of strains due to an earthquake and temporal changes of displacement and pore fluid pressure ; 2. pore pressure fluctuations due to cyclic loads such as atmospheric pressure and earth tides ; and, 3. the effects of pore pressure change on in-situ stress in a sedimentary basin. Finally, extending this theory to anisotropic materials is discussed, and the importance of anisotropy under low-effective stress conditions is emphasized.
Poroelastic properties, including hydraulic properties, govern fluid flow and deformation of geological formations. In this paper, we discuss the applicability of methods for estimating hydraulic properties from pore pressure response to cyclic loading such as atmospheric pressure, earth tide, ocean tide, and seawater wave in terms of loading frequencies, measurement depths, and physical properties of materials. We also show 2 case studies in which we estimate hydraulic diffusivity using pore pressure fluctuations to atmospheric loading at about 100 m in depth, and those to seawater wave loading at about 1 m below the sea floor.
Modeling the interaction of geomaterials with pore fluids is one of the key issues in numerical geomechanics. In 2004, many disasters caused by heavy rainfall and flooding occurred in various areas throughout Japan. These rain-related disasters were probably accompanied by geodisasters, e.g., flows of debris and slope failures in mountain areas and river dike breaks in plain areas. Although the disasters are of course very unfortunate, it is anticipated that data from them will contribute to geomechnics in terms of future geo-disaster prevention. Because most of these geo-disasters are related to the behavior of pore fluids in geomaterials, it is important to construct and use an appropriate model of the interaction of geomaterials with pore fluids. In the present paper, a newly developed evaluation method for the safety of river dike embankments during flooding is introduced first. This method consists of a deformation analysis that can simultaneously consider stability and unsaturated seepage flow. It is quite different, therefore, from the conventional evaluation method in which stability and seepage flow are considered separately. A soil skeleton-pore water coupled elasto-plastic finite element analysis is applied to the problem by incorporating the unsaturated seepage characteristics, and by assuming the pore air pressure in the unsaturated soil region to be atmospheric pressure. The deformation and the stability of a river dike embankment during a flood are investigated for various cases of initial saturation and permeability. The results of the analysis show that the existing evaluation criterion for the seepage failure of river dike embankments is not always on the safe side. Secondly, the problems to be solved are discussed. A multi-phase coupled analysis, which considers the interaction of geomaterials not only with pore water, but also with pore air, is shown. The present analysis has been applied to simulate the experimental results of triaxial tests on unsaturated silt under unexhausted and undrained conditions. This method is expected to be a useful numerical tool for predicting deformation and stability of unsaturated river dike embankments during floods in practical fields. However, more complex problems exist which are difficult to solve with the present numerical analysis method, which is based on continuum mechanics. Some examples of these unsolved problems are also introduced, namely, river dike breaks due to flooding and the progressive seepage failure of sandy deposits that include discrete air bubbles.
Dilatancy and pore fluid pressure changes in a fault zone significantly affect the mode of the nucleation process, and are key factors for understanding the physical mechanisms of some precursory phenomena. We investigate slip processes with a model of a vertical fluid-infiltrated strike-slip fault in a 3-D elastic half space using a Dietrich/Ruina rate-and state-dependent friction law and an evolution equation for plastic porosity. Due to dilatancy, porosity is thought to increase with slip velocity. In this model plastic porosity is assumed to depend logarithmically on the state variable in the friction law. The results of numerical simulations show that porosity in the fault zone increases with increasing slip velocity in the nucleation zone. Then, pore fluid pressure in the fault zone drops significantly, resulting in dilatant hardening. With an increasing dilatancy coefficient, a larger pore fluid pressure drop is observed during the nucleation process, which in turn increases the size of the nucleation zone. In the case of considerably larger dilatancy coefficients, the fault system becomes stable. The various modes of slip processes are reproduced by the spatial variation of dilatancy coefficients. We examine changes in pore-fluid pressure on the fault near the surface. When a slow event occurs in a deeper region of a seismogenic zone in the later stage of an earthquake cycle, it is observed that slip velocity on the fault near the surface increases significantly. Slip velocity also increases slightly around one year before the main rupture begins. These changes in slip velocity lead to a significant drop in the pore-fluid pressure on the fault near the surface.
It has long been recognized that unconsolidated or partially consolidated sediments can be intensely deformed during burial, either from gravity-driven subhorizontal extension, or from tectonic shortening. Such deformation can be evaluated in the context of experimental and empirical results from engineering geology literature. We try to summarize this knowledge using several submarine and on-land examples that illustrate the concepts of pore-fluid pressure, shaking, permeability, and consolidation, and are applicable to the interpretation of geologic structures that develop in unconsolidated or partially consolidated sediments. We first review dish, web, and vein structures, scaly clay and liquefaction features, which are typical of unconsolidated or partially consolidated sediments along convergent plate margins. Then, some experimental aspects of permeability test results are applied to fluid flow phenomena in and around accretionary prisms to characterize the role and the importance of fluid flow parallel to the σ2 direction. The results are further applied to explain the concentration of hot springs of a non-volcanic origin along active, strike-slip faults, as well as how seepage along convergent margins occurs in an en echelon pattern on conjugate sets of strike slip faults. Additional experimental work is suggested to develop a better understanding of these geologic structures.
Fluid behavior during the evolution of the plate boundary fault (pbf) from a trench to seismogenic depths is the central problem when evaluating the relationship between fluids and seismicity in subduction zones. Ocean Drilling Program Legs 190 and 196 at the toe region of the Nankai accretionary margin reveal that fluid-filled dilatant fractures and underconsolidated underthrust sediments lead to an elevated fluid pressure in and below the pbf, respectively. The pbf with elevated fluid pressure extends down-dip to 35 km, resulting in the absence of seismic behavior at shallow depths and mechanical decoupling between accreted and underthrust sediments. Underconsolidated underthrust sediments are primarily caused by rapid tectonic loading compared to the rate of fluid escape in underthrust sediments and secondarily by a lowpermeability cap due to the compactively deformed pbf. Fluid-filled dilatant fractures represent the overconsolidate state within the pbf, which is caused by the generation of high fluid pressure after compactive deformations. The exhumed plate boundary rocks (i.e., tectonic melange) in the Shimanto accretionary complex indicate that the underthrust sediments became rocks due to dewatering, pressure solution, and other diagenetic reactions, thus acquiring elastic strength. The pbf in the upper part of the seismogenic depths was weak due to elevated fluid pressure, this facilitated the downward step of the pbf and the underplating of underthrust rocks. The pbf under low effective stress was unlikely to nucleate the instability; however the fluid-related repeated deformations, which probably reflect the seismic cycle in the subduction zone, could be recorded. The coseismic deformations were attributed to hydraulic implosion breccias, injection of ultracataclasite, and fluid inclusion stretching in the pbf. Implosion breccias suggest rapid depressurization associated with the passage of the rupture through dilational jog. Other deformations represent shear heating and fluidization along the narrow ultracataclasite layer, which could enhance the propagation of instability at the pbf in the upper parts of the seismogenic depths.
Experimental monitoring of fluid pressures was initiated in June 2001 at 2 underwaterholes drilled during the Ocean Drilling Program (ODP) Leg 196 to investigate the relationship between deformation and fluid flow processes in the Nankai accretionary prism. ODP Leg 196 visited Sites 1173 (drilled on Leg 190) and 808 (drilled through the frontal thrust on Leg 131), installing 2 Advanced Circulation Obviation Retrofit Kits (ACORKs) to monitor fluid pressures along the walls of the drilled holes. Site 808 penetrates the frontal thrust fault and the déconement, while Site 1173 is located about 11 km seaward from the Nankai Trough deformation front. Formation water freshening around the decollement was first observed on ODP Leg 131, and geochemists have been investigating whether or not the freshening was due to the production of deep-sourced dehydration processes. We now know that the role of smectite dehydration and dehydrant quartz-cristobalite phase transition should be estimated quantitatively. One of the important findings of fluid pressure monitoring is that the formation fluid pressures seem to reflect the change in the stress state in and around the accretionary prism. We believe that such fluid pressure monitoring in the accretionary prism and in the sediments on top of the subducting oceanic plate in terms of stress field and dehydrant process of minerals is a key to deepening our knowledge in future investigations of seismogenic processes.
This paper discusses characterizing and validating potential sites for high level radioactive waste disposal along the coastal areas using newly developed geophysical prospecting tools. The specific objective of this paper is to develop an electromagnetic (EM) technology for investigating the subsurface to the depths of 1, 000 m below the seafloor in the near-shore environment. The depth to the sea floor is up to 200 m. Characterization and validation of a site for high level radioactive waste disposal require a detailed knowledge of both the geological structures and the groundwater characteristics. The commercial as well as research-based marine controlled-source and natural-source EM technologies are available, however, there has been little work done in shallow water depths environment using such technologies. Here, we report the state-of-the-art EM method with applications to coastal areas with shallow water depths environment. Marine EM instrument developed is appropriate for investigating shallow water environment near coastal area. The interpretational technology for EM data is focused on 3-D magnetotelluric (MT) and 2.5-D controlled-source electromagnetics (CSEM). This paper demonstrates the performance of the new type of instrument and software, and the field experiment that was carried out in the Monterey Bay of California, USA, in 2003 and 2004. Additionally, we demonstrate interpretation techniques applied to characterizing groundwater using MT3-D result of Horonobe, Hokkaido.
Estimating saline water intrusion into aquifers in coastal plains has been becoming an important subject in terms of site characterization for the geological disposal of radioactive waste. In addition, delineating the distribution of high-salinity groundwater zones is valuable when using groundwater for municipal, industrial, and agricultural purposes. Conventionally, the electrical conductivity of groundwater is measured in hydrogeological surveys to estimate salinity concentration. However, if there are no boreholes available, electrical and electromagnetic exploration methods are employed. In such cases, a qualitative hydrogeological interpretation of estimated electrical resistivity or conductivity distribution is usually made. To simulate the future transport of salinity, a quantitative estimation of the current salinity distribution is essential. In this paper, first I review electromagnetic methods and electrical conductivity of formation and pore water as a function of salinity concentration. Secondly, I introduce a case study of an electromagnetic investigation in the Kujukuri coastal plain, eastern Japan and evaluate the equivalent NaCl concentration of pore water from formation resistivity values obtained from the electromagnetic investigation.