Electrical properties in the Earth's interior are obtained through geoelectromagnetic observations, and are used to infer constituent materials and temperature. Because electrical properties are very sensitive to the existence of liquids, they are used to detect water and melt in the crust and the mantle. Interpreting observed data requires a good understanding of the electrical properties of crust and mantle materials. In this paper, I review the state of our understanding of rock electrical properties. First, I relate macroscopic electrical conductivity and permittivity to microscopic material parameters. Our understanding of the electrical properties of rock-forming minerals and liquids is reviewed from the viewpoint of microscopic structures, and then the properties of rocks are discussed through the effective medium theory. Finally, I point out problems that should be tackled to get a better understanding of the electrical properties of rocks. They are (1) the physics of the electrical conduction in crustal rocks, (2) the distribution of conductive minerals in rocks, (3) the fluid distribution in rocks, (4) the properties of thin fluid film, and (5) the nature of interfacial polarization of fluid-bearing rocks.
To estimate crustal rheology, and to synthesize tectonic processes such as seismic and volcanic activities beneath the island arc, crustal water volume fraction and its connectivity is one of the most important parameters. Due to significant improvements in instruments, data-processing methods and inversion schemes, we are now coming to the stage where we can obtain detailed crustal electrical conductivity structures, and can discuss the relationship between structures and crustal activities. On the other hand, dependence of effective electrical conductivity versus temperature and water volume fraction can be estimated by referring to electrical conductivity-temperature dependence for dry rocks and water from laboratory experiments, and by assuming a connection rule with interstitial water. Then, if we can estimate the temperature structure successfully, the spatial distribution of crustal water volume fraction can be estimated for a specific connection rule directly from the electrical conductivity cross-section. In this paper, as a case study, we try to estimate the spatial distribution of water volume fraction from a 2-D electrical conductivity structure beneath an active back-arc area in Tohoku district, NE Japan (Ogawa, et al., 2001), and compare the results with seismic structure (Matsubara et al., 2004). Finally, we want to emphasize that a joint analysis using both electric and seismic information will reveal more detailed features of the interstitial water, and enhance the persuasiveness of the estimation.
Partially molten regions have played a key role in material differentiation in the Earth through melt segregation. Permeability is an important parameter in melt segregation because it controls the velocity of melt migration. Electrical conductivity is a useful observable parameter for estimating melt distribution in the Earth. These transport properties are controlled by topology and geometry of liquid phases. They are controlled by interfacial energy, grain-size distribution, and mineral assemblages. In this paper, we review studies on structure, permeability, and electrical conductivity of partially molten systems. First, we summarize structure and transport properties of a simple partially molten system where the liquid phase is distributed uniformly on the grain scale. Second, studies on structure and transport properties of real systems are examined. Finally, the permeability-electrical conductivity relationship of the partially molten system is described. We emphasize that the relation between transport properties and topology and geometry of the liquid phase must be clarified.
Characterizing the transport properties of reservoir-forming rocks is one of the most important tasks in reservoir engineering. We review the relationships among permeability, porosity, electrical formation factor, and electrokinetic coupling coefficient under saturated and unsaturated conditions on the basis of the capillary tube model of porous medium, by which one can relate the microscopic physics of the transport properties to the macroscopic behaviors described by Darcy's and Ohm's laws and the cross-coupling effects. These relationships together with the recent models of clay rich sandstones provide a useful guideline for interpreting core, logging, and geophysical survey data. Among various rock properties, permeability in particular needs in situ measurements such as pressure transient tests, because in situi values are usually at least a few orders of magnitude larger than those measured for intact core samples due to the presence of discontinuities such as fractures in reservoirs. Concerning this topic, the concept of fractured rocks, i.e., the double porosity medium and how to characterize fractured reservoirs are described. Even if the results of extensive field-wide pressure transient tests are available, in addition to drilling and various exploration data, numerical models of reservoirs are never precise, due to the problem of non-uniqueness. However, once exploitation begins in earnest, additional data become available such as temporal trends in downhole flowing pressure and enthalpy (in case of geothermal reservoirs), which may be used in history-matching studies. Because uncertainty in predictions of numerical reservoir models is directly related to the amount of field data available against which the models can be tested, it is clear that the addition of repeat geophysical survey data to the list of pertinent field measurements is likely to improve the reliability of these forecasts. Recently developed computational tools such as the EKP-postprocessor, which can calculate changes in self-potential distribution through electrokinetic coupling caused by changing underground conditions computed by reservoir simulation, enable us to use geophysical monitoring data in history-matching studies.
This article reviews the mechanical properties of solid-liquid composites with special emphasis on acoustic properties. A simple method proposed by Takei (2002) to estimate porosity, pore shape, and/or liquid compressibility from data on elastic wave velocities (Vp and Vs) is summarized. Using this method, a variety of pore shapes are treated systematically in terms of the equivalent aspect ratio of an oblate spheroid model. An important application of this method is in the detection of terrestrial fluids from seismic tomographic images. Information on pore shape obtained from tomographic images is shown to provide valuable information on the migration of aqueous fluids and melts in the Earth's interior. A brief summary is also presented on dispersion and attenuation mechanisms in solid-liquid composite media to clarify the assumptions and the limitations of the present method.
On the basis of the viewpoint that rocks are typical random heterogeneous media consisting of different minerals and micro-cracks, modeling techniques for predicting seismic wave velocities in crystalline rocks are reviewed. The term seismic velocity is sometimes unclear in its physical meaning in real media that contain heterogeneity and anisotropy. The physical meaning of seismic velocity is first discussed on the basis of wave fields in heterogeneous media. Elastic constants and phase velocity in an anisotropic medium are then introduced. Further, phase velocity and group velocity in transverse isotropy are discussed because transverse isotropy is considered to be a realistic case when we consider seismic wave propagation in the Earth's crust. In most cases in actual field observations, rocks are considered to be homogeneous materials because the wavelength of seismic velocity is always longer than the scale length of heterogeneity in rock. It is thus important to understand techniques used to model the macroscopic elastic properties of rocks, which are microscopically heterogeneous. Finally, modeling techniques are shown for anisotropic rocks having lattice-preferred orientations and micro-cracks. These techniques are useful for interpreting field seismic observations, as well as for interpreting experimental results in the laboratory.
Velocity anisotropy of biotite schist and amphibolite from the Hidaka metamorphic belt was measured under confining pressures of up to 150 MPa, where most of thin cracks are closed and intrinsic anisotropy of rocks appears. In both rocks, intrinsic velocity anisotropy is basically interpreted as the result of the lattice preferred orientation of horblende and biotite. The anisotropy of biotite schist can be interpreted well by TI anisotropy with a slight modification of orthorhombic symmetry caused by crenulation. However, amphibolite showed a confusing anisotropy which cannot be explained well by either TI or orthorhombic symmetry. Employing a model consisting of a TI matrix having oriented oblate-spheroidal cracks with short axes parallel to the symmetry axis of the matrix, we interpret the velocity changes in biotite schist as a function of confining pressure. The model explains changes of velocity anisotropy in biotite schist fairly well.
We review the methods of measuring the velocities of elastic-waves in rocks and summarize the temperature-dependence of elastic-wave velocities under high-temperature and high-pressure conditions. The elastic-wave velocities in rocks are strongly affected by several phenomena such as thermal cracking, phase transition of minerals, partial melting of rocks, and dehydration of hydrous minerals. These phenomena are strongly affected by pressure-temperature conditions and chemical compositions of rocks and minerals. Thus, it is very difficult to predict the elastic wave velocities of rocks and minerals under high-pressure and high-temperature conditions theoretically. Laboratory measurements of the velocities of elastic-waves in rocks under high-pressure and high-temperature conditions have provided useful data for estimating physical and geological properties in the crust and upper mantle. We also mention the next issues to be studied in relation to the velocity of elastic waves in rocks. It is important to measure elastic-wave velocities in rocks under high-temperature and high-pressure conditions in the presence of pore-fluids.
Interpretation of a seismic reflection profile across a metamorphic belt based on acoustic impedance measurements of exhumed metamorphic rocks is illustrated with an example of the Hidaka metamorphic belt, where a part of the cross section through the ancient Kuril-arc crust (Hidaka crust) is exposed. A combined seismic reflection profile across the Hidaka metamorphic belt reveals the strongly reflective and laminated lower Hidaka crust as well as the relatively transparent upper Hidaka crust. The exposed Hidaka crust, i.e., Hidaka metamorphic Main Zone, is composed of amphibolite- to greenschist-facies felsic rocks in its upper main part, while mainly of granulite-facies amphibolite frequently intercalated with felsic rocks in its basal part. Acoustic impedance values of representative rock samples of the exposed Hidaka crust show an overall similarity in the upper main part, but significant fluctuations in the basal part due to interlayering of amphibolite and felsic rocks. The seismically transparent upper Hidaka crust is therefore attributable to the dominance of felsic rocks, while the strongly reflective and laminated lower Hidaka crust is likely due to interlayering of amphibolite and felsic rocks, which is verified by 1D seismic reflection modeling based on the acoustic impedance structures of the exposed Hidaka crust.
The ultimate goal of rock physics is to gain insights into the physical properties of a reservoir. A rock physics study makes use of measured elastic properties from seismic data to generate attributes that yield information about reservoir rocks. For example, if we measure the velocity of a seismic wave propagating through an aquifer or an oil reservoir, rock physics provides the theory to convert the measured physical value into information about the fluids present in the rock. This paper describes an application of rock physics study to the seismic monitoring of injected CO2 in geological sequestration. Laboratory experiments on porous sandstones show that the P-wave velocity reduction due to injected CO2 is typically on the order of-10%. The results of the seismic tomography show that the CO2 migration pattern is consistent with the pore space distribution within porous sandstone. Such results support the applicability of a seismic survey to CO2 monitoring in geological sequestration. Measurements on a core recovered from the Nagaoka pilot site show that the P-wave velocity reduction agreed well with sonic logging results. History matching with velocity changes of a sonic P-wave caused by a CO2 breakthrough at observation well OB-2, was successfully done based on Gassmann theory and the rock physics model in this study.
Methods for measuring contemporary stress fields are reviewed with special attention given to problems with the existing hydraulic fracturing method. Regarding reopening pressure, we have no unique solutions for questions about the water pressure inside artificial fractures just before reopening. Furthermore, based on numerical simulations, several authors suggest that reopening pressure can be equal to shut-in pressure. Observation results also show that reopening pressure is almost equal to shut-in pressure at any depth throughout most of the world. If the latter suggestion is always true, the hydraulic fracturing method cannot determine the magnitude of maximum horizontal stress. Due to longstanding disputes, many different methods have been introduced over the past twenty years. Methods for measuring contemporary stress fields are based on basic knowledge of the physical properties of rocks and rock mechanics. To get answers to these questions, it is important to re-consider the basic principles of measurement methods.
Three-dimensional measurements of the complex shapes of pores in porous rocks are essential for quantitative discussions of material transport in strata. X-ray computed tomography (CT) visualizes the three-dimensional distribution of X-ray linear absorption coefficients of rock samples, and is a useful non-destructive technique for measuring pore shape. Some examples of the application of X-ray CT to rock pore imaging are shown to facilitate petrophysical CT studies in Japan. The 3-dimensional pore images were obtained for sandy sediment, rhyolitic/andesitic lavas, and sandstone. Pore connectivity analysis and tortuosity estimate were demonstrated using sandstone image data.