Westerly Granite Samples Research Articles

Acoustic, Electromagnetic, and Photon Emission from Dynamically Fracturing Granite

Laboratory samples of Westerly granite were impacted by a falling striker, and the time series of acoustic and electromagnetic emission (AE and EME) from fracturing rock were recorded with the time resolution of 10 ns. In order to determine precisely the actual instants of beginning and termination of fracturing, the fractoluminescence (FL) signal related to chemical bond ruptures on the sample surface was recorded in parallel to both AE and EME signals. Energy distributions in all three series were characterized by the function N(E > E′), where N is the number of emitted signals characterized with the energy release E exceeding a threshold value E′. The fracture dynamics was studied using both the scaling analysis and the non-extensive Tsallis statistics. The energy distributions were approximated with both the empirical Gutenberg–Richter-type relation N(E > E′) ∝ E′−b, and the analytical function Nq(E > E′) that includes the Tsallis’ parameter of non-extensivity, q, and the released energy density. A comparative consideration of information taken from the data of EME, AE and FL emission techniques was carried out in the context of the physical sense of the parameters b and q. The AE time series reflected adequately the cracking process both at microscopic and laboratory scale levels, while the photon emission did not show a direct response on the split-off formation. Time resolution of the EME method was found to be insufficient for studying the dynamic fracture of the given kind.

Ultrasonic moduli for fluid-saturated rocks: Mavko-Jizba relations rederived and generalized

Mavko and Jizba propose a quantitative model for squirt dispersion of elastic-wave velocities between seismic and ultrasonic frequencies in granular rocks. Their central results are the expressions for the so-called unrelaxed frame bulk and shear moduli computed under an assumption that the stiff pores are drained (or dry) but the soft pores are filled with fluid. Mavko-Jizba expressions are limited to liquid-saturated rocks but become inaccurate when the fluid-bulk modulus is small (e.g., for gas-saturated rocks). We have derived new expressions for unrelaxed moduli of fluid-saturated porous rocks using Sayers-Kachanov discontinuity formalism. The derived expressions generalize the established Mavko-Jizba relations to gas-saturated rocks, reduce to Mavko-Jizba results when the pore fluid is liquid, and yield dry moduli when fluid-bulk modulus tends to zero. We tested this by comparing our model and the model of Mavko and Jizba against laboratory measurements on a sample of Westerly granite.

Premonitory acoustic emissions and stick‐slip in natural and smooth‐faulted Westerly granite

A stick‐slip event was induced in a cylindrical sample of Westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 MPa. Continuously recorded acoustic emission (AE) data and computer tomography (CT)‐generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault. The dynamic stick‐slip event, considered to be a laboratory analog of an earthquake, generated an ultrasonic signal that was recorded as a large‐amplitude AE event. First arrivals of this event were inverted to determine the nucleation site of slip, which is associated with a geometric asperity on the fault surface. CT images and AE locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault. This experiment is compared with a stick‐slip experiment on a sample prepared with a smooth, artificial saw‐cut fault surface. Nearly a thousand times more AE were observed for the natural fault, which has a higher friction coefficient (0.78 compared to 0.53) and larger shear stress drop (140 compared to 68 MPa). However at the measured resolution, the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments, suggesting a similar dynamic rupture process. We propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field‐scale fault system in terms of the presence of asperities, fault surface heterogeneity, and interaction of branching faults.

Electroseismic and seismoelectric measurements of rock samples in a water tank

An electromagnetic (EM) or seismic wave can induce seismic or EM waves because of the electrokinetic conversion based on the electric double layer in a fluid-saturated porous medium. We tested a new method for observing electroseismic and seismoelectric conversions in rock samples. Our method is designed to overcome the shortcoming of previous attempts to separate signals generated by a continuous electric or seismic source in a small container. We first observed acoustic fields around electrodes excited by an electric pulse in a water tank or in a water-saturated porous sample. In our approach, we immersed rock samples in a water tank and measure the seismic or electric responses using electric or acoustic pulses conveyed through the immersed electrodes or hydrophone. We measured electroseismic- and seismoelectric-frequency responses in Berea Sandstone and Westerly Granite samples at a frequency range of [Formula: see text]. The measurements clearly separate the effects of the electric source, background noises, and signals. We calculate the normalized electroseismic and seismoelectric coupling coefficients for rock samples and confirm our results by comparing them with those theoretically simulated in a closed-capillary model. The variation of normalized coupling coefficients in the frequency domain is similar to that in theoretical predictions. Our measurement method can be used to investigate electroseismic and seismoelectric properties for petroleum exploration applications.

Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated Westerly granite

A series of 20 chevron cracked notched Brazilian disc (CCNBD) samples of Westerly granite were failed in a standard Mode I tensile test at room temperature in order to evaluate the effect of thermal damage on fracture toughness. The heat treatment involved slowly heating four sets of four samples to 250, 450, 650 and 850 °C. The fifth set of samples was not thermally treated. Thermal cracking not only induced a substantial decrease of the mechanical strength, but also of the dynamic elastic properties of Westerly granite. In particular, normalized P-wave compressional velocities matched remarkably well the decreasing trend of normalized fracture toughness ( K IC). Above 450 °C, grain boundary opening and cracking, intragranular cracking and mineral grain dissection linked to the quartz α–β phase transition induced a significant increase in the total crack density. Fracture path interaction with various mineral–mineral contact types showed that fracture branching and total fracture length increased with the amount of temperature of heat treatment. Using non-interactive crack theories, dimensionless crack densities were obtained from wave velocity inversion, up to unusually high values of ∼10 at 850 °C. This geophysical analysis showed to be in close agreement with crack parameters determined optically, such as optical crack density determination, crack aspect ratio evolutions, and the measured sample porosity with temperature. Our results also show that only the non-interactive crack theory can predict K IC relatively well at high crack density, by simply using dimensionless crack densities inverted from velocities. A decrease of 50% for crack densities larger than 1, 80% for crack densities larger than 5 is predicted, in close agreement with our observed experimental variation of K IC. At the microscale, this can be interpreted by the fact that the main fracture is strongly interacting with the pre-existing microcrack fabric. These combined experimental and modeling results illustrate the importance of understanding the details of how the rock microstructure is changing in response to an external stimulus, in order to predict the simultaneous evolution of physical and mechanical properties of rock.

The rock fracture experiment with a drive control: A spatial aspect

The statistical analysis of the space–time distributions of acoustical signals emitted from the compressed samples of Westerly granite is presented. The samples were uniaxially loaded in two regimes, one of which included a feedback loop controlled by the current acoustical emission (AE) activity. The distribution of distances between newly-appeared hypocenters follows the power law with the exponent that does not depend on the mode of loading. In both regimes, a transient period of the decorrelated accumulation of damaged sites was revealed between the initial and focal stages of the fracture process. The Hurst analysis indicates a more pronounced trend to planar organization of final damage structure in the case of AE-controlled loading.

Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation

New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity (as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments. It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation.

A viscoelastic damage model with applications to stable and unstable fracturing

SUMMARY A viscoelastic damage rheology model is presented that provides a generalization of Maxwell viscoelasticity to a non-linear continuum mechanics framework incorporating material degradation and recovery, transition from stable to unstable fracturing and gradual accumulation of non-reversible deformation. The model is a further development of the damage rheology framework of Lyakhovsky et al. for evolving effective elasticity. The framework provides a quantitative treatment for macroscopic effects of evolving distributed cracking with local density represented by an intensive state variable. The formulation, based on thermodynamic principles, leads to a system of kinetic equations for the evolution of damage. An effective viscosity inversely proportional to the rate of damage increase is introduced to account for gradual accumulation of irreversible deformation due to dissipative processes. A power-law relation between the damage variable and elastic moduli leads to a non-linear coupling between the rate of damage evolution and the damage variable itself. This allows the model to reproduce a transition from stable to unstable fracturing of brittle rocks and the Kaiser effect. 3-D numerical simulations based on the model formulation for homogeneous and heterogeneous materials account for the main features of rock behaviour under large strain. The model coefficients are constrained, using triaxial laboratory experiments with low-porosity Westerly granite and high-porosity Berea sandstone samples.

Anisotropic Changes in P-Wave Velocity and Attenuation during Deformation and Fluid Infiltration of Granite

Fluid infiltration and pore fluid pressure changes are known to have a significant effect on the occurrence of earthquakes. Yet, for most damaging earthquakes, with nucleation zones below a few kilometers depth, direct measurements of fluid pressure variations are not available. Instead, pore fluid pressures are inferred primarily from seismic-wave propagation characteristics such as V p / V s ratio, attenuation, and reflectivity contacts. We present laboratory measurements of changes in P -wave velocity and attenuation during the injection of water into a granite sample as it was loaded to failure. A cylindrical sample of Westerly granite was deformed at constant confining and pore pressures of 50 and 1 MPa, respectively. Axial load was increased in discrete steps by controlling axial displacement. Anisotropic P -wave velocity and attenuation fields were determined during the experiment using an array of 13 piezoelectric transducers. At the final loading steps (86% and 95% of peak stress), both spatial and temporal changes in P -wave velocity and peak-to-peak amplitudes of P and S waves were observed. P -wave velocity anisotropy reached a maximum of 26%. Transient increases in attenuation of up to 483 dB/m were also observed and were associated with diffusion of water into the sample. We show that velocity and attenuation of P waves are sensitive to the process of opening of microcracks and the subsequent resaturation of these cracks as water diffuses in from the surrounding region. Symmetry of the orientation of newly formed microcracks results in anisotropic velocity and attenuation fields that systematically evolve in response to changes in stress and influx of water. With proper scaling, these measurements provide constraints on the magnitude and duration of velocity and attenuation transients that can be expected to accompany the nucleation of earthquakes in the Earth9s crust.

Three-dimensional observations of faulting process in Westerly granite under uniaxial and triaxial conditions by X-ray CT scan

Observations of spatial fault development in granite undergoing compression provide new insights into the process of faulting. Dry intact Westerly granite samples were loaded under a confining pressure of 100 MPa (triaxial conditions) and 5 MPa (∼ uniaxial conditions), and the progress of faulting was controlled by maintaining the increment of circumferential displacement at a constant rate, which apparently stiffened the machine. The samples were unloaded after they experienced some degree of stress drop and were successfully recovered before faulting progressed further. A conventional medical X-ray CT scanning system was used to image the sample interiors. Three-dimensional fault systems were detected with sequential X-ray CT images. It was found that three-dimensional reconstruction by X-ray CT images yields not only three-dimensional images of the fault system, but also provides fault cross-section images with much less artificial noise (artifacts) than does direct X-ray CT imaging. Three-dimensional images show that a fault system that developed under uniaxial conditions is much more complicated than a fault system produced by triaxial conditions. In addition, the fault plane produced under uniaxial conditions is inclined at a lower angle to the maximum compressive axis than under triaxial conditions. Comparing X-ray CT images, we show that a fault nucleates locally on the sample surface just after peak stress, then develops into the final fault plane in the residual stress stage of the complete stress–strain relationship under triaxial conditions.

Rigorous connection between physical properties of porous rocks

Rigorous cross‐property bounds that connect the effective thermal conductivity k* (or the electrical conductivity σ*) and the effective bulk modulus K* of any isotropic, two‐phase composite were recently derived by the authors. Here we reformulate these bounds and apply them to porous rocks with dry or fluid‐filled pores. It is shown that knowledge of the effective conductivity can yield sharp estimates of the effective bulk modulus (and vice versa), even in cases where there is a wide disparity in the phase properties. The bounds yield, in particular, relations between the formation factor and the bulk modulus of the porous medium. By using the same approach we obtain new relations between the bulk moduli of a dry porous material and the bulk modulus of the same material with fluid‐filled pores that are more general than the traditional Gassmann equation. The Gassmann formula for the bulk modulus of the fluid‐saturated porous medium is shown to correspond to a lower bound on this quantity. Limiting cases that we consider include cracked materials with dry and fluid‐saturated pores. Theoretical results are tested against experimental measurements of the effective bulk modulus of dry and water‐saturated Westerly granite and sandstone samples. We found good agreement between our cross‐property bounds and the experimental data, even when the experimental data depart from the Gassmann formula. Our results add new insight to understanding of the properties of the porous media. They show that the Gassmann approximation works well for rocks with high porosity but needs to be corrected for rocks with high crack‐type porosity.

Hydraulic fracturing stress measurement using a true triaxial apparatus

Observations of the spatial development of cracks in granite undergoing triaxial compression provide new insights into the proces of fault formation. Dry intact Westerly granite samples were loaded under confining pressure of 100 MPa, and the progress of fault formation was controlled by maintaining the increment of circumferential displacement at a constant rate. The samples were unloaded when they experienced stress drops and were successfully recovered without severe crushing. A stereo microscope was used to observe the spatial pattern of cracks on the sample surfaces, and an X-ray CT scanning system was used to non-destructively image the sample interiors. Microscope observations show that several fault-like linear cracks developed on the surfaces of all the samples and have an angle of about 30 degrees to the maximum compression axis. This suggests that fault nucleation began just after peak stress. The X-ray CT images show that a fault-like linear crack grew inward toward the sample interior as axial stress decreased. It is concluded that the fault nucleated locally on the sample surface just after peak stress, and developed into an apparent fault by the plateau stage of the complete stress-strain relationship.

The observations of faulting in westerly granite under triaxial compression by X-ray CT scan

Abstract Observations of the spatial development of cracks in granite undergoing triaxial compression provide new insights into the proces of fault formation. Dry intact Westerly granite samples were loaded under confining pressure of 100 MPa, and the progress of fault formation was controlled by maintaining the increment of circumferential displacement at a constant rate. The samples were unloaded when they experienced stress drops and were successfully recovered without severe crushing. A stereo microscope was used to observe the spatial pattern of cracks on the sample surfaces, and an X-ray CT scanning system was used to non-destructively image the sample interiors. Microscope observations show that several fault-like linear cracks developed on the surfaces of all the samples and have an angle of about 30 degrees to the maximum compression axis. This suggests that fault nucleation began just after peak stress. The X-ray CT images show that a fault-like linear crack grew inward toward the sample interior as axial stress decreased. It is concluded that the fault nucleated locally on the sample surface just after peak stress, and developed into an apparent fault by the plateau stage of the complete stress-strain relationship.

Complexities of rock fracture and rock friction from deformation of Westerly granite

A series of rock friction experiments has been carried out to study the complexities in rock fracture and rock friction. Intact Westerly granite samples were loaded to shear failure in a laboratory polyaxial loading apparatus. The resultant fractured samples were reloaded to cause frictional sliding. Both polyaxial loading (σ1 > σ2 > σ3 > 0) and equal confining condition (σ1 > σ2 = σ3 > 0) were used. The deformation processes were monitored by macroscopic axial stress-strain, optical holography, and ultrasonic velocity measurements.

Comparison of fluid inclusion decrepitation and acoustic emission profiles of Westerly granite and Sioux quartzite

Acoustic emissions recorded during slow heating of rocks at 1 atm have generally been attributed to intergranular stresses generated by differences in thermal expansion coefficients and elastic moduli of neighboring grains. These experiments have considered only the solid phases in analysis of the results, and little attention has been given to the role of intragranular or grain boundary fluid inclusions in initiating or facilitating fracturing. In principle, fluid inclusions may be thought of as a special case in which the “ neighboring grains” are fluids rather than the usual solid mineral phases. The major differences are that (1) the number of inclusions per unit volume is usually 10 2 to 10 6 times greater than the number of mineral grains in that same volume and that (2) the volume increase with temperature for most fluids is 1–2 orders of magnitude greater than for minerals. Thus, when fluid inclusion “grains” are considered, the absolute number of grain-to-grain contacts in a given sample volume is several orders of magnitude greater than if the inclusions are not considered. Further, the magnitude of the stresses between any two adjacent grains may be much higher if one of the “grains” is a fluid inclusion, owing to the larger coefficient of thermal expansion of the fluid relative to a solid phase. As a result, substantial overpressures may be generated at the fluid inclusion “grain”-mineral grain contact, during heating experiments. These overpressures result in brittle failure when the stress around the inclusion exceeds the local strength of the host crystal. In fluid inclusion terminology, this fracturing event is referred to as decrepitation. Samples of Westerly granite and Sioux quartzite were heated at 2° C/min in a gas-flow stage in order to determine the decrepitation profiles (number of decrepitations as a function of temperature) of these rocks. Decrepitation was monitored visually during heating experiments, and was evidenced by the explosive loss of fluids from the inclusion during a fracturing event. Decrepitation produces microfractures which originate at decrepitated fluid inclusions and propagate outward into the surrounding mineral. Four different types of inclusions, based on room temperature phase ratios and distribution within the host phase, were noted in Westerly granite. Type (1) inclusions contain a single aqueous liquid, type (2) are two-phase, liquid-rich inclusions, type (3) are mostly one-phase fluid inclusions that occur in dense clusters in plagioclase and type (4) are two-phase, moderate-density, H 2O-CO 2-“salt” inclusions. Type (5) inclusions are one-phase, aqueous liquid inclusions observed in Sioux quartzite. Fluid inclusions in Westerly granite decrepitate between 75° and 573° C with peaks at 275–300°C and 400–450°C. Type (4) inclusions in Westerly granite tend to decrepitate at lower temperatures (75–300°C) than other inclusion types. Initial fracturing of the quartz host surrounding type (1) inclusions (primary decrepitation) generally occurs over the range 100–400°C and the fractures continue to propogate through the quartz (secondary decrepitation) during further heating from 400° to 573°C. Fluid inclusions in Sioux quartzite decrepitate between 188° and 573°C with a broad maximum at 250–450°C. Primary decrepitation generally occurs at 200–450°C while secondary decrepitation occurs at 350°–573°C in Sioux quartzite. Some small inclusions in both samples do not decrepitate until the quartz α-β transition temperature (573°C) is reached. Decrepitation profiles determined in this study display many similarities and differences when compared to previously published acoustic emission profiles of Westerly granite and Sioux quartzite. Owing to fundamental differences in the techniques used to measure acoustic emission and decrepitation, and the potentially different energies associated with the two events, it is unclear whether decrepitating fluid inclusions are recorded during acoustic emission studies. However, if decrepitating fluid inclusions are being recorded during acoustic emission measurements, then our results suggest that: (1) decrepitation contributes more to low temperature portions of acoustic emission profiles than to high temperature portions for both Sioux quartzite and Westerly granite; (2) compared to Westerly granite, a larger percentage of acoustic emissions in the low temperature decrepitation profile of Sioux quartzite is due to fluid inclusions; (3) decrepitating fluid inclusions, not stresses generated by solid-solid interactions, cause the majority of intragranular fracturing in quartz below the temperature of the α-β transition in both these rocks.

Micromechanics of thermally induced cracking in three crustal rocks

Samples of Westerly granite, Frederick diabase, and Oak Hall limestone were thermally cracked at room pressure to various peak temperatures. Scanning electon microscopy (SEM) was performed on ion‐thinned samples. The thermally induced crack density is dependent upon the temperature, thermal expansion mismatch, thermal expansion anisotropy, initial crack porosity, and grain size. The mode of propagation is different for grain boundary and intragranular cracks. Crack densities in the granite were quantified using stereological techniques. The thermally induced crack surface area per unit volume apparently has a quadratic dependence on the temperature increase, a physical interpretation for which can be formulated on the basis of energetic balance. Fracture mechanics models are developed to interpret thermal cracking. The predictions concerning thermal crack initiation temperature and crack propagation and arrest behavior agree well with observations for the granite and the diabase. The model predicts significant thermal cracking for the Oak Hall limestone, which contradicts the SEM observation. A possible explanation for this discrepancy is that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.

Role of cracks in progressive permeability reduction during flow of heated aqueous fluids through granite

Permeability was measured continuously during an experiment in which a heated, aqueous fluid was pumped through a cylindrical sample of Westerly Granite. Fluid flowed radially outward from an axial borehole down a temperature gradient to the outer edge of the sample. The fluid temperature in the borehole was maintained at 300°C, while a pore pressure difference of 0.5 MPa provided the driving force for fluid flow. Permeability decreased by a factor of 25 during the 2‐week experiment. Scanning electron microscope examination of this altered sample indicates that about half of the grain boundaries involving quartz are cracked or were cracked at some time during the experiment and now contain a filling material. Grain boundaries between the two feldspars are closed in the starting material and in all areas of the altered sample that were examined. Intragranular cracks in all three major minerals are open in the starting material. These observations suggest that initially, the quartz forms a more effective fluid pathway than do the feldspars. About half of the intragranular cracks near the borehole are open, whereas the other half contain a massive deposit which is either Si‐rich or Ca‐rich. In most cracks at the outer edge of the altered sample a Si‐rich filling is present. In crack intersections in this area the filling has a platy texture. At several times during the experiment the discharged fluids were sampled and chemically analyzed. The silica content of fluids passing through the sample was calculated as a function of radius, using rate equations for the dissolution and precipitation of silica. The calculated silica concentrations for the discharged fluids are approximately in agreement with measured values. A separate calculation of the reduction of crack porosity due to uniform dissolution and reprecipitation of quartz indicates that these processes caused a maximum of 10% crack porosity change for an experiment with a borehole temperature of 300°C. Assuming that reprecipitation of quartz was homogeneous, this porosity change corresponds to a permeability reduction of 27%, whereas the observed value was 96%. Therefore other processes besides the simple homogeneous precipitation of a layer of quartz must be operating to reduce permeability. Four possible processes are (1) nonhomogeneous precipitation, (2) precipitation in critical narrow places in cracks, (3) precipitation of other minerals in addition to quartz, and (4) crack healing.

Changes in complex resistivity during creep in granite

A sample of Westerly granite was deformed under constant stress conditions: a pore pressure of 5 MPa, a confining pressure of 10 MPa, and an axial load of 170 MPa. Pore volume changes were determined by measuring the volume of pore fluid (0.01M KClaq) injected into the sample. After 6 days of creep, characterized by accelerating volumetric stain, the sample failed along a macroscopic fault. Measurements of complex resistivity over the frequency range 0.001–300 Hz, taken at various times during creep, showed a gradual increase in both conductivity and permittivity. When analysed in terms of standard induced polarization (IP) techniques, the changing complex resistivity resulted in systematic changes in such parameters as percent frequency effect and chargeability. These results suggest that it may be possible to monitor the development of dilatancy in the source region of an impending earthquake through standard IP techniques.

Complex resistivity measurements of confined rock

Complex resistivity was measured for samples of Westerly granite and Berea sandstone saturated with 0.01 M KClaq. A four‐electrode technique was employed to make measurements over a frequency range of 10−3 to 106 Hz and a confining pressure range of 0.2 to 200 MPa. When the electrical response was analyzed in terms of equivalent conductivity σ′ and relative permittivity K′, both of these quantities varied more with confining pressure for the granite samples than they did for the sandstone. In addition, both rock types exhibited large low‐frequency dispersion of permittivity. While previous studies have measured the response of near‐dc conductivity to changes in pressure, measurements of pressure dependence of the full complex resistivity ρ*(ω) provide greater insight into earthquake‐related phenomena as well as induced polarization measurements and other remote sensing techniques.

The effect of fluid flow on the magnetic field in low porosity crystalline rock

Changes in the local geomagnetic field have been observed and documented in several seismically active areas prior to, during, and following earthquakes. Mizutani and Ishido (1975) proposed that groundwater may diffuse through a hypocentral region and that the fluid flow can produce an electric potential by means of electrokinetic phenomena. The resulting current flow, in turn, will generate a magnetic field.Fluid flow tests conducted on samples of Westerly granite demonstrated that the diffusion of an electrolytic fluid causes a measurable magnetic field change. At high fluid flow rates, a magnetic field change was observed that was in agreement with theoretically predicted and numerically calculated values. In addition, the sign of the magnetic field changes as the fluid flow direction through the sample was reversed. Moreover, the tests showed that the magnitude of the magnetic field change is dependent upon (1) the porosity of the rock; (2) the pore wall surface area; (3) the permeability of the rock; and (4) zeta potential of the fluid‐pore wall system.