Relating pore fabric geometry to acoustic and permeability anisotropy in Crab Orchard Sandstone: A laboratory study using magnetic ferrofluid

Pore fabric anisotropy is a common feature of many sedimentary rocks. In this paper we report results from a comparative study on the anisotropy of a porous sandstone (Crab Orchard) using anisotropy of magnetic susceptibility (AMS), acoustic wave velocity and fluid permeability techniques. Initially, we characterise the anisotropic pore fabric geometry by impregnating the sandstone with magnetic ferro-fluid and measuring its AMS. The results are used to guide subsequent measurements of the anisotropy of acoustic wave velocity and fluid permeability. These three independent measures of anisotropy are then directly compared. Results show strong positive correlation between the principal directions given from the AMS, velocity anisotropy and permeability anisotropy. Permeability parallel to the macroscopic crossbedding observed in the sandstone is 240% higher than that normal to it. P and S-wave velocity anisotropy and AMS show mean values of 19.1%, 4.8% and 3.8% respectively, reflecting the disparate physical properties measured

Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography

We have deformed basalt from Mount Etna (Italy) in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice (40 MPa), and at a constant strain rate of 5 similar to 10(-6) s(-1). Despite containing a high level of pre-existing microcrack damage, Etna basalt retains a high strength of 475 MPa. We have monitored the complete deformation cycle through contemporaneous measurements of axial strain, pore volume change, compressional wave velocity change and acoustic emission (AE) output. We have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation. Locations of AE events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm. s(-1). We also find excellent agreement between AE locations and post-test images from X-ray microtomography scanning that delineates deformation zone architecture

Analysis of laboratory simulations of volcanic hybrid earthquakes using empirical Green's functions

[1] Here we present a new analysis of experimental simulations of the seismic signals characteristically observed in volcanic environments. We examine the waveforms of laboratory microseismic events generated during two rock deformation experiments performed on samples of Mt. Etna basalt to determine their source characteristics and establish evidence for a mode of failure. Events were recorded during deformation under (a), unsaturated (dry) conditions, and (b), samples saturated with water. We employ an empirical Green's function approach to isolate the acoustic emission event source spectra from attenuation and travel path effects, and estimate the spectral corner frequency using a least squares fit to a Brune spectral model. Spectral fits indicate that the acoustic emission events occurring under dry conditions follow the expected scaling of moment and corner frequency for standard brittle‐failure in an elastic medium with constant stress drop, namely M0 ∝ fc−3. However, the events occurring during the fluid decompression phase of the saturated experiment have estimated corner frequencies which are not easily described by any simple scaling relationship. The implication of the observed scaling is that the events occurring under dry conditions must result from a standard stick‐slip (i.e., brittle‐failure) source. The observed moment‐corner frequency scaling also suggests that event durations change in a predictable way with increasing moment for the events occurring under dry conditions. Conversely, events occurring under wet conditions do not show any distinctive relationship between duration and event size. The specific dependence of duration on event size exhibited by the events in the dry experiment must consequently rule out fluid‐flow as a source, as there is no plausible reason for the driving pressure for fluid‐flow to be dependent on duration in such a specific way. We compare laboratory observations of brittle‐failure scaling (M0 ∝ fc−3) to previous observations of volcanic hybrid events in a field environment. Scaling dissimilarities between field observations and the wet laboratory events suggest that hybrid seismic signals observed in a volcanic environment do not always require fluid‐flow to explain their signal

Imaging compaction band propagation in Diemelstadt sandstone using acoustic emission locations

We report results from a conventional triaxial test performed on a specimen of Diemelstadt sandstone under an effective confining pressure of 110 MPa; a value sufficient to induce compaction bands. The maximum principal stress was applied normal to the visible bedding so that compaction bands propagated parallel to bedding. The spatio-temporal distribution of acoustic emission events greater than 40 dB in amplitude, and associated with the propagation of the first compaction band, were located in 3D, to within +/- 2 mm, using a Hyperion Giga-RAM recorder. Event magnitudes were used to calculate the seismic b- value at intervals during band growth. Results show that compaction bands nucleate at the specimen edge and propagate across the sample at approximately 0.08 mm s(-1). The seismic b-value does not vary significantly during deformation, suggesting that compaction band growth is characterized by small scale cracking that does not change significantly in scale

Fracture, fluid and saturation effects on the seismic attributes of rocks from the Southern Negros geothermal field, Philippines

Seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential. However, interpreting geophysical data (such as velocities and dynamic modulus and fracture density/alignment) generated from geothermal reservoirs remains difficult. Here we present the results of a new laboratory study measuring seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (Southern Negros Geothermal Project - SNGP) during triaxial deformation. Two types of rocks were obtained by sub-coring samples of low porosity (~1%) andesite and higher porosity (~10%) volcaniclastic samples from the SNGP. Samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture. An embedded array of Acoustic Emission (AE) sensors allows elastic wave and induced microseismic data to be collected. We measure a significant reduction in elastic wave velocities and moduli, with the exception of Poisson's ratio, after shear fracture development. An initially pre-fractured permeability of approximately 10−17 m2 is measured. We find that the permeability decreases from 2.0 × 10−14 m2 to lower than 7.4 × 10−15 m2 as the confining pressure is increased from 5 MPa to 30 MPa. A concomitant increase in P and S-wave velocities, dynamic bulk and Young's moduli are also measured. Finally, we simulate a geothermal ‘venting’ situation by intentionally releasing the high pore fluid (water) pressure from 10 to 50 MPa to ambient pressure, generating a swarm of AE that increases in duration with higher pore pressure. We postulate that this is due to fluid phase change (liquid to gas) and movement along the natural fracture plane and damage zone

Role of void space geometry in permeability evolution in crustal rocks at elevated pressure

[1] A key consequence of the presence of void space within rock is their significant influence upon fluid transport properties. In this study, we measure changes in elastic wave velocities (P and S) contemporaneously with changes in permeability and porosity at elevated pressure for three rock types with widely different void space geometries: a high‐porosity sandstone (Bentheim), a tight sandstone (Crab Orchard), and a microcracked granodiorite (Takidani). Laboratory data are then used with the permeability models of Guéguen and Dienes and Kozeny‐Carman to investigate the characteristics that different void space geometries impart to measured permeabilities. Using the Kachanov effective medium theory, elastic wave velocities are inverted, permitting the recovery of crack density evolution with increasing effective pressure. The crack densities are then used as input to the microcrack permeability model of Guéguen and Dienes. The classic Kozeny‐Carman approach of Walsh and Brace is also applied to the measured permeability data via a least squares fit in order to extract tortuosity data. We successfully predict the evolution of permeability with increasing effective pressure, as directly measured in experiments, and report the contrast between permeability changes observed in rock where microcracks or equant pores dominate the microstructure. Additionally, we show how these properties are affected by anisotropy of the rock types via the measured anisotropic fabrics in each rock. The combined experimental and modeling results illustrate the importance of understanding the details of how rock microstructure changes in response to an external stimulus in predicting the simultaneous evolution of different rock physical properties

Laboratory simulation of fluid-driven seismic sequences in shallow crustal conditions

[1] We report new laboratory simulations of fluid‐induced seismicity on pre‐existing faults in sandstone. By introducing pore pressure oscillations, faults were activated or reactivated to generate seismic sequences. These sequences were analysed using a slip‐forecast model. Furthermore, field data from the Monticello reservoir was used to verify the model. Our results suggest that short‐term forecasting is reliant upon the final stages when crack communication begins, limiting reservoir‐induced seismicity (RIS) forecasting strategies to short periods. In addition, our laboratory data confirms the general accuracy and robustness of short‐term forecast techniques dealing with natural crack‐linkage processes, whether strain driven or fluid driven, ranging from volcanic hazard mitigation to episodic tremors and slips. Finally, oscillating pore pressure can prolong the period of fluid‐induced seismicity, and the aftershock decay rate is slower than that without oscillations

Laboratory simulation of fluid-driven seismic sequences in shallow crustal conditions

[1] We report new laboratory simulations of fluid‐induced seismicity on pre‐existing faults in sandstone. By introducing pore pressure oscillations, faults were activated or reactivated to generate seismic sequences. These sequences were analysed using a slip‐forecast model. Furthermore, field data from the Monticello reservoir was used to verify the model. Our results suggest that short‐term forecasting is reliant upon the final stages when crack communication begins, limiting reservoir‐induced seismicity (RIS) forecasting strategies to short periods. In addition, our laboratory data confirms the general accuracy and robustness of short‐term forecast techniques dealing with natural crack‐linkage processes, whether strain driven or fluid driven, ranging from volcanic hazard mitigation to episodic tremors and slips. Finally, oscillating pore pressure can prolong the period of fluid‐induced seismicity, and the aftershock decay rate is slower than that without oscillations

Laboratory Simulation of Volcano Seismicity

The physical processes generating seismicity within volcanic edifices are highly complex and not fully understood. We report results from a laboratory experiment in which basalt from Mount Etna volcano (Italy) was deformed and fractured. The experiment was monitored with an array of transducers around the sample to permit full-waveform capture, location, and analysis of microseismic events. Rapid post-failure decompression of the water-filled pore volume and damage zone triggered many low-frequency events, analogous to volcanic long-period seismicity. The low frequencies were associated with pore fluid decompression and were located in the damage zone in the fractured sample; these events exhibited a weak component of shear (double-couple) slip, consistent with fluid-driven events occurring beneath active volcanoes

Laboratory simulations of fluid/gas induced micro-earthquakes: application to volcano seismology

Understanding different seismic signals recorded in active volcanic regions allows geoscientists to derive insight into the processes that generate them. A key type is known as Low Frequency or Long Period (LP) event, generally understood to be generated by different fluid types resonating in cracks and faults. The physical mechanisms of these signals have been linked to either resonance/turbulence within fluids, or as a result of fluids “sloshing” due to a mixture of gas and fluid being present in the system. Less well understood, however, is the effect of the fluid type (phase) on the measured signal. To explore this, we designed an experiment in which we generated a precisely controlled liquid to gas transition in a closed system by inducing rapid decompression of fluid-filled fault zones in a sample of basalt from Mt. Etna Volcano, Italy. We find that fluid phase transition is accompanied by a marked frequency shift in the accompanying microseismic dataset that can be compared to volcano seismic data. Moreover, our induced seismic activity occurs at pressure conditions equivalent to hydrostatic depths of 200–750 m. This is consistent with recently measured dominant frequencies of LP events and with numerous models