5:15 PM - 6:45 PM
[SSS07-P14] Effect of internal fracture distribution on frequency dependence of elastic constants of granite
Keywords:low frequency, elastic constants
Introduction
The elastic properties of subsurface rocks are estimated from seismic surveys and natural seismic waves (0.1 ~ 100 Hz). Based on these observations, estimates of subsurface rock type, porosity and fracture distribution, and stress state are commonly based on laboratory seismic wave measurements (100 k ~ 1 MHz). However, care must be taken when comparing elastic properties of rocks that are known to be dependent on elastic wave frequencies and estimating materials in frequency bands that are many orders of magnitude apart. For example, one factor that is known to cause the frequency dependence of elastic properties of rocks is the interaction between pore fluids and rocks (Mavko & Jizba, 1991). Therefore, it is important to investigate the nature of the internal fracture state, as it is thought to influence its frequency dependence. Therefore, the objective of this study is to investigate the elastic constants of granites with different fracture distributions and their frequency dependence using the dynamic stress-strain method. In the high frequency band (1 MHz), Poisson's ratio and Young's modulus were derived from the Vp/Vs ratio by transmission method measurements using an AE sensor.
Method
A cylindrical sample of Aji granite (from Kagawa Prefecture, Japan), 40 mm in diameter and 80 mm in length, was used in the experiment. First, two types of specimens were prepared for the purpose of obtaining different crack distribution conditions: an unheated specimen (porosity 0.30%) and a specimen that was heated to 550℃ and cooled slowly by placing it in a furnace (porosity 1.45%). Each sample was subjected to low-frequency (0.13 - 13 Hz) elastic vibration in the axial direction, and axial stress and axial and circumferential strain were measured. These measurements were made in the dry and saturated state of the samples, respectively. A piezo actuator (PI Japan P-045.20P) was used to apply elastic vibration to the samples. Sine and square waves were used as input voltage signals to the piezo actuator. The elastic constants were derived from the stress and axial and circumferential strain amplitude values and frequency spectra of the measurement data, and their frequency dependence was investigated. For high-frequency (1 MHz) measurements, elastic wave velocities were measured by the transmission method using AE sensors. The terminals of the AE sensor were glued to both bottoms of the specimen. An electrical signal of 1 MHz frequency, 10 Vp-p voltage, and square wave was input to the AE sensor, and the waveform of the square wave transmitted through the rock was checked with an oscilloscope.
Results
For both saturated samples, a positive correlation was observed between Poisson's ratio, Young's modulus, and P-wave velocity and frequency. For the saturated unheated sample (0.3% porosity), Young's modulus increased to 58.9, 65, and 65.7 GPa and Poisson's ratio increased to 0.24, 0.26, and 0.28 as the frequency increased from 0.13, 1.3, and 13 Hz, respectively. Young's modulus increased to 41.5, 41.8, and 43.6 GPa and Poisson's ratio increased to 0.25, 0.28, and 0.31 for the saturated post-heated samples (porosity 1.45%). The higher Poisson's ratio values for the post-heated samples than for the unheated samples are consistent with the results of Pimienta et al. (2018). The increased values of these elastic properties also suggest that the interaction between rock and porewater may have been at work. The measurements did not show any difference in the frequency dependence of the elastic properties with different porosities. Future measurements under confining pressure and observation of internal fracture conditions are being considered.
References
Mavko, G. and Jizba, D.(1991)Estimating grain scale fluid effects on velocity dispersion in rocks. Geophysics, 56, 1940-1949.
Pimienta, L et al.(2018)Anomalous Vp/Vs Ratios at Seismic Frequencies Might Evidence Highly Damaged Rocks in Subduction Zones. Geophysical Research Letters, 45, 12210-12217
The elastic properties of subsurface rocks are estimated from seismic surveys and natural seismic waves (0.1 ~ 100 Hz). Based on these observations, estimates of subsurface rock type, porosity and fracture distribution, and stress state are commonly based on laboratory seismic wave measurements (100 k ~ 1 MHz). However, care must be taken when comparing elastic properties of rocks that are known to be dependent on elastic wave frequencies and estimating materials in frequency bands that are many orders of magnitude apart. For example, one factor that is known to cause the frequency dependence of elastic properties of rocks is the interaction between pore fluids and rocks (Mavko & Jizba, 1991). Therefore, it is important to investigate the nature of the internal fracture state, as it is thought to influence its frequency dependence. Therefore, the objective of this study is to investigate the elastic constants of granites with different fracture distributions and their frequency dependence using the dynamic stress-strain method. In the high frequency band (1 MHz), Poisson's ratio and Young's modulus were derived from the Vp/Vs ratio by transmission method measurements using an AE sensor.
Method
A cylindrical sample of Aji granite (from Kagawa Prefecture, Japan), 40 mm in diameter and 80 mm in length, was used in the experiment. First, two types of specimens were prepared for the purpose of obtaining different crack distribution conditions: an unheated specimen (porosity 0.30%) and a specimen that was heated to 550℃ and cooled slowly by placing it in a furnace (porosity 1.45%). Each sample was subjected to low-frequency (0.13 - 13 Hz) elastic vibration in the axial direction, and axial stress and axial and circumferential strain were measured. These measurements were made in the dry and saturated state of the samples, respectively. A piezo actuator (PI Japan P-045.20P) was used to apply elastic vibration to the samples. Sine and square waves were used as input voltage signals to the piezo actuator. The elastic constants were derived from the stress and axial and circumferential strain amplitude values and frequency spectra of the measurement data, and their frequency dependence was investigated. For high-frequency (1 MHz) measurements, elastic wave velocities were measured by the transmission method using AE sensors. The terminals of the AE sensor were glued to both bottoms of the specimen. An electrical signal of 1 MHz frequency, 10 Vp-p voltage, and square wave was input to the AE sensor, and the waveform of the square wave transmitted through the rock was checked with an oscilloscope.
Results
For both saturated samples, a positive correlation was observed between Poisson's ratio, Young's modulus, and P-wave velocity and frequency. For the saturated unheated sample (0.3% porosity), Young's modulus increased to 58.9, 65, and 65.7 GPa and Poisson's ratio increased to 0.24, 0.26, and 0.28 as the frequency increased from 0.13, 1.3, and 13 Hz, respectively. Young's modulus increased to 41.5, 41.8, and 43.6 GPa and Poisson's ratio increased to 0.25, 0.28, and 0.31 for the saturated post-heated samples (porosity 1.45%). The higher Poisson's ratio values for the post-heated samples than for the unheated samples are consistent with the results of Pimienta et al. (2018). The increased values of these elastic properties also suggest that the interaction between rock and porewater may have been at work. The measurements did not show any difference in the frequency dependence of the elastic properties with different porosities. Future measurements under confining pressure and observation of internal fracture conditions are being considered.
References
Mavko, G. and Jizba, D.(1991)Estimating grain scale fluid effects on velocity dispersion in rocks. Geophysics, 56, 1940-1949.
Pimienta, L et al.(2018)Anomalous Vp/Vs Ratios at Seismic Frequencies Might Evidence Highly Damaged Rocks in Subduction Zones. Geophysical Research Letters, 45, 12210-12217