Seismic attenuation: Effects of pore fluids and frictional‐sliding
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Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in terms of wave induced pore fluid flow. The ratio of compressional to shear attenuation is found to be a more sensitive and reliable indicator of partial gas saturation than is the corresponding velocity ratio. Potential applications may exist in exploration for natural gas and geothermal steam reservoirs.Keywords:
Anelastic attenuation factor
Saturation (graph theory)
Shear waves
Seismic waves are significantly affected by the presence of fractures and faults. Fractures alter the arrival time of a seismic wave and the amplitude of the seismic wave. Attenuation of a seismic wave is the reduction of wave amplitude due to the presence of e.g. fractures. Attenuation of acoustic compressional Pand shear S-waves have been measured in laboratory studies on different rock types. These studies generally show a decrease in attenuation with an increase in stress. This decrease in attenuation is attributed to progressive crack closure of preexisting cracks. The stress-dependent decrease in attenuation reported in these studies all occur within the elastic deformation field, i.e. below yield stress levels and thus no additional cracks/micro-fractures have yet been formed.
Anelastic attenuation factor
Shear waves
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Anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves. Understanding the causes of compressional wave attenuation is important in the acquisition, processing, and interpretation of high‐resolution seismic data, and in deducing the physical properties of rocks from seismic data. We have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 MPa (equivalent to a depth of burial of about 1.5 km) in a frequency range from 0.5 to 1.5 MHz. The compressional wave measurements were made using a pulse‐echo method in which the sample (5 cm diameter, 1.8 cm to 3.5 cm long) was sandwiched between perspex (lucite) buffer rods inside the high‐pressure rig. The attenuation of the sample was estimated from the logarithmic spectral ratio of the signals (corrected for beam spreading) reflected from the top and base of the sample. The results show that for these samples, compressional wave attenuation (α, dB/cm) at 1 MHz and 40 MPa is related to clay content (C, percent) and porosity (ϕ, percent) by α=0.0315ϕ+0.241C−0.132 with a correlation coefficient of 0.88. The relationship between attenuation and permeability is less well defined: Those samples with permeabilities less than 50 md have high attenuation coefficients (generally greater than 1 dB/cm) while those with permeabilities greater than 50 md have low attenuation coefficients (generally less than 1 dB/cm) at 1 MHz at 40 MPa. These experimental data can be accounted for by modifications of the Biot theory and by consideration of the Sewell/Urick theory of compressional wave attenuation in porous, fluid‐saturated media.
Anelastic attenuation factor
Overburden pressure
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Results of a modeling study to examine the effect of shear on compressional wave attenuation are presented. This study also investigates a shear inversion algorithm based on interface waves using synthetic data. Recent studies suggest that inclusion of shear speed is necessary to explain the correct frequency dependence of attenuation. Synthetic data will be generated for elastic bottom, with different shear speeds, and these data will be inverted for compressional wave attenuation. This could provide insight into the effect of shear speed on the attenuation coefficient obtained from inversion at various frequencies. In addition to investigating this effect, we also develop inversion algorithms for shear speed. One of the most promising approaches is to invert the relation between seismo-acoustic interface waves (Scholte waves) that travel along boundaries between media and shear wave speed. The propagation speed and attenuation of the Scholte wave are closely related to shear-wave speed and attenuation over a depth of 1–2 wavelengths into the seabed. The dispersion characteristics of the Scholte wave has been successfully used for inversion of sediment shear properties. Synthetic data will be used in our study to develop an inversion scheme. [Work supported by Office of Naval Research.]
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Shear waves
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Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in terms of wave induced pore fluid flow. The ratio of compressional to shear attenuation is found to be a more sensitive and reliable indicator of partial gas saturation than is the corresponding velocity ratio. Potential applications may exist in exploration for natural gas and geothermal steam reservoirs.
Anelastic attenuation factor
Saturation (graph theory)
Shear waves
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Anelastic attenuation factor
Dispersive body waves
Vertical seismic profile
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Ultrasonic compressional‐ and shear‐wave attenuation in water‐saturated Carrara Marble increase with increasing crack density and decreasing effective pressure. Between 0.4 and 1.0 MHz, empirical linear relationships between 1/Q and crack density CD were found to be: CD = 1.96 ± 0.63 × 1/Q, for compressional waves and CD = 6.7 ± 1.5 × 1/Q, for shear waves.
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Calculations for an unconsolidated sand with partial gas saturation show a 20 percent increase in compressional wave velocity between 1 and 100 hz and attenuation of 27 db/1000 ft at 31 hz and 82 db/1000 ft at 123 hz. Shear velocity and attenuation are not affected. Fluid‐flow waves are shown to be responsible for the dispersion and attenuation at low frequencies; relations are derived by extending Gassmann’s viewpoint to include coupling between fluid‐flow waves and seismic body waves. This appears to be an important loss mechanism for heterogeneous porous rocks.
Saturation (graph theory)
Anelastic attenuation factor
Velocity dispersion
Shear waves
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Shear waves
Anelastic attenuation factor
Seismic anisotropy
Vertical seismic profile
S-wave
Coda
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It is believed that seismic anisotropy represents dynamics of the Earth's interior, directly reflecting either instantaneous stress, cumulative strain, or deformation of in-situ rocks. A shear wave passing through an anisotropic elastic medium splits into two orthgonally polarized quasi-shear waves with different propagation speeds. This phenomenon is called shear wave splitting, and is useful to understand the Earth's anisotropic fabric, with potential advantages as high lateral resolving power and relatively low sensitivity to seismic wave velocity heterogeneities. During the past decade, a variety of anisotropy-induced shear wave splitting has been observed in many different fields of seismology, indicating that anisotropy is an ubiquitous feature in the Earth's crust and upper mantle. In this review I summarize recent observations of shear wave splitting, with special emphases on their geophysical implications. I also discuss several problems concerned with shear wave splitting analyses, which are expected to be solved in the near future.
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Seismic wave tomography is a potentially powerful tool for detecting and delineating nonaqueous phase liquid (NAPL) contaminants in the shallow subsurface. To develop this application, we are conducting laboratory and numerical studies to understand the mechanisms of P-wave transmission through NAPL-water‐sand systems. P-wave measurements of traveltime and amplitude were taken in the 100–900 kHz frequency range through saturated sand with variable NAPL content. To simulate the stress conditions of the shallow surface, a low confining and axial pressure of 60 and 140 kPa, respectively, was applied. The measurements show a significant change in the traveltime and amplitude of the primary arrival as a function of NAPL saturation. To simulate the laboratory measurements, we performed numerical calculations of P-wave propagation through a 1-D medium. The results show that the main behavior of traveltime and amplitude variation can be explained by P-wave scattering. This represents an alternative explanation to the theories that describe local fluid flow as the dominant mechanism for seismic wave attenuation and velocity dispersion.
Saturation (graph theory)
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