Attenuation Mechanism of Seismic Wave in Chamoli Region of Garhwal-Kumaun Himalayas
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In this study the S-wave attenuation mechanism is analyzed and intrinsic and scattering attenuation effects are separated. This helps in understanding the physical mechanisms governing attenuation properties of the crust of this region. It has been observed that the Qs−1, Qd−1 and Qc−1 values at all frequencies are comparable. The estimated Le−1, B0, Qi−1, Qs−1, Qd−1 and Qc−1 values show that S-wave attenuation is primarily controlled by scattering attenuation in the source zone of the 1999 Chamoli earthquake. My results also show that for the source zone of 1999 Chamoli earthquake in the Garhwal Himalayas seismic albedo is very high, i.e. attenuation is mainly controlled by scattering attenuation. In conclusion, The extinction length Le varies between about 12 km to about 125 km in the study region. The seismic albedo (B0) values vary between 0.65 and 0.8. B0 values are very large (>0.5) showing that at all frequencies scattering attenuation is the dominating factor causing attenuation of seismic waves. This shows that the degree of heterogeneity is very high for the frequency range considered in this area.Keywords:
Anelastic attenuation factor
Albedo (alchemy)
Seismic reflections at an interface are often regarded as the variation of the acoustic impedance (product of seismic velocity and density) in a media. In fact, they can also be generated due to the difference in absorption of the seismic energy. In this work, we investigate the impacts of attenuation on seismic reflections based on the diffusive-viscous wave equation, which is used to investigate seismic attenuation and frequency-dependent seismic anomalies related to hydrocarbon reservoirs. The results show that the reflections are significantly affected by the diffusive attenuation but they are insensitive to the viscous attenuation in an acoustic dispersive medium. In an elastic dispersive medium, the attenuation parameter in P wave equation has a big impact on both PP and PS reflections, however, the attenuation parameter in S wave equation has little effect on the PP reflection but it strongly affects the PS reflections. Furthermore, the PP and PS reflections in the dispersive medium are dependent on the frequency, and the effect of attenuation on PP and PS reflections at lower frequencies is bigger than those at higher frequencies.
Anelastic attenuation factor
Reflection
Dispersive body waves
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Seismic waves propagating through the earth are attenuated by the conversion of some fraction of the elastic energy to heat. Using terminology analogous to that used for the elastic properties that control seismic velocities, attenuation properties are characterized as anelastic properties. Attenuation data complement other physical measurements for characterizing rock properties. In seismic studies, attenuation data can at least double the information obtained from velocities alone. An understanding of the attenuative properties of the earth has two major motivations. First, seismic wave amplitudes are reduced as waves propagate through an anelastic medium, and this reduction is generally frequency dependent. Second, attenuation characteristics reveal much information, such as lithology, physical state, and degree of saturation of rocks. The phenomenon of attenuation is much more complex than the elastic aspects of seismic wave propagation, Both laboratory and field measurements are difficult to make. The mechanisms contributing to attenuation are numerous, and small changes in some conditions can affect attenuation significantly. However, sensitivities to some parameters, su ch as fluid saturation, make the measurement and understanding of attenuation highly important for many applications. The realization of the need and promise for specific data and models has prompted astrong resurgence of interest and research concerning attenuation in the fields of both seismology and rock physics. Laboratory measurements of attenuation in rock samples under varying pressures, temperatures, strain amplitudes, frequencies, and saturation conditions are presently being carried out. Detailed theoretical modeling of processes that may be responsible for attenuation is being undertaken. Measurements of attenuation in the earth using direct and refracted compressional and shear waves, surface waves, reflection seismograms, vertical seismic profiling, and full-wave acoustic well logs are being explored intensively. The net result of these field, laboratory, and theoretical studies will be a rapid expansion of our knowledge concerning the attenuation of seismic waves in the earths crust. We have undertaken the editing of this volume to help the broad-range research effort gain abetter understanding of attenuation and its applications to seismic exploration problems.
Anelastic attenuation factor
Saturation (graph theory)
Degree of saturation
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Seismic attenuation is defined as the loss of the seismic wave amplitude as the wave propagates excluding losses strictly due to geometric spreading. Information gleaned from seismic waves can be utilized to solve for the attenuation properties of the earth. One method of solving for earth attenuation properties is called t*. This report will start by introducing the basic theory behind t* and delve into inverse theory as it pertains to how the algorithm called tstarTomog inverts for attenuation properties using t* observations. This report also describes how to use the tstarTomog package to go from observed data to a 3-D model of attenuation structure in the earth.
Anelastic attenuation factor
Vertical seismic profile
Earth model
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C-28 FREQUENCY AND TRAVEL-DISTANCE CHARACTERISTICS OF SCATTERING ATTENUATION OBSERVED FROM SEISMIC PRIMARIES T.M. MÜLLER C.M.A. SICK and S.A. SHAPIRO FreieUniversität Berlin FachrichtungGeophysik Malteserstr. 74-100 12249Berlin Germany Summary We further develop and numerically verify a recently proposed scattering attenuation model which predicts the frequency- and traveldistance-dependent attenuation of seismic primaries propagating through weakly inhomogeneous solids. Such attenuation estimates are useful for seismic modeling and petrophysical interpretations of rocks. To test the theory we perform numerical simulations of seismic wave propagation in random media using a finite-difference solution of the elastodynamic wave equation. From the synthetic seismograms we determine the quality factor
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Anelastic attenuation factor
Petrophysics
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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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The amplitude of a seismic wavefront travelling through a heterogeneous rock matrix decreases due to (1) absorption or intrinsic attenuation, i.e. conversion of seismic energy to heat, and (2) apparent attenuation by scattering, i.e . transfer of seismic energy to later arrival times and other propagation directions. For a lithologic interpretation of seismic data it is highly desireable to separate both effects, for example by analyzing VSP or cross-hole data.
Anelastic attenuation factor
Dispersive body waves
Matrix (chemical analysis)
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Our detailed study of the crust and upper mantle of the South Baikal basin focused on seismic coda and seismic S-waves attenuation and estimated seismic quality factor ( Q S and Q C ), frequency parameter ( n ), attenuation coefficient (δ), total attenuation ( Q T ), and the ratio of two components the total attenuation: intrinsic attenuation ( Q i ), and attenuation due to scattering caused by the inhomogeneities of the medium ( Q SC ). We calculated the sizes of inhomogeneities revealed in the block medium, which put their effect on the attenuation of seismic waves in different frequency ranges. The seismic wave attenuation field was analyzed in comparison with the geological and geophysical characteristics of the medium, and a direct relationship was established between attenuation, composition and active processes in the crust and upper mantle of the studied area. According to the estimated intrinsic attenuation ( Q i ) and scattering attenuation ( Q SC ) contributions into the total attenuation, intrinsic attenuation is generally dominant in the studied area, while the Q SC component increases in the areas of large active faults.
Anelastic attenuation factor
Coda
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Seismic imaging, a technique in which the reflections of a source seismic wave are recorded as it passes through the earth, is a major tool for geophysical exploration. Seismic imaging can be used to reconstruct a profile of the material properties of the earth below the surface, and is thus widely used for locating hydrocarbons.
The problem presented by Husky Energy concerns seismic attenuation: the loss of energy as a seismic wave propagates through the earth. As an exploration tool, attenuation effects have only recently attracted attention. These effects can prove useful in two ways: as a means of correcting seismic data to enhance resolution of standard imaging techniques, and as a direct hydrocarbon indicator. Theoretically, a subsurface reservoir full of hydrocarbons will tend to be acoustically softer than a porous rock filled only with water; Kumar et al show that attenuation is highest in a partially fluid-saturated rock.
Many physical processes can lead to the attenuation of a seismic trace. In the present work, we ignore attenuation effects such as spherical divergence or scattering, and concentrate on intrinsic attenuation effects exclusively. The latter are caused by friction, particularly in porous rocks between fluid and solid particles.
The goal of the workshop was to find a means of computing seismic attenuation from relatively short windows of seismic imaging data, and particularly be able to identify regions of anomalous attenuation.
Anelastic attenuation factor
Vertical seismic profile
Geophysical Imaging
Seismic to simulation
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Thermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100 km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data.
Anelastic attenuation factor
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