S-wave Quality Factor Research Articles

Complex seismic amplitude inversion for P-wave and S-wave quality factors

Stratum quality factors (P-wave and S-wave quality factors, Q p and Q s ) have gradually been utilized in the study of physical state of crust and uppermost mantle, tectonic evolution, hydrogeololgy, gas hydrates, petroleum exploration, etc. Different opinions of the seismic attenuation mechanism result in various approaches to estimate the P-wave and S-wave quality factors. Considering the viscoelasticity of the underground medium, the constitutive matrix of the Earth medium is written as the superposition of homogeneous background medium, elastic perturbation medium and viscoelastic perturbation medium. Under the hypothesis of Born integral and stationary phase approximation, the seismic reflectivity is initially raised in terms of P-wave and S-wave moduli, density, P-wave and S-wave quality factors. Furthermore, incorporating the complex seismic traces with the seismic wavelets at different offsets, a two-step inversion approach is proposed to estimate the P-wave and S-wave quality factors. The AVO/AVA Bayesian inversion approach is suggested to estimate the P-wave modulus and S-wave modulus with the real component of the pre-stack seismic data initially. Taking the estimated P-wave and S-wave moduli as prior information, the P-wave and S-wave quality factors are further estimated with the imaginary component of the complex pre-stack seismic data, which is the quadrature of the original data. Finally, synthetic examples demonstrate that the proposed approach is able to estimate P-wave and S-wave quality factors stably and properly, and two field data examples demonstrate that the proposed approach may work as an efficient approach to fluid identification.

Quality Factor Characteristics of Six Metamorphic Kinds of Coal in China

AbstractLab ultrasonic measurements to metamorphic kinds of coal reveal the relation between the maximum vitrinite reflectance and ultrasonic quality factor. And then the relations between ultrasonic velocities and quality factors, including the anisotropic characteristics of quality factors are analyzed. The results show that there are good linear correlations between the maximum vitrinite reflectance and P‐ and S‐wave quality factors, between velocities in direction perpendicular to seams and quality factors, and between the P‐ wave quality and S‐wave's quality factors in direction vertical to seams. Moreover, there are stronger anisotropies in seismic quality factors than velocities. Comparison of the measured S‐wave Q values with estimated values, based on a theoretical formula, indicates an only 7.2% relative error in this method. It proves that this theoretical formula can be applied to attenuation compensation of the converted shear wave in processing of multi‐component seismic data.

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

ABSTRACTWe investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective QS/QP ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small.

Attenuation Tomography Based on Strong Motion Data: Case Study of Central Honshu Region, Japan

Three-dimensional frequency dependent S-wave quality factor (Qβ(f)) value for the central Honshu region of Japan has been determined in this paper using an algorithm based on inversion of strong motion data. The method of inversion for determination of three-dimensional attenuation coefficients is proposed by Hashida and Shimazaki (J Phys Earth. 32, 299–316, 1984) and has been used and modified by Joshi (Curr Sci. 90, 581–585, 2006; Nat Hazards. 43, 129–146, 2007) and Joshi et al. (J. Seismol. 14, 247–272, 2010). Twenty-one earthquakes digitally recorded on strong motion stations of Kik-net network have been used in this work. The magnitude of these earthquake ranges from 3.1 to 4.2 and depth ranging from 5 to 20 km, respectively. The borehole data having high signal to noise ratio and minimum site effect is used in the present work. The attenuation structure is determined by dividing the entire area into twenty-five three-dimensional blocks of uniform thickness having different frequency-dependent shear wave quality factor. Shear wave quality factor values have been determined at frequencies of 2.5, 7.0 and 10 Hz from record in a rectangular grid defined by 35.4°N to 36.4°N and 137.2°E to 138.2°E. The obtained attenuation structure is compared with the available geological features in the region and comparison shows that the obtained structure is capable of resolving important tectonic features present in the area. The proposed attenuation structure is compared with the probabilistic seismic hazard map of the region and shows that it bears some remarkable similarity in the patterns seen in seismic hazard map.

Simultaneous estimation of earthquake source parameters and crustal Q value from broadband data of selected aftershocks of the 2001 M w 7.7 Bhuj earthquake

This paper presents the simultaneous estimation of source parameters and crustal Q values for small to moderate-size aftershocks (M w 2.1–5.1) of the $M_{w }$ 7.7 2001 Bhuj earthquake. The horizontal-component S-waves of 144 well located earthquakes (2001–2010) recorded at 3–10 broadband seismograph sites in the Kachchh Seismic Zone, Gujarat, India are analyzed, and their seismic corner frequencies, long-period spectral levels and crustal Q values are simultaneously estimated by inverting the horizontal component of the S-wave displacement spectrum using the Levenberg–Marquardt nonlinear inversion technique, wherein the inversion scheme is formulated based on the ω-square source spectral model. The static stress drops (Δσ) are then calculated from the corner frequency and seismic moment. The estimated source parameters suggest that the seismic moment (M 0) and source radius (r) of aftershocks are varying from 1.12 × 1012 to 4.00 × 1016 N-m and 132.57 to 513.20 m, respectively. Whereas, estimated stress drops (Δσ) and multiplicative factor (E mo) values range from 0.01 to 20.0 MPa and 1.05 to 3.39, respectively. The corner frequencies are found to be ranging from 2.36 to 8.76 Hz. The crustal S-wave quality factor varies from 256 to 1882 with an average of 840 for the Kachchh region, which agrees well with the crustal Q value of the seismically active New Madrid region, USA. Our estimated stress drop values are quite large compared to the other similar size Indian intraplate earthquakes, which can be attributed to the presence of crustal mafic intrusives and aqueous fluids in the lower crust as revealed by the earlier tomographic study of the region.

Anisotropic attenuation analysis of crosshole data generated during hydraulic fracturing

Attenuation of seismic waves is sensitive to the physical properties of the subsurface and has been observed in vertical seismic profiling (VSP) and reflection data. De et al. (1994) report measurements of the P- and S-wave quality factors from VSP surveys and sonic logs. Klimentos (1995) measures compressional and shear attenuation from sonic logs in sandstone formations with variable oil, water, and gas saturation and observes that [Formula: see text] and [Formula: see text] can be used for pore-fluid discrimination. Maultzsch et al. (2007) evaluate P-wave azimuthal attenuation anisotropy from 3D VSP data acquired over a fractured hydrocarbon reservoir and infer fracture directions from attenuation analysis. Attenuation anisotropy has also been observed in P-wave reflection data (Clark et al., 2009; Vasconcelos and Jenner, 2005). Seismic attenuation is most commonly measured using the spectral-ratio method. Zhu et al. (2007) extend the spectral-ratio method to anisotropic media and apply it to physical modeling data acquired for a transversely isotropic (TI) sample. Computing spectral ratios helps eliminate the source spectrum and can be used to obtain accurate effective and interval attenuation coefficients of P- and S-waves in layered anisotropic media (Behura and Tsvankin, 2009a; Shekar and Tsvankin, 2011).

Estimation of near-surface quality factors by constrained inversion of Rayleigh-wave attenuation coefficients

Quality factors (Q) of near-surface materials are as important as velocities of the materials in many applications. Only phase information of surface-wave data is utilized when high-frequency (≥ 2 Hz) surface-wave data are routinely inverted to determine near-surface shear (S)-wave velocities. Amplitude information of high-frequency surface-wave data can be used to determine quality factors of near-surface materials. Given S-wave velocity, compressional (P)-wave velocity, and Rayleigh-wave phase velocities , it is feasible to solve for S-wave quality factor Q S and P-wave quality factor Q P (for some specific velocity models) down to 30 m below the ground surface in many settings by inverting high-frequency Rayleigh-wave attenuation coefficients in a layered earth model. Amplitude of seismic data is an exponential function of attenuation coefficients. When calculating attenuation coefficients from changes in amplitude, this nonlinear nature would result in that small variations in amplitude cause huge changes in attenuation coefficients. This result suggests data (attenuation coefficients) that normally possess large errors could eventually transfer to a model (quality factors); therefore, constraints (or a priori information) on models are necessary. Because an inversion system to solve this problem is unstable, a regularization parameter must be introduced into an inversion algorithm to stabilize the inversion. These characteristics of the inversion problem allow us to solve the problem as a constrained and regularized linear system . Usually, a set of models that meet the defined constraints can be obtained by solving the system. Based on the linear nature of the inversion system, a smooth model can be selected from the set of models as a solution of the inversion using the L-curve method. This approach is a trade-off solution between data misfit and model length. Several real-world examples demonstrate the importance of constraints in finding acceptable realistic quality factors from empirical data. ► Amplitude of seismic data is an exponential function of attenuation coefficients. ► Small variations in amplitude cause huge changes in attenuation coefficients. ► Large errors in attenuation coefficients could transfer to quality factors. ► Constraints (or a priori information) on models are necessary. ► A realistic model can be selected using the L-curve method.

Assessing the monitorability of CO2 saturation in subsurface saline aquifers

We propose a method to assess the geophysical monitorability of petrophysical parameters (in this case CO2 saturation) in subsurface geological reservoirs. The approach is based on measuring and inverting up to six geophysical parameters: P- and S-wave impedances and quality factors (1/attenuation), density and electrical resistivity. We use Shannon's information measure to quantify the information obtained from the inversion of different combinations of geophysical parameters. We thus develop a generic approach to assess the joint monitorability of supercritical CO2 saturation when CO2 is stored in subsurface reservoirs. Uncertainty analysis shows that expected information is in general nonlinearly related to the level of a priori or measurement uncertainty in the geophysical and petrophysical parameters, to the true CO2 saturation, and also to the measurement frequency in the case of seismic measurements. Prior uncertainties in petrophysical parameters such as porosity have a significant effect on the monitorability, highlighting the need for accurate benchmark (pre-injection) measurements and reservoir characterization in the case of time-lapse (4D) monitoring. We show that estimates of P-wave seismic attenuation contribute most to the overall information obtained, and that in principle the joint application of different geophysical methods can significantly improve monitorability. We applied the approach to assess the monitorability of the CO2 saturation after 10 years of simulated injection into a saline aquifer reservoir in the near-shore UK North Sea. Application of the approach to design optimal data combinations for hydrocarbon saturation assessment and monitoring is straightforward.

Shallow seismic structure of Kunlun fault zone in northern Tibetan Plateau, China: implications for the 2001Ms8.1 Kunlun earthquake

SUMMARY The shallow seismic velocity structure of the Kunlun fault zone (KLFZ) was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions. The data were collected after the 2001 Kunlun Ms8.1 earthquake in the northern Tibetan Plateau. Seismic phases for the in-line record sections (26 records up to a distance of 15 km) along the fault zone were analysed, and 1-D P- and S-wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method. Sixteen seismic stations were deployed along the off-line profile perpendicular to the fault zone. Fault-zone trapped waves appear clearly on the record sections, which were simulated with a 3-D finite difference algorithm. Quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the Kunlun fault-zone width is 300 m, and S-wave quality factor Q within the fault zone is 15. Significantly, S-wave velocities within the fault zone are reduced by 30–45 per cent from surrounding rocks to a depth of at least 1–2 km, while P-wave velocities are reduced by 7–20 per cent. A fault-zone with such Pand S-low velocities is an indication of high fluid pressure because Vs is affected more than Vp. The low-velocity and low-Q zone in the KLFZ model is the effect of multiple ruptures along the fault trace of the 2001 Ms8.1 Kunlun earthquake.

Theory and modeling of constant-Q P- and S-waves using fractional time derivatives

I have developed and solved the constant-[Formula: see text] model for the attenuation of P- and S-waves in the time domain using a new modeling algorithm based on fractional derivatives. The model requires time derivatives of order [Formula: see text] applied to the strain components, where [Formula: see text] and [Formula: see text], with [Formula: see text] the P-wave or S-wave quality factor. The derivatives are computed with the Grünwald-Letnikov and central-difference fractional approximations, which are extensions of the standard finite-difference operators for derivatives of integer order. The modeling uses the Fourier method to compute the spatial derivatives, and therefore can handle complex geometries and general material-property variability. I verified the results by comparison with the 2D analytical solution obtained for wave propagation in homogeneous Pierre Shale. Moreover, the modeling algorithm was used to compute synthetic seismograms in heterogeneous media corresponding to a crosswell seismic experiment.

3‐D Tomography for the S‐Wave Quality Factor of the Crust and Upper Mantle Beneath the Chinese Mainland

AbstractWe select 223 pairs of Rayleigh wave from 2681 waveforms recorded at 58 stations in the Eurasia continent and west Pacific region. These data are two‐path records of the rays which pass through the Chinese mainland and meet the requirements of our study. Using the method combining phase‐matched filter and Winner filter in frequency domain, we calculate the attenuation coefficient of the mixed path between two stations. We make a mesh of 4° x 4° to the Chinese mainland and invert the attenuation coefficient of pure path for each grid element. Then we perform depth inversion for each element based on the velocity and density models to obtain the S wave quality factor (Qβ) in the depth range of about 10~350km. In this way the Qβ distribution of the whole Chinese mainland is acquired. The result shows that the lateral and vertical change of the Qβ distribution is very evident. The asthenosphere structure can be clearly identified from the image. The Qβ structure manifests remarkable variations between tectonic units. This result provides helpful information for the study on the material property, viscidity structure, and dynamic process of the Earth interior beneath the Chinese mainland.

A shallow fault-zone structure illuminated by trapped waves in the Karadere-Duzce branch of the North Anatolian Fault, western Turkey

SUMMARY We discuss the subsurface structure of the Karadere‐Duzce branch of the North Anatolian Fault based on analysis of a large seismic data set recorded by a local PASSCAL network in the 6 months following the Mw = 7.4 1999 Izmit earthquake. Seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long-period oscillations in both P- and S-wave trains that do not exist in nearby off-fault stations. Examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone. The anomalous features in fault-zone seismograms produced by events not necessarily in the fault may be referred to generally as fault-zone-related site effects. The oscillatory shear wave trains after the direct S arrival in these seismograms are analysed as trapped waves propagating in a low-velocity fault-zone layer. The time difference between the S arrival and trapped waves group does not grow systematically with increasing source‐receiver separation along the fault. These observations imply that the trapping of seismic energy in the Karadere‐Duzce rupture zone is generated by a shallow fault-zone layer. Traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3‐4 km. The synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m, a velocity decrease relative to the surrounding rock of approximately 50 per cent and an S-wave quality factor of 10‐15. The results are supported by large 2-D and 3-D parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones. The inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower-type structure. The motion amplification associated with fault-zone-related site effects increases the seismic shaking hazard near fault-zone structures. The effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large.

Determining Q of near-surface materials from Rayleigh waves

High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity ( V S), compressional (P)-wave velocity ( V P), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor Q P and S-wave quality factor Q S in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting Q S from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from Q P cannot be ignored when Vs/ V P reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert Q P from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to Q P than to Q S. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.

Estimation on site-amplification from different methods using strong motion data obtained in Tangshan, China

A seismic observation array for strong motions was deployed to estimate seismic source, propagation path and local site effects in Tangshan, China. We compared site response from the S-wave inversion and those from other techniques, such as traditional direct spectral ratios of S waves and receiver-function of S waves. From the inversion, we found that S-wave quality factor,i.e. Q S-value, is approximately satisfied with the relation ofQ S=67f 1.1 in the range of frequency from 0.5 Hz to 32 Hz and that the source spectra follow theω −2 model of seismic source for low frequencies less than about 12 Hz. From the comparison of site responses estimated by the different methods for each soil site, we found that all the methods can extract the same predominant peaks from the responses, the amplifications from direct S-wave spectral ratios are well correlated with those from the S-wave inversion within a factor of 2 to 3, while the correlation between the amplifications from S-wave receiver-function and those from the S-wave inversion is not good, especially for high frequencies more than 8 Hz.

DEVELOPMENT OF LOCALIZED IDENTIFICATION OF S-WAVE VELOCITY AND QUALITY FACTOR IN SUBSURFACE LAYERS OF GROUND

筆者らは, これまで鉛直アレー観測記録を用いて地盤各層のS波速度とQ値を同定する手法を提案してきた. この方法では, 地盤を構成する層の数が増加すると, 同定すべきパラメータが増加するため, 同定精度が低下するという問題点が生じた. 本研究は, この問題の1つの解決策として, ある特定のいくつかの層のみを独立に同定する, いわゆる部分同定法を開発し, その妥当性を検討したものである. 具体的には, 地中の連続する3点で得られた鉛直アレー観測記録を用いて, これらの記録が得られた最上点と最下点の間の各層のS波速度とQ値を独立に同定する手法を提案した.

Qβ estimates from spectral ratios and multiple lapse time window Analysis: Results from an underground research laboratory in granite

An experiment to measure quality factor (Q) in‐situ was carried out at the Underground Research Laboratory in Manitoba, Canada using the Spectral Ratio technique and a form of the Mulitple Lapse Time Window Analysis technique. The volume of rock examined measures approximately 30 × 30 × 30 m at a depth of 420 m in Lac Du Bonnet Granite. The site encompases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls. The source for the seismic pulses was a mechanical hammer placed at the bottom of specially located boreholes. Results from the Spectral Ratio experiment gave S‐wave quality factor (Qβ) of 114 ± 18. Based upon the short duration of the first arrival pulses and low scattered wave energy in the signal coda, we conclude that the Spectral Ratio technique measured the intrinsic attenuation of the rock mass. Further analysis using the more recently developed Multiple Lapse Time Window Analysis gave results for intrinsic Qβ (Qi) of between 85 and 100. Conversely, values for scattering Qβ (Qs) were greater than 800. The high correlation for Qi values between the two methods help to confirm the nature and absolute values of the attenuation, and also support the use of the later technique where spectral ratios cannot be applied.

Crustal wave propagation anomaly across the Pyrenean Range. Comparison between observations and numerical simulations

Lg records analysis and numerical modelling of Lg propagation are used to find out to what extent this phase can be seen as a marker of unidentified structural anomalies in the crust. This study is based on Lg propagation through the Pyrenean range from earthquakes located in Spain. We have first evaluated the mean value of the S-wave quality factor for central Spain. We have computed simultaneously the seismic station responses and the source functions. The correction for propagation effects, assuming a homogeneous attenuation and the theoretical calculation of the Lg excitation, lead to the seismic moment of each event. The moment magnitude obtained correlates well with the magnitude proposed by the local networks. This gives a confirmation of the Q model in the low-frequency range (1-5 Hz). As we intended to compare traces of different Spanish earthquakes recorded in France at different epicentral distances, we had to make amplitudes independent of propagation and source effects. Therefore, we corrected the spectral amplitudes for geometrical spreading, anelastic attenuation and normalized them to equal seismic moment. We then plotted the records as a function of group velocity, in order to make up a fan profile along the Pyrenean axis. The resulting section reveals that in the central and the eastern parts of the range, neither the North Pyrenean Fault, nor the Moho jump deduced from seismic-refraction experiments and vertical seismics, seem to affect Lg propagation. However, there is an extinction of the Lg phase in the western part of the chain. The lateral extent of this area is correlated with a zone of positive gravity anomaly, probably linked to the presence of dense material of mantle origin. A numerical simulation in the low-frequency band indicates that the Moho topography inferred from deep seismic soundings does not explain the strength of the observed attenuation. Ray-tracing seismograms show that, at high frequency, the conclusion is the same. The attenuation effect due to lateral variation of structure should not be so strong. We, therefore, think that attenuation of guided waves is not due to large-scale geometry effects, but is due to local properties of the crustal materials, possibly apparent attenuation due to scattering on small-scale heterogeneities.

Fault‐zone attenuation of high‐frequency seismic waves

We have developed a technique to measure seismic attenuation within an active fault‐zone at seismogenic depths. Utilizing a pair of stations and pairs of earthquakes, spectral ratios are performed to isolate attenuation produced by wave‐propagation within the fault‐zone. This empirical approach eliminates common source, propagation, instrument and near‐surface site effects. The technique was applied to a cluster of 19 earthquakes recorded by a pair of downhole instruments located within the San Andreas fault‐zone, at Parkfield California. Over the 1‐40 Hz bandwidth used in this analysis, amplitudes are found to decrease exponentially with frequency. Furthermore, the fault‐zone propagation distance correlates with the severity of attenuation. Assuming a constant Q attenuation operator, the S‐wave quality factor within the fault‐zone at a depth of 5‐6 kilometers is 31 (+7,−5).If fault‐zones are low‐Q environments, then near‐source attenuation of high‐frequency seismic waves may help to explain phenomenon such as fmax. Fault‐zone Q may prove to be a valuable indicator of the mechanical behavior and rheology of fault‐zones. Specific asperities can be monitored for precursory changes associated with the evolving stress‐field within the fault‐zone. The spatial and temporal resolution of the technique is fundamentally limited by the uncertainty in earthquake location and the interval time between earthquakes.