P-wave Quality Factor Research Articles

The testing of Qp and QsQp equation in Kujung I Formation, East Java, Indonesia

Hydrocarbon-bearing reservoir shows attenuation on seismic section, but the use of attenuation in hydrocarbon exploration is less popular than other attributes. This is due to the sophisticated and advanced technology required to measure attenuation. Meanwhile, some studies were designed to estimate attenuation by formulating rock quality factor equations from a wire-line log, although others run on sandstone samples. Therefore, this study aims to estimate rock quality factors in carbonate rock of Kujung Formation using three different methods, discriminate hydrocarbon-bearing reservoir, and identifying the impact of carbonate facies. The results showed the P-wave quality factor method is sensitive to discriminate high gas saturation on facies A. Meanwhile, the QsQp ratio is sensitive to discriminate shale and carbonate. The SQp and SQs methods are sensitive to discriminate hydrocarbon-bearing reservoirs and less affected by carbonate facies. The novelty of this is to estimate the rock quality factor from various methods and identify the impact of facies in hydrocarbon identification, especially for carbonate reservoirs in Northeast Java Basin. Therefore, a new understanding of facies impact on rock quality factor contributes to choosing the best methods in discriminating pore-filling fluid identification.

Viscoelastic Wave Propagation Simulation Using New Spatial Variable-Order Fractional Laplacians

ABSTRACTTime-domain constant-Q (CQ) viscoelastic wave equations have been derived to efficiently model Q, but are known to break down in accuracy in describing CQ attenuation at low Q. In view of this, a new time-domain viscoelastic wave equation for modeling wave propagation in anelastic medium is evaluated based on Kjartansson’s CQ model to improve the accuracy in describing CQ attenuation at low Q. We use an approximate frequency-domain viscoelastic wave equation to replace the accurate frequency-domain viscoelastic wave equation. Then, a new time-domain wave equation is derived by converting the approximate viscoelastic wave equation from the frequency domain to the time domain. The newly derived viscoelastic wave equation consists of several Laplacian differential operators with variable fractional order. We use an arbitrary-order Taylor series expansion (TSE) to approximate the derived mixed domain fractional Laplacian operators, and realize the decoupling of the wavenumber and fractional order. Then, the proposed viscoelastic wave equation can be solved directly using the staggered-grid pseudospectral method (SGPSM). We evaluate the precision of the new viscoelastic wave equation by comparing the numerical solutions with the analytical solutions in homogeneous medium. Theoretical curve analysis and numerical results indicate that the proposed fractional viscoelastic wave equation has higher precision in describing CQ attenuation than that of the traditional fractional viscoelastic wave equation, especially for cases that P-wave quality factor QP is less than 10, and S-wave quality factor QS is less than 8. Furthermore, we use two numerical examples to verify the effectiveness of the TSE SGPSM in heterogeneous media. The discussion shows that the advantage of using our fractional viscoelastic wave equation over the traditional fractional viscoelastic wave equation is the higher precision in describing CQ attenuation at different frequency.

Seismic wave attenuation in carbonate rock and its implication to hydrocarbon-bearing reservoir identification, East Java, Indonesia

The heterogeneity of carbonate rock properties triggers propagated seismic wave to get some attenuations. This mechanism allows that attenuation can be applied in hydrocarbon exploration to identify lithology, porosity, fluid content and fracture identification. Unfortunately, to measure the degree of attenuation, the sophisticated tools and advanced processing technology are required. Thus, some methods were formulated to estimate attenuation. This study is aimed to estimate and compare attenuation from wireline log interpretation by using four different methods. The study is also supported by a series of exploration wells penetrating Kujung I Formation in northwestern edge of North East Java Basin, Indonesia. The result shows that only Qs from Zhang et al method is overestimate. P-wave quality factor of Klimentos and Zhang methods and scaled quality factor (SQp and SQs) method are sensitive to identify hydrocarbon. Furthermore, lithofacies also affects estimated rock quality factor. The novelty of study is to estimate and compare the attenuation from different method and review its result and application in hydrocarbon identification, especially for carbonate reservoir in North East Java basin, Indonesia. So that, a new understanding of attenuation characteristic in carbonate reservoir contributes to discriminate pore fluid identification in hydrocarbon exploration.

Seismic Attenuation Tomography From 2018 Lombok Earthquakes, Indonesia

Local earthquake data was used to determine a three-dimensional (3D) seismic attenuation structure around the aftershock source region of the 2018 Lombok earthquake in Indonesia. The aftershocks were recorded by 13 seismic stations from August 4 to September 9, 2018. The selected data consist of 6,281 P-wave t∗ values from 914 events, which had good t∗ quality in at least four stations. Our results show that the two aftershock clusters northwest and northeast of Lombok Island have different attenuation characteristics. A low P-wave quality factor (low-Qp), low P-wave velocity (Vp), and high ratio of P-wave velocity and S-wave velocity (Vp/Vs), which coincide with a shallower earthquake (<20 km) northwest of Lombok Island, might be associated with a brittle area of basal and imbricated faults influenced by high fluid content. At the same time, the high-Qp, low Vp, and low Vp/Vs, which coincide with a deeper earthquake (>20 km) northeast of Lombok Island, might be associated with an area that lacks fluid content. The difference in fluid content between the northwest and northeast regions might be the cause of the early generation of aftershocks in the northwest area. The significant earthquake that happened on August 5, 2018, took place in a region with moderate Qp, close to the contrast of high and low-Qp and high Vp, which suggests that the earthquake started in a strong material before triggering the shallower aftershocks occurring in an area affected by fluid content. We also identified an old intrusive body on the northeast flank of the Rinjani volcano, which was characterized by a high-Qp, high-velocity, and a high Bouguer anomaly.

Ultrasonic velocity, attenuation, and mechanical behavior of Longmaxi bedded shale under uniaxial compressive tests

Knowledge of ultrasonic wave velocity and attenuation related with bedding orientations plays an important role for successful downhole, hydraulic fracturing, and surface seismic data interpretation. Real-time ultrasonic wave measurements in load direction have been conducted on Longmaxi shale specimens during compressive fractured process in this study. The relations between mechanical properties and ultrasonic response of shale with various bedding angles are investigated. The P- and S-wave velocities increase with increasing stress to the peak strength, which can be ascribed to the closure of microcracks and pores perpendicular to the wave propagation direction. And then they decrease with sudden fall of stress at the post-peak stage that is controlled by the coalescence of newly generated microcracks to macroscopic fractures. The P-wave velocity is more sensitive to the bedding angles and axial stress than S-wave velocity. The axial stress weakens the velocity anisotropy of shale. The P-wave attenuations decrease (quality factor values increase) for 0°, 15°, and 30° shale specimens under pressure, whereas the first increment and then less variation occur for 45°, 60°, 75°, and 90° specimens. The difference may be caused by the bedding scattering that is more pronounced when the beddings are more inclined and perpendicular with respect to the wave propagation direction. The P-wave quality factor has a close relationship with the peak strength for each inclination specimen. What’ s more, the failure modes have been illustrated based on the wave velocity and attenuation anisotropy.

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

The quality factor Q is highly sensitive to the medium properties related to the fluid presence, like porosity, permeability and level of rock saturation. Therefore, it is very useful to image the subsoil of volcanoes in terms of anelastic images, in addition to more common elastic tomographic models. In this geological structure, indeed, fluids play a key role in controlling and governing the evolution of magmatic processes, so their accurate identification and tracking is crucial. Here, we describe a methodology that allows to obtain accurate 1-D and 3-D attenuation models from seismic active data. It has been applied to a small-scale volcanic volume of the shallow Solfatara crater, in southern Italy. Our purposes were achieved in three steps: 1/ we measured the attenuation parameter t* from active survey data, first by cross-correlating the signal with the sweep function and then by using the spectral decay method; 2/ we determined a reference 1-D attenuation model for the area through a grid model search procedure; 3/ we reconstructed the 3-D attenuation model through an iterative inversion of t* data. In order to validate the t* catalogue, we analyzed the map of t* residuals with respect to the reference 1-D model... The shallowest subsoil of Solfatara is characterized as a volume with a P-wave quality factor, QP, which increases with depth and has a high lateral anelastic heterogeneity. QP can be therefore considered as a proxy for the detection of strong variations both in fluid saturations and in the type of fluids permeating the shallowest volcanic sediments of the investigated volume.

Amplitude-based misfit functions in viscoelastic full-waveform inversion applied to walk-away vertical seismic profile data

Viscoelastic full-waveform inversion is applied to walk-away vertical seismic profile data acquired at a producing heavy-oil field in Western Canada for the determination of subsurface velocity models (P-wave velocity [Formula: see text] and S-wave velocity [Formula: see text]) and attenuation models (P-wave quality factor [Formula: see text] and S-wave quality factor [Formula: see text]). To mitigate strong velocity-attenuation trade-offs, a two-stage approach is adopted. In Stage I, [Formula: see text] and [Formula: see text] models are first inverted using a standard waveform-difference (WD) misfit function. Following this, in Stage II, different amplitude-based misfit functions are used to estimate the [Formula: see text] and [Formula: see text] models. Compared to the traditional WD misfit function, the amplitude-based misfit functions exhibit stronger sensitivity to attenuation anomalies and appear to be able to invert [Formula: see text] and [Formula: see text] models more reliably in the presence of velocity errors. Overall, the root-mean-square amplitude-ratio and spectral amplitude-ratio misfit functions outperform other misfit function choices. In the final outputs of our inversion, significant drops in the [Formula: see text] to [Formula: see text] ratio (~1.6) and Poisson’s ratio (~0.23) are apparent within the Clearwater Formation (depth ~0.45–0.50 km) of the Mannville Group in the Western Canada Sedimentary Basin. Strong [Formula: see text] (~20) and [Formula: see text] (~15) anomalies are also evident in this zone. These observations provide information to help identify the target attenuative reservoir saturated with heavy-oil resources.

A downscaling strategy from FWI to microscale reservoir properties from high-resolution images

Extracting detailed earth information from an ensemble of seismic traces is a challenge facing full-waveform inversion. So far, success on synthetic and real data has been accomplished primarily for the twin purposes of complex structural imaging and geologic interpretation. An ongoing issue for the seismic-imaging community, in addition to building high-resolution images, is the reliable extraction of acoustic and shear velocities, anisotropic parameters, quality factors, and density. Such extractions, performed at the seismic resolution scale, should help greatly with quantitative interpretation and estimation of rock properties. A step toward this goal is described here. A generic rock-physics model is assumed, which upscales microscale rock-physics properties to mesoscale (effective-medium) poroelastic quantities to be recovered from macroscale estimates of seismic attributes. It is shown on simple synthetic examples that quantitative multiparameter reconstruction, when it is possible, can reduce ambiguities in mesoscale parameter estimation dramatically, using a semiglobal search. Successful estimation of these effective-medium quantities will narrow the range of possible rock-physics estimations to be considered for seismic imaging target zones. For example, estimating the P-wave quality factor along with P-wave velocity from full-waveform inversion is shown to change the estimation of mesoscale parameters significantly, assuming that the upscaling of the rock-physics model and the recovered macroscale parameters are well constrained. In addition, shear-wave information is shown to be crucial for pressure-saturation discrimination. The inferred information at the reservoir level, resulting from full-waveform inversion and subsequent mesoscale estimation, can be useful for reservoir characterization.

Ultrasonic elastic characteristics of five kinds of metamorphic deformed coals under room temperature and pressure conditions

The calibration of the elastic characteristics of deformed coals is essential for seismic inversion of such units, because the prediction of coal deformation is essential for both mining safety and methane production. Therefore, many samples of broken and mylonitic deformed coal were tested with ultrasonic waves in the laboratory. These samples came from four mining areas: the Huainan, Pingdingshan, Hebi and Jiaozuo coal mines, which present five different metamorphic ranks shown as cylinders striking across circular limits of steel. Under normal pressures and temperatures, ultrasonic P- and S-wave tests show that the velocities, quality factors, and elastic moduli of the deformed coals were greatly reduced compared with undeformed coals. Also, some correlation was found between the P- and S-wave velocities in the deformed coals. However, there is no evidence of linear correlations between velocity and density, velocity and quality factor, or the quality factors of P- and S-waves. Compared with the elastic characteristics of undeformed coals, such as P- and S-wave velocity ratios or Poisson’s ratio, those of deformed coals generally decrease and the P-wave quality factors are less than those of S-waves. Moreover, the analysis of the relationship between pore structure and elastic modulus shows a better correlation between the P- and S-wave velocities and effective porosity, pore volume and specific surface area. Also, there are similar relationships between the pore structure and the Young’s and shear moduli. However, there are no such correlations with other moduli. Correlations between these elastic moduli, pore structure, coal rank and density were not found for the various samples of deformed coals, which is consistent with only structural destruction occurring in the deformed coals with other physical properties remaining unchanged. The experimental results show that it is possible to predict the deformation of coals with multi-component seismic elastic inversion.

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.

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.

Estimation of interval velocity and attenuation anisotropy from reflection data at Coronation Field

Attenuation can be extremely valuable in characterizing gas accumulations in shales and sandstones. Laboratory studies clearly indicate that attenuation is closely related to fluid saturation and mobility. Well-log analysis shows that P-wave attenuation is higher in gas-bearing rocks than in those saturated with oil or water. Extremely low P-wave quality factors (Q), ranging between 5 and 10, have been observed in some gas reservoirs. For example, such low Q values in gas sands are reported by Mavko and Dvorkin (2005) in well logs from the Gulf Coast.

High-resolution passive seismic tomography for 3D velocity, Poisson’s ratioν, and P-wave qualityQPin the Delvina hydrocarbon field, southern Albania

We have studied using traveltimes of P- and S-waves and initial seismic-pulse rise-time measurements from natural microearthquakes to derive 3D P-wave velocity VPinformation (mostly structural) as well as P- and S-wave velocity VP/VSand P-wave quality factor QPinformation (mostly lithologic) in a known hydrocarbon field in southern Albania. During a 12-month monitoring period, 1860 microearthquakes were located at a 50-station seismic network and were used to obtain the above parameters. The data set included earthquakes with magnitudes ranging from –0.1 to 3.0 R (Richter scale) and focal depths typically occurring between 2 and 10 km. Kohonen neural networks were implemented to facilitate the lithological classification of the passive seismic tomography (PST) results. The obtained results, which agreed with data from nearby wells, helped delineate the structure of the reservoir. Two subregions of the investigated area, one corresponding to an oil field and one to a gas field, were correlated with the PST results. This experiment showed that PST is a powerful new geophysical technique for exploring regions that present seismic penetration problems, difficult topographies, and complicated geologies, such as thrust-belt regions. The method is economical and environmentally friendly, and it can be used to investigate very large regions for the optimal design of planned 2D or 3D conventional geophysical surveys.

Seismic wave attenuation in the uppermost glacier ice of Storglaciären, Sweden

AbstractWe conducted seismic refraction surveys in the upper ablation area of Storglaciären, a small valley glacier located in Swedish Lapland. We estimated seismic-wave attenuation using the spectral-ratio method on the energy travelling in the uppermost ice with an average temperature of approximately −1 °C. Attenuation values were derived between 100 and 300 Hz using the P-wave quality factor, QP, the inverse of the internal friction. By assuming constant attenuation along the seismic line we obtained mean QP = 6 ± 1. We also observed that QP varies from 8 ± 1 to 5 ± 1 from the near-offset to the far-offset region of the line, respectively. Since the wave propagates deeper at far offsets, this variation is interpreted by considering the temperature profile of the study area; far-offset arrivals sampled warmer and thus more-attenuative ice. Our estimates are considerably lower than those reported for field studies in polar ice (∼500–1700 at −28°C and 50–160 at −10°C) and, hence, are supportive of laboratory experiments that show attenuation increases with rising ice temperature. Our results provide new in situ estimates of QP for glacier ice and demonstrate a valuable method for future investigations in both alpine and polar ice.

PS-waveQestimation based on the P-waveQvalues

Through assumption of the equivalent velocity and equivalent quality factor of the PS-wave, in visco-elastic media, PS-wave Q estimation can be realized with the P-wave quality factor and P- to S-wave velocity ratio. For sedimentary rock, which has strong agglutination, the internal friction is mainly contributed by the shear friction along crevices or inter-granular crevices, so that a relationship between PS-wave Q values and P-wave Q values can be built up even when the S-wave Q values are unknown. In the estimation of the PS-wave quality factor, the P- to S-wave velocity ratio can be computed based on two-way traveltimes of PP and PS events respectively, in order to avoid the influence of the inaccuracy of P- and S-wave velocities. The method is demonstrated with a zero-offset VSP data in a coal-mining field. The results of P-wave and PS-wave Q values estimated show a consistency with lithology revealed from drilling.

P-wave seismic attenuation by slow-wave diffusion: Numerical experiments in partially saturated rocks

P-wave attenuation by slow-wave diffusion is a significant loss mechanism at seismic frequencies. This effect is known as mesoscopic loss, because it is a consequence of fluid flow at mesoscopic-scale inhomogeneities. These are larger than the pore size but smaller than the wavelength, typically tens of centimeters, and are due to local variations in lithological properties or to patches of immiscible fluids. Basically, a P-wave traveling in a porous medium induces a fluid-pressure gradient in regions of different properties, such as patches saturated with different fluids, generating slow P-waves, which diffuse away from the interfaces separating the fluids. This mechanism can be explained by the combined effect of mesoscopic-scale inhomogeneities and mode conversion at interfaces. We consider a periodically stratified medium and perform numerical experiments to determine the P-wave quality factor in partially saturated rocks. The modeling method is an iterative domain-decomposition 2D finite-element algorithm for solving Biot equations of motion in a parallel computer, which is a requirement to run the numerical experiments at seismic frequencies. The simulated pulses show evidence of the mesoscopic-loss mechanism, and the quality factors estimated with the spectral-ratio and frequency-shift methods are in good agreement with the theoretical values predicted by the White theory. Errors in the estimation of the quality factor are less than 5% (spectral ratio) and 3% (frequency shift).

Source parameters of microearthquakes at Mount St Helens (USA)

SUMMARY We estimate the source parameters for a selection of microearthquakes that occurred at Mount St Helens in the period 1995–1998. Excluding the activity of 2004 September, this time period includes the most intense episode of earthquake activity since the last dome-building eruption in 1986 October. 200 seismograms were processed to obtain seismic moments, source radii, stress drops and average fault slip. The source parameters were determined from the spectral analysis of P waves, after correction for attenuation and site effects. In particular, P-wave quality (Qp) and site (S) factors have been previously calculated in the frequency ranges 2–7 Hz and 18–30 Hz. Because it was impossible to perform corrections for Qp and S over the whole spectrum we applied a new approach, based on the notion of ‘holed spectrum’, to estimate spectral parameters. The term ‘holed spectrum’ indicates a spectrum lacking corrected spectral amplitude values at certain frequencies. We carried out a statistical study to verify that dealing with the ‘holed spectrum’ does not lead to significant differences in the estimates of spectral parameters. We also investigated the dependence of spectral parameters (low-frequency level, corner frequency and high-frequency decay) on the bandwidth of spectral hole, and defined the threshold values for three different spectral models. Displacement ‘holed spectra’, corrected by attenuation and site response, are then used to determine spectral parameters in order to calculate seismic source parameters. Seismic moments range from 1017 to 1019 dyne-cm, source dimensions from 100 to 350 m, and average fault slip from 0.003 to 0.1 cm. Self-similarity seems to break down in that stress drops are very low (0.1–1 bars). We postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime.

Three-dimensional tomographic imaging of the Taranaki volcanoes, New Zealand

3-D models of the P-wave velocity (Vp), the ratio of P- to S-wave velocity (Vp/Vs), and the P-wave quality factor (Qp) are determined for the crust in Taranaki, New Zealand, using local tomography and data from a dense network (average spacing 5 km) of 68 three-component, broad-band seismographs. Vp and Vp/Vs models were determined by jointly inverting P traveltimes and S–P traveltime intervals, and a Qp model by inverting t* (t/Qp) observations derived from modelling the velocity amplitude spectrum of P wave arrivals. An approximately 5 km diameter region of high Vp and low Vp/Vs beneath the Taranaki volcanoes extends from the surface to a depth of about 10 km and images the roots of the volcanoes formed by successive magmatic intrusions. At Mt Taranaki this will probably be the path through which future magma intrusions will reach the surface. We are unable to image any magma storage within the upper 16 km of the crust as the volumes involved are probably smaller than the 5 km resolution of our models. The volcanoes sit in the Taranaki basin, a large sedimentary basin characterized by low Vp (c. 4 km s−1) and high Vp/Vs (≥1.9), attributed to unconsolidated, water-saturated sediments, and these are underlain by basement rocks with Vp≥ 5 km s−1 and Vp/Vs≤ 1.7. There is a c. 20 per cent contrast in Vp across the Taranaki fault at the eastern boundary of the Taranaki basin, and a small, shallow, high Vp/Vs (≥2.0) anomaly above the seismically active Cape Egmont Fault Zone, west of the Taranaki volcanoes. Although the Qp model is of significantly lower resolution than the Vp and Vp/Vs models, it shows a low Qp anomaly (c. 100) between 4 and 10 km depth that corresponds to the deepest onshore part of the Taranaki basin.

Hemispherical transition of seismic attenuation at the top of the earth's inner core

In contrast to the liquid outer core, the earth's inner core is mostly solid, and its composition is more pure iron. Based on dynamic arguments related to the freezing process of the inner core, and the observation of much lower P-wave quality factor in the inner core ( Q α<450) than in the outer core ( Q α>10,000), it has been suggested that a mushy layer with liquid inclusions may exist at the top of the inner core. On the other hand, seismic measurements indicate that Q α increases towards the center of the inner core. We here present estimates of Q α in the depth range 32–110 km beneath the Inner Core Boundary (ICB), based on the measurement of PKIKP/PKiKP amplitude ratios after a narrow band-pass filtering (0.7–2.0 Hz). Our measurements indicate that there are pronounced hemispherical differences in the values of Q α (∼335 and ∼160 in the western (180°W to 40°E) and eastern (40°E to 180°E) hemispheres, respectively), and in the depth of transition from decreasing to increasing Q α (<32 km beneath the ICB in the eastern hemisphere and ∼85 km in the western hemisphere). Below 85 km, the hemispherical pattern disappears. We also confirm the existence of a correlated hemispherical pattern in P velocity down to 85 km. The P velocity and Q α variations are compatible with an interpretation in terms of small hemispherical variations of temperature at the top of the inner core and their influence on the morphology of porosity and connectivity of liquid inclusions in the mushy zone. The disappearance of the differences in Q α beneath 85 km provide constraints on the likely depth extent of the mushy zone.

Dispersion Measurements of P Waves and their Implications for Mantle Q p

We analyze the anelasticity of the earth using group delays of P-body waves of deep (>200 km) events in the period range 4–32 s for epicentral distances of 5–85 degrees. We show that Time Frequency Analysis (TFA), which is usually applied to very dispersive surface waves, can be applied to the much less dispersive P-body waves to measure frequency-dependent group delays with respect to arrival times predicted from the CMT centroid location and PREM reference model. We find that the measured dispersion is due to: (1) anelasticity (described by the P-wave quality factor Q p ), (2) ambient noise, which results in randomly distributed noise in the dispersion measurements, (3) interference with other phases (triplications, crustal reverberations, conversions at deep mantle boundaries), for which the total dispersion depends on the amplitude and time separation between the different phases, and (4) the source time function, which is dispersive when the wavelet is asymmetrical or contains subevents. These mechanisms yield dispersion ranging in the order of one to 10 seconds with anelasticity responsible for the more modest dispersion. We select 150 seismograms which all have small coda amplitudes extending to ten percent of the main arrival, minimizing the effect of interference. The main P waves have short durations, minimizing effects of the source. We construct a two-layer model of Q p with an interface at 660 km depth and take Q p constant with period. Our data set is too small to solve for a possible frequency dependence of Q p . The upper mantle Q 1 is 476 [299–1176] and the lower mantle Q 2 is 794 [633–1064] (the bracketed numbers indicate the 68 percent confidence range of Q p −1). These values are in-between the AK135 model (Kennett et al., 1995) and the PREM model (Dziewonski and Anderson, 1981) for the lower mantle and confirm results of Warren and Shearer (2000) that the upper mantle is less attenuating than PREM and AK135.