Seismic quality factor measured for compressional and shear waves in the firn column of Korff Ice Rise, West Antarctica

<p><span>Comprehensive descriptions of the seismic properties of glaciers and ice masses require that both compressional (P-) and shear (S-) wave components are considered. Among these properties is the seismic attenuation, expressed by the Quality Factor (Q). Q is valuable for two reasons: first, to correct measurements of seismic amplitude for wavelet propagation effects, as in reflection amplitude-versus-angle (AVA) studies. Second, Q is an indicator of ice properties such as temperature and impurity content, and laboratory/field studies of soils and geological materials suggests that the ratio of the compressional- and shear-wave quality factors, Qp/Qs, may indicate fluid saturation (particularly when considered jointly with the velocity ratio Vp/Vs). Thus, a measurement of Qp/Qs could usefully inform the hydrological structure of the firn and indicate variations in the density of the firn column. </span></p><p><span>Despite its importance, few studies appear to have measured Qp in firn columns and none appear to have measured Qs in firn. Doing so for either compressional- or shear-wave arrivals is challenging, due to the ray paths followed by the diving wave first arrivals and their accurate representation in attenuation measurement methods. In preparation for an AVA study of bed properties at Korff Ice Rise, West Antarctica, we have used spectra of diving wave first arrivals and a modified spectral-ratio method to measure Qp and Qs as a function of depth in the firn column. Shot gathers with vertically oriented geophones at offsets of 2.5 - 1000m were used to measure Qp. For detecting the shear component, the geophones were oriented horizontally; in this configuration, diving and reflected shear phases were recorded with high signal-to-noise ratios. The variation of Q with depth is represented as discrete constant-Q layers with thicknesses between 6 and 27 m. Qp shows progressive increases in depth from 21 ± 3 in the uppermost 20 m (where Vp < </span><span>30</span><span>00 m/s), to 246 ± 30 between 74 and 80 m depth (3750 m/s < Vp < 3770 m/s). Qs increases from 14 ± 4 in the uppermost 20m, to 80 ± 6 between 80 and 90m depth. The ratio Qp/Qs varies throughout the depths measured, from Qp/Qs ~ 1.5 at the surface, to Qp/Qs ~ 3 at 80 m.</span><span> This is broadly consistent with previously quoted values, but the variation may imply that Qp/Qs is influenced by firn structure.<br></span></p><p><span>Similar measurements at a variety of sites could help to inform a relationship between Qp, Qs and firn properties. In the immediate future, the measurement of Q in the firn will aid measurements of bed reflectivity, and help to determine the material properties of the ice-bed interface.</span></p>

Fracture-generated frequency-dependent seismic Q measured from a VSP in granite

<div> <div> <div> <p>We integrate two topics – seismic characterisation of fractures, and seismic attenuation quantified as the frequency-dependent Seismic Quality Factor Q, Q(f). The former is vital for predicting and monitoring fluid movement and containment in energy-related settings (hydrocarbons; geothermal; CO<sub>2</sub>, hydrogen or compressed air storage; radwaste). Fractures control the fluid flow and structural behaviour of a rock mass, yet their expression in Q is poorly studied and not well understood despite it typically being more sensitive than wavespeeds as a rock physics parameter. The latter is long-recognised, little-studied, and a paradigm shift from frequency-independent Q (‘constant-Q’, a routine signal-processing and image enhancement tool in hydrocarbon exploration), despite theory, laboratory, and field data showing that Q must be frequency dependent due to varying scale-lengths of the physical-mechanical phenomena causing attenuation.</p> <p>We therefore measure Q(f) from the downgoing direct P-wave arrival in a near-offset vertical seismic profile in granite at a former geothermal test site in Cornwall, SW England, where vertical and horizontal fracturing is seen at surface: horizontal fractures are confirmed at depth by well-log data. Sensors were 3-component 15Hz geophones at 15m depth spacing: the source was a single vibrator, linear 8-100Hz up-sweep, 30m offset from the wellhead in the azimuth of well deviation: record length was 1000ms at 1ms sample interval. We analyse only the deeper cased interval, from 700m to 1735m. Pre-processing was geometric spreading correction, hodogram-based component rotation toward the source, and wavefield separation using a 7-point median filter to suppress interference from upgoing energy. Measured attenuation Q<sub>eff</sub> is the harmonic sum of intrinsic Q, Q<sub>int</sub>, and apparent attenuation, Q<sub>app</sub>. Q<sub>int</sub> in massive granite is typically 500-1000, yet we find Q<sub>eff</sub>(f) is 50-70 at >60Hz and only ≈30 at <30-35Hz, features masked in the constant-Q result of 55±11 over our working bandwidth of 25-90Hz.</p> <p>One contribution to Q<sub>app</sub> is ‘stratigraphic attenuation’, forward-scattering interference of short-path internal multiple reflections superimposed on direct arrivals, and quantifiable from sonic and density well-logs using O’Doherty-Anstey-Shapiro methodology. We find it is indeed frequency-dependent (peaking at ≈50-60Hz, 10-40% lower at our bandwidth limits) but its absolute magnitude is insignificant (Q≈20,000-30,000) and unable to explain the measured Q<sub>eff</sub>(f). We therefore investigate the effect of fracturing directly using finite difference models in which fractures are defined explicitly as displacement discontinuities with opposing surfaces connected by a normal and shear stiffness. An individual fracture acts somewhat like a low pass filter: more complex frequency behaviour emerges from multiple fractures, particularly when fracture stiffness, spacing and size can vary. We concentrate first on large horizontal fractures perpendicular to the borehole receiver array, and find that these can indeed influence effective attenuation within the 25-90Hz bandwidth. We then discuss the range of fracture spacings and stiffnesses capable of explaining the data and whether they are sufficiently physically credible as an explanation of the observed Q(f).</p> </div> </div> </div>

Elastic wave velocity and attenuation in granular material

Discrete Elements Method simulations are carried out to investigate waves propagation in isotropic, frictional granular media. The focus is on the effects of confining pressure, microstructure and input frequency on both wave velocity and attenuation. The latter is described via the seismic quality factor Q and three different measurement approaches are compared, in time and frequency domain. The simulation data validate previous findings on the scaling of wave velocity with confining pressure and coordination number. The quality factor Q shows a non-monotonic behavior with input frequency.

Modulation of seismic noise near the San Jacinto fault in Southern California: Origin and observations of the cyclical time dependence and associated crustal properties

Summary We examine the cyclic amplitude variation of seismic noise recorded by continuous three-component broadband seismic data with durations spanning 91 to 713 days (2008–2011) from three different networks: Anza seismic network, IDA network and the Transportable seismic array. These stations surround the San Jacinto Fault Zone (SJFZ) in southern California. We find the seismic noise amplitudes exhibit a cyclical variation between 0.3 and 7.2 Hz. The high frequency (≥ 0.9 Hz) noise variations can be linked to human activity and are not a concern. Our primary interest is signals in the low frequencies (0.3–0.9 Hz), where the seismic noise is modulated by semi-diurnal tidal mode M2. These long-period (low frequency) variations of seismic noise can be attributed to a temporal change of the ocean waves breaking at the shoreline, driven by ocean tidal loading. We focus on the M2 variation of seismic noise at f = 0.6 Hz, travelling distances of ∼92 km through the crust from offshore California to the inland Anza, California, region. Relative to the shoreline station, data from the inland stations show a phase lag of ∼ –12°, which we attribute to the cyclic change in M2 that can alter crustal seismic attenuation. We also find that for mode M2 at 0.6 Hz, the amplitude variations of the seismic quality factor (Q) depend on azimuth and varies from 0.22 per cent (southeast to northwest) to 1.28 per cent (northeast to southwest) with Q = 25 for Rayleigh waves. We propose the direction dependence of the Q variation at 0.6 Hz reflects the preferred orientation of sub-faults parallel to the main faulting defined by the primarily N45° W strike of the SJFZ.

Estimation of seismic quality factor in the time-frequency domain using variational mode decomposition

Seismic attenuation as represented by the seismic quality factor [Formula: see text] has a substantial impact on seismic reflection data. To effectively eliminate the interference of reflection coefficients for [Formula: see text] estimation, a new method is proposed based on the stationary convolutional model of a seismic trace using variational mode decomposition (VMD). VMD is conducted on the logarithmic spectra extracted from the time-frequency distribution of the seismic reflection data generated from the generalized S transform. For the intrinsic mode functions after VMD, mutual information and correlation analysis are used to reconstruct the signals, which effectively eliminates the influence of the reflection coefficients. The difference between the two reconstructed logarithmic spectra within the selected frequency band produces a better linear property, and it is more suitably approximated with the linear function compared to the conventional spectral-ratio method. Least-squares fitting is finally applied for [Formula: see text] estimation. Application of this method to synthetic and real data examples demonstrates the stabilization and accuracy for [Formula: see text] estimation.

Investigation of Mars Seismic Attenuation Using InSight SEIS Data.

<p> NASA InSight (the Interior Exploration using Geodesy and Heat Transport) has placed the first broadband seismometer (SEIS) on the Martian surface and now continuously monitoring Martian seismic activity. Since the first detection of a marsquake in March 2019, SEIS detected more than 200 marsquakes and Mars has been revealed to be a seismically active planet. The dataset can now be used to perform the seismic investigation of the Mars interior and interpret this in a comparative manner by referring to the examples from the Earth and the Moon.</p><p>In this study, we investigate the seismic attenuation on Mars and compare this with the Earth and the Moon. Attenuation can be described as a combination of inelastic absorption and elastic diffusion of energy. Such properties will give important constraints on the composition of the Mars interior and also its thermal state. Another interesting aspect will be to discuss the water content with respect to the attenuation. Given the large variety of water content for the Earth, the Moon and Mars, the attenuation feature will be likely to differ significantly between these planets and satellite. Here we use the seismic dataset obtained by InSight SEIS and construct a 1D structure of seismic attenuation on Mars. Then we refer to the values obtained for the Earth and the Moon to discuss the possible implication on their differences and similarities.</p><p>　The presentation aims to summarize the results from different approaches taken by the authors. The approach includes; 1) spectral analyses of seismic signals and spectral decay fitting, 2) seismic coda analyses with coda rise time and decay, 3) numerical coda simulation with diffusion theory on seismic energy. With these approaches we will be constraining seismic quality factor Q and diffusivity D for different depth range. Different approaches have sensitivities to different depth and prarameters and we aim to provide our view on the martian attenuation and diffusion to date by summarizing the obtained results.</p>