Geophysical Journal International

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Evolution of the statistical distribution of crystal orientations in time- and space-varying viscous flows

Fri, 05/17/2019 - 00:00
SummaryMagmas and other viscously deforming fluids in the Earth frequently contain embedded crystals or other solid inclusions. These inclusions generally rotate about their own axis and, under certain conditions, align themselves in a direction dictated by the details of the flow. This rotational behavior has been studied extensively for homogeneous flows. Here, we couple the crystal rotation dynamics with the fluid mechanical Navier-Stokes equations for the large-scale flow, thus allowing the analysis of crystal rotations in settings that are variable in both space and time. The solution is valid provided that the inter-crystal spacing is sufficiently large to preclude interaction between crystals. Additionally, we derive an evolution equation for the probability density function (PDF) of crystal orientations based on the fundamental concept of conservation of generic properties in continuum mechanics. The resulting system of equations is extensively tested against previous analytical and numerical solutions. Given the focus on method validation, we limit the fluid mechanics to simple systems with analytical solutions for the velocity field. Even for the simple examples computed, all of which are characterized by fluid flow that is constant in time, the crystal orientation patterns are spatially complex and change in time. Pressure-driven flow in a channel results in coherent bands of crystal orientations with band thickness decreasing towards the channel walls. In corner flow constrained by two mutually perpendicular walls, the pattern of crystal orientations does not exhibit any significant similarity with the flow field. Given that there is no local one-to-one correspondence between the flow and the PDF pattern, a combined and larger-scale solution of the two systems is generally required. The simple flow examples shown demonstrate the viability of this new approach. Application to more complex flow geometries which may commonly occur in nature is deferred to future studies.

Square-Root Variable Metric based elastic full-waveform inversion – Part 1: Theory and validation

Fri, 05/17/2019 - 00:00
SummaryFull-waveform inversion (FWI) has become a powerful tool in inverting subsurface geophysical properties. The estimation of uncertainty in the resulting Earth models and parameter trade-offs, although equally important to the inversion result, has however often been neglected or became prohibitive for large-scale inverse problems. Theoretically, the uncertainty estimation is linked to the inverse Hessian (or posterior covariance matrix), which for massive inverse problems becomes impossible to store and compute. In this study, we investigate the application of the square-root variable metric (SRVM) method, a quasi-Newton optimisation algorithm, to FWI in a vector version. This approach allows us to reconstruct the final inverse Hessian at an affordable storage memory cost. We conduct SRVM based elastic FWI on several elastic models in regular, free-surface and practical cases. Comparing the results with those obtained by the state-of-the-art L-BFGS algorithm, we find that the proposed SRVM method performs on a similar, highly-efficient level as L-BFGS, with the advantage of providing additional information such as the inverse Hessian needed for uncertainty quantification.

A new formulation for coupled magma/mantle dynamics

Fri, 05/17/2019 - 00:00
SummaryModelling the porous flow of melt through a viscously deforming solid rock matrix is a useful tool for interpreting observations from the Earth’s surface, and advances our understanding of the dynamics of the Earth’s interior. However, the system of equations describing this process becomes mathematically degenerate in the limit of vanishing melt fraction. Numerical methods that do not consider this degeneracy or avoid it solely by regularising specific material properties generally become computationally expensive as soon as the melt fraction approaches zero in some part of the domain. Here, we present a new formulation of the equations for coupled magma/mantle dynamics that addresses this problem, and allows it to accurately compute large-scale 3-D magma/mantle dynamics simulations with extensive regions of zero melt fraction. We achieve this by rescaling one of the solution variables, the compaction pressure, which ensures that for vanishing melt fraction, the equation causing the degeneracy becomes an identity and the other two equations revert to the Stokes system. This allows us to split the domain into two parts: In mesh cells where melt is present, we solve the coupled system of magma/mantle dynamics. In cells without melt, we solve the Stokes system as it is done for mantle convection without melt transport and constrain the remaining degrees of freedom. We have implemented this formulation in the open source geodynamic modelling code ASPECT and illustrate the improved performance compared to the previous three-field formulation, showing numerically that the new formulation is robust in terms of problem size and only slightly sensitive to model parameters. Beyond that, we demonstrate the applicability to realistic problems by showing large-scale 2-D and 3-D models of mid-ocean ridges with complex rheology. Hence, we believe that our new formulation and its implementation in ASPECT will prove a valuable tool for studying the interaction of melt segregating through and interacting with a solid host rock in the Earth and other planetary bodies using high-resolution, three-dimensional simulations.

Magnetic mineral assemblage as a potential indicator of depositional environment in gas-bearing Silurian shales from Northern Poland

Fri, 05/17/2019 - 00:00
SummaryOrganic matter preservation and associated conditions during deposition, important in the context of fossil fuel exploration, are commonly determined by advanced geochemical analyses. However, the relation between organic matter preservation and magnetic mineral composition remains poorly constrained. The aim of the studies was to check the potential of magnetic mineral differentiation between facies containing various amounts of organic matter as a factor to better understand the processes which influence water chemistry at the bottom of sedimentary basins, and thus to better understand factors controlling the preservation of organic matter. To determine the composition and the properties of magnetic minerals, detailed low-temperature measurements of Saturation Isothermal Remanent Magnetization and hysteresis loops were performed on two types of rocks, Silurian shales from the Baltic Basin (northern Poland). The analyzed shale facies are characterized by similar thermal evolution, but different amounts of organic matter: the Pelplin Formation, containing a modest content of organic matter, in which we also examined early diagenetic carbon concretions; and the Jantar Formation, which represents an organic-rich ‘sweet spot’ layer. In both facies, the results indicate the presence of multi- or pseudo-single domain magnetite, which is interpreted as detrital in origin. However, the main observation gained from this study is the relation between magnetic mineral assemblage in the studied shales and the amount of organic matter: in the rocks with modest amounts of organic matter we observed hematite, while in organic-rich layers hematite was absent. Hematite (mostly single-domain grains) preserved in the Pelplin Formation suggests that stable oxygen-rich conditions were present at the bottom of the sedimentary basin continuously during deposition, concretion cementation, and compaction. In turn, its absence in the Jantar Formation suggests that during sedimentation and early diagenesis more anoxic conditions appeared. Generally, findings show that the presence of hematite is related to the significantly lower amount of organic matter in sedimentary rocks. Thus, presence of this mineral may be a useful indicator of organic matter preservation.

Repeatability of land-based controlled-source electromagnetic measurements in industrialised areas and including vertical electric fields

Thu, 05/16/2019 - 00:00
SummaryOver the last decade, an increasing number of numerical studies have proposed controlled-source electromagnetic (CSEM) techniques for monitoring of fluid flow in reservoirs, e.g. in the framework of hydrocarbon production or CO2 storage scenarios. A fundamental prerequisite for any monitoring application in practise is repeatability of the measurements, particularly in areas with high noise levels.Here, we report on CSEM data acquired across a producing oil field on land in three consecutive surveys between 2014 and 2016. As major conductivity changes in the reservoir structure are not expected for this time frame, the data sets provide an excellent basis to study accuracy and repeatability of such measurements over a time span of 2.5 years.Our results show that uncertainties of single CSEM measurements lie between 0.1 and 10 per cent with a focus around 1 per cent in all surveys. For source-receiver offsets < 2 km uncertainties are in the range of ∼0.1 to 0.3 per cent, proportional to the transfer function amplitudes, and are dominated by intrinsic noise of the measuring system. At source-receiver distances > 4 km external noise resulting from natural electromagnetic field variations and powered installations dominates uncertainties which assume minimum absolute values of 10−9 to 10−10 V/(Am) with lowest values at frequencies between 0.1–10 Hz.Overall, repeatability of CSEM measurements depends on a range of factors, including source-receiver distances, component of the transfer function, source-polarisation, and relocation errors, in particular at sites close to the source, where the geometry and characteristics of the source fields vary rapidly in space. Best repeatability was observed for receiver stations at 2–4 km distance from the source and frequencies < 20 Hz. At these stations, phases and amplitudes of transfer functions usually agreed within ± 1° and ± 5 per cent between measurements. Such values are in a range as expected from time-lapse signals due to resistivity changes in target (reservoir) formations. Hence, precise surveying procedures are essential.We also measured the vertical electric field (Ez) with a newly developed receiver chain in a 200 m deep observation borehole. The vertical electric field component shows generally higher sensitivity to resistivity changes in reservoir structures than the horizontal electric fields measured at surface. Although amplitudes of Ez are about one to two orders of magnitude smaller than amplitudes of horizontal electric fields, recordings of Ez are stable. More importantly, Ez transfer functions of three measurements between 2015 and 2016 show excellent quality and repeatability within < ± 2° and < ± 5 per cent, similar as horizontal electric fields indicating that noise conditions at depth improve when compared with sensors at surface.

Quality parameters for Passive Image Interferometry tested at the Groningen network

Thu, 05/16/2019 - 00:00
SummaryPassive Image Interferometry is widely used to retrieve velocity variations as function of time. The cross-coherence (spectrally normalized cross-correlation) of ambient noise recorded at two receivers is an estimate of the Green’s function that due to velocity changes will be stretched or compressed in time. The relative velocity change (Δv/v) is determined by the time stretching (-Δt/t) that yields highest correlation between the reference cross-coherence and stretched lapse cross-coherence. The estimation of Δv/v could be used, e.g., to warn for hazardous situations developing in a volcano setting or due to degradation of a civil engineering structure. Before a warning would be issued, however, one would like to have a handle on the quality of the medium-change estimate. In the Groningen area in the Netherlands a well sampled network of seismometers exists. This allows direct assessment of the quality of the Δv/v estimation by many different receivers paths sampling the same medium. We use this quality assessment of Δv/v to test other possible quality parameters that could also be used in settings with only a sparsely sampled seismic network. The quality of the Δv/v determination appears to be well described by the correlation coefficient which is used to determine the velocity change. Consequently, in order to measure medium changes with a high certainty, it is important to have a high consistency between the lapse and the reference cross-coherence. A quality estimate based on this correlation coefficient can also be applied when there is only one receiver pair available. If the quality is insufficient, it can be improved at the cost of temporal resolution. This study also investigates possible causes of the measured medium changes: variations in temperature, soil moisture, air pressure, water table, gas production rate and subsidence. We find a weak anti-correlation with temperature, and a weak correlation with the gas production rate and subsidence. The observed medium change is likely a complicated combination of different processes taking place.

A generalized theory for full microtremor horizontal-to-vertical [H/V(z, f)] spectral ratio interpretation in offshore and onshore environments

Thu, 05/16/2019 - 00:00
SummaryAdvances in the field of seismic interferometry have provided a basic theoretical interpretation to the full spectrum of the microtremor horizontal-to-vertical spectral ratio [H/V(f)]. The interpretation has been applied to ambient seismic noise data recorded both at the surface and at depth. The new algorithm, based on the diffuse wavefield assumption, has been used in inversion schemes to estimate seismic wave velocity profiles that are useful input information for engineering and exploration seismology both for earthquake hazard estimation and to characterize surficial sediments. However, until now, the developed algorithms are only suitable for on land environments with no offshore consideration. Here, the microtremor H/V(z, f) modeling is extended for applications to marine sedimentary environments for a 1D layered medium. The layer propagator matrix formulation is used for the computation of the required Greenâs functions. Therefore, in the presence of a water layer on top, the propagator matrix for the uppermost layer is defined to account for the properties of the water column. As an application example we analyze eight simple canonical layered earth models. Frequencies ranging from 0.2 to 50 Hz are considered as they cover a broad wavelength interval and aid in practice to investigate subsurface structures in the depth range from a few meters to a few hundreds of meters. Results show a marginal variation of 8 percent at most for the fundamental frequency when a water layer is present. The water layer leads to variations in H/V peak amplitude of up to 50 percent atop the solid layers.

Global finite-frequency S-wave delay-times: how much crust matters

Thu, 05/16/2019 - 00:00
SummaryWe investigate the influence of crust on time residual measurements made by cross-correlation in the 10–51 s filtering period range on a global scale, considering two crustal models: CRUST2.0 and CRUST1.0. This study highlights, in a quantitative way, crust-related time corrections. One part of this correction is directly linked to the body wave travel time through the crust as predicted by the ray theory , whereas a second part is related to interferences with multiple crustal reflections. This second component, called finite-frequency crustal correction, is frequency-dependent unlike the ray-theory based correction. We show that if this frequency-dependent crust-related correction is not taken into account in cross-correlation measurements, it may lead to a dispersive effect in S-wave delay-times that could ultimately bias tomographic models. On average, this finite-frequency correction increases with the filtering period. Comparisons between the two crustal models highlight the significant dispersive effect of the crust, which has complex patterns depending on geological contexts, with an important role of the sediment thickness. Although ray crustal corrections remain important, finite-frequency crustal effects may lead to a bias in measurements if not properly taken into account; on average they may reach 0.9–1.6 s for CRUST2.0 and 0.5–1.6 s for CRUST1.0, for period ranging from 10–51 s, respectively.

Passive seismic imaging of subsurface natural fractures: application to Marcellus shale microseismic data

Thu, 05/16/2019 - 00:00
SummaryImaging and characterizing subsurface natural fractures that are common in the Earth crust has been a long-sought goal in seismology. We present an application of a three-dimensional passive seismic fracture imaging method applied to Marcellus shale microseismic data for mapping natural fractures. Unlike conventional seismic imaging methods that need source information, the proposed imaging method does not require source information and is flexible enough to apply to any passive seismic data where the source location is unknown or inaccurate. We first test our imaging approach using surface microseismic monitoring array data in three-dimensional synthetic examples. The finite-aperture fractures are designed by an open-source discrete fracture network software. Compared to conventional source-dependent fracture imaging, the proposed source-independent imaging approach produces superior images of fractures with less ambiguity. These tests also illustrate that the proposed method is less sensitive to the accuracy of background velocity and less affected by the sparse and irregular acquisition geometry which often cause acquisition-footprint issues in convention imaging methods. The final test in the field microseismic data from the Marcellus Shale (Pennsylvania) demonstrates the applicability of the proposed imaging method. Field data results indicate two clusters of east-northeast fractures existed above and below the hydraulic fracturing zone, which corroborates previous work that found two main types of faults in the study area.

Characterisation of crustal structure by comparing reflectivity patterns of wide-angle and near vertical seismic data from the Parnaíba Basin, Brazil

Thu, 05/16/2019 - 00:00
SummaryRecently an ambitious experiment combining deep seismic surveys from near-vertical and wide-angle acquisition methods was carried out in Brazil. The seismic lines are essentially coincident and crossed the Parnaíba Basin from west to east near latitude 5° S. Here, the wide-angle reflection and refraction (WARR) and deep seismic reflection (DSR) results, which were previously interpreted independently, are compared by directly correlating WARR interfaces converted to TWTT with the major reflective horizons identified in the zero-offset image and by considering coincident reflectivity patterns displayed in both data sets. This integrated WARR and DSR analysis allowed a spatial association of the apparently acoustically featureless crust imaged in the DSR profile to the high reflectivity observed in the WARR data. Numerical tests and elastic modelling show that variations of the elastic properties of the crust, particularly as they are characterised by low Vp and Vs contrasts with a possible increase of the Vp/Vs ratio, can only weakly explain the observed reflectivity patterns but that fine-scale lithological heterogeneity within the crust is capable of replicating the observed contrasting seismic responses. The segment of the Parnaíba Basin crust that is characterised by fine-scale lithological heterogeneity lies directly above a mafic crustal underplate defined by the WARR model and was named as the Grajaú domain on the basis of WARR-derived velocity model. The applied methodologies allow added value to be taken from the independent seismic datasets and provide new information about crustal structure that may have important implications for overlying intracontinental basin evolution.

Analysis of surface and seismic sources in dense array data with Match Field Processing and Markov Chain Monte Carlo sampling

Thu, 05/16/2019 - 00:00
SummaryWe introduce a methodology based on array processing to detect and locate weak seismic events in a complex fault zone environment. The method is illustrated using data recorded by a dense array of 1108 vertical component geophones in a 600 m x 600 m area on the Clark branch of the San Jacinto Fault. Because surface and atmospheric sources affect weak ground motion, it is necessary to discriminate them from weak seismic sources at depth. Source epicentral positions and associated apparent velocities are extracted from continuous seismic waveforms using Match Field Processing (MFP). We implement MFP at specific frequencies targeting surface and subsurface sources, using for computational efficiency a forward model of acoustic source in a homogenous medium and Markov-Chain Monte Carlo sampling. Surface sources such as Betsy gun shots and a moving vehicle are successfully located. Weak seismic events are also detected outside of the array, and their back-azimuth angle is retrieved and found to be consistent with the fault geometry. We also show that the homogeneous acoustic model does not yield satisfying results when extracting micro-seismic event depth, because of the ambiguity between depth and the apparent velocity based on surface data.

Numerical simulation of tsunami waves generated by the underwater landslide for the northern coast of the black sea (Dzhubga Area)

Thu, 05/16/2019 - 00:00
SummaryThe new method of computation of the underwater landslide motion in the framework of the elastoplastic model is presented. This model, with seismic or aseismic action, makes it possible to model the nonlinear behavior of a porous saturated sedimentary mass under conditions of its plastic flow beyond the yield strength. The computation method is formulated for the example of a possible landslide process in the region of Dzhubga (north-west coast of the Black Sea). A model for numerical simulation of a tsunami wave generation upon the slipping of the underwater landslide is presented. The numerical simulation of tsunami wave is based on a nonlinear system of shallow water equations. The results of numerical modeling of the motion of a landslide along the underwater slope and computation of generation, propagation and runup of a tsunami, induced by the underwater landslide are presented. It is noted that when the tsunami wave is generated by a landslide computed within the framework of an elastoplastic model, the problem changes significantly in comparison with the methods for computation of a landslide as the motion of a rigid body or a viscous fluid. The evolution of the form of surface water wave generated by the underwater landslide with time, for the values of the maximum friction angle, determining the properties of the landslide body, is studied. The process of arising of a local tsunami—the process of formation of a crest climbing the beach, from which the landslide came down, is studied in details.

The Dulmage-Mendelsohn Permutation in Seismic Tomography

Tue, 05/14/2019 - 00:00
SummarySeismic tomography inverse problems are among the largest high-dimensional parameter estimation tasks in Earth science. Although iterative algorithms can be used to efficiently solve these problems, their size gives rise to several issues such as the intractability of the computation of the model resolution and the model posterior covariance matrices that provide the means of assessing the robustness of the solution. In this work, we utilize methods from combinatorics and graph theory to study the structure of typical regional seismic body-wave tomography problems, and to effectively decompose them into subsets that can be solved efficiently by means of the least squares method. In combination with recent high performance direct sparse algorithms, this reduction in dimensionality allows for an efficient computation of the model resolution and covariance matrices using limited resources. We apply this methodology to a moderate size imaging of the structure of the crust and the upper mantle beneath Japan using deep local earthquakes recorded by the High Sensitivity Seismograph Network stations. Among the prominent features that are being imaged is a strong low-velocity region beneath the subducting Pacific slab along the entire Japan trench.

Effect of capillarity and relative permeability on Q anisotropy of hydrocarbon source rocks

Sat, 05/11/2019 - 00:00
SummaryShale – reservoir formations are porous rocks of low permeability composed of fluid saturated illite-smectite and kerogen layers, which behave as viscoelastic transversely isotropic (VTI) media at long wavelengths, i.e., much larger than the average layer thickness. Seismic waves travelling across these heterogeneous materials induce fluid flow (WIFF) and Biot slow waves generating energy loss (mesoscopic loss) and velocity dispersion. When these formations are saturated by two-phase fluids, the presence of capillary forces – interfacial tension – and interaction between the two fluids as they move within the pore space need to be taken into account. This can be achieved by using a Biot model of a poroelastic solid saturated by a two-phase fluid that includes capillary pressure and relative permeability functions and supports the existence of two slow waves. An upscaling finite element method is used to analize the WIFF, which determines an effective VTI medium predicting higher attenuation and (Q) anisotropy than the classical single-phase (single fluid) models.

Love wave amplitude decay from rotational ground motions

Fri, 05/10/2019 - 00:00
SummaryThe broadband surface wave magnitude equation assigns magnitudes based on source-receiver distance and peak surface wave amplitude. It is standard practice to use the vertical component of peak ground velocity to determine magnitude, such that only contributions from the vertical motion of Rayleigh waves are present in the surface wave train. With the advent of rotational ground motion observations from instruments such as ring laser gyroscopes, it is possible to measure rotational ground motions about three orthogonal axes. For surface waves, observations of rotations about the vertical axis are theoretically only sensitive to the transverse nature of Love waves, unaffected by either component of Rayleigh waves. We use this concept to separate and study the amplitude information of surface waves independently. With a large database of recorded seismic waveforms for co-located translations and rotations, collected in Wettzell, Germany, we empirically define magnitude scale attenuation constants as a method for quantifying amplitude decay. Through this differential analysis, we determine a necessity for separate surface wave magnitude equations through measurements of translations and rotations. Synthetic seismograms were concurrently produced using an open-source spectral-element wave propagation code, for comparisons against observations. Though synthetically derived amplitude decays agree for translations, they do not accurately predict the decay found for rotations. Synthetics also overpredict amplitudes for both rotations and translations. Results from observations imply that rotation amplitudes decay faster over distance with respect to velocity amplitudes, and that the current surface wave magnitude equation is insufficient for predicting observed translation and rotation amplitudes. We attribute variations in amplitude decay characteristics to the different effects of attenuation on Love and Rayleigh waves, with potential influence from local velocity structure, and scattering effects. The lack of agreement in synthetics is attributed to the insufficiency of synthetic attenuation and velocity structure to replicate the effects seen in observations.

Multiscale and Layer-Stripping Wave-Equation Dispersion Inversion of Rayleigh Waves

Fri, 05/10/2019 - 00:00
SummaryThe iterative wave-equation dispersion inversion can suffer from the local minimum problem when inverting seismic data from complex Earth models. We develop a multiscale, layer-stripping method to alleviate the local minimum problem of wave-equation dispersion inversion of Rayleigh waves and improve the inversion robustness. We first invert the high-frequency and near-offset data for the shallow S-velocity model, and gradually incorporate the lower-frequency components of data with longer offsets to reconstruct the deeper regions of the model. We use a synthetic model to illustrate the local minima problem of wave-equation dispersion inversion and how our multiscale and layer-stripping wave-equation dispersion inversion method can mitigate the problem. We demonstrate the efficacy of our new method using field Rayleigh-wave data.

An adaptive Topography Correction Method of Gravity field and Gradient Measurements by Polyhedral Bodies

Thu, 05/09/2019 - 00:00
SummaryThis paper presents a new adaptive algorithm for topographic correction of gravity measurements based on a triangulated polyhedral representation of topographic surfaces. The adaptive grid resampling of topography is driven by the actual gravity effect on the station, which depends on the topographic mass and distance. This is a major improvement for accuracy and computational speed compared to some old algorithms that account only for distance. Today, global availability of gravity data allows the regional investigation (∼100 km anomaly wavelengths) of the lithosphere. The variety of different datasets (from terrestrial and aero surveys to satellite data), the size of possible investigation areas and the availability of high-resolution, digital terrain models (DTM) ask for new approaches of topographic correction. Specifically, an algorithm for topographic correction should be useable on large-scale investigation areas, i.e. correctly model the long wavelength signal of topography, work in spherical domain, be able to handle high-resolution topography data available, and accurately represent topography. The new algorithm does consider sphericity of the earth, calculates gravity gradients, deals with large datasets and uses an adaptive approach for resampling topography to save computation time significantly. The resampling uses an efficient quadtree representation of the topography grid. High resolution of the topography grid is only considered if it has a significant influence on the gravity effect at the station. This leads to an accurate representation of distant terrain and a massive speed up of computation time. The new algorithm is tested in an area of South America and Central Asia. The results are almost identical to calculations with constant grid resolutions, but need only 2 per cent of calculation time. This method enables to correct all gravity data sources (terrestrial, aero, satellite data) with the necessary topographic resolution with huge saving in computation time without presumptions by the user.

The coupling between inertial and rotational eigenmodes in planets with liquid cores

Thu, 05/09/2019 - 00:00
SummaryThe Earth is a rapidly rotating body. The centrifugal pull makes its shape resemble a flattened ellipsoid and Coriolis forces support waves in its fluid core, known as inertial waves. These waves can lead to global oscillations, or modes, of the fluid. Periodic variations of the Earth’s rotation axis (nutations) can lead to an exchange of angular momentum between the mantle and the fluid core and excite these inertial modes. In addition to viscous torques that exist regardless of the shape of the boundaries, the small flattening of the core-mantle boundary (CMB) allows inertial modes to exert pressure torques on the mantle. These torques effectively couple the rigid-body dynamics of the Earth with the fluid dynamics of the fluid core. Here we present the first high resolution numerical model that solves simultaneously the rigid body dynamics of the mantle and the Navier-Stokes equation for the liquid core. This method takes naturally into account dissipative processes in the fluid that are ignored in current nutation models. We find that the Free Core Nutation (FCN) mode, mostly a toroidal fluid flow if the mantle has a large moment of inertia, enters into resonance with nearby modes if the mantle’s moment of inertia is reduced. These mode interactions seem to be completely analogous to the ones discovered by Schmitt (2006) in a uniformly rotating ellipsoid with varying flattening.

Multi-channel quality factor Q estimation

Thu, 05/09/2019 - 00:00
SummaryThe estimation of quality factor, Q, plays an important role in many geophysical problems, including Q-compensated seismic imaging, geophysical interpretation, fluid characterization, ground motion predictions, and seismic hazard estimations. One of the most widely used approaches for estimating Q is the spectral ratio method (SRM). However, the spectral division in SRM may not be stable due to the spectral nulls. The shaping regularized inversion that treats the spectral division as a regularized least-squares inversion problem can help solving the spectral-nulls problem and make the spectral division stable. In the case of very noisy seismic data, the time-frequency maps can not be optimally obtained and thus the Q estimation will be strongly affected and unstable even with the regularized inversion method. We propose a multi-channel Q estimation approach that takes advantage of the multi-channel spatial coherence to constrain the inversion so that the estimated Q is spatially continuous. We use a set of synthetic and real data examples to demonstrate the performance of the multi-channel Q estimation method. Results show that the proposed method can obtain accurate and more importantly stable Q estimation result even in the case of strong random noise.

Study of the regional pattern of intrinsic and scattering seismic attenuation in Eastern Sicily (Italy) from local earthquakes

Wed, 05/08/2019 - 00:00
SummaryThe mechanism of seismic attenuation for shear waves was studied through the separate measure of the intrinsic dissipation and scattering coefficients. A modified version of the Multiple Lapse Time Window Analysis (MLTWA) was applied to about 5,000 local earthquakes with magnitude 1.5 ≤ Ml ≤ 4.8 occurred in the period 2006–2017 in both the tectonic (Peloritani Mountains and Hyblean Plateau) and volcanic (Aeolian Island and Mt. Etna) areas of eastern Sicily. Observed data, processed via direct Fourier Transform instead of the filtering and averaging procedures utilized in the ordinary MLTWA technique, were fit to the correspondent Paasschens solution of the Energy Transport model in 3D. Model parameters B0, the seismic albedo, and Le, the extinction length, in turn associated with the intrinsic (Qi) and scattering (Qs) quality factors for S-waves, are thus obtained from the fit in the frequency bands centered at 1.5, 3, 6, and 12 Hz. Finally, the corrections to these estimates in case of an opaque layer (the crust) overlying a uniform and transparent mantle, were applied in order to obtain more realistic estimates of the seismic attenuation in the areas investigated. Obtained results are first interpreted in terms of the characteristics of each domain and then compared by the same point of view with a huge set of measurements of the same parameters carried out in other tectonic and volcanic domains through the world. Results show how the rock heterogeneity characterizes the tectonic domain and determines the value of total attenuation coefficient, which, on the other hand, is a crucial parameter in the estimation of the seismic hazard.

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