Topside reverberations off mantle discontinuities are commonly observed at long periods, but their interpretation is complicated because they include both near‐source and near‐receiver reflections. ...We have developed a method to isolate the stationside reflectors in large data sets with many sources and receivers. Analysis of USArray transverse‐component data from 3,200 earthquakes, using direct S as a reference phase, shows clear reflections off the 410‐ and 660‐km discontinuities, which can be used to map the depth and brightness of these features. Because our results are sensitive to the impedance contrast (velocity and density), they provide a useful complement to receiver‐function studies, which are primarily sensitive to the S velocity jump alone. In addition, reflectors in our images are more spread out in time than in receiver functions, providing good depth resolution. Our images show strong discontinuities near 410 and 660 km across the entire USArray footprint, with intriguing reflectors at shallower depths in many regions. Overall, the discontinuities in the east appear simpler and more monotonous with a uniform transition zone thickness of 250 km compared to the western United States. In the west, we observe more complex discontinuity topography and small‐scale changes below the Great Basin and the Rocky Mountains, and a decrease in transition‐zone thickness along the western coast. We also observe a dipping reflector in the west that aligns with the top of the high‐velocity Farallon slab anomaly seen in some tomography models, but which also may be an artifact caused by near‐surface scattering of incoming S waves.
Key Points
Common‐reflection‐point stacking applied to long‐period teleseismic SH waves images upper‐mantle discontinuities under USArray
These images show the 410‐ and 660‐km discontinuities as well as a negative polarity reflection between 50 and 100 km under much of the United States
A steeply dipping structure is imaged in the western United States that could be related to the Farallon Slab
Rock mass characterization requires a deep geometric understanding of the discontinuity sets affecting rock exposures. Recent advances in Light Detection and Ranging (LiDAR) instrumentation currently ...allow quick and accurate 3D data acquisition, yielding on the development of new methodologies for the automatic characterization of rock mass discontinuities. This paper presents a methodology for the identification and analysis of flat surfaces outcropping in a rocky slope using the 3D data obtained with LiDAR. This method identifies and defines the algebraic equations of the different planes of the rock slope surface by applying an analysis based on a neighbouring points coplanarity test, finding principal orientations by Kernel Density Estimation and identifying clusters by the Density-Based Scan Algorithm with Noise. Different sources of information – synthetic and 3D scanned data – were employed, performing a complete sensitivity analysis of the parameters in order to identify the optimal value of the variables of the proposed method. In addition, raw source files and obtained results are freely provided in order to allow to a more straightforward method comparison aiming to a more reproducible research.
•• We present a method for the analysis of plane surfaces in rocky slopes.• The method uses the 3D data obtained by LiDAR or photogrammetry.• The method obtains the discontinuity sets (DS), its orientations and the clusters.• Finally, each point has a cluster and a DS assigned. Each cluster is defined by its best-fit plane equation.
Phase‐field modeling of rock fractures with roughness Fei, Fan; Choo, Jinhyun; Liu, Chong ...
International journal for numerical and analytical methods in geomechanics,
04/2022, Letnik:
46, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Phase‐field modeling—a continuous approach to discontinuities—is gaining popularity for simulating rock fractures due to its ability to handle complex, discontinuous geometry without an explicit ...surface tracking algorithm. None of the existing phase‐field models, however, incorporates the impact of surface roughness on the mechanical response of fractures—such as elastic deformability and shear‐induced dilation—despite the importance of this behavior for subsurface systems. To fill this gap, here we introduce the first framework for phase‐field modeling of rough rock fractures. The framework transforms a displacement‐jump‐based discrete constitutive model for discontinuities into a strain‐based continuous model, without any additional parameter, and then casts it into a phase‐field formulation for frictional interfaces. We illustrate the framework by constructing a particular phase‐field form employing a rock joint model originally formulated for discrete modeling. The results obtained by the new formulation show excellent agreement with those of a well‐established discrete method for a variety of problems ranging from shearing of a single discontinuity to compression of fractured rocks. It is further demonstrated that the phase‐field formulation can well simulate complex crack growth from rough discontinuities. Consequently, our phase‐field framework provides an unprecedented bridge between a discrete constitutive model for rough discontinuities—common in rock mechanics—and the continuous finite element method—standard in computational mechanics—without any algorithm to explicitly represent discontinuity geometry.
A new class of back-to-back integrated aperture- and gap-coupled discontinuities is proposed for substrate-integrated waveguide band-pass filter design. The developed structure is shown to take ...advantage of both discontinuities in the design of cavity and/or planar resonators with an optimum performance including higher quality factor accompanied by transmission zero realisation, wider upper stop-band with second harmonic suppression, and a considerable size reduction. The measured unloaded quality factor has been increased by a ratio of 60% in comparison to the conventional gap-coupled structures.
The regression kink (RK) design is an increasingly popular empirical method for estimating causal effects of policies, such as the effect of unemployment benefits on unemployment duration. Using ...simulation studies based on data from existing RK designs, we empirically document that the statistical significance of RK estimators based on conventional standard errors can be spurious. In the simulations, false positives arise as a consequence of nonlinearities in the underlying relationship between the outcome and the assignment variable, confirming concerns about the misspecification bias of discontinuity estimators pointed out by Calonico, Cattaneo, and Titiunik. As a complement to standard RK inference, we propose that researchers construct a distribution of placebo estimates in regions with and without a policy kink and use this distribution to gauge statistical significance. Under the assumption that the location of the kink point is random, this permutation test has exact size in finite samples for testing a sharp null hypothesis of no effect of the policy on the outcome. We implement simulation studies based on existing RK applications that estimate the effect of unemployment benefits on unemployment duration and show that our permutation test as well as inference procedures proposed by Calonico, Cattaneo, and Titiunik improve upon the size of standard approaches, while having sufficient power to detect an effect of unemployment benefits on unemployment duration. Supplementary materials for this article are available online.
We propose a new class of estimators for a jump discontinuity on nonparametric regression. While there is a vast literature in econometrics that addresses this issue (e.g., Hahn et al., 2001; Porter, ...2003; Imbens and Lemieux, 2008; Cattaneo and Escanciano, 2017), the main approach in these studies is to use local polynomial (linear) estimators on both sides of the discontinuity to produce an estimator for the jump that has desirable boundary properties. Our approach extends the regression from both sides of the discontinuity using a theorem of Hestenes (1941). The extended regressions are then estimated and used to construct an estimator for the jump discontinuity that solves the boundary problems normally associated with classical Nadaraya–Watson estimators. We provide asymptotic characterizations for the jump estimators, including bias and variance orders, and asymptotic distributions after suitable centering and normalization. Monte Carlo simulations show that our jump estimators can outperform those based on local polynomial (linear) regression.
•A new class of estimators for a jump discontinuity on regression is defined.•Estimators have good boundary behavior and are asymptotically normally distributed.•Monte Carlo simulations reveal desirable finite sample properties of the estimators.
Geologic structure characterization is a key step for seismic structure interpretation, such as fluvial channels, faults, and fractures. The coherence attribute is a widely used tool for describing ...seismic discontinuities, which is usually calculated based on the similarity and dissimilarity of the adjacent seismic traces. However, accurately extracting coherence attribute is a difficult task in field data applications because seismic signal is one of the typical nonstationary, non-Gaussian, and wideband signals. To describe seismic discontinuities at different scales, we propose a workflow to extract the multiscale coherence (MSC) attribute. We first decompose seismic data into several band-limited intrinsic mode functions (IMFs) with different dominant frequencies via the multichannel variational mode decomposition (MVMD). Afterward, we develop a Cauchy kernel correlation-based coherence algorithm to extract the coherence attributes at different scales based on the decomposed IMFs. Finally, we can compute the MSC attribute by utilizing the calculated coherence attributes. Field data applications demonstrate that the proposed MSC attribute characterizes seismic discontinuities, such as faults and fluvial channels, more accurately and more clearly than the traditional coherence attribute and the 1-D variational mode decomposition (VMD)-based coherence attribute.
Lithospheric discontinuities are elusive, with properties that are strongly frequency dependent. Results from a temporary deployment of broadband stations along a north-south transect through Central ...Australia, and the permanent arrays ASAR and WRA, are used to evaluate the spatial coherence of lithospheric features, particularly mid-lithospheric discontinuities. We exploit stacked station autocorrelograms that provide an estimate of P-wave reflectivity beneath stations, with imaging methods exploiting teleseismic arrivals. We use both common conversion point (CCP) stacking from Ps receiver functions and reflection point imaging using the autocorrelation of the P wavetrain. The results tie well for the Moho and have a good general correspondence for deeper levels. Although indications of mid-lithospheric discontinuities from changes in the frequency of reflectivity occur at similar depths, the spatial continuity of specific features at high frequency (around 2 Hz) is of the order of 10–15 km. Broader trends can be tracked across the profile, but no strong lithospheric interfaces can be mapped, except for a south dipping feature traversing the lithosphere on the southern part of the profile that is likely to be a former mantle detachment zone.
Display omitted
•Lithospheric structure along north-south profile through Central Australia•MLD at high frequencies on both portable stations and across permanent arrays•Complementary imaging using autocorrelograms and receiver functions•Dipping structure may be former mantle detachment.
Seismic body waves from distant earthquakes, which propagate near vertically beneath recording stations, provide tools for imaging shallow Earth structures with high vertical resolution. The most ...commonly used techniques such as P and S wave receiver functions utilize mode conversions from P to S waves or vice versa to retrieve information on the gradients of elastic properties in the crust and upper mantle. Here we demonstrate the feasibility and advantage of utilizing reflection signals through an improved method of teleseismic P wave coda autocorrelation. We recover clear reflections independently on vertical and radial components, which provide complementary constraints on the subsurface structures. Field data from two stations from different geological settings are analyzed, one of which is an ice station in Antarctica and the other is a bedrock station on the Kaapvaal craton in South Africa. The results from both analyses show the feasibility of the method to unveil P and S wave reflection signals from the ice‐rock interface and the Moho discontinuity. Extensive synthetic experiments are set up to corroborate our results.
Key Points
High‐frequency P and S wave reflections extracted using station autocorrelations
Improved processing allows access to shallow reflections
Successful application to ice and bedrock situations
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