arXiv: Geophysics最新文献

{"title":"Full-waveform inversion by source extension: Why it works","authors":"W. Symes","doi":"10.1190/SEGAM2020-3424509.1","DOIUrl":"https://doi.org/10.1190/SEGAM2020-3424509.1","url":null,"abstract":"An extremely simple single-trace transmission example shows how an extended source formulation of full waveform inversion can produce an optimization problem without spurious local minima (\"cycle skipping\"). The data consist of a single trace recorded at a given distance from a point source. The velocity or slowness is presumed homogeneous, and the target source wavelet is presumed quasi-impulsive or focused at zero time lag. The source is extended by permitting energy to spread in time, and the spread is controlled by adding a weighted mean square of the extended source wavelet to the data misfit, to produce the extended inversion objective. The objective function and its gradient can be computed explicitly, and it is easily seen that all local minimizers must be within a wavelength of the correct slowness. The derivation shows several important features of all similar extended source algorithms. For example, nested optimization, with the source estimation in the inner optimization (variable projection method), is essential. The choice of the weight operator, controlling the extended source degrees of freedom, is critical: the choice presented here is a differential operator, and that property is crucial for production of an objective immune from cycle-skipping.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131281946","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Magnetic Induction in Convecting Galilean Oceans","authors":"S. Vance, B. Bills, C. Cochrane, K. Soderlund, N. Gómez-Pérez, M. Styczinski, C. Paty","doi":"10.1002/essoar.10502420.1","DOIUrl":"https://doi.org/10.1002/essoar.10502420.1","url":null,"abstract":"To date, analyses of magnetic induction in putative oceans in Jupiter's large icy moons have assumed uniform conductivity in the modeled oceans. However, the phase and amplitude response of the ind...","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129910261","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"From constant- to variable-density inverse extended Born modeling","authors":"M. Farshad, H. Chauris","doi":"10.1190/geo2019-0489.1","DOIUrl":"https://doi.org/10.1190/geo2019-0489.1","url":null,"abstract":"For quantitative seismic imaging, iterative least-squares reverse time migration is the recommended approach. The existence of an inverse of the forward modelling operator would considerably reduce the number of required iterations. In the context of the extended model, such a pseudo-inverse exists, built as a weighted version of the adjoint and accounts for the deconvolution, geometrical spreading and uneven illumination. The application of the pseudo-inverse Born modelling is based on constant density acoustic media, which is a limiting factor for practical applications. To consider density perturbation, we propose and investigate two approaches. The first one is a generalization of a recent study proposing to recover acoustic perturbations from angle-dependent response of the pseudo-inverse Born modelling operator. The new version is based on weighted least-squares objective function. The method not only provides more robust results, but also offers the flexibility to include constrains in the objective function in order to reduce the parameters cross-talk. We also propose an alternative approach based on Taylor expansion that does not require any Radon transform. Numerical examples based on simple and the Marmousi2 models using correct and incorrect background models for the variable density Born modelling, verify the effectiveness of the weighted least-squares method when compared with the other two approaches. The Taylor expansion approach appears to contain too many artifacts for a successful applicability.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-01-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129956207","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Learning to Invert Pseudo-Spectral Data for Seismic Waveform Inversion","authors":"C. Zerafa, P. Galea, C. Sebu","doi":"10.7423/XJENZA.2019.1.01","DOIUrl":"https://doi.org/10.7423/XJENZA.2019.1.01","url":null,"abstract":"Full-waveform inversion (FWI) is a widely used technique in seismic processing to produce high resolution Earth models that fully explain the recorded seismic data. FWI is a local optimisation problem which aims to minimise in a least-squares sense the misfit between recorded and modelled data. The inversion process begins with a best-guess initial model which is iteratively improved using a sequence of linearised local inversions to solve a fully non-linear problem. Deep learning has gained widespread popularity in the new millennium. At the core of these tools are Neural Networks (NN), in particular Deep Neural Networks (DNN) are variants of these original NN algorithms with significantly more hidden layers, resulting in efficient learning of a non-linear functional between input and output pairs. The learning process within DNN involves iteratively updating network neuron weights to best approximate input-to-output mappings. There is clearly similarity between FWI and DNN. Both approaches attempt to solve for a non-linear mapping in an iterative sense, however they are fundamentally different in that the former is knowledge-driven whereas the latter is data-driven. This article proposes a novel approach which learns pseudo-spectral data-driven FWI. We test this methodology by training a DNN on 1D multi-layer, horizontally-isotropic data and then apply this to previously unseen data to infer the surface velocity. Results are compared against a synthetic model and successfulness and failures of this approach are hence identified.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"12 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134020151","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Logjams are not jammed: measurements of log motions in Big Creek, Idaho","authors":"N. Deshpande, B. Crosby","doi":"10.31223/osf.io/x9s27","DOIUrl":"https://doi.org/10.31223/osf.io/x9s27","url":null,"abstract":"Colloquially, a \"logjam\" indicates a kinematic arrest of movement. Taken literally, it refers to a type of dense accumulation of wood in rivers widely recognized as bestowing numerous biological and physical benefits to the system but also present serious hazards to infrastructure. Despite this, no in-situ field measurements have assessed the degree of arrest in a naturally-formed logjam. Using time-lapse photography, repeat total station surveys and water level loggers, we provide an unprecedented perspective on the evolution of a logjam in central Idaho. Despite the namesake, we find that the logjam is not jammed. The ensemble of logs progressively deforms in response to shear and buoyant lift of flowing water, modulated by the rising limb, peak and falling limb of the snowmelt hydrograph. As water rises and log drag against the bed and banks decreases, they collectively translate downstream, generating a heterogeneous pattern of deformation. As streamflow recedes and the logs reconnect with the bed and banks, the coherent deformation pattern degrades as logs settle opportunistically amongst their neighbors. Field observations of continuous movement at a low rate are qualitatively similar to creep and clogging, behaviors that are common to a wide class of disordered materials. These similarities open the possibility to inform future studies of environmental clogging, wood-laden flows, logjams, hazard mitigation and the design of engineered logjams by bridging these practices with frontier research efforts in soft matter physics and granular rheology.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"37 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115913326","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dynamic emergence of domino effects in systems of interacting tipping elements in ecology and climate","authors":"A. Klose, Volker Karle, R. Winkelmann, J. Donges","doi":"10.5194/egusphere-egu2020-4490","DOIUrl":"https://doi.org/10.5194/egusphere-egu2020-4490","url":null,"abstract":"In ecology, climate and other fields, systems have been identified that can transition into a qualitatively different state when a critical threshold or tipping point in a driving process is crossed. An understanding of those tipping elements is of great interest given the increasing influence of humans on the biophysical Earth system. Tipping elements are not independent from each other as there exist complex interactions, e.g. through physical mechanisms that connect subsystems of the climate system. Based on earlier work on such coupled nonlinear systems, we systematically assessed the qualitative asymptotic behavior of interacting tipping elements. We developed an understanding of the consequences of interactions on the tipping behavior allowing for domino effects and tipping cascades to emerge under certain conditions. The application of these qualitative results to real-world examples of interacting tipping elements shows that domino effects with profound consequences can occur: the interacting Greenland ice sheet and thermohaline ocean circulation might tip before the tipping points of the isolated subsystems are crossed. The eutrophication of the first lake in a lake chain might propagate through the following lakes without a crossing of their individual critical nutrient input levels. The possibility of emerging domino effects calls for the development of a unified theory of interacting tipping elements and the quantitative analysis of interacting real-world tipping elements.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"74 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115702286","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Topographic uncertainty quantification for flow-like landslide\u0000models via stochastic simulations","authors":"Hu Zhao, J. Kowalski","doi":"10.5194/nhess-2019-358","DOIUrl":"https://doi.org/10.5194/nhess-2019-358","url":null,"abstract":"Topography representing digital elevation models (DEMs) are essential inputs for computational models capable of simulating the run-out of flow-like landslides. Yet, DEMs are often subject to error, a fact that is mostly overlooked in landslide modeling. We address this research gap and investigate the impact of topographic uncertainty on landslide-run-out models. In particular, we will describe two different approaches to account for DEM uncertainty, namely unconditional and conditional stochastic simulation methods. We investigate and discuss their feasibility, as well as whether DEM uncertainty represented by stochastic simulations critically affects landslide run-out simulations. Based upon a historic flow-like landslide event in Hong Kong, we present a series of computational scenarios to compare both methods using our modular Python-based workflow. Our results show that DEM uncertainty can significantly affect simulation-based landslide run-out analyses, depending on how well the underlying flow path is captured by the DEM, as well as further topographic characteristics and the DEM error's variability. We further find that in the absence of systematic bias in the DEM, a performant root mean square error based unconditional stochastic simulation yields similar results than a computationally intensive conditional stochastic simulation that takes actual DEM error values at reference locations into account. In all other cases the unconditional stochastic simulation overestimates the variability of the DEM error, which leads to an increase of the potential hazard area as well as extreme values of dynamic flow properties.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"182 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132182384","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Toward Cloud Tomography from Space Using MISR and MODIS: Locating the “Veiled Core” in Opaque Convective Clouds","authors":"Linda Forster, A. Davis, D. Diner, B. Mayer","doi":"10.1175/JAS-D-19-0262.1","DOIUrl":"https://doi.org/10.1175/JAS-D-19-0262.1","url":null,"abstract":"For passive satellite imagers, current retrievals of cloud optical thickness and effective particle size fail for convective clouds with 3D morphology. Indeed, being based on 1D radiative transfer (RT) theory, they work well only for horizontally homogeneous clouds. A promising approach for treating clouds as fully 3D objects is cloud tomography, and this has been demonstrated for airborne observations. For cloud tomography from space, however, more efficient forward 3D RT solvers are required. Here, we present a path forward, acknowledging that optically thick clouds have \"veiled cores.\" Photons scattered into and out of this deep region do not contribute significant information to the observed imagery about the inner structure of the cloud. We investigate the location of the veiled core for the MISR and MODIS imagers. While MISR provides multi-angle imagery in the visible and near-IR, MODIS includes channels in the short-wave IR, albeit at a single view angle. This combination will enable future 3D retrievals to disentangle the cloud's effective particle size and optical thickness. We find that, in practice, the veiled core is located at an optical distance of $approx$5 starting from the cloud boundary along the line-of-sight. For MODIS' absorbing wavelengths the veiled core covers a larger volume, starting at smaller optical distances. This result makes it possible to reduce the number of unknowns for the cloud tomographic reconstruction, and opens up new ways to increase the efficiency of the 3D RT solver at the heart of the reconstruction algorithm.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"42 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132871730","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Signature of supershear transition seen in damage and aftershock pattern","authors":"H. Bhat, J. Jara, L. Bruhat, S. Antoine, Kurama Okubo, M. Thomas, E. Rougier, A. Rosakis, C. Sammis, Y. Klinger, R. Jolivet","doi":"10.5194/egusphere-egu2020-7109","DOIUrl":"https://doi.org/10.5194/egusphere-egu2020-7109","url":null,"abstract":"\u0000 <p>Supershear earthquakes are rare but powerful ruptures with devastating consequences. How quickly an earthquake rupture attains this speed, or for that matter decelerates from it, strongly affects high-frequency ground motion and the spatial extent of coseismic off-fault damage.&#160;Traditionally, studies of supershear earthquakes have focused on determining which fault segments sustained fully-grown supershear ruptures. Knowing that the rupture first propagated at subshear rupture speeds, these studies usually guessed&#160;an approximate location for the transition from subshear to supershear regimes.&#160;The rarity of confirmed supershear ruptures, combined with the fact that conditions for supershear transition are still debated, complicates the investigation of supershear transition in real earthquakes.&#160;Here, we find a unique signature of the location of a supershear transition: we show that, when a rupture accelerates towards supershear speed, the stress concentration abruptly shrinks, limiting the off-fault damage and aftershock productivity.&#160;First, we use theoretical fracture mechanics to demonstrate that, before transitioning to supershear, the stress concentration around the rupture tip shrinks, confining the region where damage & aftershocks are expected. Then, employing two different dynamic rupture modeling approaches, we confirm such reduction in stress concentration, further validating the expected signature in the transition region. We contrast these numerical and theoretical results with high-resolution aftershock catalogs for three natural supershear earthquakes, where we identify a small region with lower aftershock density near the supershear transition. Finally, using satellite optical image correlation techniques, we show that, for a fourth event, the transition zone is characterized by a diminution in the width of the damage zone.&#160;Our results demonstrate that the transition from subshear to supershear rupture can be clearly identified by a localized absence of aftershocks, and a decrease in off-fault damage, due to a transient reduction of the stress intensity at the rupture tip.</p>\u0000","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"2 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130886760","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dislocation interactions during low-temperature plasticity of olivine strengthen the lithospheric mantle","authors":"D. Wallis, L. Hansen, K. Kumamoto, C. Thom, O. Plümper, M. Ohl, W. Durham, D. Goldsby, D. Armstrong, C. Meyers","doi":"10.31223/osf.io/hmp3k","DOIUrl":"https://doi.org/10.31223/osf.io/hmp3k","url":null,"abstract":"The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10$^{14}$ m$^{-2}$ and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples.","PeriodicalId":390991,"journal":{"name":"arXiv: Geophysics","volume":"10 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2019-09-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130641712","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}