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The seawater column is typically taken as a homogeneous velocity layer in wide-angle crustal seismic surveys in marine environments. However, heterogeneities in salinity and temperature throughout the seawater layer result insignificant lateral variations in its seismic velocity, especially in deep marine environments. Failure to compensate for these velocity inhomogeneities will introduce significant artifacts in constructing crustal velocity models using seismic tomography. In this study, we conduct numerical experiments to investigate the impact of heterogeneous seismic velocities in seawater on tomographic inversion for crustal velocity models. Experiments that include lateral variation in seawater velocity demonstrated that the modeled crustal velocities were contaminated by artifacts from tomographic inversions when assuming a homogeneous water layer. To suppress such artifacts, we propose two strategies: 1) simultaneous inversion of water velocities and the crustal velocities; 2) layer-stripping inversion during which to first invert for seawater velocity and then correct the travel times before inverting for crustal velocities. The layer-stripping inversion significantly improves the modeling of variation in seawater velocity when preformed with seismic sensors deployed on the ocean bottom and in the water column. Such strategies improve crustal modeling via wide-angle seismic surveys in deep-marine environment. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed a global tomographic model for the Tibet–Himalayan collision zone, which indicates that the Indian lithospheric slab has been subducted subhorizontally beneath nearly the entire Tibetan plateau to depths of 165–260km. Tibetan velocity structure is low in its crust and high in its lithospheric mantle at depths between 75 and 120km. We interpret an asthenospheric layer positioned above the subducted Indian slab at depths between 120 and 165km beneath the Tibetan plateau. Beneath the central portion of the plateau a low-velocity anomaly exists from the crust down to 310km depth, indicating mantle upwelling through a weakened part of the subducted slab. We present a model, which explains that, the uplift history and low relief of the Tibetan plateau is a result of subhorizontal subduction and heating of Indian lithosphere that is separated from Tibetan lithosphere by a thin channel of asthenosphere. Two predictions made by our model are: (1) the amount of shortening in the Himalayas is equivalent to the amount of underthrusted Indian mantle lithosphere; and (2) a young mantle geochemical signature should be present along the entire southern portion of the Tibetan plateau. [Copyright &y& Elsevier]

Accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion. We describe the application of deformable-layer tomography (DLT) to invert for the geometry of salt flank using travel times of transmitted P- and S-waves from surface sources to downhole receivers. The DLT allows us to take advantage of a common situation that the salt velocity is known and generally invariant, but we need to determine the variable geometry of salt flank. We demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical-seismic-profiling (VSP) survey. We first use picked P-wave arrivals to invert for the salt flank geometry; the sinusoid shape of the salt flank is estimated fairly by the DLT due to the uneven P-wave raypath coverage. As an improvement, we further incorporate the S-wave arrivals in the DLT. The picking of S-wave arrivals is assisted with modeled S-wave arrivals based on the P-wave DLT model. The DLT solution using both P- and S-wave arrivals delineates the salt flank geometry more accurately than that using P-wave arrivals alone. The salt flank delineation using DLT can complement with the conventional salt proximity and migration methods. It could also serve as constraints or the initial model for the full-waveform inversion of salt geometry. [ABSTRACT FROM AUTHOR]

Mapping the crustal structure beneath the South Yellow Sea (SYS), which includes the collision zone between the North China Block and the South China Block, will help to reveal the geologic history and relationship between these geologic blocks. Previous interpretations of the crustal structure of the SYS did not use detailed seismic velocity structure as a constraint. We produced an upper crustal velocity model of the northern SYS using a new wide-angle seismic dataset. Velocity anomalies resolved by our models correlate well with known features such as sedimentary basins and the tectonic uplifts in the SYS as low- and high-velocity anomalies, respectively. As might be expected, not only major faults and/or lithologic boundaries were found to be associated with large velocity gradients, but also our models exhibited resolvable deeply penetrating vertical low-velocity anomalies associated with the locations of strike-slip faults that were poorly resolved in an older seismic reflection profile. Our interpretations of the locations of major faults were based on the stacked seismic reflection profile and velocity modelling of wide-angle seismic array data and were extended to neighbouring regions by following trends in gravity, magnetic and earthquake location data. Faults traversing the North Basin of the SYS converged with the onshore Jia-Xiang Fault (JXF), suggesting that the branching of the JXF fault system controlled the formation of the North Basin. The distribution of the strike-slip faults resolved by our modelling of the JXF leads us to propose a model for the North Basin as a strike-slip stress regime that is consistent with the known tectonic framework. These models will be useful for future analysis of the regional tectonic evolution of the SYS region. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]