A plane-wave least-squares reverse-time migration (LSRTM) is formulated with a new parameterization, where the migration image of each shot gather is updated separately and an ensemble of prestack images is produced along with common image gathers. The merits of plane-wave prestack LSRTM are the following: (1) plane-wave prestack LSRTM can sometimes offer stable convergence even when the migration velocity has bulk errors of up to 5%; (2) to significantly reduce computation cost, linear phase-shift encoding is applied to hundreds of shot gathers to produce dozens of plane waves. Unlike phase-shift encoding with random time shifts applied to each shot gather, plane-wave encoding can be effectively applied to data with a marine streamer geometry. (3) Plane-wave prestack LSRTM can provide higher-quality images than standard reverse-time migration. Numerical tests on the Marmousi2 model and a marine field data set are performed to illustrate the benefits of plane-wave LSRTM. Empirical results show tha...

/pdf/plane-wave-least-squares-reverse-time-migration-354k99bbeg.pdf

Plane-wave least-squares reverse-time migration

USArray, a pioneering project for the dense acquisition of earthquake data, provides a semi-uniform sampling of the seismic wavefield beneath its footprint and greatly advances the understanding of the structure and dynamics of Earth. Despite continuing efforts in improving the acquisition design, network irregularity still causes spatial sampling alias and incomplete, noisy data, which imposes major challenges in array-based data analysis and seismic imaging. Here we employ an iterative rank-reduction method to simultaneously reconstruct the missing traces and suppress noise, i.e., obtaining free USArray recordings as well as enhancing the existing data. This method exploits the spatial coherency of three-dimensional data and recovers the missing elements via the principal components of the incomplete data. We examine its merits using simulated and real teleseismic earthquake recordings. The reconstructed P wavefield enhances the spatial coherency and accuracy of tomographic travel time measurements, which demonstrates great potential to benefit seismic investigations based on array techniques. The USArray of EarthScope is a seismic broadband network acquiring global seismic data. Here, the authors apply an iterative rank-reduction method to obtain free earthquake data at locations where no seismic stations are available as well as enhancing existing data recorded by the USArray.

/pdf/obtaining-free-usarray-data-by-multi-dimensional-seismic-4nmdy90z6c.pdf

Obtaining free USArray data by multi-dimensional seismic reconstruction.

PURPOSE: An apparatus, a method and a recording medium for imaging an underground structure using waveform inversion of a Laplace-Fourier domain are provided to stably obtain a speed model with long wavelength information and middle wavelength information. CONSTITUTION: An apparatus for imaging an underground structure using waveform inversion of a Laplace-Fourier domain includes a receiver and a signal processing unit. The signal processing unit comprises an input unit, a converting unit(202), a modeling data generating unit(203), and a parameter update unit(206). The converting unit converts measurement data in a time domain into measurement data in a Laplace-Fourier domain using an attenuation constant of a complex number. The modeling data generating unit sets an equation including a parameter by receiving a parameter showing a feature of a measurement target region. The modeling data generating unit calculates the equation in the Laplace-Fourier domain to calculate modeling data.

Apparatus, methods and recording medium for imaging a subsurface using the waveform inversion in the laplace-fourier domain

Numerical seismic modeling by finite-difference methods usually works with a global time-step size. Because of stability considerations, the time-step size is determined essentially by the highest seismic velocity, i.e., the higher the highest velocity, the smaller the time step needs to be. Therefore, if large velocity contrasts exist within the numerical grid, domains of low velocity are oversampled temporally. Using different time-step sizes in different parts of the numerical grid can reduce computational costs considerably.

Seismic Finite-difference Modeling With Spatially Varying Time Steps

Research progress on seismic imaging technology

Dense arrays allow sampling of seismic wavefield without significant aliasing, and surface wave tomography has benefitted from exploiting wavefield coherence among neighbouring stations. However, explicit or implicit assumptions about wavefield, irregular station spacing and noise still limit the applicability and resolution of current surface wave methods. Here, we propose to apply the theory of compressive sensing (CS) to seek a sparse representation of the surface wavefield using a plane-wave basis. Then we reconstruct the continuous surface wavefield on a dense regular grid before applying any tomographic methods. Synthetic tests demonstrate that wavefield CS improves robustness and resolution of Helmholtz tomography and wavefield gradiometry, especially when traditional approaches have difficulties due to sub-Nyquist sampling or complexities in wavefield.

/pdf/application-of-wavefield-compressive-sensing-in-surface-wave-k9rlgtbjf2.pdf

Application of wavefield compressive sensing in surface wave tomography

/pdf/finite-difference-seismic-forward-modeling-method-for-fluid-4wq15qvj3b.pdf

Finite difference seismic forward modeling method for fluid–solid coupled media with irregular seabed interface

Optimized equivalent staggered-grid FD method for elastic wave modelling based on plane wave solutions

The least-squares reverse time migration (LSRTM) method with higher image resolution and amplitude is becoming increasingly popular. However, the LSRTM is not widely used in field land data processing because of its sensitivity to the initial migration velocity model, large computational cost and mismatch of amplitudes between the synthetic and observed data. To overcome the shortcomings of the conventional LSRTM, we propose a cross-correlation least-squares reverse time migration algorithm in pseudo-time domain (PTCLSRTM). Our algorithm not only reduces the depth/velocity ambiguities, but also reduces the effect of velocity error on the imaging results. It relieves the accuracy requirements on the migration velocity model of least-squares migration (LSM). The pseudo-time domain algorithm eliminates the irregular wavelength sampling in the vertical direction, thus it can reduce the vertical grid points and memory requirements used during computation, which makes our method more computationally efficient than the standard implementation. Besides, for field data applications, matching the recorded amplitudes is a very difficult task because of the viscoelastic nature of the Earth and inaccuracies in the estimation of the source wavelet. To relax the requirement for strong amplitude matching of LSM, we extend the normalized cross-correlation objective function to the pseudo-time domain. Our method is only sensitive to the similarity between the predicted and the observed data. Numerical tests on synthetic and land field data confirm the effectiveness of our method and its adaptability for complex models.

Cross-correlation least-squares reverse time migration in the pseudo-time domain

The mapping method is a forward-modeling method that transforms the irregular surface to horizontal by mapping the rectangular grid as curved; moreover, the wave field calculations move from the physical domain to the calculation domain. The mapping method deals with the irregular surface and the low-velocity layer underneath it using a fine grid. For the deeper high-velocity layers, the use of a fine grid causes local oversampling. In addition, when the irregular surface is transformed to horizontal, the flattened interface below the surface is transformed to curved, which produces inaccurate modeling results because of the presence of ladder-like burrs in the simulated seismic wave. Thus, we propose the mapping method based on the dual-variable finite-difference staggered grid. The proposed method uses different size grid spacings in different regions and locally variable time steps to match the size variability of grid spacings. Numerical examples suggest that the proposed method requires less memory storage capacity and improves the computational efficiency compared with forward modeling methods based on the conventional grid.

Variable-coordinate forward modeling of irregular surface based on dual-variable grid

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