A review of the wave gradiometry method for seismic imaging

As dense seismic arrays at different scales are deployed,the techniques to make full use of array data with low computing cost become increasingly needed.The wave gradiometry method(WGM)is a new branch in seismic tomography,which utilizes the spatial gradients of the wavefield to determine the phase velocity,wave propagation direction,geometrical spreading,and radiation pattern.Seismic wave propagation parameters obtained using the WGM can be further applied to invert 3D velocity models,Q values,and anisotropy at lithospheric(crust and/or mantle)and smaller scales(e.g.,industrial oilfield or fault zone).Herein,we review the theoretical foundation,technical development,and major applications of the WGM,and compared the WGM with other commonly used major array imaging methods.Future development of the WGM is also discussed.

Passive Seismic Structure Imaging of a Coal Mine by Ambient Noise Seismic Interferometry on a Dense Array

Active source seismic method is generally used to image subsurface structures for resource exploration,including oil,gas and coal.Although it can provide highresolution subsurface structures,due to some economic and environmental restrictions,it is not suitable in some cases.In recent 20 years,passive seismic survey based on ambient noise seismic interferometry(ANSI)has started to be widely used in imaging subsurface structures.In comparison,ANSI does not need active sources and can image subsurface structures at different depths as a lowcost alternative to active seismic exploration.

Seismic fine imaging and its significance for natural gas hydrate exploration in the Shenhu Test Area, South China Sea

Shenhu area in South China Sea includes extensive collapse and diapir structures,forming high-angle faults and vertical fracture system,which functions as a fluid migration channel for gas hydrate formation.In order to improve the imaging precision of natural gas hydrate in this area,especially for fault and fracture structures,the present work propose a velocity stitching technique that accelerates effectively the convergence of the shallow seafloor,indicating seafloor horizon interpretation and the initial interval velocity for model building.In the depth domain,pre-stack depth migration and residual curvature are built into the model based on high-precision grid-tomography velocity inversion,after several rounds of tomographic iterations,as the residual velocity field converges gradually.Test results of the Shenhu area show that the imaging precision of the fault zone is obviously improved,the fracture structures appear more clearly,the wave group characteristics significantly change for the better and the signal-to-noise ratio and resolution are improved.These improvements provide the necessary basis for the new reservoir model and field drilling risk tips,help optimize the favorable drilling target,and are crucial for the natural gas resource potential evaluation.

Seismic Imaging and 3D Architecture of Yongle Atoll of the Xisha Archipelago, South China Sea

Yongle atoll in the Xisha(Paracel) Archipelago is an isolated carbonate platform developed on Precambrian metamorphic and Mesozoic volcanic rocks since the early Miocene. To identify the 3D stratigraphic architecture and evolution of this platform, 13 high-resolution seismic profiles and shallow-to-deep water multi-beam data were processed and analyzed to reveal seismic facies, sequence boundary reflectors, seismic units, and platform architecture. Nine types of seismic facies were recognized based on their geometry, which included seismic amplitude, continuity, and termination patterns;additionally, six reflections, i.e., Tg, T60, T50, T40, T30, and T20, were identified in the Cenozoic strata. Five seismic units, SQ1(lower Miocene), SQ2(middle Miocene), SQ3(upper Miocene), SQ4(Pliocene), and SQ5(Quaternary), were identified from bottom to top across the platform. The platform grew rapidly in the middle Miocene and backstepped in the late Miocene–Pliocene. Here, we discuss the developmental characteristics and evolution of the Yongle Atoll, in combination with drilling wells, which can be divided into four stages: the initiation stage in the early Miocene, the flourishing stage in the middle Miocene, the partial-drowning stage in the late Miocene–Pliocene, and modern atoll in the Quaternary.

3-D Seismic Imaging and Morpho-tectonical Interpretation of Saucerlike Dykes of the Tarim Flood Basalt Province, NW China

The detailed structures of the plumbing system of the early Permian Tarim flood basalt were investigated by 3-D seismic imaging.The images show that the Tarim flood basalt mainly erupted from central volcanoes

Structures of the Bohai Petroliferous Area,Bohai Bay Basin

This paper, for the first time, deals with a more systematic study of the structures in the Bohaipetroliferous area that covers nearly one third of the Bohai Bay basin. The study mainly involves the effects of preexisting basement faults on the basin formation, the characteristics of basin geometry and kinetics, the modelling of the tectonic-thermal history, the polycyclicity and heterogeneity in the structural evolution and the natural seismic tomographic images of the crust and upper mantle. The authors analyze the features of the dynamic evolution of the basin in the paper and point out that the basin in the Bohai petroliferous area is an extensional pull-apart basin.

A case study of 3D RTM-TTI algorithm on multicore and many-core platforms

3D reverse time migration in tiled transversly isotropic(3D RTM-TTI) is the most precise model for complex seismic imaging.However,vast computing time of 3D RTM-TTI prevents it from being widely used,which is addressed by providing parallel solutions for 3D RTM-TTI on multicores and many-cores.After data parallelism and memory optimization,the hot spot function of 3D RTMTTI gains 35.99 X speedup on two Intel Xeon CPUs,89.75 X speedup on one Intel Xeon Phi,89.92 X speedup on one NVIDIA K20 GPU compared with serial CPU baseline.This study makes RTM-TTI practical in industry.Since the computation pattern in RTM is stencil,the approaches also benefit a wide range of stencil-based applications.

Amplitude Compensation for One-Way Wave Propagators in Inhomogeneous Media and its Application to Seismic Imaging

TheWKBJ solution for the one-waywave equations inmediawith smoothly varying velocity variation with depth,c(z),is reformulated from the principle of energy flux conservation for acoustic media.The formulation is then extended to general heterogeneous media with local angle domain methods by introducing the concepts of Transparent Boundary Condition(TBC)and Transparent Propagator(TP).The influence of the WKBJ correction on image amplitudes in seismic imaging,such as depth migration in exploration seismology,is investigated in both smoothly varying c(z)and general heterogeneous media.We also compare the effect of the propagator amplitude compensation with the effect of the acquisition aperture correction on the image amplitude.Numerical results in a smoothly varying c(z)medium demonstrate that theWKBJ correction significantly improves the one-way wave propagator amplitudes,which,after compensation,agree very well with those from the full wave equation method.Images for a point scatterer in a smoothly varying c(z)medium show that the WKBJ correction has some improvement on the image amplitude,though it is not very significant.The results in a general heterogeneous medium(2D SEG/EAGE salt model)show similar phenomena.When the acquisition aperture correction is applied,the image improves significantly in both the smoothly varying c(z)medium and the 2D SEG/EAGE saltmodel.The comparisons indicate that although theWKBJ compensation for propagator amplitude may be important for forward modeling(especially for wide-angle waves),its effect on the image amplitude in seismic imaging is much less noticeable compared with the acquisition aperture correction for migration with limited acquisition aperture in general heterogeneous media.

Dreamlet:A New Representation and Migration of Seismic Wavefield in Full Local Domains

Seismic events have limited time duration,vary with space/traveltime and interact with the local subsurface medium during propagation.Partitioning is a valu-able strategy for nonstationary seismic data analysis,processing and wave propagation.It has the potential for sparse data representation,flexible data operation and highly accurate local wave propagation.Various local transforms are powerful tools for seismic data segmentation and representation.In this paper,a detailed description of a multi-dimensional local harmonic transformed domain wave propagation and imaging method is given.Using a tensor product of a Local Exponential Frame(LEF)vector as the time-frequency atom(a drumbeat)and a Local Cosine Basis(LCB)function as the space-wavenumber atom(a beamlet),we construct a time-frequency-space-wavenumber local atom-dreamlet,which is a combination of drumbeat and beamlet.The dreamlet atoms have limited spatial extension and temporal duration and constitute a complete set of frames,termed as dreamlet frames,to decompose and represent the wavefield.The dreamlet transform first partitions the wavefields using time-space supporting functions and then the data in each time-space blocks is repre-sented by local harmonic bases.The transformed wavefield is downward-continued by the dreamlet propagator,which is the dreamlet atom evolution weightings deduced from the phase-shift one-way propagator.The dreamlet imaging method is formulated with a local background propagator for large-scale medium propagation and com-bined with a local phase-screen correction for small-scale perturbations.The features of dreamlet migration and imaging include sparse seismic data representation,accurate wave propagation and the flexibility of localized time operations during migration.Numerical tests using Sigsbee 2A synthetic data set and real marine seismic data demonstrate the validity and accuracy of this method.With time-domain localization being involved,the dreamlet method can also be applied effectively to target-oriented migration and imaging.

Acoustic waveform inversion in frequency domain:Application to a tunnel environment

Waveform inversion is an approach used to find an optimal model for the velocity field of a ground structure such that the dynamic response is close enough to the given seismic data.First,a suitable numerical approach is employed to establish a realistic forward computer model.The forward problem is solved in the frequency domain using higher-order finite elements.The velocity field is inverted over a specific number of discrete frequencies,thereby reducing the computational cost of the forward calculation and the nonlinearity of the inverse problem.The results are presented for different frequency sets and with different source and receiver locations for a twodimensional model.The influence of attenuation effects is also investigated.The results of two blind tests are presented where only the seismic records of an unknown synthetic model with an inhomogeneous material parameter distribution are provided to mimic a more realistic case.Finally,the result of the inversion in a three-dimensional space is illustrated.

Performance Analysis of a High-Order Discontinuous Galerkin Method Application to the Reverse Time Migration

This work pertains to numerical aspects of a finite element method based discontinuous functions.Our study focuses on the Interior Penalty Discontinuous Galerkin method(IPDGM)because of its high-level of flexibility for solving the full wave equation in heterogeneousmedia.We assess the performance of IPDGMthrough a comparison study with a spectral element method(SEM).We show that IPDGM is as accurate as SEM.In addition,we illustrate the efficiency of IPDGM when employed in a seismic imaging process by considering two-dimensional problems involving the Reverse Time Migration.

Pseudo-Depth/Intercept-Time Monotonicity Requirements in the Inverse Scattering Algorithm for Predicting Internal Multiple Reflections

In this paper we discuss the inverse scattering algorithm for predicting internal multiple reflections(reverberation artefacts),focusing our attention on the construction mechanisms.Roughly speaking,the algorithm combines amplitude and phase information of three different arrivals(sub-events)in the data set to predict one multiple reflection.The three events are conditioned by a certain relation which requires that their pseudo-depths,defined as the depths of their turning points relative to the constant background velocity,satisfy a lower-higher-lower relationship.This implicitly assumes a pseudo-depth monotonicity condition,i.e.,the relation between the actual depths and the pseudo-depths of any two sub-events is the same.We study this relation in pseudo-depth and show that it is directly connected with a similar relation between the vertical or intercept times of the sub-events.The paper also provides the first multidimensional analysis of the algorithm(for a vertically varying acoustic model)with analytical data.We show that the construction of internal multiples is performed in the plane waves domain and,as a consequence,the internal multiples with headwaves sub-events are also predicted by the algorithm.Furthermore we analyze the differences between the time monotonicity condition in vertical or intercept time and total travel time and show a 2D example which satisfies the former but not the latter.Finally we discuss one case in which the monotonicity condition is not satisfied by the sub-events of an internal multiple and discuss ways of lowering these restrictions and of expanding the algorithm to address these types of multiples.