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.

An improved reverse time migration for subsurface imaging over complex geological structures:A numerical study

In seismic exploration,it is a critical task to image and interpret different seismic signatures over complex geology due to strong lateral velocity contrast,steep reflectors,overburden strata and dipping flanks.To understand the behavior of these seismic signatures,nowadays Reverse Time Migration(RTM)technique is used extensively by the oil&gas industries.During the extrapolation phase of RTM,the source wavefield needs to be saved,which needs high storage memory and large computing time.These two are the main obstacles of RTM for production use.In order to overcome these disadvantages,in this study,a second-generation improved RTM technique is proposed.In this improved form,a shift operator is introduced at the time of imaging condition of RTM algorithm which is performed automatically both in space and time domain.This effort is made to produce a better-quality image by minimizing the computational time as well as numerical artefacts.The proposed method is applied over various benchmark models and validated by implementing over one field data set from the Jaisalmer Basin,India.From the analysis,it is observed that the method consumes a minimum of 45%less storage space and reduce the execution time by 20%,as compared to conventional RTM.The proposed RTM is found to work efficiently in comparison to the conventional RTM both in terms of imaging quality and minimization of numerical artefacts for all the benchmark models as well as field data.

Wide/narrow azimuth acquisition footprints and their effects on seismic imaging

Acquisition footprint is a new concept to describe the seismic noise in three-dimensional seismic exploration and it is closely related to geometry and observation shuttering. At present, the study on acquisition footprints has become a hot spot. In partnership with the Dagang Oilfield, we used the channel sand body seismic physical model to study the characteristics of wide/narrow azimuth acquisition footprints and analyzed and compared the two types of footprints and their effects on target imaging. In addition, the footprints caused by data processing of the normal moveout offset （NMO） stretching aberration were discussed. These footprints are located only in the shallow or middle layer in the time slice, and possibly affect the imaging of shallow target layers, and have no influence on deep target imaging. Seismic physical modeling has its advantages in the study of acquisition footprints.

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.

Method of high-density seismic imaging exploration and application samples

The paper introduces the method of high-density seismic imaging exploration, discusses its features different from conventional shallow seismic reflection wave technique, and illustrates the application effect of the method using three samples of engineering geological explorations on land and in water exploration of underground cavity, location survey of sunk ship and investigation of channel silt depth.

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 distributed

A seismic modeling analysis of wide and narrow 3D observation systems for channel sand bodies

The effect of the wide and narrow azimuth 3D observation systems on seismic imaging precision is becoming a hot area for studies of high precision 3D seismic acquisition methods in recent years. In this paper we utilize 3D physical seismic modeling experiments. A 3D channel sand body physical seismic model is constructed and two acquisition systems are designed with wide azimuth （16 lines） and narrow azimuth （8 lines） to model 3D seismic data acquisition and processing seismic work flows. From analysis of migrated time slice data with high quality and small size, we conclude that when the overlying layers are smooth and lateral velocities have little change, both wide and narrow azimuth observation systems in 3D acquisition can be used for obtaining high precision imaging and equivalent resolution of the channel sand body.

Numerical simulation scattered imaging in deep mines

Conventional seismic exploration,mostly based on reflection theory,hardly has accurate imaging results for disaster geologic bodies which have small scale,steep dip,or complex structure.In this paper,we design two typical geologic models for analyzing the characteristics of scattered waves in mines for forward modeling by finite difference and apply the equivalent offset migration（EOM）and EOM-based interference stack migration methods to mine prospecting.We focus on the analysis of scatted imaging’s technological superiority to reflection imaging.Research shows：1）scattered imaging can improve fold and make the best of weak scattered information,so it shows better results than post-stack migration imaging and 2）it can utilize the diffraction stack migration method-based ray path theory for mine seismic advanced prediction,so it provides an new efficient imaging method for improving resolution of mine seismic exploration.

Study and application analysis of random noise adaptive morphological fi lter algorithm reconstruction for seismic signals

In this study,a new adaptive morphological filter is developed based on the mathematical morphology algorithm and characteristics of the subtle differences in the waveform morphology in seismic data.The algorithm improves the traditional morphological dilation and corrosion operations.In this study,we propose a multiscale adaptive operator based on the principle of morphological structural“probes”and present the corresponding mathematical proof.Simulation experiments and actual seismic data processing results show that compared with traditional morphological filters,the constructed OCCO-based multistructure adaptive morphological filter can suppress noise to the greatest extent.Moreover,it can effectively improve the SNR of the images,and offers great application prospects.

Application of High-Density Seismic Image in Nondestructive Exploration of Cut-off Cement-Soil Wall in Earth Dam

In order to quickly explore the quality of cut-off wall in dams, a new method of high-density seismic image was adopted and estimated by model and in-situ wall tests.The vibration exciter was employed and several parameters such as hypocentral distance, length of signal record and sampling space in signal collection were determined, which are 8 m, 0.25 ms and 128 ms respectively. Through time and frequency field signal analyses, it is concluded that, the smaller arrival times of reflected longitudinal and surface waves, and the higher their main frequencies, the higher the strength of the wall, vice versa. Accordingly the construction quality of the wall can be evaluated quickly by high-density seismic image.

Fracture network characterisation of the Balmuccia peridotite using drone-based photogrammetry,implications for active-seismic site survey for scientific drilling

The presence of discontinuities(e.g.faults,fractures,veins,layering)in crystalline rocks can be challenging for seismic interpretations because the wide range of their size,orientation,and intensity,which controls the mechanical properties of the rock and elastic wave propagation,resulting in equally varying seismic responses at different scales.The geometrical characterisation of adjacent outcrop discontinuity networks allows a better understanding of the nature of the subsurface rocks and aids seismic interpretation.In this study,we characterise the discontinuity network of the Balmuccia peridotite(BP)in the IvreaeVerbano Zone(IVZ),northwestern Italy.This geological body is the focus of the Drilling the Ivrea eVerbano zonE(DIVE),an international continental scientific drilling project,and two active seismic surveys,SEismic imaging of the Ivrea ZonE(SEIZE)and high-resolution SEIZE(Hi-SEIZE),which aim to resolve the subsurface structure of the DIVE drilling target through high-resolution seismic imaging.For fracture characterisation,we developed two drone-based digital outcrop models(DOMs)at two different resolutions(10^(-3)-10 m and 10^(-1)-10^(3)m),which allowed us to quantitatively characterise the orientation,size,and intensity of the main rock discontinuities.These properties affect the seismic velocity and consequently the interpretation of the seismic data.We found that(i)the outcropping BP discontinuity network is represented by three more sets of fractures with respect to those reported in the literature;(ii)the discontinuity sizes follow a power-law distribution,indicating similarity across scales,and(iii)discontinuity intensity is not uniformly distributed along the outcrop.Our results help to explain the seismic behaviour of the BP detected by the SEIZE survey,suggesting that the low P-wave velocities observed can be related to the discontinuity network,and provide the basic topological parameters(orientation,density,distribution,and aperture)of the fracture network unique to the BP.These,in turn,can be used for interpretation of the Hi-SEIZE seismic survey and forward modelling of the seismic response.

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 Bohai petroliferous 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.

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.

Seismic Imaging of Complex Structures in the Tarim Basin

Conventional time imaging techniques are not capable of producing accurate seismic imaging of the subsurface in the mountain front of the Tarim Basin, China. Their imaged structures have led to some major drilling failures before, bearing a disrepute that ＂their structural closures have wheels and their structural highs have springs＂. This article first lists the imaging challenges, and explains in a schematic why the time imaging techniques fail in this area. Then through a series of real data examples, it demonstrates that when there exist lateral velocity variations, depth imaging is the only solution to tackle the imaging challenges in this area. Depth imaging accounts for the complexity of the wavefield, therefore produces superior and geological plausible images. The core task in properly performing depth imaging is building the velocity model. This article stresses some the main aspects in this regard.

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.

On the Layering Artifacts in Seismic Imageries

A common feature in seismic imageries of the crust and mantle is a layering pattern.Layering structures do exist in multiple scales,such as layered strata and unconformities in local and regional scales,and undulating seismic discontinuities in the crust and mantle.However,layering arti-fact also exists due to limitations in seismic processing and data coverage.There is a tendency for seis-mic stacking methods to over-map reflectors and scatters into along-isochron layers.In contrast,seis-mic tomography methods tend to under-map sub-horizontal layers with along-raypath smears and ar-tifacts of various de-mean processes.To better identify signals and artifacts in seismic imageries,it is necessary to understand the origins of various artifacts and make careful comparison between the solu-tions of different data and methods.

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.

Migration of multiples from the South China Sea

The South China Sea where water depth is up to 5000 m is the most promising oil and gas exploration area in China in the future.The seismic data acquired in the South China Sea contain various types of multiples that need to be removed before imaging can be developed.However,compared with the conventional reflection migration,multiples carry more information of the underground structure that helps provide better subsurface imaging.This paper presents a method to modify the conventional reverse time migration so that multiple reflections can migrate to their correct locations in the subsurface.This approach replaces the numerical impulsive source with the recorded data including primaries and multiples on the surface,and replaces the recorded primary reflection data with multiples.In the reverse time migration process,multiples recorded on the surface are extrapolated backward in time to each depth level,while primaries and multiples recorded on the surface are extrapolated forward in time to the same depth levels.By matching the difference between the primary and multiple images using an objective function,this algorithm improves the primary resultant image.Synthetic tests on Sigsbee2 B show that the proposed method can obtain a greater range and better underground illumination.Images of deep water in the South China Sea are obtained using multiples and their matching with primaries.They demonstrate that multiples can make up for the reflection illumination and the migration of multiples is an important research direction in the future.