Migration Imaging Research Articles

Subsurface offset behaviour in velocity analysis with extended reflectivity images

ABSTRACTMigration velocity analysis with the constant‐density acoustic wave equation can be accomplished by the focusing of extended migration images, obtained by introducing a subsurface shift in the imaging condition. A reflector in a wrong velocity model will show up as a curve in the extended image. In the correct model, it should collapse to a point. The usual approach to obtain a focused image involves a cost functional that penalizes energy in the extended image at non‐zero shift. Its minimization by a gradient‐based method should then produce the correct velocity model. Here, asymptotic analysis and numerical examples show that this method may be too sensitive to amplitude peaks at large shifts at the wrong depth and to artefacts. A more robust alternative is proposed that can be interpreted as a generalization of stack power and maximizes the energy at zero‐subsurface shift. A real‐data example is included.

Stretch-free migration imaging condition

Prestack migration has angle-dependent wavelet stretch effects, which lowers the image resolution at large reflection angles. Most current stretch correction methods operate on the migrated images. We develop a new stretch-free imaging condition, which does a shrink-and-shift operation on the extracted propagation wavelet after extrapolation, but before the imaging condition is applied. The algorithm is illustrated with the excitation amplitude imaging condition; the new images show successful stretch corrections over wide angle apertures, and preserve amplitude and phase.

The effects of coating structure and water-holding capacity on the oxygen-scavenging ability of enzymes embedded in the coating layer

Enzymes catalyzing oxygen scavenging were embedded in latex-based coatings with and without barrier kaolin clay to produce material for active packages. The clay was used to create a porous structure, and the closed-structure matrix consisted of a biopolymer comprising either starch or gelatin to increase the water uptake of the coating. The effects of the porous open structure and of the water uptake of the coated layer on the oxygen-scavenging ability of the embedded enzymes were examined at both 75% and 100% relative humidity. The results showed that the porous clay structure led to higher oxygen-scavenging capacity than that of a closed structure at both test conditions by enabling a high diffusion rate for oxygen and glucose to the active sites of the enzymes. The addition of a water-holding biopolymer did not always significantly affect the oxygen-scavenging capacity. However for a less-porous layer at 100% relative humidity, an increase in the amount of biopolymer resulted in an increase in oxygen-scavenging capacity. The results were treated statistically using multiple-factor analysis where the most important factor for the oxygen-scavenging ability was found to be the addition of clay. The coatings were also characterized with respect to water vapor uptake, overall migration, porosity, and scanning electron microscopy images.

An algorithm for 3D acoustic time-Laplace-Fourier-domain hybrid full waveform inversion

When full waveform inversion (FWI) is performed in the Laplace-Fourier domain, Laplace-Fourier-domain wave modeling, using finite-difference or finite-element methods, is performed to compute the gradient direction. In this case, a complex impedance matrix is composed and factored by direct matrix solvers. Alternatively, iterative matrix solvers may be used. However, solving 3D problems with such methods requires excessive computer memory or computing time, which causes problems in the application of the Laplace-Fourier-domain FWI. To avoid computational overburden in 3D problems, we propose a time-Laplace-Fourier hybrid FWI, in which forward and backward modeling are performed in the time domain and other procedures are conducted in the Laplace-Fourier domain. Our hybrid FWI is applied to two groups of frequencies. Inversions for the first and second groups of frequencies correspond to the Laplace-domain FWI and the Laplace-Fourier-domain FWI, respectively. The graphic processing unit is used to speed up the hybrid inversion algorithm. To verify the feasibility of this technique, the 3D hybrid FWI is applied to the data recorded along the A1 line of the synthetic SEG/EAGE 3D salt model and 3D wide-azimuth real exploration data. Numerical examples show that the hybrid FWI yields reasonable subsurface velocity structures that contribute to the enhancement of reverse-time migration images.

Plane-wave least-squares reverse-time migration

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 that LSRTM in the plane-wave domain, compared to standard reverse-time migration, produces images efficiently with fewer artifacts and better spatial resolution. Moreover, the prestack image ensemble accommodates more unknowns to makes it more robust than conventional least-squares migration in the presence of migration velocity errors.

Analysis of subspace migrations in limited-view inverse scattering problems

In this paper, we analyze the subspace migration that occurs in limited-view inverse scattering problems. Based on the structure of singular vectors associated with the nonzero singular values of the multi-static response matrix, we establish a relationship between the subspace migration imaging function and Bessel functions of integer order of the first kind. The revealed structure and numerical examples demonstrate why subspace migration is applicable for the imaging of small scatterers in limited-view inverse scattering problems.

Optimization of the seismic processing phase-shift plus finite-difference migration operator based on a hybrid genetic and simulated annealing algorithm

Although the phase-shift seismic processing method has characteristics of high accuracy, good stability, high efficiency, and high-dip imaging, it is not able to adapt to strong lateral velocity variation. To overcome this defect, a finite-difference method in the frequency-space domain is introduced in the migration process, because it can adapt to strong lateral velocity variation and the coefficient is optimized by a hybrid genetic and simulated annealing algorithm. The two measures improve the precision of the approximation dispersion equation. Thus, the imaging effect is improved for areas of high-dip structure and strong lateral velocity variation. The migration imaging of a 2-D SEG/EAGE salt dome model proves that a better imaging effect in these areas is achieved by optimized phase-shift migration operator plus a finite-difference method based on a hybrid genetic and simulated annealing algorithm. The method proposed in this paper is better than conventional methods in imaging of areas of high-dip angle and strong lateral velocity variation.

Macrophage coculture enhanced invasion of gastric cancer cells via TGF-β and BMP pathways

Objective. Transforming growth factor β (TGF-β) superfamily plays an important role in regulating gastric cancer progression. As previously demonstrated, tumor-associated macrophages (TAMs) promoted the invasion of gastric cancer cells in Matrigel. However, the role of TGF-β superfamily signaling between TAMs and gastric cancer remains unclear. Material and methods. Three-dimensional dynamic migration imaging system was used to detect gastric cancer invasion rate cocultured with macrophages in Matrigel before or after TGF-β receptor 1 or bone morphogenic protein (BMP) receptor 1A and 1B inhibition; real-time RT-PCR was used to quantitatively investigate gene expression (TGF-β1, TGF-β2, BMP4, and BMP7, ADAM9, MMP9, TIMP3, VEGF-A, and VEGF-C). Results. TGF-β1, TGF-β2, BMP4, and BMP7 expressions were increased significantly in macrophages grown with cancer cells as compared to macrophages grown alone. The invasion rate and invasion-related genes expressions of both AGS and Hs-746T gastric cancer cell lines were upregulated by macrophages, although the expression profile was different. Invasion rate and invasion-related genes' expressions of AGS cells cocultured with macrophages were downregulated significantly after TGF-βR1 and BMPR1 inhibition. Conclusions. Macrophages associated with tumor might promote gastric cancer cells invasion though enhancing TGF-β/BMPs signal pathway. Inhibiting TGF-β/BMPs signal between TAMs and gastric cancer cells might provide a new therapeutic method of gastric cancer.

Variable-depth streamer acquisition: Broadband data for imaging and inversion

Conventional marine acquisition uses a streamer towed at a constant depth. The resulting receiver ghost notch gives the maximum recoverable frequency. To push this limit, the streamer must be towed at a quite shallow depth, but this compromises the low frequencies. Variable-depth streamer (VDS) acquisition is an acquisition technique aimed at achieving the best possible signal-to-noise ratio at low frequencies by towing the streamer very deeply, but by using a depth profile varying with offset in order not to limit the high-frequency bandwidth by notches as in conventional constant-depth streamer acquisition. The idea is to use notch diversity, each receiver having a different notch, so that the final result, combining different receivers, will have no notches. The key step to process VDS acquisitions is the receiver deghosting. We found that the optimal receiver deghosting, instead of being a preprocessing step, should be done postimaging, by using a dual-input, migration and mirror migration, and a new joint deconvolution algorithm that produces a 3D real amplitude deghosted output. This method can be applied poststack, the inputs being the migration and mirror migration images and the output being the deghosted image. Using a multichannel joint deconvolution, the inputs are the migrated and mirror migrated image gathers and the outputs are the prestack deghosted image gathers. This method preserves the amplitude-versus-offset behavior, as the deghosted output can be seen on synthetic examples to be equal to a reference computed by migrating the data modeled without any reflecting water surface. A real data set was used to illustrate this method, and another one was used to check the possibility of performing prestack elastic inversion on the deghosted gathers.

Estimated source wavelet‐incorporated reverse‐time migration with a virtual source imaging condition

ABSTRACTMany geophysicists perform reverse‐time migration using a variety of artificial sources to obtain the source wavefield. Upon processing the seismic data, however, it is difficult to recover the original phase and amplitude of the source wavelet used for seismic exploration, regardless of the source. We have therefore used several artificial source wavelets such as Ricker or the first derivative Gauss wavelets expressed by well‐known functions. There are some differences between these artificial source wavelets and the original source wavelets, resulting in imperfect migration images. Artificial source wavelets tend to distort the exact location of subsurface reflectors and they create noise around the boundary of the stratum. To solve this problem, we applied the source estimation technique to the reverse‐time migration algorithm. The source estimation technique approximates the source wavelet to the original exploration source wavelet by a deconvolution method. This technique is used in full waveform inversion and provides better inversion results as demonstrated by other studies. To prove the effect of reverse‐time migration with source estimation, we tested this algorithm on the Sigsbee2a model, SEG/EAGE 3D salt model and 3D real field land data. Using the resulting images of these three models, we found that the source estimation technique can yield better migration images. To suppress the artefacts produced in the migration image, we used a wavenumber filter and Laplacian filter on 2D and 3D examples, respectively. Furthermore, we used the pseudo‐Hessian similar to the source illumination to scale the migration image because the virtual source imaging condition was used for reverse‐time migration.

Finite Difference Migration Imaging of Magnetotellurics

we put forward a new migration imaging technique of Magnetotellurics (MT) data based on improved finite difference method, which increased the accuracy of difference equation and imaging resolution greatly. We also discussed the determination of background resistivity and reimaging. The processing results of theoretical model and case study indicated that this method was a more practical and effective for MT imaging. Finally the characteristics of finite difference migration imaging were summarized and the factors which can affect the migration imaging were analyzed.

Application of Inverse Scattering Series Method for Internal Multiple Attenuation – a case study

Internal multiples due to a series of subsurface impedance contrasts are commonly observed in seismic data acquired in various places such as the Gippsland Basin of Australia and the Santos Basin of Brazil. The origin of these impedance contrasts can be due to coal seams as in the case of the Gippsland Basin or geological unconformities as in the Santos Basin. Regardless of how they are generated, internal multiple reflections often pose problems to the interpretation of geological structures. They are not easily handled by conventional de-multiple methods such as Radon Transform because internal multiples often have little difference in move out than their corresponding primary events, or predictive deconvolution because the multiple generators, and hence the multiple period, are often not known. In this paper, we present a case study of applying inverse scattering series (ISS) based method for attenuating internal multiples. The ISS method is a data-driven approach that can predict all internal multiples of a given order without any priori knowledge of the multiple generators. Moreover, it does not require any information about the subsurface velocity field. We discuss the application of this method for handling internal multiples in Santos Basin data, and present results on both the synthetic and field data from the Tupi oilfield. The results show that the internal multiples are well predicted and, with a suitably constrained subtraction technique, the migration artifacts caused by these multiples are greatly suppressed resulting in much clearer migration image for interpretation.

Both macrophages and hypoxia play critical role in regulating invasion of gastric cancer in vitro

Background. As previously demonstrated, tumor associated macrophages (TAMs) infiltration is associated with some cancers invasion and metastasis. However, the role of TAMs in the gastric cancer remains unclear. Methods Three- dimensional dynamic migration imaging system and real time RT-PCR were used to quantitatively investigate the effect of macrophages on the cancer cell mobility and gene expression related to cancer invasion and metastasis, including ADAM8, ADAM9, MMP9, TIMP3, VEGF-A and IL8 genes, in AGS, HGC-27, Hs-746T and NCI-N87 gastric cancer cell lines under normal or hypoxic conditions. Results. Under normal conditions, the cancer cell invasion rate was increased significantly and all six gene expressions were upregulated in all four cancer cell lines by macrophages. Under hypoxia the changes in the cancer cell invasion rate induced by macrophages was negatively correlated to the TIMP3 expression. In non- metastatic cell line AGS, the increase in migration rate induced by macrophages was further elevated under hypoxia with increased ADAM8 and ADAM9 expression and decreased MMP9 and TIMP3 expressions. Under hypoxia, the induction by macrophages for IL-8 expression was increased significantly in distant metastatic cell lines NCI-N87 and HS-746T, VEGF-A was increased in HGC-27 cell line. Conclusions. Both macrophages and hypoxia play an indispensable role in regulating the invasion of gastric cancer cells in vitro; ADAMs, MMP9 and TIMP3 might be involved in TAM induced invasive power of gastric cancer cells.

Acceleration of stable TTI P-wave reverse-time migration with GPUs

When a pseudo-acoustic TTI (tilted transversely isotropic) coupled wave equation is used to implement reverse-time migration (RTM), shear wave energy is significantly included in the migration image. Because anisotropy has intrinsic elastic characteristics, coupling P-wave and S-wave modes in the pseudo-acoustic wave equation is inevitable. In RTM with only primary energy or the P-wave mode in seismic data, the S-wave energy is regarded as noise for the migration image. To solve this problem, we derive a pure P-wave equation for TTI media that excludes the S-wave energy. Additionally, we apply the rapid expansion method (REM) based on a Chebyshev expansion and a pseudo-spectral method (PSM) to calculate spatial derivatives in the wave equation. When REM is incorporated with the PSM for the spatial derivatives, wavefields with high numerical accuracy can be obtained without grid dispersion when performing numerical wave modeling. Another problem in the implementation of TTI RTM is that wavefields in an area with high gradients of dip or azimuth angles can be blown up in the progression of the forward and backward algorithms of the RTM. We stabilize the wavefields by applying a spatial-frequency domain high-cut filter when calculating the spatial derivatives using the PSM. In addition, to increase performance speed, the graphic processing unit (GPU) architecture is used instead of traditional CPU architecture. To confirm the degree of acceleration compared to the CPU version on our RTM, we then analyze the performance measurements according to the number of GPUs employed.

Building starting models for full waveform inversion from wide‐aperture data by stereotomography

ABSTRACTBuilding an accurate initial velocity model for full waveform inversion (FWI) is a key issue to guarantee convergence of full waveform inversion towards the global minimum of a misfit function. In this study, we assess joint refraction and reflection stereotomography as a tool to build a reliable starting model for frequency‐domain full waveform inversion from long‐offset (i.e., wide‐aperture) data. Stereotomography is a slope tomographic method that is based on the inversion of traveltimes and slopes of locally‐coherent events in a data cube. One advantage of stereotomography compared to conventional traveltime reflection tomography is the semi‐automatic picking procedure of locally‐coherent events, which is easier than the picking of continuous events, and can lead to a higher density of picks. While conventional applications of stereotomography only consider short‐offset reflected waves, we assess the benefits provided by the joint inversion of reflected and refracted arrivals. Introduction of the refracted waves allows the construction of a starting model that kinematically fits the first arrivals, a necessary requirement for full waveform inversion. In a similar way to frequency‐domain full waveform inversion, we design a multiscale approach of stereotomography, which proceeds hierarchically from the wide‐aperture to the short‐aperture components of the data, to reduce the non‐linearity of the stereotomographic inversion of long‐offset data. This workflow which combines stereotomography and full waveform inversion, is applied to synthetic and real data case studies for the Valhall oil‐field target. The synthetic results show that the joint refraction and reflection stereotomography for a 24‐km maximum offset data set provides a more reliable initial model for full waveform inversion than reflection stereotomography performed for a 4‐km maximum offset data set, in particular in low‐velocity gas layers and in the deep part of a structure below a reservoir. Application of joint stereotomography, full waveform inversion and reverse‐time migration to real data reveals that the FWI models and the reverse‐time migration images computed from the stereotomography model shares several features with FWI velocity models and migrated images computed from an anisotropic reflection‐traveltime tomography model, although stereotomography was performed in the isotropic approximation. Implementation of anisotropy in joint refraction and reflection stereotomography of long‐offset data is a key issue to further improve the accuracy of the method.

The influence of sea water velocity variation on seismic traveltimes, ray paths, and amplitude

The main factors affecting seismic exploration is the propagation velocity of seismic waves in the medium. In the past, during marine seismic data processing, the propagation velocity of sea water was generally taken as a constant 1500 m/s. However, for deep water exploration, the sound velocity varies with the season, time, location, water depth, ocean currents, and etc.. It also results in a layered velocity distribution, so there is a difference of seismic traveltime, ray paths, and amplitude, which affect the migration imaging results if sea water propagation velocity is still taken as constant for the propagation wavefield. In this paper, we will start from an empirical equation of seismic wave velocity in seawater with changes of temperature, salinity, and depth, consider the variation of their values, build a seawater velocity model, and quantitatively analyze the impact of seawater velocity variation on seismic traveltime, ray paths, and amplitude in the seawater velocity model.

Investigation of Air/Ground Reflection and Antenna Beamwidth for Compressive Sensing SFCW GPR Migration Imaging

For stepped frequency continuous wave ground penetrating radar (SFCW GPR), the image of buried targets is usually reconstructed by a combination of point-like scatters whose number is much smaller than that of pixels of target space image. The intrinsic sparseness of target space offers a migration imaging method to make the high-quality image of underground region based on compressive sensing (CS) theory. In this paper, the effects of air/ground interface and antenna beamwidth on CS-based SFCW GPR migration imaging are presented and analyzed. It is shown that the presence of the strong air/ground interface reflection and finite antenna beamwidth usually challenges the robust CS migration imaging algorithm in practice. To overcome this problem in the context of CS migration imaging, an improved CS migration imaging method for SFCW GPR system is proposed in this paper. Experimental results show that the approach is robust to deliver a high-quality image of underground region.

From Health Check to Muslim Test: The Shifting Politics of Governing Migrant Masculinity

Racialised notions of ‘foreign masculinity’ play a crucial role in migration discourses. This paper argues that such constructs are firmly embedded in political rationalities of migration control. Analysing Austrian post-war migration policies, the paper shows that shifting constructs of ‘foreign masculinity’ were used to legitimate restrictive migration laws. The early ‘guest worker’ regime was mainly interested in questions of bodily health, strength and resilience, as migrant men were primarily seen as work objects. When migration was later reframed as a security threat to the nation and its people, new images of dangerous migrant masculinity arose. Contemporary politics of governing migration and integration construct images of archaic migrant masculinity to define unwanted populations and legitimate differentiated techniques of evaluation, inquiry and exclusion. The analysis leads to a call for critically reflecting the role that the social sciences play in constructing particular notions of migrant masculinity and its entanglements with migration control.

Multisource least‐squares migration of marine streamer and land data with frequency‐division encoding

ABSTRACTMultisource migration of phase‐encoded supergathers has shown great promise in reducing the computational cost of conventional migration. The accompanying crosstalk noise, in addition to the migration footprint, can be reduced by least‐squares inversion. But the application of this approach to marine streamer data is hampered by the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by the finite‐difference modelling method. This leads to a strong mismatch in the misfit function and results in strong artefacts (crosstalk) in the multisource least‐squares migration image. To eliminate this noise, we present a frequency‐division multiplexing (FDM) strategy with iterative least‐squares migration (ILSM) of supergathers. The key idea is, at each ILSM iteration, to assign a unique frequency band to each shot gather. In this case there is no overlap in the crosstalk spectrum of each migrated shot gather m(x, ωi), so the spectral crosstalk product m(x, ωi)m(x, ωj) =δi, j is zero, unless i=j. Our results in applying this method to 2D marine data for a SEG/EAGE salt model show better resolved images than standard migration computed at about 1/10th of the cost. Similar results are achieved after applying this method to synthetic data for a 3D SEG/EAGE salt model, except the acquisition geometry is similar to that of a marine OBS survey. Here, the speedup of this method over conventional migration is more than 10. We conclude that multisource migration for a marine geometry can be successfully achieved by a frequency‐division encoding strategy, as long as crosstalk‐prone sources are segregated in their spectral content. This is both the strength and the potential limitation of this method.

Implementation of elastic reverse-time migration using wavefield separation in the frequency domain

SUMMARY Considerable effort has been devoted to the migration of multicomponent data in elastic media with wavefield separation techniques being the most successful. Most of this work has been carried out in the time domain. In this paper, we formulate a multicomponent migration technique in the frequency domain. Reverse-time migration can be viewed as the zero-lag cross-correlation between virtual source and back-propagated wavefields. Cross-correlating the Helmholtz decomposed wavefields rather than directly correlating the vector displacement fields results in sharper, more interpretable images, contaminated by fewer crosstalk artefacts. The end products are separate P and S wave (and if desired, PS and SP) migration images. We test our migration algorithm on synthetic seismic data generated using the SEG/EAGE salt-dome, Overthrust and Marmousi-2 models. We correctly image the location and shape of the target zone for oil exploration using these data sets. Furthermore, we demonstrate that our new migration technique provides good images even when the initial velocity model is only approximate.