Common Midpoint Research Articles

Using seismic noise derived from fluid injection well for continuous reservoir monitoring

To construct a reliable and cost-effective monitoring system for injected [Formula: see text] in carbon capture and storage projects, we have considered a seismic monitoring approach using seismic noise from a fluid injection well. The passive seismic interferometry continuously monitors injected [Formula: see text], enabling the detection of associated accidental incidents (e.g., [Formula: see text] leakage). We have applied three approaches: (1) crosscorrelation, (2) crosscoherence, and (3) deconvolution, to the passive seismometer data acquired during a fluid-injection experiment in Svalbard in the Norwegian Arctic. The crosscoherence approach enabled the construction of shot gathers similar to active-source data. Reflectors from the reservoir could be identified on common-midpoint (CMP) gathers constructed via seismic interferometry, and seismic velocity could be estimated from the time-lapse CMP gathers. High-frequency noise from fluid injection operations and low-amplitude background ambient noise were suitable for reconstructing virtual seismic data. However, we clearly found that the time variation characteristics of the noise influenced monitoring results, and thus the stable part of the noise should be used for monitoring. We further applied surface-wave analysis to the virtual shot gathers derived from seismic interferometry and investigated variations in S-wave velocity structure in a shallow formation. We observed clear time variations in seismic velocity in the shallow part of permafrost regions. The information derived from the surface-wave analysis is useful in evaluating the influence of shallow formations on monitoring results of deep reservoirs.

Effective parameters in ground roll attenuation using FO CRS stacking

Ground roll is a coherent noise in land seismic data that has high energy, high amplitude, low frequency and low velocity. It has to be attenuated in the seismic data processing as it may mask reflections in the zone of ground roll. In this study, we employed common reflection surface for finite offset (FO CRS) to attenuate the ground roll. The FO CRS stacking operator is a hyperbola; therefore, it fits the hyperbolic reflections in the prestack data. Conversely, the ground roll is linear in the common-midpoint (CMP) and common-shot (CS) gathers and can be distinguished and attenuated by the FO CRS operator. Thus, we search for the dip and curvature of the reflections in the CMP section and CS gather prior to the finite-offset section. When the algorithm is specified, the ground roll and reflections have low and high coherency values, respectively. So, any event with non-hyperbolic traveltime, like the linear traveltime ground roll can be removed.We applied the proposed method on a synthetic and an oilfield data from the west of Iran. Results showed that the FO CRS stacking method properly attenuated the ground roll. Further investigations were the effects of spatial aliasing, frequency content, random noise, ground roll dip, the range of dip and curvature scans and reflection amplitudes on ground roll attenuation by the FO CRS stacking. From mentioned parameters, spatial aliasing, frequency content, and random noise had no significant effects. On the contrary, the proposed method turned out to be strongly dependent upon ground roll dip, the range of dip and curvature scans and reflection amplitudes.

Constraining attenuation uncertainty in common midpoint radar surveys of ice sheets

For common-offset radar data, there is no clear way to disentangle path effects from reflector characteristics, so efforts to determine the physical properties at the bed using reflection amplitude are inherently limited by the constraints on englacial attenuation. In this study, we identify the theoretical considerations required for interpreting bistatic radar surveys, and use data collected on the Northeast Greenland Ice Stream (NEGIS) and Kamb Ice Stream to compute local attenuation profiles. We found that failing to correct for angle-dependent controls on return power (including antenna directivity, the reflection coefficient, and refractive focusing) can bias the computed attenuation rates as much as 30 dB/km for reflectors at 1 km depth. Because the radiation characteristics are the dominant source of uncertainty in our data, we recommend either a simplified survey design for the future (where the antennae are decoupled from the ice surface), or additional data collection to constrain the near-field permittivity and its effect on the radiation pattern. Depth-averaged attenuation rates computed using CMP methods for deep reflectors yield values >10 dB/km higher than attenuation rates computed using common-offset techniques with the same data. We attribute these anomolously high attenuation rates to additional wavenumber (and therefore, angle) dependent interferences between sub-wavelength reflectors.

Increasing signal-to-noise ratio of marine seismic data: A case study from offshore Korea

Subsurface imaging is difficult without removing the multiples intrinsic to most marine seismic data. Choosing the right multiple suppression method when working with marine data depends on the type of multiples and sometimes involves trial and error. A major amount of multiple energy in seismic data is related to the large reflectivity of the surface. Surface-related multiple elimination (SRME) is effective for suppressing free-surface-related multiples. Although SRME has some limitations, it is widely used because it requires no assumptions about the subsurface velocities, positions, and reflection coefficients of the reflector causing the multiples. The common reflector surface (CRS) stacking technique uses CRS reflectors rather than common mid-point (CMP) reflectors. It stacks more traces than conventional stacking methods and increases the signal-to-noise ratio. The purpose of this study is to address a process issue for multiple suppression with SRME and Radon filtering, and to increase the signal-to-noise ratio by using CRS stacking on seismic data from the eastern continental margin of Korea. To remove free surface multiples, SRME and Radon filtering are applied to attenuate the interbed multiples. Results obtained using synthetic data and field data show that the combination of SRME and Radon filtering is effective for suppressing free-surface multiples and peg-leg multiples. Applying CRS stacking to seismic data in which multiples have been eliminated increases the signal-to-noise ratio for the area examined, which is being considered for carbon dioxide capture and storage.

Migration velocity analysis using residual diffraction moveout: a real-data example

Unfocused seismic diffraction events carry direct information about errors in the migration-velocity model. The residual-diffraction-moveout (RDM) migration-velocity-analysis (MVA) method is a recent technique that extracts this information by means of adjusting ellipses or hyperbolas to uncollapsed migrated diffractions. In this paper, we apply this method, which has been tested so far only on synthetic data, to a real data set from the Viking Graben. After application of a plane-wave-destruction (PWD) filter to attenuate the reflected energy, the diffractions in the real data become interpretable and can be used for the RDM method. Our analysis demonstrates that the reflections need not be completely removed for this purpose. Beyond the need to identify and select diffraction events in post-stack migrated sections in the depth domain, the method has a very low computational cost and processing time. To reach an acceptable velocity model of comparable quality as one obtained with common-midpoint (CMP) processing, only two iterations were necessary.

Surface wave attenuation in the shallow subsurface from multichannel–multishot seismic data: a new approach for detecting fractures and lithological discontinuities

Surface wave analysis generally neglects amplitude information, instead using phase information to delineate near-surface S-wave velocity structures. To effectively characterize subsurface heterogeneities from amplitude information, we propose a method of estimating lateral variation of attenuation coefficients of surface waves from multichannel–multishot (multifold) seismic data. We extend the concept of the common midpoint cross-correlation method, used for phase velocity estimation, to the analysis of attenuation coefficients. Our numerical experiments demonstrated that when used together, attenuation coefficients and phase velocities could characterize a lithological boundary as well as fracture zone. We applied the proposed method to multifold seismic reflection data acquired in Shikoku Island, Japan. We clearly observed abrupt changes in lateral variation of estimated attenuation coefficients around fault locations associated with a lithological boundary and with well-developed fractures, whereas phase velocity results could detect only the lithological boundary. Our study demonstrated that simultaneous interpretation of attenuation coefficients and phase velocities has the potential to distinguish localized fractures from lithological boundaries.

Common-reflection-point time migration

Since the early days of seismic processing, time migration has proven to be a valuable tool for a number of imaging purposes. Main motivations for its widespread use include robustness with respect to velocity errors, as well as fast turnaround and low computation costs. In areas of complex geology, in which it has well-known limitations, time migration can still be of value by providing first images and also attributes, which can be of much help in further, more comprehensive depth migration. Time migration is a very close process to common-midpoint (CMP) stacking and, more recently, to zero-offset commonreflection- surface (CRS) stacking. In fact, Kirchhoff time migration operators can be readily formulated in terms of CRS parameters. In the nineties, several studies have shown advantages in the use of common-reflection-point (CRP) traveltimes to replace conventional CMP traveltimes for a number of stacking and migration purposes. In this paper, we follow that trend and introduce a Kirchhoff-type prestack time migration and velocity analysis algorithm, referred to as CRP time migration. The algorithm is based on a CRP operator together with optimal apertures, both computed with the help of CRS parameters. A field-data example indicates the potential of the proposed technique.

Fully automated near-surface analysis by surface-consistent refraction method

Industry practices for near-surface analysis indicate difficulties in coping with the increased number of channels in seismic acquisition systems, and new approaches are needed to fully exploit the resolution embedded in modern seismic data sets. To achieve this goal, we have developed a novel surface-consistent refraction analysis method for low-relief geology to automatically derive near-surface corrections for seismic data processing. The method uses concepts from surface-consistent analysis applied to refracted arrivals. The key aspects of the method consist of the use of common midpoint (CMP)-offset-azimuth binning, evaluation of mean traveltime and standard deviation for each bin, rejection of anomalous first-break (FB) picks, derivation of CMP-based traveltime-offset functions, conversion to velocity-depth functions, evaluation of long-wavelength statics, and calculation of surface-consistent residual statics through waveform crosscorrelation. Residual time lags are evaluated in multiple CMP-offset-azimuth bins by crosscorrelating a pilot trace with all the other traces in the gather in which the correlation window is centered at the refracted arrival. The residuals are then used to build a system of linear equations that is simultaneously inverted for surface-consistent shot and receiver time shift corrections plus a possible subsurface residual term. All the steps are completely automated and require a fraction of the time needed for conventional near-surface analysis. The developed methodology was successfully performed on a complex 3D land data set from Central Saudi Arabia where it was benchmarked against a conventional tomographic work flow. The results indicate that the new surface-consistent refraction statics method enhances seismic imaging especially in portions of the survey dominated by noise.

High lateral resolution exploration using surface waves from noise records

Determination of the shear-wave velocity structure at shallow depths is a constant necessity in engineering or environmental projects. Given the sensitivity of Rayleigh waves to shear-wave velocity, subsoil structure exploration using surface waves is frequently used. Methods such as the spectral analysis of surface waves (SASW) or multi-channel analysis of surface waves (MASW) determine phase velocity dispersion from surface waves generated by an active source recorded on a line of geophones. Using MASW, it is important that the receiver array be as long as possible to increase the precision at low frequencies. However, this implies that possible lateral variations are discarded. Hayashi and Suzuki (2004) proposed a different way of stacking shot gathers to increase lateral resolution. They combined strategies used in MASW with the common mid-point (CMP) summation currently used in reflection seismology. In their common mid-point with cross-correlation method (CMPCC), they cross-correlate traces sharing CMP locations before determining phase velocity dispersion. Another recent approach to subsoil structure exploration is based on seismic interferometry. It has been shown that cross-correlation of a diffuse field, such as seismic noise, allows the estimation of the Green’s Function between two receivers. Thus, a virtual-source seismic section may be constructed from the cross-correlation of seismic noise records obtained in a line of receivers.In this paper, we use the seismic interferometry method to process seismic noise records obtained in seismic refraction lines of 24 geophones, and analyse the results using CMPCC to increase the lateral resolution of the results. Cross-correlation of the noise records allows reconstructing seismic sections with virtual sources at each receiver location. The Rayleigh wave component of the Green’s Functions is obtained with a high signal-to-noise ratio. Using CMPCC analysis of the virtual-source seismic lines, we are able to identify lateral variations of phase velocity inside the seismic line, and increase the lateral resolution compared with results of conventional analysis.

Ground-roll attenuation using modified common-offset–common-reflection-surface stacking

We modified the common-offset–common-reflection-surface (COCRS) method to attenuate ground roll, the coherent noise typically generated by a low-velocity, low-frequency, and high-amplitude Rayleigh wave. The COCRS operator is based on hyperbolas, thus it fits events with hyperbolic traveltimes such as reflection events in prestack data. Conversely, ground roll is linear in the common-midpoint (CMP) and common-shot gathers and can be distinguished and attenuated by the COCRS operator. Thus, we search for the dip and curvature of the reflections in the common-shot gathers prior to the common-offset section. Because it is desirable to minimize the damage to the reflection amplitudes, we only stack the multicoverage data in the ground-roll areas. Searching the CS gathers before the CO section is another modification of the conventional COCRS stacking. We tested the proposed method using synthetic and real data sets from western Iran. The results of the ground-roll attenuation with the proposed method were compared with results of the f–k filtering and conventional COCRS stacking after f–k filtering. The results show that the proposed method attenuates the aliased and nonaliased ground roll better than the f–k filtering and conventional CRS stacking. However, the computation time was higher than other common methods such as f–k filtering.

Review of multi-offset GPR applications: Data acquisition, processing and analysis

GPR and reflection seismics share common physical and methodological bases but are sensitive to different subsurface physical properties. The peculiarities of the electromagnetic case impact data acquisition, processing and interpretation. We review multi-offset techniques in GPR applications focusing on similarities and differences through examples taken from different subsurface and target conditions.GPR multi-offset data acquisition methods basically involve common-offset and common midpoint geometries: accuracy and work load are the main factors that drive the choice, together with effectiveness of the solution for the objectives of the study.Multi-fold data processing algorithms can bring remarkable signal-to-noise ratio enhancement and offer the opportunity to extract additional information from field data. Velocity field and related dielectric constants distribution, attenuation and related conductivity variations, changes in the GPR response with offset are some of the examples. Coherent noise suppression and velocity analysis are key features in GPR multi-fold processing sequences and we review the relevant methods with examples of application in addition to technical aspects.Multi-channel acquisitions, full wave-form inversion, pre-stack depth migration, azimuthal and polarimetric analysis, are among the many topics in current and future research that are briefly reviewed to provide some highlights of the forthcoming developments in GPR methods.

Weighted stacking of seismic AVO data using hybrid AB semblance and local similarity

The common-midpoint (CMP) stacking technique plays an important role in enhancing the signal-to-noise ratio (SNR) in seismic data processing and imaging. Weighted stacking is often used to improve the performance of conventional equal-weight stacking in further attenuating random noise and handling the amplitude variations in real seismic data. In this study, we propose to use a hybrid framework of combining AB semblance and a local-similarity-weighted stacking scheme. The objective is to achieve an optimal stacking of the CMP gathers with class II amplitude-variation-with-offset (AVO) polarity-reversal anomaly. The selection of high-quality near-offset reference trace is another innovation of this work because of its better preservation of useful energy. Applications to synthetic and field seismic data demonstrate a great improvement using our method to capture the true locations of weak reflections, distinguish thin-bed tuning artifacts, and effectively attenuate random noise.

Seismic random noise attenuation using artificial neural network and wavelet packet analysis

In this paper, we present a method for attenuating background random noise and enhancing resolution of seismic data, which takes advantage of semi-automatic training of feed forward back propagation (FFBP) artificial neural network (ANN) in a multiscale domain obtained from wavelet packet analysis (WPA). The images of approximations and details of the input seismic sections are calculated and utilized to train neural network to model coherent events by an automatic algorithm. After the modeling of coherent events, the remainder data are assumed to be related to background random noise. The proposed method is applied on both synthetic and real seismic data. The results are compared with that of the adaptive Wiener filter (AWF) in synthetic shot gather and real common midpoint gather and also with that of band-pass filtering on real common offset gather. The comparison indicates substantially higher efficiency of the proposed method in attenuating random noise and enhancing seismic signals.

Simultaneous-Source Separation Using Iterative Seislet-Frame Thresholding

The distance-separated simultaneous-sourcing (DSSS) technique can make the smallest interference between different sources. In a distance-separated simultaneous-source acquisition with two sources, we propose the use of a novel iterative seislet-frame thresholding approach to separate the blended data. Because the separation is implemented in common shot gathers, there is no need for the random scheduling that is used in conventional simultaneous-source acquisition, where random scheduling is applied to ensure the incoherent property of blending noise in common midpoint, common receiver, or common offset gathers. Thus, DSSS becomes more flexible. The separation is based on the assumption that the local dips of the data from different sources are different. We can use the plane-wave destruction algorithm to simultaneously estimate the conflicting dips and then use seislet frames with two corresponding local dips to sparsify each signal component. Then, the different signal components can be easily separated. Compared with the FK-based approach, the proposed seislet-frame-based approach has the potential to obtain better separated components with less artifacts because the seislet frames are local transforms while the Fourier transform is a global transform. Both simulated synthetic and field data examples show very successful performance of the proposed approach.

Common midpoint versus common offset acquisition geometry in seismic imaging

We compare and contrast the qualitative nature of backprojected imagesobtained in seismic imaging when common offset data are usedversus when common midpoint data are used. Our results showthat the image obtained using common midpoint data contains artifactswhich are not present with common offset data. Although there aresituations where one would still want to use common midpoint data,this result points out a shortcoming that should be kept in mind wheninterpreting the images.

Affine Diameters of a Convex Body

We prove that each convex body in ℝn has n pairwise orthogonal affine diameters d1, . . . , dn such that it is possible to shift each of them through a linear combination of direction vectors of the diameters with smaller numbers so that their translates will intersect at their common midpoint.

Accounting for Surface Refraction in Velocity Semblance Analysis With Air-Coupled GPR

The aim of this study is to evaluate the velocity semblance analysis technique for air-coupled common midpoint (CMP) measurements with a small antenna offset. The technique was originally developed for seismic surveys, assuming the small spread approximation. Owing to the strong refraction at the surface and shallow investigation depth, this assumption is not valid in the case of air-coupled ground-penetrating radar (GPR). To overcome this assumption, a modification to the method is proposed accounting for the refraction at the surface. Synthetic experiments were executed to demonstrate that the traditional method resulted in a persistent overestimation, whereas the modified method improved the results significantly. Two field experiments have been conducted to test the method under different field conditions. In a first experiment on a road test site, the modified method improved the estimation of depth and propagation velocity significantly. However, the technique failed to estimate the propagation velocity and depth or objects in a second field test, due to rough terrain conditions and noise in the data. Therefore, an additional modification was proposed, by incorporating in-line data as well. This improved the depth and velocity estimations significantly. Overall, this study demonstrates that the traditional velocity semblance analysis (TRAD) is not valid for air-coupled GPR. By accounting for the refraction at the surface and incorporating inline data, it is possible to successfully estimate depth and propagation velocity with small offset air-coupled GPR configurations.

Progress on Ultra-Wideband (UWB) Multi-Antenna radar imaging for MIGA

Progress on the development of the multi-channel, ground penetrating radar imaging system is presented from hardware and software perspectives. A new exponentially tapered slot antenna, with an operating bandwidth from 100 MHz to 1.5 GHz was fabricated and tested using the eight-port vector network analyzer, designed by Rhode and Schwarz Incorporated for this imaging project. An eight element antenna array mounted on two carts with automatic motor drive, was designed for optimal common midpoint (CMP) data acquisition. Data acquisition scenarios were tested using the acoustic version of the NORSAR2D seismic ray-tracing software. This package enables the synthesis and analysis of multi-channel, multi-offset data acquisitions comprising more than a hundred thousand traces. Preliminary processing is in good agreement with published bistatic ground-penetrating radar images obtained in the tunnels of the Low-noise Underground Laboratory (LSBB) at Rustrel, France.

Soil Formation Lithological Profiling Using Ground Penetrating Radar

The paper presents a modified technique for generation of the high-contrast sub-horizontally layered earth depth sections which is based on the common offset and common midpoint GPR methods. The iterative algorithm for fitting and correction of the reflection and refraction time-distance curves, as well as identification of the basic layer parameters, is suggested. Some results of the technique's implementation on railroads and highways in the Russian Far East are summarized.

Improving homomorphic wavelet estimation by compensating for residual NMO stretching on stack section

Wavelet estimation is a common step in seismic data processing and inversion. Homomorphic wavelet estimation has long utilized as a method that uses a seismic stack section with no phase presumption. Forming a stack section, normal move-out (NMO) correction must be applied on common midpoint (CMP) gathers, although it introduces NMO stretching. After stacking, residual of the NMO stretching may affect the stack section even after muting the highly stretched zone of the NMO corrected CMP gather. Presence of significant residual NMO stretching changes the spectral characteristics of data in time direction, by different degrees. Considering that in homomorphic process the wavelet is estimated based on the spectral characteristics of data, compensating for the residual NMO stretching, can improve the accuracy of the process. Here, we introduce a fast method of calculating the amount of residual NMO stretching and compensating for its effect on wavelet estimation. The proposed method needs limited prestack information like offsets and velocity function and include no prestack processing. We apply the proposed method on synthetic and real datasets and demonstrate the improvement of the estimated wavelet.