Stacked Sections Research Articles

Passive Seismic Ambient Noise Correlation: An Example from the Ketzin Experimental CO2 Storage Site, Germany

Passive seismic ambient noise correlation is potentially a new tool to monitor carbon dioxide storage sites. Unlike conventional monitoring tools, such as time-lapse active seismic surveys, the passive seismic ambient noise correlation method is a relatively low cost method and can be performed together with microseismic and reservoir monitoring. In this study, we test the method at the Ketzin CO2 storage site, Germany. A new passive seismic survey was performed in August 2013 together with an active survey in conjunction with a source test. The survey consisted of two profiles, Line 1 and Line 2. Line 1 contained 42 10Hz geophones spaced at 24 m intervals and Line 2 contained 23 DSU3 (3-component MEMS) sensors spaced at 5 m intervals. In total, 6 nights and 3 daytime series of ambient noise data were recorded. First, we applied autocorrelation to the passive data records to reconstruct the reflection response. The results show good structural correlation with a stacked section from the active data. Then we used passive seismic interferometry to reconstruct common shot gathers for each receiver location. Surface waves were observed in some retrieved gathers from Line 1. However, for Line 2 we also observed refracted and reflected waves in some retrieved shot gathers. Here we review ambient noise correlation methods and compare our results with active seismic data.

On optimal NMO and generalised Dix equations for velocity determination and depth conversion

Abstract The classical Earth model used to process seismic data assumes plane layers and a velocity analysis based on a hyperbolic approximation of the reflection events, where basically one parameter (the stacking velocity) is used to perform the normal move-out (NMO) correction to obtain the stacked section. We explore the possibility of using alternative approaches, other than the standard Dix equation based on the root-mean-square (RMS) velocities, to obtain the interval velocities and perform the depth conversion in order to locate the interfaces. Specifically, we consider traveltime equations as a function of offset using different NMO approximations, based on the average, RMS and root-mean-quartic (RMQ) velocities. A generalised form of Dix's equation is used for this purpose. Moreover, we analyse the model-dependency of the different NMO equations. We consider a simple 4-layer model and a model based on data from the Cooper basin, South Australia, to test the NMO equations. We build an elastic-velocity model and compute a common midpoint (CMP) synthetic seismogram. The reflection events are identified and traveltimes are picked to perform a non-linear inversion with the conventional (one parameter) hyperbolic approximation, the 3-term Taner and Koehler equation and approximations based on the average, RMQ and RMS velocities, also using two parameters. The model has a velocity inversion which poses a challenge to the approximations. The performance of the approximations is model dependent, so a-priori information of the velocity profile can be useful to perform a suitable inversion, or an optimal stack of the reflection events is required to test the NMO correction. Moreover, the results show that the inversion based on the RMS velocities yields better results than those based on the RMQ velocities, but this is not always the case. On the other hand, the inversion using the average velocity performs a worse velocity–depth estimation, even compared to the RMS results from the hyperbolic approximation.

CRS stack based seismic imaging - a case study from St Ives mining camp, Kambalda, Western Australia

Seismic exploration in hard rock environments is challenging due to complex geological conditions that reduce the signal-to-noise (S/N) ratio of seismic data and, thus, an accurate velocity model recovery likelihood. This limits the application of conventional imaging methods, e.g., normal moveout (NMO), dip moveout (DMO) stacking or pre-stack migration. The common reflection surface (CRS) stacking technique can be used in complex environments to increase the fold and therefore the signal quality and produces reliable stacked sections with high resolution. On the other hand, CRS stacking approach does not depend on a velocity model. The CRS stacking approach was applied to 2D and 3D datasets acquired across the St Ives mining camp in Kambalda, Western Australia located within the core of a regional-scale Kambalda Dome. The main objective of this study is to review the application of the CRS approach at the St Ives mining camp in Kambalda, Western Australia. Stacked 2D seismic sections and a 3D seismic volume obtained using the CRS approach are superior to those obtained by conventional DMO/NMO processing. They are characterized by a higher S/N ratio and improved continuity of seismic reflection events. Parameters (wavefield attributes) estimated using the CRS approach have a clear geophysical interpretation and will be used for building velocity models. The target area and the existing faults and fractures were imaged clearly and the high grade of tectonic displacement necessary to ensure a sufficiently large production rate was verified. The CRS approach is now adopted as a part of the standard processing flow for hard rock seismic.

Broadband marine seismic, how much difference does acquisition and or processing make? A case study from Southeast Asia

In this paper, we present a case study of a consistent, experimental offshore dataset in Southeast Asia. This data consists of a single boat pass of multiple cables at different depth configurations. The resultant sub lines were then processed with their appropriate deghosting methods and results compared. The key determinants of the eventual bandwidth of surface seismic data are the convolution of the source pulse with near surface effects (free surface source and receiver ghosts in the marine situation), the overall earth attenuation, and the level of additive environmental noise. Some of these effects can be modified by changes in the field acquisition geometry or, at least, deterministically compensated in processing. The noise level may constrain or limit the capacity of signal processing tools to compensate for the “field” effects. When evaluating the raw and processed data it is wise to use various types of analysis and displays. Simple seismic amplitude stack sections and associated spectra can be misleading. Spectral “split” plots and inversion of the seismic data are often more indicative of success. In marine seismic acquisition, the free-surface ghost effect is one factor that can strongly impact the data bandwidth dependant on the streamer and source depths. These are also easily adjusted in the survey design. Shallow towing favours the higher frequency response at the expense of low frequencies, whilst deeper towing favours the lower frequencies, at the expense of higher frequencies. Moreover, a deeper tow typically has lower levels of swell noise. At very low frequencies, the critical two octaves between two to eight Hz, we are presented with a “triple threat”; the level of ambient environmental noise rises, the “DC notch” strengthens, and the power output of airgun sources declines. This combination provides both the biggest challenge and the biggest opportunity for bandwidth expansion. METHOD AND RESULTS

Deep seismic image enhancement with the common reflection surface (CRS) stack method: evidence from the Aravalli–Delhi fold belt of northwestern India

Imaging of deep crustal features from narrow-angle deep seismic reflection data, especially from fold belt region, has been a challenging task. The common reflection surface (CRS) stack is an alternative seismic imaging technique for multicoverage reflection data. It is an automatic stacking process, which does not require explicit knowledge of stacking velocity. This CRS stack is especially useful when the data quality is poor and foldage is low. In this paper, we demonstrate an application of the CRS stack to a deep seismic reflection data set acquired across the Aravalli–Delhi fold belt of the northwestern India, which provides a seismic stack section with much improved signal-to-noise ratio. Comparing the conventional common mid-point (CMP) Stack with the CRS stack, we find that the Moho and other crustal reflections have been resolved clearly and the continuity of the reflectors has also been enhanced with the CRS stack method. The major findings from our CRS processing include clear image of the Moho discontinuity below the Marwar Basin and Sandmata Complex, and prominent upper and mid-crustal reflections. Our study for the first time images an extension of crustal-scale Jahazpur thrust below the Sandmata Complex, which becomes listric at the Moho. Some of the crustal features derived in this study were not identified in the earlier investigations using the CMP stack. Our study clearly demonstrates that the CRS stacking method is more appropriate for imaging the crustal and subcrustal structures of the thrust fold belt region than the conventional CMP method, where limited velocity information is available. Crustal thickness across the Proterozoic orogenic Aravalli–Delhi fold belt varies between 38 and 50 km. Global correlation of the seismic results suggests no relation between crustal thickness and age of the crustal block, but it depends on the thermorheological and tectonic history of the region. Palaeosignatures of the Proterozoic subduction and collision processes are still preserved in major portion of the crust in this study area as well as in several parts of the world.

Evaluation of performance and operation viability of non-uniform potential solid oxide fuel cell fueled by reformed methane

The performance and the operation viability of solid oxide fuel cells (SOFC) fueled by reformed methane using the concept of non-uniform potential operation were evaluated by one-dimensional finite difference analysis. Two thermal constraints including maximum acceptable stack temperature gradient and maximum stack temperature were the criteria for considering the operation viability. The average power density was used as a performance indicator. In addition, two major process parameters: fuel and air inlet temperatures and the operating voltage were examined to determine the suitable values for different viabilities. The results revealed that non-uniform potential SOFC with two stack sections operating at different voltages could not offer a significantly higher average power density compared with the uniform potential SOFC owing to thermal constraint limitations. The installation of coolers between the stacks is important and it could offer 12.8% improvement in average power density.

Seismic exploration‐scale velocities and structure from ambient seismic noise (>1 Hz)

The successful surface waves retrieval in solid‐Earth seismology using long‐time correlations and subsequent tomographic images of the crust have sparked interest in extraction of subsurface information from noise in the exploration seismology. Subsurface information in exploration seismology is usually derived from body‐wave reflections > 1 Hz, which is challenging for utilization of ambient noise. We use 11 h of noise recorded in the Sirte basin, Libya. First, we study the characteristics of the noise. We show that the bulk of the noise is composed of surface waves at frequencies below 6 Hz. Some noise panels contain nearly vertically traveling events. We further characterize these events using a beamforming algorithm. From the beamforming, we conclude that these events represent body‐wave arrivals with a fairly rich azimuthal distribution. Having body‐wave arrivals in the noise is a prerequisite for body‐wave reflections retrieval. We crosscorrelate and sum the recorded ambient‐noise panels to retrieve common‐source gathers, following two approaches—using all the noise and using only noise panels containing body‐wave arrivals likely to contribute to the reflections retrieval. Comparing the retrieved gathers with active seismic data, we show that the two‐way traveltimes at short offsets of several retrieved events coincide with those of reflections in the active data and thus correspond to apexes of reflections. We then compare retrieved stacked sections of the subsurface from both approaches with the active‐data stacked section and show that the reflectors are consistent along a line. The results from the second approach exhibit the reflectors better.

New seismic images of the Central Indian Suture Zone and their tectonic implications

The Central Indian Suture (CIS), a mega shear zone, located in the central part of the Indian shield exhibits a complex structure with compositional heterogeneity. Multifold deep seismic reflection data acquired across the shear zone have been previously processed using the conventional common midpoint (CMP) method. In this study, we show results from imaging this shear zone using the common reflection surface (CRS) stack method, an alternative to the CMP stack. Our new images reveal geological features that are either poorly resolved or entirely absent in the earlier CMP stack section. Our major findings include a well‐imaged Moho throughout the study area, existence of different crustal blocks each with distinct dipping reflection fabrics on the northern and southern sides of the CIS, a well‐defined 8 km of Moho offset beneath the CIS, and a high‐amplitude reflective zone representing the CIS. The Moho located at a depth of 48 km in the Bastar craton, to the south of the CIS, is also clearly resolved in our CRS stack, as well as in the time‐migrated section. A deeply penetrating crustal‐scale imbricated structure imaged up to a depth of 16.0 s two‐way time (TWT), to the south of the CIS, suggests that the Bastar craton is subducting toward north. An oppositely dipping reflection fabric, a Moho offset, a positive‐negative gravity anomaly pair, and the geological data indicate that the CIS represents a collision zone developed due to the interaction of the Bastar and Bundelkhand cratons with the evolution of the Sausar orogeny at ~1000 Ma. This orogeny is contemporaneous with the Grenvillian orogeny and Rodinia assembly. Another thrust fault extending from 4 to 14 s TWT observed to the north of the CIS may represent the earlier, pre‐Sausar orogenic activity at 1.6–1.5 Ga. The available seismic characteristics suggest that the CIS, the collisional suture, is also likely to represent a strike‐slip fault.

Zero-offset vertical seismic profiling survey and estimation of gas hydrate concentration from borehole data from the Ulleung Basin, Korea

Gas hydrate has been receiving much attention in recent years as the demand for energy has increased. Before an economic evaluation of the feasibility of producing gas hydrate can be made, an assessment of reserves must be completed. To evaluate a reserve of gas hydrate, we should accurately know the depth interval of the gas hydrate-bearing region and the concentration of gas hydrate in it. Because vertical seismic profiling (VSP) data can provide higher resolution than surface seismic data, we can improve the reliability of the characterization of the gas hydrate-bearing region using VSP data along with surface seismic data.In this study, we obtained interval velocities and corridor stacked sections from zero-offset VSP data acquired at the UBGH2-6 and UBGH2-10 wells in gas hydrate-bearing zones of the Ulleung Basin, East Sea of Korea. The interval velocities obtained from the zero-offset VSP data agreed well overall with sonic logs and showed a significant decrease at the depth beneath the gas hydrate stability zone. Most reflection events in the corridor stacked section matched well those in the surface migrated seismic section. Additionally, we computed the concentration of the gas hydrate from the log data using a three-phase Biot-type equation, and the result agreed well with the concentration calculated from the resistivity log using Archie's equation.

Recovering diffractions in CRS stacked sections

The common reflection surface (CRS) method has been proposed as an alternative to the classical common midpoint method to further improve the signal-to-noise ratio, as well as to obtain additional kinematic parameters that are useful for a number of imaging purposes. In the CRS method, reflected events are enhanced by stacking along a generalized hyperbolic moveout, referred to as the CRS moveout. However, during the process of CRS stacking, diffractions are likely to be attenuated or even suppressed. Diffracted events are important since they carry high-resolution information about the subsurface structure. By the use of a modified version of the CRS technique, diffractions can be enhanced in the same way as reflections. This paper proposes a combined approach where the CRS stack is superimposed on the CRS diffraction-enhanced stack. In this way we can recover the diffractions in CRS stacked sections. The potential of the method has been demonstrated using marine seismic data acquired offshore Brazil.

High-resolution gathers by inverse Q filtering in the wavelet domain

We have developed and applied an inverse [Formula: see text] filter formulation using the continuous wavelet transform (CWT), which provides a natural domain for time-variant operations, such as compensation for propagation in attenuating and dispersive media. The well-known linear attenuation model, given as a function of time and frequency, was applied very efficiently over wavelet coefficients in the time-frequency domain to correct for amplitude and phase distortions, as necessary. The inverse CWT yields the recovered trace with a broader bandwidth. The process works on a trace-by-trace basis, making no distinction if the data is pre- or poststack. Our motivation was to develop gather conditioning tools to enhance prestack interpretation techniques such as amplitude variation with offset (AVO) analysis and inversion — a technique that is often compromised by tuning and other propagation related issues that degrade seismic resolution. Thus, we investigated the AVO fidelity of our filter and the sensitivity of the results to incorrect values of [Formula: see text], using real and synthetic data. Our synthetic data experiments clearly showed that AVO anomalies are recovered and preserved in a stable manner, even with values of [Formula: see text] off by 50% of its correct value. The application in time-migrated gathers shows a very natural increase in the vertical definition of the events, especially due to the partial elimination of the tuning effect. The benefits for imaging are also evidenced by comparing stacked sections before and after inverse [Formula: see text] filtering. The higher resolution of seismic sections leads to a better definition of smaller scale stratigraphic and structural features.

Near surface S-wave seismic reflection profiling– new approaches and insights

Multi-component high resolution seismic reflection profiling has been extensively tested over a wide variety of ground surfaces across the southern provinces of Canada, showing new potential for applications of the method in groundwater and natural hazards research. The near-surface shear-wave reflection method using vibratory sources and short spacing land streamers equipped with three-component receivers is an excellent tool for accurately characterizing shear-wave velocities and recording optimal, non-aliased shear-wave data in the most polarized direction. A small portable multi-component vibrator developed at the Geological Survey of Canada (GSC) named ‘Microvibe’ provides higher frequency S-wave and P-wave signals than can be acquired with a Minivib I. In this paper we show that the shear-wave polarization can vary with depth and it may be necessary to combine multiple components together to achieve an optimized stacked section. Significant velocity anisotropies of up to 15% have been observed between the horizontal and vertical directions when using this multicomponent Microvibe source. We make key recommendations based on time and space sampling recording windows for successful near surface PP-wave, PS-wave and SS-wave seismic reflection surveys. Using field examples and velocity measurements, we show the complexity of velocities in non-homogeneous media in the near surface.

Pre-discovery seismic modelling and prediction of the Deep Panuke Late Jurassic carbonate bank gas discovery, offshore Nova Scotia

Abstract Deposition of the Abenaki Formation carbonate complex began during the Middle Jurassic, shortly after the interpreted onset of western Atlantic sea-floor spreading, and continued through Late Jurassic time, resulting in thick carbonate deposits along the Jurassic shelf edge. This carbonate complex was the exploration target of 16 wells drilled from 1970 to 1989. Although there were no discoveries in the carbonate objectives, oil was discovered in Cretaceous siliciclastics in structural drape closures overlying the carbonate margin. The Province of Nova Scotia decided to undertake a technical study to further the interest in the carbonate bank objective while the shallow oil play was being developed. Subsequently, in 1989, two studies were undertaken to investigate a porosity play in the Jurassic Baccaro Member of the Abenaki Formation because significant porosity had been encountered in a number of the dry holes that targeted the carbonate margin, particularly the Demascota G-32 well. The first study modelled porosity in several wells utilizing one-dimensional (1D) and wedge models, and amplitude v. offset (AVO) models. The Demascota G-32 well was used to seismically identify and model several types of potential hydrocarbon-bearing prospects along the bank. Modelling indicated that zones of 11–14% porosity with a minimum thickness of 10 m could be detected. However, a thickness of 50 m was required to resolve the top and base of the porosity. The AVO model for the Demascota well demonstrated that an increase in amplitude with offset (AVO effect) in the presence of gas and fracturing (which dramatically decreases Poisson’s ratio) should be detectable and may be useful in identifying prospects in this type of play. The assumption of a fractured reservoir was reasonable due to the number of seismically identified faults at the front of the carbonate bank and the description of fractures in conventional cores taken in the Baccaro Member. The second study involved acquiring a pair of dip and strike seismic lines adjacent to the Demascota well and reprocessing them to preserve relative amplitudes while providing a high-resolution, stacked section, especially at the Baccaro Member level. Interpretation of these lines was undertaken in the AVO domain. An increase in amplitude with offset was observed up-dip of the Demascota G-32 well that indicated a fractured, gas-filled porous zone could exist up-dip of this well. Following these studies, 3D seismic data was acquired over the shallower Panuke Oil Field and similar studies were conducted on that dataset in 1997. As a result, Encana drilled a structural high containing bright amplitude reflectors and encountered over 100 m of net gas pay in vuggy and cavernous limestones and dolomites in 1999. This validated the earlier studies, confirming that these reservoirs could be seen on seismic data and proved that economic gas reservoirs exist in the carbonate bank. These results should encourage exploration along the extensive, sparsely drilled Late Jurassic carbonate margins bounding offshore eastern North America and its conjugates in the Central Atlantic Region.

Scaling behavior and the effects of heterogeneity on shallow seismic imaging of mineral deposits: A case study from Brunswick No. 6 mining area, Canada

Abstract We have studied the scaling behavior of compressional-wave velocity and density logs from an exploration borehole that extends down to about 700 m depth in the Brunswick No. 6 mining area, Bathurst Mining Camp, Canada. Using statistical methods, vertical and horizontal scale lengths of heterogeneity were estimated. Vertical scale length estimates from the velocity, density and calculated acoustic impedance are 14 m, 33 m, and about 20 m, respectively. Although the estimated scale length for the acoustic impedance implies a weak scattering environment, elastic finite difference modeling of seismic wave propagation in 2D heterogeneous media demonstrates that even this weak scattering medium can mask seismic signals from small, but yet economically feasible, massive sulfide deposits. Further analysis of the synthetic seismic data suggests that in the presence of heterogeneity, lenticular-shaped targets may only exhibit incomplete diffraction signals whereby the down-dip tails of these diffractions are mainly visible on the stacked sections. Therefore, identification of orebody generated diffractions is much easier on the unmigrated stacked sections than on migrated stacked sections. The numerical seismic modeling in 2D heterogeneous media indicates that in the presence of large horizontal, but small vertical scale lengths (structural anisotropy), identification of massive sulfide deposits is possible, but their delineation at depth requires detailed velocity modeling and processing algorithms which can handle the anisotropy.

Investigation of the weathering layer using seismic refraction and highresolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16 m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18 m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.

F-X 필터와 중앙값 필터를 연속적으로 사용한 파랑잡음 제거

High-amplitude swell noises (HASN) are very difficult to eliminate from the marine seismic data. In this paper, we applied F-X filter and median filter in order to suppress HASN. Test data have been acquired on the northern offshore of the South Shetland Islands in December, 2010. Parts of data have been contaminated by HASN caused by bad weather during the cruise. We applied F-X filter and median filter to test data with HASN. After F-X filtering, most of non-coherent noises and small-amplitude swell noises are eliminated effectively but HASN are still remained significantly. With median filter, HASN was suppressed better than F-X filter, however some of non-coherent noises are still remains. We applied a cascade of two filters and results show HASN and non-coherent noises are suppressed effectively. After the cascade of two filtering, it is possible to define reflection layers clearly on the velocity spectrum and to produce better stacked section with a good signal-to-noise ratio.

Principle and application of high density spatial sampling in seismic migration

To avoid spatial aliasing problems in broad band high resolution seismic sections, I present a high density migration processing solution. I first analyze the spatial aliasing definition for stack and migration seismic sections and point out the differences between the two. We recognize that migration sections more often show spatial aliasing than stacked sections. Second, from wave propagation theory, I know that migration output is a new spatial sampling process and seismic prestack time migration can provide the high density sampling to prevent spatial aliasing on high resolution migration sections. Using a 2D seismic forward modeling analysis, I have found that seismic spatial aliasing noise can be eliminated by high density spatial sampling in prestack migration. In a 3D seismic data study for Daqing Oilfield in the Songliao Basin, I have also found that seismic sections obtained by high-density spatial sampling (10 × 10 m) in prestack migration have less spatial aliasing noise than those obtained by conventional low density spatial sampling (20 × 40 m) in prestack migration.

Imaging a near‐surface feature using cross‐correlation analysis of multi‐channel surface wave data

In this study, we demonstrate that cross‐correlation gathers of multi‐channel and multi‐shot configurations provide accurate estimations of shear‐wave velocity (Vs) perturbations from Rayleigh wave data for the reconstruction of two‐dimensional (2D), high‐resolution velocity distributions without requiring the systematic calculation of surface wave dispersion, as in the spectral analysis of surface waves (SASW). Data acquisition for cross‐correlation analysis is similar to that for a 2D seismic common midpoint reflection survey. The data processing involved is similar to the coda wave interferometry used for seismological data but differs in the sense that the cross‐correlation of the original waveform is calculated for active source seismic data. Data processing in cross‐correlation analysis consists in the following three steps: first, cross‐correlations are calculated for every trace in each shot gather with the same offset trace of a reference shot to flatten the linearly sloping events of surface waves. A common receiver location stack section can then be obtained, which allows for the assessment of lateral variations in the elastic properties of the medium. Second, the maxima of the time‐shifted cross‐correlation gathers and the maxima of the shot gathers’ envelope traces are picked for different frequencies and trace values having the same receiver location are averaged to calculate Vs perturbations, as described in the theory of coda wave interferometry. Finally, a 2D Vs profile is reconstructed by applying the lateral Vs perturbation to a homogeneous Vs velocity profile obtained by inversion of the surface wave dispersion of the reference shot gather. Analyses of waveform data from numerical modelling and field observations indicate that this new method is valid and greatly improves the accuracy and resolution of near‐surface imagery using surface waves and reconstructed subsurface velocity distributions compared with a conventional SASW or multi‐channel application.

Common-reflection-surface imaging of shallow and ultrashallow reflectors

We analyzed the feasibility of the common-reflection-surface (CRS) stack for near-surface surveys as an alternative to the conventional common midpoint (CMP) stacking procedure. The data-driven, less user-interactive CRS method could be more cost efficient for shallow surveys, where the high sensitivity to velocity analysis makes data processing a critical step. We compared the results for two field data sets collected to image shallow and ultrashallow reflectors: an example of shallow P-wave reflection for targets in the first few hundred meters, and an example of SH-wave reflection for targets in the first 10 m. By processing the shallow P-wave records using the CMP method, we imaged several nearly horizontal reflectors with onsets from 60 to about 250 ms. The CRS stack produced a stacked section more suited for a subsurface interpretation, without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency and lateral continuity. With CMP processing of the SH-wave records, we imaged a dipping bedrock interface below four horizontal reflectors in unconsolidated, very low velocity sediments. The vertical and lateral resolution was very high, despite the very shallow depth: the image showed the pinchout of two layers at less than 10 m depth. The numerous traces used by the CRS stack improved the continuity of the shallowest reflector, but the deepest overburden reflectors appear unresolved, with not well-imaged pinchouts. Using the kinematic wavefield attributes determined for each stacking operation, we retrieved velocity fields fitting the stacking velocities we had estimated in the CMP processing. The use of CRS stack could be a significant step ahead to increase the acceptance of the seismic reflection method as a routine investigation method in shallow and ultrashallow seismics.

Global optimisation by simulated annealing for common reflection surface stacking and its application to low-fold marine data in southwest Japan

The common reflection surface (CRS) stack is an alternative method of producing a zero-offset stacked section with a higher signal-to-noise ratio (SNR) than the conventional normal moveout (NMO)/dip moveout (DMO) stack method. Since, however, it is difficult to determine global optimal parameters for the CRS stack method by the conventional three-step search method, especially for complex structures and low-fold data, we investigate the ability of simulated annealing (SA) to optimise our estimation of these parameters. We show a detailed but practical procedure for the application of SA to the CRS stack method. We applied the CRS stack method with SA to numerically modelled seismic reflection data, and to multichannel marine seismic data over complicated geological structures around the Median Tectonic Line (MTL) in Japan. We used the results of the conventional three-step search algorithm as the initial model for the SA search and showed that with this approach SA can estimate CRS parameters accurately within a reasonable number of calculations. The CRS stack method with this approach provided a clearer seismic profile with a higher SNR than either a conventional NMO stack method or a conventional CRS stack method.