Seismic Acquisition Geometries Research Articles

Horizontal resolution of seismic acquisition geometries

      , , ,  , SUMMARY Spatial sampling has a crucial influence on the horizontal resolution of seismic imaging, but how to quantify the influence is still controversial especially in complex media. Most of the studies on horizontal resolution focus on the measurement of wavelet widths for seismic migration, but neglect to evaluate the effect of side-lobe perturbations on spatial resolution. The side-lobe effect, as a migration noise, is important for seismic imaging in complex media. In this article, with focal beam analysis, we define two parameters to represent the horizontal resolution of an acquisition geometry: the width of the main lobe (WML) along the inline and crossline directions and the ratio of the main-lobe amplitude to the total amplitude (RMT) in a focal beam. We provide examples of typical acquisition geometries to show that how spatial sampling affects the horizontal resolution, measured in terms of WML and RMT values. WML defines the horizontal resolution to image the target, whereas RMT describes the clarity of the imaging. Migration noise reduces with increasing RMT, indirectly improving both the vertical and horizontal resolutions of seismic imaging. Case studies of seismic migration with 3D seismic data demonstrate how the acquisition geometries with different WML and RMT values influence the performance of seismic imaging. A prior WML and RMT analysis to predict the quality of acquired datasets can optimize acquisition geometries before the implementation of seismic acquisition.

A feasibility and efficiency study of seismic waveform inversion for time-lapse monitoring of onshore CO2 geological storage sites using reflection seismic acquisition geometries

Monitoring of the CO2 distribution at depth is imperative for onshore geological storage of CO2. Seismic methods are effective monitoring tools during and after the injection process, but are generally expensive and time consuming to perform. In this paper we perform a series of synthetic experiments in order to compare the seismic waveform inversion method with conventional seismic monitoring methods for time-lapse monitoring at CO2 geological storage sites. We mainly focus our study on seismic data sets with typical reflection acquisition geometries that is far offset limited. Two synthetic seismic data sets (consisting of one baseline and repeat) were generated by a third party 2D forward modeling code to compare the waveform inversion method with a conventional time-lapse procedure. The velocity models are based on a simplified structure of the Heletz site in Israel. The area into which CO2 is injected is on the order of a few hundred meters wide and a few 10s of meters high. We compare the waveform inversion results and conventional time-lapse results to determine how sparse spatial sampling affects the results by increasing both the shot and receiver spacing. We test the waveform inversion method for a synthetic time-lapse data set with different inversion strategies. First, we invert each data set independently by using both phase and amplitude information. Then we invert them by using only phase information. We also test the influence of the starting model. Finally, we test the double difference inversion method which is equivalent to only inverting the difference between the baseline and repeat data sets. During the test a relative coarse starting model and higher starting frequency were used in order to better represent real seismic reflection data. Our results show that, under certain noise conditions, it may be possible to use a combination of sparse spatial sampling geometries and seismic waveform inversion to monitor the velocity changes caused by CO2 injection. We conclude that seismic waveform inversion may be a good complement to standard CDP processing when monitoring CO2 injection.

Depth domain inversion to improve the fidelity of subsalt imaging: a Gulf of Mexico case study

Accurate geophysical reservoir characterisation in complex geologic environments remains a challenge. In particular, conventional methods of amplitude inversion assume that amplitudes in the seismic image are correctly located and can be inverted to elastic parameters from which a true representation of rock proper¬ties can be derived. However, complex geology, often com¬bined with limitations imposed by surface seismic acquisition geometries, can lead to inadequate illumination of subsurface targets, which can have detrimental effects on the amplitudes and phase of the migrated image. Conventional amplitude inversion techniques do not compensate for these amplitude and phase variations. Consequently, imprints of various non-geological effects and a complex overburden will manifest themselves in the results of seismic inversion, leading to less reliable estimation of acoustic and elastic parameters. An additional challenge to accurate amplitude inversion in complex geologic environments is that depth imaging is normally required to obtain a reliable image of the subsurface, while current amplitude inversion techniques are usually implemented in the time domain. This difference in approach between the imaging and inversion steps can compromise the fidelity of the attributes derived from seismic inversion. In order to improve consistency between structural imag¬ing and rock property estimation, a technique has been developed to perform amplitude inversion directly in the depth domain. The inversion workflow uses point spread functions to capture and correct for space-, depth- and dip-dependent illumination effects resulting from the acquisition geometry and complex geology. The amplitude inversion is performed in the depth domain and the output is a reflectivity image and associated acoustic impedance volume corrected for illumina¬tion effects, thus creating more consistent and reliable imaging products and rock property attributes from depth-migrated datasets. This article presents the application of the depth domain inversion workflow to a long-offset full-azimuth (FAZ) dataset from the Green Canyon area of the Gulf of Mexico (GoM) (Letki et al., 2015).

Feasibility of CO2 plume detection using 4D seismic: CO2CRC Otway Project case study — Part 2: Detectability analysis

A key objective of stage 2 of the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway Project is to explore the ability of the seismic reflection method to detect and monitor injection of a small amount of greenhouse gas into a saline formation. Development of a seismic monitoring program requires an understanding of expected time-lapse (TL) seismic signals. Hence, before such an injection experiment is undertaken, we assessed the feasibility of seismic monitoring in a modeling study. Considering realistic gas distributions inferred from reservoir simulations, we analyzed the influence of various factors (injection volume, time after injection, and realizations of the reservoir flow model) on the TL seismic signal. However, the applicability of seismic monitoring depends not only on the strength of the TL seismic signal but also on the noise level of the seismic data. Hence, to estimate the detectability of gas in the subsurface, we have developed a workflow that integrated actual data repeatability observed at the Otway test site into the seismic feasibility study. Although we observed differences between the considered scenarios, all of the scenarios indicated a high likelihood of successful plume detection with the observed noise level and surface 4D seismic acquisition geometry used in stage 1 of the CO2CRC Otway Project at the same site. However, a thin layer of gas spreading out from the edges of the main plume below the seal in all scenarios would be a challenge for surface seismic monitoring.

Optimization on the Hierarchical Tucker manifold – Applications to tensor completion

In this work, we develop an optimization framework for problems whose solutions are well-approximated by Hierarchical Tucker (HT) tensors, an efficient structured tensor format based on recursive subspace factorizations. By exploiting the smooth manifold structure of these tensors, we construct standard optimization algorithms such as Steepest Descent and Conjugate Gradient for completing tensors from missing entries. Our algorithmic framework is fast and scalable to large problem sizes as we do not require SVDs on the ambient tensor space, as required by other methods. Moreover, we exploit the structure of the Gramian matrices associated with the HT format to regularize our problem, reducing overfitting for high subsampling ratios. We also find that the organization of the tensor can have a major impact on completion from realistic seismic acquisition geometries. These samplings are far from idealized randomized samplings that are usually considered in the literature but are realizable in practical scenarios. Using these algorithms, we successfully interpolate large-scale seismic data sets and demonstrate the competitive computational scaling of our algorithms as the problem sizes grow.

Eliminating 3D pre‐stack migration artefacts by 6D filtering

ABSTRACTState‐of‐the‐art 3D seismic acquisition geometries have poor sampling along at least one dimension. This results in coherent migration noise that always contaminates pre‐stack migrated data, including high‐fold surveys, if prior‐to‐migration interpolation was not applied. We present a method for effective noise suppression in migrated gathers, competing with data interpolation before pre‐stack migration. The proposed technique is based on a dip decomposition of common‐offset volumes and a semblance‐type measure computation via offset for all constant‐dip gathers. Thus the processing engages six dimensions: offset, inline, crossline, depth, inline dip, and crossline dip. To reduce computational costs, we apply a two‐pass (4D in each pass) noise suppression: inline processing and then crossline processing (or vice versa). Synthetic and real‐data examples verify that the technique preserves signal amplitudes, including amplitude‐versus‐offset dependence, and that faults are not smeared.

On horizontal resolution for seismic acquisition geometries in complex 3D media

Abstract Spatial sampling has a crucial influence on the horizontal resolution of seismic imaging, but how to quantify the influence is still controversial especially in complex media. Most of the studies on horizontal resolution focus on the measurement of wavelet widths for seismic migration, but neglect to evaluate the effect of side-lobe perturbations on spatial resolution. The side-lobe effect, as a migration noise, is important for seismic imaging in complex media. In this article, with focal beam analysis, we define two parameters to represent the horizontal resolution of an acquisition geometry: the width of the main lobe (WML) along the inline and crossline directions and the ratio of the main-lobe amplitude to the total amplitude (RMT) in a focal beam. We provide examples of typical acquisition geometries to show how spatial sampling affects the horizontal resolution, measured in terms of WML and RMT values. WML defines the horizontal resolution to image the target, whereas RMT describes the clarity of the imaging. Migration noise reduces with increasing RMT, indirectly improving both the vertical and horizontal resolutions of seismic imaging. Case studies of seismic migration with 3D seismic data from an oil field of China, demonstrate how the acquisition geometries with different WML and RMT values influence the performance of seismic imaging. Prior WML and RMT analyses to predict the quality of acquired datasets can optimize acquisition geometries before the implementation of seismic acquisition.

Quantitative regularity analysis of offset-vector sampling for seismic acquisition geometry

Symmetric acquisition geometry consisting of identical sampling of shots and receivers, can maintain the spatial continuity of the wavefield automatically, according to symmetric sampling theory. However, asymmetric geometry is often adopted in practical seismic exploration applications. Such geometry can cause uneven sampling and is necessary to be assessed for its sampling performance prior to acquisition. In conventional survey design, based on the common mid-point (CMP) analysis for a horizontally layered earth or common reflection point (CRP) analysis for a complex subsurface structure, the quality of acquisition geometry is generally judged by such bin properties as effective fold, offset scalar and azimuth distributions. However, these conventional approaches are limited by an incomplete understanding of the offset-vector sampling. Therefore, we propose a new method for quantitatively evaluating the continuity of offset-vector sampling including four spatial coordinates of shot and receiver. On the basis of physical potential energy and force-balance principle, it analyzes the regularity coefficient of offset-vector sampling as a whole using potential function model and takes into account fold, offset-scalar and azimuth distribution factors. The combination of regularity coefficients of every bin can produce spatial continuity distribution of offset-vector sampling. Similar to symmetric sampling, this approach emphasize the spatial relationships between adjacent bins rather than single bin attribute, since it aims to maintain the spatial continuity of the wavefield which allows the faithful reconstruction of the underlying continuous wavefield. Using this method, we can quantitatively compare spatial continuity distribution for different seismic acquisition geometries, and then choose the better acquisition scheme.

On the influence of model parametrization in elastic full waveform tomography

SUMMARY Elastic Full Waveform Tomography (FWT) aims to reduce the misfit between recorded and modelled data, to deduce a very detailed model of elastic material parameters in the underground. The choice of the elastic model parameters to be inverted affects the convergence and quality of the reconstructed subsurface model. Using the Cross-Triangle-Squares (CTS) model three elastic parametrizations, Lame parameters m1 = [λ, μ, ρ], seismic velocities m2 = [Vp, Vs, ρ] and seismic impedances m3 = [Ip, Is, ρ] for far-offset reflection seismic acquisition geometries with explosive point sources and free-surface condition are studied. In each CTS model the three elastic parameters are assigned to three different geometrical objects that are spatially separated. The results of the CTS model study reveal a strong requirement of a sequential frequency inversion from low to high frequencies to reconstruct the density model. Using only high-frequency data, cross-talk artefacts have an influence on the quantitative reconstruction of the material parameters, while for a sequential frequency inversion only structural artefacts, representing the boundaries of different model parameters, are present. During the inversion, the Lame parameters, seismic velocities and impedances could be reconstructed well. However, using the Lame parametrization -artefacts are present in the λ model, while similar artefacts are suppressed when using seismic velocities or impedances. The density inversion shows the largest ambiguity for all parametrizations. However, the artefacts are again more dominant, when using the Lame parameters and suppressed for seismic velocity and impedance parametrization. The afore mentioned results are confirmed for a geologically more realistic modified Marmousi-II model. Using a conventional streamer acquisition geometry the P-velocity, S-velocity and density models of the subsurface were reconstructed successfully and are compared with the results of the Lame parameter inversion.

Fast multifrequency focal beam analysis for 3D seismic acquisition geometry

A method for the efficient computation of multifrequency focal beams for 3D seismic acquisition geometry analysis has been developed. By computing them for all the frequency components of seismic data, single-frequency focal beams can be extended to multifrequency focal beams. However, this straightforward method involves considerable computer time and memory requirements, especially in complex media settings. Therefore, we propose a rapid 3D multifrequency focal beam method in which only a few single-frequency focal beam computations are followed by a number of smart interpolations. The 3D wavefield extrapolation in the focal beam analysis is conducted by the combined applications of a 3D degenerate Fourier migrator and a 3D Born-Kirchhoff interpolation operator, a process that reduces the computational cost for complex media. The multifrequency focal beam analysis is applied to a 3D model from an oil field of China, demonstrating how spatial sampling differences affect seismic imaging.

Introduction to this special section: Seismic modeling

Seismic modeling plays an important role in data acquisition, processing, interpretation, and reservoir characterization, making the results more effective and reliable. The simplest form of seismic modeling is the generation of synthetic seismograms from well logs (forward modeling) and its subsequent comparison with seismic data. Another common process is the generation of pseudologs from seismic data (inverse modeling) and using the results in those parts of the seismic volume where there is not enough well control. Seismic modeling is important, therefore, for both forward and inverse problems. One straightforward application of seismic forward modeling is the development of models to address problems of structure and stratigraphy during the interpretation of seismic data. This helps an interpreter relate the modeled seismic response generated from a hypothesized geologic model with the seismic data being interpreted. Another application is in the design of seismic acquisition geometries and the simul...

3D surface-related multiple elimination using parabolic sparse inversion

The basic theory of surface-related multiple elimination (SRME) can be formulated easily for 3D seismic data. However, because standard 3D seismic acquisition geometries violate the requirements of the method, the practical implementation for 3D seismic data is far from trivial. A major problem is to perform the crossline-summation step of 3D SRME, which becomes aliased because of the large separation between receiver cables and between source lines. A solution to this problem, based on hyperbolic sparse inversion, has been presented previously. This method is an alternative to extensive interpolation and extrapolation of data. The hyperbolic sparse inversion is formulated in the time domain and leads to few, but large, systems of equations. In this paper, we propose an alternative formulation using parabolic sparse inversion based on an efficient weighted minimum-norm solution that can be computed in the angular frequency domain. The main advantage of the new method is numerical efficiency because solving many small systems of equations often is faster than solving a few big ones. The method is demonstrated on 3D synthetic and real data with reflected and diffracted multiples. Numerical results show that the proposed method gives improved results compared to 2D SRME. For typical seismic acquisition geometries, the numerical cost running on 50 processors is [Formula: see text] per output trace. This makes production-scale processing of 3D seismic data feasible on current Linux clusters.

Controlled illumination and seismic acquisition geometry for target-oriented imaging

The conventional method of seismic data acquisition geometry design is based on the assumption of horizontal subsurface reflectors, which often is not suitable for complex structure. We start from a controlled illumination analysis and put forward a method of seismic survey geometry design for target-oriented imaging. The method needs a velocity model obtained by a preliminary seismic interpretation. The one-way Fourier finite-difference wave propagator is used to extrapolate plane wave sources on the target layer to the surface. By analyzing the wave energy distribution at the surface extrapolated from the target layer, the shot or receiver locations needed for target layer imaging can be determined. Numerical tests using the SEG-EAGE salt model suggest that this method is useful for confirming the special seismic acquisition geometry layout for target-oriented imaging.

Infilling of sparse 3D data for 3D focusing operator estimation

ABSTRACTSeismic migration can be formulated in terms of two consecutive downward extrapolation steps: refocusing the receivers and refocusing the sources. Applying only the first focusing step with an estimate of the focusing operators results in a common focal point (CFP) gather for each depth point at a reflecting boundary. The CFP gathers, in combination with the estimates of the focusing operators, can be used in an iterative procedure to obtain the correct operators. However, current 3D seismic data acquisition geometries do not contain the dense spatial sampling required for calculation of full 3D CFP gathers. We report on the construction of full 3D CFP gathers using a non‐full 3D acquisition geometry. The proposed method uses a reflector‐orientated data infill procedure based on the azimuthal redundancy of the reflection data. The results on 3D numerical data in this paper show that full 3D CFP gathers, which are kinematically and dynamically correct for the target event, can be obtained. These gathers can be used for iterative updating of the 3D focusing operators.

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

The lowermost section of the continental superdeep drill hole German Continental Deep Drilling Program (KTB) (south Germany) has been investigated for the first time by vertical seismic profiling (VSP). The new VSP samples the still accessible range of 6–8.5 km depth. Between 7 and 8.5 km depth, the drill hole intersects a major cataclastic fault zone which can be traced back to the Earth's surface where it forms a lineament of regional importance, the Franconian line. To determine the seismic properties of the crust in situ, in particular within and around this deep fault zone, was one of the major goals of the VSP. For the measurements a newly developed high‐pressure/high‐temperature borehole geophone was used that was capable of withstanding temperatures and pressures up to 260°C and 140 MPa, respectively. The velocity‐depth profiles and reflection images resulting from the VSP are of high spatial resolution due to a small geophone spacing of 12.5 m and a broad seismic signal spectrum. Compared to the upper part of the borehole, we found more than 10% decrease of the P wave velocity in the deep, fractured metamorphic rock formations. P wave velocity is ∼5.5 km/s at 8.5 km depth compared to 6.0–6.5 km/s at more shallow levels above 7 km. In addition, seismic anisotropy was observed to increase significantly within the deep fracture zone showing more than 10% shear wave splitting and azimuthal variation of S wave polarization. In order to quantify the effect of fractures on the seismic velocity in situ we compared lithologically identical rock units at shallow and large depths: Combining seismic velocity and structural logs, we could determine the elastic tensors for three gneiss sections. The analysis of these tensors showed that we need fracture porosity in the percent range in order to explain seismic velocity and anisotropy observed within the fault zone. The opening of significant pore space around 8 km depth can only be maintained by differential tectonic stress combined with intense macroscopic fracturing. VSP reflection imaging based on PP and PS converted reflected waves showed that the major fault system at the KTB site is wider and more complex than previously known. The so‐called SE1 reflection previously found in two‐ and three‐dimensional surface seismic surveys corresponds to the top of an ∼1 km wide fault system. Its lower portion was not illuminated by surface seismic acquisition geometry. VSP imaging shows that the fault zone comprises two major and a number of smaller SE dipping fault planes and several conjugate fracture planes. The previously recognized upper fault plane is not associated with a strong velocity anomaly but indicates the depth below which the dramatic velocity decrease starts. Regarding the complexly faulted crustal section of the KTB site as a whole, we found that fluctuation spectra of rock composition and seismic velocity show similar patterns. We could verify that a significant amount of P wave energy is continuously converted into shear energy by forward scattering and that multipathing plays an important role in signal formation. The media behaves effectively smoothly only at wavelength larger than 150 m. It was shown by moving source profiling that the media is orthorhombic on a regional scale. The tilt of the symmetry axes of anisotropy varies with depth following the dip of the geological structure.

Comprehensive assessment of seismic acquisition geometries by focal beams—Part II: Practical aspects and examples

The acquisition geometry of a 3-D seismic survey should be designed in such a way that it allows high‐quality images and fulfills economical constraints. Focal source beams and focal detector beams are used to analyze and design field geometries. An attractive property is that the focal source and detector beams can be computed and evaluated separately. Seismic quality attributes such as resolution, noise suppression rate, angle‐averaged amplitude, and angle‐dependent amplitude information can be obtained efficiently from those beams. Examples confirm the theory that source and detector distributions may complement each other to achieve a high resolution. This important property can be used to design cost‐effective geometries. The acquisition design criteria for reliable amplitude versus ray parameter information turn out to be more stringent and may require a compromise in resolution.

Comprehensive assessment of seismic acquisition geometries by focal beams—Part I: Theoretical considerations

The starting point for a relatively simple approach to data acquisition design can be found in the common focus point (CFP) philosophy, which describes seismic migration as a double focusing process. The migration output is presented as the combined result of focused source beams and focused detector beams for a given velocity model, revealing the potential amplitude accuracy and spatial resolution of a specific field geometry. In addition, any noise model can be fed into the input, and the subsequent beam‐forming operations can be applied to predict the potential noise suppression rate. The economical optimization comes from the possibility of balancing the source and detector efforts as well as the acquisition and processing efforts.

AVO-A response of an anisotropic half-space bounded by a dipping surface for P–P, P–SV and P–SH data

We analyze amplitude variations with offsets and azimuths (AVO-A) of an anisotropic half-space bounded by a dipping surface. By analyzing the response of a dipping reflector instead of a horizontal one, we integrate the fundamental problem of lateral heterogeneity vs. anisotropy into our study. This analysis is limited to the three scattering modes that dominate ocean bottom seismic (OBS) data: P–P, P–SV and P–SH. When the overburden is assumed isotropic, the AVO-A of each of these three scattering modes can be cast in terms of a Fourier series of azimuths, φ, in general form, R avoa (φ)=F 0+ ∑ n=1 4 [F n cos(nφ)+G n sin(nφ)], where F 0, F n and G n are the functions that describe the seismic amplitude variations with offsets (AVO) for a given azimuth. The forms of AVO functions are similar to those of classical AVO formulae; for instance, the AVO functions corresponding to the P–P scattering mode can be interpreted in terms of the intercept and gradient, although the resulting numerical values can differ significantly from those of isotropic cases or horizontal reflectors. One of the benefits of describing the AVO-A as a Fourier series is that the contribution of amplitude variations with azimuths (AVAZ) is distinguishable from that of AVO. The AVAZ is characterized by the functions {1, cos φ, sin φ, cos2 φ, sin2 φ, cos3 φ, sin3 φ, cos4 φ, sin4 φ}, that are mutually orthogonal. Thus, the AVO–A inversion can be formulated as a series of AVO inversions where the AVO behaviors are represented by the functions F 0, F n and G n . When the coordinate system of seismic acquisition geometry coincides with the symmetry planes of the rock formations, the series corresponding to P–P and P–SV simplify even further; they reduce to F 0 for azimuthally isotropic symmetry and to F 0, F 2, F 4, G 2 and G 4 for orthorhombic symmetry. The series corresponding to P–SH scattering is reduced to G 2 and G 4 for these two symmetries. Unfortunately, the coordinate system of seismic acqusition geometry rarely coincides with the symmetry planes of the rock formations; therefore, the other terms are rarely zero. In particular, the functions F 1, F 3, G 1 and G 3 become important for large dips and are actually largely dependent on the angle of the dipping reflector. For P–P scattering, these functions are zero if the reflector is horizontal, irrespective of the anisotropic behavior. For P–SV and P–SH scattering, these functions are not necessarily zero for horizontal reflectors because they are affected by the asymmetry of the P–S reflection in addition to the effect of dip.

Resistivity imaging of shallow salt with magnetotellurics as an aid to prestack depth migration

Currently NAM (Nederlandse Aardolie Maatschappij B.V.) is undertaking a large Pre-Stack Depth Migration (Pre-SDM) campaign over several salt-related prospects in the Netherlands, two of which, Broek and Ommelanderwijk, are located in fig. 1. To build proper Pre-SDM models of salt domes it is essential to have accurate knowledge of the geology in the shallow subsurface. Using data acquired in a conventional 3D seismic acquisition geometry, with typical 400-500 m line spacing, it is often difficult to image the top of shallow salt domes accurately in the 0-500 m depth range. This problem can be seen clearly in Fig. 2, where a seismic section through the Broek salt dome is displayed.