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- Volume 67, Issue 2, 2019
Geophysical Prospecting - Volume 67, Issue 2, 2019
Volume 67, Issue 2, 2019
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Three‐dimensional curvature analysis of seismic waveforms and its interpretational implications
Authors Haibin Di, Motaz Alfarraj and Ghassan AlRegibABSTRACTThe idea of curvature analysis has been widely used in subsurface structure interpretation from three‐dimensional seismic data (e.g., fault/fracture detection and geomorphology delineation) by measuring the lateral changes in the geometry of seismic events. However, such geometric curvature utilizes only the kinematic information (two‐way traveltime) of the available seismic signals. While analysing the dynamic information (waveform), the traditional approaches (e.g., complex trace analysis) are often trace‐wise and thereby fail to take into account the seismic reflector continuity and deviate from the true direction of geologic deposition, especially for steeply dipping formations. This study proposes extending the three‐dimensional curvature analysis to the waveforms in a seismic profile, here denoted as the waveform curvature, and investigates the associated implications for assisting seismic interpretation. Applications to the F3 seismic dataset over the Netherlands North Sea demonstrate the added values of the proposed waveform curvature analysis in four aspects. First, the capability of the curvature operator in differentiating convex and concave bending allows automatic decomposition of a seismic image by the reflector types (peaks, troughs and zero crossings), which can greatly facilitate computer‐aided horizon interpretation and modelling from three‐dimensional seismic data. Second, the signed minimum curvature offers a new analytical approach for estimating the fundamental and important reflector dip attribute by searching the orientation associated with least waveform variation. Third, the signed maximum curvature makes it possible to analyse the seismic signals along the normal direction of the reflection events. Finally, the curvature analysis promotes the frequency bands of the seismic signals and thereby enhances the apparent resolution on identifying and interpreting subtle seismic features.
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Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis
Authors Veronica Omofoma, Colin MacBeth and Hamed AminiABSTRACTDuring the time taken for seismic data to be acquired, reservoir pressure may fluctuate as a consequence of field production and operational procedures and fluid fronts may move significantly. These variations prevent accurate quantitative measurement of the reservoir change using 4D seismic data. Modelling studies on the Norne field simulation model using acquisition data from ocean‐bottom seismometer and towed streamer systems indicate that the pre‐stack intra‐survey reservoir fluctuations are important and cannot be neglected. Similarly, the time‐lapse seismic image in the post‐stack domain does not represent a difference between two states of the reservoir at a unique base and monitor time, but is a mixed version of reality that depends on the sequence and timing of seismic shooting. The outcome is a lack of accuracy in the measurement of reservoir changes using the resulting processed and stacked 4D seismic data. Even for perfect spatial repeatability between surveys, a spatially variant noise floor is still anticipated to remain. For our particular North Sea acquisition data, we find that towed streamer data are more affected than the ocean‐bottom seismometer data. We think that this may be typical for towed streamers due to their restricted aperture compared to ocean‐bottom seismometer acquisitions, even for a favourable time sequence of shooting and spatial repeatability. Importantly, the pressure signals on the near and far offset stacks commonly used in quantitative 4D seismic inversion are found to be inconsistent due to the acquisition timestamp. Saturation changes at the boundaries of fluid fronts appear to show a similar inconsistency across sub‐stacks. We recommend that 4D data are shot in a consistent manner to optimize aerial time coverage, and that additionally, the timestamp of the acquisition should be used to optimize pre‐stack quantitative reservoir analysis.
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Advanced three‐dimensional seismic imaging of deep supercritical geothermal rocks in Southern Tuscany
Authors Tomi Jusri, Ruggero Bertani and Stefan BuskeABSTRACTThe seismic K‐Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K‐Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal‐to‐noise ratio. Our study aims to reveal high‐resolution seismic images of the K‐Horizon below a geothermal field in Southern Tuscany, using an advanced three‐dimensional seismic depth imaging approach. The key seismic pre‐processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre‐Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three‐dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal‐to‐noise ratio data set, the migration results with the heterogeneous three‐dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K‐Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K‐Horizon with better reflection coherency and higher signal‐to‐noise ratio than standard Kirchhoff Pre‐Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K‐Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K‐Horizon.
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Application of common reflection angle migration for imaging deformation structures in an inner accretionary wedge, Nankai Trough, Japan
Authors Kazuya Shiraishi, Masako Robb, Karl Hosgood and Yasuhiro YamadaABSTRACTTo better image deformation structures within the inner accretionary wedge of the Nankai Trough, Japan, we apply common reflection angle migration to a legacy two‐dimensional seismic data set acquired with a 6 km streamer cable. In this region, many seismic surveys have been conducted to study the seismogenic zone related to plate subduction. However, the details of the accreted sediments beneath the Kumano forearc basin are still unclear due to the poor quality of seismic images caused by multiple reflections, highly attenuated signals, and possibly complex geological structures. Generating common image gathers in the subsurface local angle domain rather than the surface offset domain is more advantageous for imaging geological structures that involve complex wave paths and poor illumination. By applying this method, previously unseen structures are revealed in the thick accreted sediments. The newly imaged geometric features of reflectors, such as the folds in the shallow part of the section and the deep reflectors with stepwise discontinuities, imply deformation structures with multiple thrust faults. The reflections within the deep accreted sediments (approximately 5 km) are mainly mapped to far angles (30°–50°) in the common reflection angles, which correspond to the recorded offset distances greater than 4.5 km. This result indicates that the far offset/angle information is critical to image the deformation structures at depth. The new depth image from the common reflection angle migration provides seismic evidence of multiple thrust faults and their relationship with the megathrust fault that is essential for understanding the structure and evolution of the Nankai Trough seismogenic zone.
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Using orthogonal Legendre polynomials to parameterize global geophysical optimizations: Applications to seismic‐petrophysical inversion and 1D elastic full‐waveform inversion
More LessABSTRACTWe use Legendre polynomials to reparameterize geophysical inversions solved through a particle swarm optimization. The subsurface model is expanded into series of Legendre polynomials that are used as basis functions. In this framework, the unknown parameters become the series of expansion coefficients associated with each polynomial. The aim of this peculiar parameterization is threefold: efficiently decreasing the number of unknowns, inherently imposing a 1D spatial correlation to the recovered subsurface model and searching for maximally decoupled parameters. The proposed approach is applied to two highly non‐linear geophysical optimization problems: seismic‐petrophysical inversion and 1D elastic full‐waveform inversion. In this work, with the aim to maintain the discussion at a simple level, we limit the attention to synthetic seismic data. This strategy allows us to draw general conclusions about the suitability of this peculiar parameterization for solving geophysical problems. The results demonstrate that the proposed approach ensures fast convergence rates together with accurate and stable final model predictions. In particular, the proposed parameterization reveals to be effective in reducing the ill conditioning of the optimization problem and in circumventing the so‐called curse‐of‐dimensionality issue. We also demonstrate that the implemented algorithm greatly outperforms the outcomes of the more standard approach to global inversion in which each subsurface parameter is considered as an independent unknown.
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Comparison of acoustic and elastic full‐waveform inversion of 2D towed‐streamer data in the presence of salt
Authors Niklas Thiel, Thomas Hertweck and Thomas BohlenABSTRACTOver the last years, full‐waveform inversion has become an important tool in the list of processing and imaging technologies available to the industry. For marine towed‐streamer data, full‐waveform inversion is typically applied using an acoustic approximation because S‐waves do not propagate in water and elastic effects in recorded data are generally assumed to be small. We compare acoustic and elastic modelling and full‐waveform inversion for a field data set acquired offshore Angola over sediments containing a salt body with significant topology. Forward modelling tests reveal that such geological structures lead to significant mode conversions at interfaces and, consequently, to significant relative amplitude differences when elastically and acoustically modelled traces are compared. Using an acoustic approach for modelling in full‐waveform inversion therefore leads to problems matching the synthetic data with the field data, even for recorded pressure data and with trace normalization applied. Full‐waveform inversion is unable to find consistent model updates. Applying elastic full‐waveform inversion leads to more consistent and reliable model updates with less artefacts, at the expense of additional computation cost. Although two‐dimensional marine towed‐streamer data are least favourable for the application of full‐waveform inversion compared to three‐dimensional data or ocean‐bottom data, it is recommended to check on the existence of elastic effects before deciding on the final processing and imaging approach.
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S‐wave splitting intensity analysis and inversion
Authors Daniele Boiero and Claudio BagainiABSTRACTAnalysing S‐wave splitting has become a routine step in processing multicomponent data. Typically, this analysis leads to determining the principal directions of a transversely isotropic medium with a horizontal symmetry axis, which is assumed to be responsible for azimuthal anisotropy, and to the time delays between the fast and slow S‐waves. These parameters are commonly estimated layer‐by‐layer from the top. Errors in layer stripping occurring in shallow layers might propagate to deeper layers.
We propose a method for S‐wave splitting analysis and compensation that consists of inverting interval values of splitting intensity to obtain a model of anisotropic parameters that vary with time and/or depth. Splitting intensity is a robust attribute with respect to structural variations and is commutative, which means that it can be summed along a ray (or throughout a sensitivity kernel volume) and can be linearly related to anisotropic perturbations at depth. Therefore, it is possible to estimate anisotropic properties within a geological formation (e.g. the reservoir) by analysing the differences of splitting intensity measured at the top and at the bottom of the layer. This allows us to avoid layer stripping, in particular, for shallow layers where anisotropic parameters are difficult to estimate due to poor coverage, and it makes S‐wave splitting analysis simpler to apply. We demonstrate this method on synthetic and real data.
Because the splitting intensity attribute shows usefulness in S‐wave splitting analysis in transversely isotropic media, we extend the splitting intensity theory to lower symmetry classes. It enables the characterization of tilted transversely isotropic and tilted orthorhombic media, opening new opportunities for anisotropic model building.
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Iterated extended Kalman filter method for time‐lapse seismic full‐waveform inversion
Authors Kjersti Solberg Eikrem, Geir Nævdal and Morten JakobsenABSTRACTTime‐lapse seismic data is useful for identifying fluid movement and pressure and saturation changes in a petroleum reservoir and for monitoring of CO2 injection. The focus of this paper is estimation of time‐lapse changes with uncertainty quantification using full‐waveform inversion. The purpose of also estimating the uncertainty in the inverted parameters is to be able to use the inverted seismic data quantitatively for updating reservoir models with ensemble‐based methods. We perform Bayesian inversion of seismic waveform data in the frequency domain by combining an iterated extended Kalman filter with an explicit representation of the sensitivity matrix in terms of Green functions (acoustic approximation). Using this method, we test different strategies for inversion of the time‐lapse seismic data with uncertainty. We compare the results from a sequential strategy (making a prior from the monitor survey using the inverted baseline survey) with a double difference strategy (inverting the difference between the monitor and baseline data). We apply the methods to a subset of the Marmousi2 P‐velocity model. Both strategies performed well and relatively good estimates of the monitor velocities and the time‐lapse differences were obtained. For the estimated time‐lapse differences, the double difference strategy gave the lowest errors.
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Three‐parameter Radon transform in layered transversely isotropic media
Authors Mohammad Mahdi Abedi, Mohammad Ali Riahi and Alexey StovasABSTRACTRadon transform is a powerful tool with many applications in different stages of seismic data processing, because of its capability to focus seismic events in the transform domain. Three‐parameter Radon transform can optimally focus and separate different seismic events, if its basis functions accurately match the events. In anisotropic media, the conventional hyperbolic or shifted hyperbolic basis functions lose their accuracy and cannot preserve data fidelity, especially at large offsets. To address this issue, we propose an accurate traveltime approximation for transversely isotropic media with vertical symmetry axis, and derive two versions of Radon basis functions, time‐variant and time‐invariant. A time‐variant basis function can be used in time domain Radon transform algorithms while a time‐invariant version can be used in, generally more efficient, frequency domain algorithms. Comparing the time‐variant and time‐invariant Radon transform by the proposed basis functions, the time‐invariant version can better focus different seismic events; it is also more accurate, especially in presence of vertical heterogeneity. However, the proposed time‐invariant basis functions are suitable for a specific type of layered anisotropic media, known as factorized media. We test the proposed methods and illustrate successful applications of them for trace interpolation and coherent noise attenuation.
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Fast hyperbolic deconvolutive Radon transform using generalized Fourier slice theorem
Authors Ahmadreza Mokhtari, Ali Gholami and Hamid Reza SiahkoohiABSTRACTThe hyperbolic Radon transform has a long history of applications in seismic data processing because of its ability to focus/sparsify the data in the transform domain. Recently, deconvolutive Radon transform has also been proposed with an improved time resolution which provides improved processing results. The basis functions of the (deconvolutive) Radon transform, however, are time‐variant, making the classical Fourier based algorithms ineffective to carry out the required computations. A direct implementation of the associated summations in the time–space domain is also computationally expensive, thus limiting the application of the transform on large data sets. In this paper, we present a new method for fast computation of the hyperbolic (deconvolutive) Radon transform. The method is based on the recently proposed generalized Fourier slice theorem which establishes an analytic expression between the Fourier transforms associated with the data and Radon plane. This allows very fast computations of the forward and inverse transforms simply using fast Fourier transform and interpolation procedures. These canonical transforms are used within an efficient iterative method for sparse solution of (deconvolutive) Radon transform. Numerical examples from synthetic and field seismic data confirm high performance of the proposed fast algorithm for filling in the large gaps in seismic data, separating primaries from multiple reflections, and performing high‐quality stretch‐free stacking.
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Transmission + reflection anisotropic wave‐equation traveltime and waveform inversion
Authors Shihang Feng and Gerard T. SchusterABSTRACTA transmission + reflection wave‐equation traveltime and waveform inversion method is presented that inverts the seismic data for the anisotropic parameters in a vertical transverse isotropic medium. The simultaneous inversion of anisotropic parameters and ε is initially performed using transmission wave‐equation traveltime inversion method. Transmission wave‐equation traveltime only provides the low‐intermediate wavenumbers for the shallow part of the anisotropic model; in contrast, reflection wave‐equation traveltime estimates the anisotropic parameters in the deeper section of the model. By incorporating a layer‐stripping method with reflection wave‐equation traveltime, the ambiguity between the background‐velocity model and the depths of reflectors can be greatly mitigated. In the final step, multi‐scale full‐waveform inversion is performed to recover the high‐wavenumber component of the model. We use a synthetic model to illustrate the local minima problem of full‐waveform inversion and how transmission and reflection wave‐equation traveltime can mitigate this problem. We demonstrate the efficacy of our new method using field data from the Gulf of Mexico.
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Effects of aperture on Marchenko focussing functions and their radiation behaviour at depth
Authors Yanadet Sripanich and Ivan VasconcelosABSTRACTA focussing function is a specially constructed field that focusses on to a purely downgoing pulse at a specified subsurface position upon injection into the medium. Such focussing functions are key ingredients in the Marchenko method and in its applications such as retrieving Green's functions, redatuming, imaging with multiples and synthesizing the response of virtual sources/receiver arrays at depth. In this study, we show how the focussing function and its corresponding focussed response at a specified subsurface position are heavily influenced by the aperture of the source/receiver array at the surface. We describe such effects by considering focussing functions in the context of time‐domain imaging, offering explicit connections between time processing and Marchenko focussing. In particular, we show that the focussed response radiates in the direction perpendicular to the line drawn from the centre of the surface data array aperture to the focussed position in the time‐imaging domain, that is, in time‐migration coordinates. The corresponding direction in the Cartesian domain follows from the sum (superposition) of the time‐domain direction and the directional change due to time‐to‐depth conversion. Therefore, the result from this study provides a better understanding of focussing functions and has implications in applications such as the construction of amplitude‐preserving redatuming and imaging, where the directional dependence of the focussed response plays a key role in controlling amplitude distortions.
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Filtering potential field data by modifying its amplitude and phase in the space domain
More LessABSTRACTPotential field datasets are commonly broken down in the space domain into amplitude (total gradient, or analytic signal amplitude) and phase (tilt angle) components as part of the data processing and interpretation procedure. However, it is possible to reconstruct the data again in the space domain from the amplitude and phase, and if they have been modified then a filtered dataset will be produced. For example, modified derivatives and filters which are based on them (such as the tilt angle and the theta map) can be produced. In addition, the modification of the data amplitude prior to the reconstruction of the signal allows controllable automatic gain control filters to be designed. The procedures are demonstrated on aeromagnetic and gravity data from Southern Africa.
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Impact of three‐dimensional attitude variations of an unmanned aerial vehicle magnetometry system on magnetic data quality
Authors Callum A. Walter, A. Braun and G. FotopoulosABSTRACTOptically pumped vapour magnetometers have an orientation dependency in measuring the scalar component of the ambient magnetic field which leads to challenges for integration with mobile platforms. Quantifying the three‐dimensional attitude variations (yaw, pitch and roll) of an optically pumped vapour magnetometer, while in‐flight and suspended underneath a rotary unmanned aerial vehicle, aids in the successful development of reliable, high‐resolution unmanned aerial vehicle magnetometry surveys. This study investigates the in‐flight three‐dimensional attitude characteristics of a GEM Systems Inc. GSMP‐35U potassium vapour magnetometer suspended 3 m underneath a Dà‐Jiāng Innovations S900 multi‐rotor unmanned aerial vehicle. A series of unmanned aerial vehicle‐borne attitude surveys quantified the three‐dimensional attitude variations that a simulated magnetometer payload experienced while freely (or semi‐rigidly) suspended underneath the unmanned aerial vehicle in fair weather. Analysis of the compiled yaw, pitch and roll data resulted in the design of a specialized semi‐rigid magnetometer mount, implemented to limit magnetometer rotation about the yaw axis. A subsequent unmanned aerial vehicle‐borne magnetic survey applying this specialized mount resulted in more than 99% of gathered GSMP‐35U magnetic data being within industry standards. Overall, this study validates that maintaining magnetometer attitude variations within quantified limits (±5° yaw, ±10° pitch and roll) during flight can yield reliable, continuous and high‐resolution unmanned aerial vehicle‐borne magnetic measurements.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 69 (2021)
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Volume 67 (2019)
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Volume 66 (2018)
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