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- Volume 68, Issue 5, 2020
Geophysical Prospecting - Volume 68, Issue 5, 2020
Volume 68, Issue 5, 2020
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Tutorial: unified 1D inversion of the acoustic reflection response
Authors Evert Slob, Kees Wapenaar and Sven TreitelABSTRACTAcoustic inversion in one‐dimension gives impedance as a function of travel time. Inverting the reflection response is a linear problem. Recursive methods, from top to bottom or vice versa, are known and use a fundamental wave field that is computed from the reflection response. An integral over the solution to the Marchenko equation, on the other hand, retrieves the impedance at any vertical travel time instant. It is a non‐recursive method, but requires the zero‐frequency value of the reflection response. These methods use the same fundamental wave field in different ways. Combining the two methods leads to a non‐recursive scheme that works with finite‐frequency bandwidth. This can be used for target‐oriented inversion. When a reflection response is available along a line over a horizontally layered medium, the thickness and wave velocity of any layer can be obtained together with the velocity of an adjacent layer and the density ratio of the two layers. Statistical analysis over 1000 noise realizations shows that the forward recursive method and the Marchenko‐type method perform well on computed noisy data.
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Nonstationary deconvolution using maximum kurtosis optimization
Authors Javad Jamali and Abdorrahim JavaherianABSTRACTDeconvolution is an essential step for high‐resolution imaging in seismic data processing. The frequency and phase of the seismic wavelet change through time during wave propagation as a consequence of seismic absorption. Therefore, wavelet estimation is the most vital step of deconvolution, which plays the main role in seismic processing and inversion. Gabor deconvolution is an effective method to eliminate attenuation effects. Since Gabor transform does not prepare the information about the phase, minimum‐phase assumption is usually supposed to estimate the phase of the wavelet. This manner does not return the optimum response where the source wavelet would be dominantly a mixed phase. We used the kurtosis maximization algorithm to estimate the phase of the wavelet. First, we removed the attenuation effect in the Gabor domain and computed the amplitude spectrum of the source wavelet; then, we rotated the seismic trace with a constant phase to reach the maximum kurtosis. This procedure was repeated in moving windows to obtain the time‐varying phase changes. After that, the propagating wavelet was generated to solve the inversion problem of the convolutional model. We showed that the assumption of minimum phase does not reflect a suitable response in the case of mixed‐phase wavelets. Application of this algorithm on synthetic and real data shows that subtle reflectivity information could be recovered and vertical seismic resolution is significantly improved.
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Semi‐auto horizon tracking guided by strata histograms generated with transdimensional Markov‐chain Monte Carlo
Authors Yongchae Cho, Daein Jeong and Hyunggu JunABSTRACTAlthough horizon interpretation is a routine task for building reservoir models and accurately estimating hydrocarbon production volumes, it is a labour‐intensive and protracted process. Hence, many scientists have worked to improve the horizon interpretation efficiency via auto‐picking algorithms. Nevertheless, the implementation of a classic auto‐tracking method becomes challenging when addressing reflections with weak and discontinuous signals, which are associated with complicated structures. As an alternative, we propose a workflow consisting of two steps: (1) the computation of strata histograms using transdimensional Markov‐chain Monte Carlo and (2) horizon auto‐tracking using waveform‐based auto‐tracking guided by those strata histograms. These strata histograms generate signals that are vertically sharper and more laterally continuous than original seismic signals; therefore, the proposed workflow supports the propagation of waveform‐based auto‐picking without terminating against complicated geological structures. We demonstrate the performance of the novel horizon auto‐tracking workflow through seismic data acquired from the Gulf of Mexico, and the Markov‐chain Monte Carlo inversion results are validated using log data. The auto‐tracked results show that the proposed method can successfully expand horizon seed points even though the seismic signal continuity is relatively low around salt diapirs and large‐scale faults.
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Extension of tube wave detection for quality evaluation of pile foundation
Authors Jun Yang, Xue‐wen Li and Xiao‐li SunABSTRACTBased on the theory of tube wave propagation in a fluid‐filled borehole, this paper proposes a new method for detecting the integrity of pile foundation and overcomes some limitations of the existing pile integrity tests. The basic principle of detecting the pile integrity by the tube wave, testing equipment and concrete quality evaluation of piles are introduced. The feasibility of the tube wave detection for geological exploration of a bridge pile in a karst development area is verified by field tests. Through some test cases of prestressed high strength concrete pipe piles, cast‐in‐place piles and rock‐socketed piles with defects, the accuracy of the tube wave detection and other methods is compared. Then, some issues of the application of the tube wave for detecting pile defects are discussed. Finally, the content of this paper is summarized, and some suggestions on further research are put forward.
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Impact of user parameters in inversion velocity analysis
Authors T. Zhou, H. Chauris and F. AudebertABSTRACTMigration velocity analysis is a method devoted to the evaluation of both reflectivity and background velocity models, associated with the high and low wavenumber components of the model, respectively. Inversion velocity analysis is one of its improved versions, leading to more stable background velocity updates. Still, the impact of the user parameters should be understood for an optimal update of the background velocity. We show that a sign reversal of the background velocity gradient could occur when the selected surface offset range or the space lag range is too small. We derive the theoretical limits and check their consistency through simulations in a simple model with a single interface. These guidelines determine the necessary ranges of surface offsets and space lags for a proper update of the background velocity model. We discuss their applicability on the Marmousi model. Artefacts in the retrieved background velocity model are observed when the guidelines are not satisfied.
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Structural complexity‐guided predictive filtering
Authors Bin Liu, Chao Fu, Yuxiao Ren, Qingsong Zhang, Xinji Xu and Yangkang ChenABSTRACTRandom noise attenuation utilizing predictive filtering achieves great performance in denoising seismic data. Conventional predictive filtering methods are based on fixed filter operators and neglect the complexity of structures. In this way, the denoised data cannot meet the requirement of balancing the signal preservation and noise removal. In this study, we proposed a structural complexity‐guided predictive filtering method that utilizes an adapted filter operator to adjust the changes of structural complexity. The proposed structural complexity‐guided predictive filtering mainly consists of two stages. A slope field information is acquired according to plane‐wave destruction to assess the structural complexity. In addition, an adaptive filter operator is obtained to denoise the seismic data according to the adaptive factor. Both synthetic data and real seismic profiles are employed to examine the denoising capacity and flexibility of the refined predictive filtering using adaptive lengths. The analysis of the predicted results shows that adaptive predictive filtering is powerful and has the ability to eliminate random noises with negligible distortions.
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Local outlier factor as part of a workflow for detecting and attenuating blending noise in simultaneously acquired data
Authors Woodon Jeong, Constantinos Tsingas and Mohammed S. AlmubarakABSTRACTA number of deblending methods and workflows have been reported in the past decades to eliminate the source interference noise recorded during a simultaneous shooting acquisition. It is common that denoising algorithms focusing on optimizing coherency and weighting down/ignoring outliers can be considered as deblending tools. Such algorithms are not only enforcing coherency but also handling outliers either explicitly or implicitly. In this paper, we present a novel approach based on detecting amplitude outliers and its application on deblending based on a local outlier factor that assigns an outlier‐ness (i.e. a degree of being an outlier) to each sample of the data. A local outlier factor algorithm quantifies outlier‐ness for an object in a data set based on the degree of isolation compared with its locally neighbouring objects. Assuming that the seismic pre‐stack data acquired by simultaneous shooting are composed of a set of non‐outliers and outliers, the local outlier factor algorithm evaluates the outlier‐ness of each object. Therefore, we can separate the data set into blending noise (i.e. outlier) and signal (i.e. non‐outlier) components. By applying a proper threshold, objects having high local outlier factors are labelled as outlier/blending noise, and the corresponding data sample could be replaced by zero or a statistically adequate value. Beginning with an explanation of parameter definitions and properties of local outlier factor, we investigate the feasibility of a local outlier factor application on seismic deblending by analysing the parameters of local outlier factor and suggesting specific deblending strategies. Field data examples recorded during simultaneous shooting acquisition show that the local outlier factor algorithm combined with a thresholding can detect and attenuate blending noise. Although the local outlier factor application on deblending shows a few shortcomings, it is consequently noted that the local outlier factor application in this paper obviously achieves benefits in terms of detecting and attenuating blending noise and paves the way for further geophysical applications.
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Seismic modelling for reservoir studies: a comparison between convolutional and full‐waveform methods for a deep‐water turbidite sandstone reservoir
Authors Hamed Amini, Colin MacBeth and Asghar ShamsABSTRACTTwo seismic modelling approaches, that is, two‐dimensional pre‐stack elastic finite‐difference and one‐dimensional convolution methods, are compared in a modelling exercise over the fluid‐flow simulation model of a producing deep‐water turbidite sandstone reservoir in the West of Shetland Basin. If the appropriate parameterization for one‐dimensional convolution is used, the differences in three‐dimensional and four‐dimensional seismic responses from the two methods are negligible. The key parameters to ensure an accurate seismic response are a representative wavelet, the distribution of common‐depth points and their associated angles of incidence. Conventional seismic images generated by the one‐dimensional convolutional model suffer from lack of continuity because it only accounts for vertical resolution. After application of a lateral resolution function, the convolutional and finite‐difference seismic images are very similar. Although transmission effects, internal multiples and P‐to‐S conversions are not included in our convolutional modelling, the subtle differences between images from the two methods indicates that such effects are of secondary nature in our study. A quantitative comparison of the (normalized root‐mean‐square) amplitude attributes and waveform kinematics indicates that the finite‐difference approach does not offer any tangible benefit in our target‐oriented seismic modelling case study, and the potential errors from one‐dimensional convolution modelling are comparatively much smaller than the production‐induced time‐lapse changes.
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Rock physics model of tight oil siltstone for seismic prediction of brittleness
Authors Wenhui Tan, Jing Ba, Tobias Müller, Gang Fang and Haibo ZhaoABSTRACTTight oil siltstones are rocks with complex structure at pore scale and are characterized by low porosity and low permeability at macroscale. The production of tight oil siltstone reservoirs can be increased by hydraulic fracturing. For optimal fracking results, it is desirable to map the ability to fracture based on seismic data prior to fracturing. Brittleness is currently thought to be a key parameter for evaluating the ability to fracture. To link seismic information to the brittleness distribution, a rock physics model is required. Currently, there exists no commonly accepted rock physics model for tight oil siltstones. Based on the observed correlation between porosity and mineral composition and known microstructure of tight oil siltstone in Daqing oilfield of Songliao basin, we develop a rock physics model by combining the Voigt–Reuss–Hill average, self‐consistent approximation and differential effective medium theory. This rock physics model allows us to explore the dependence of the brittleness on porosity, mineral composition, microcrack volume fraction and microcrack aspect ratio. The results show that, as quartz content increases and feldspar content decreases, Young's modulus tends to increase and Poisson ratio decreases. This is taken as a signature of higher brittleness. Using well log data and seismic inversion results, we demonstrate the versatility of the rock physics template for brittleness prediction.
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A new contribution to the geology of the Egyptian Red Sea shelf using geophysical data
Authors Ahmad Azab, Ali Ali El‐Khadragy and Saada Ahmed SaadaABSTRACTThe study examines the Egyptian Red Sea shelf and throws more light on the structural set‐up and tectonics controlling the general framework of the area and nature of the crust. Herein, an integrated study using gravity and magnetic data with the available seismic reflection lines and wells information was carried out along the offshore area. The Bouguer and reduced‐to‐pole aeromagnetic maps were processed and reinterpreted in terms of rifting and plate tectonics. The qualitative interpretation shows that the offshore area is characterized by positive gravity everywhere that extremely increases towards the centre of the graben, supporting the presence of an intrusive zone below the axial/main trough. The gravity data were confirmed by the presence of high magnetic amplitudes, magnetic linearity and several dipoles concentrated along the rift axis for at least 250 km. The lineament analysis indicates widespread of the Erythrean (Red Sea) trend that was offset/cut by transform faults in the NE direction (Aqaba). The tectonic model suggests the presence of one tensional (N65°E) and two compressional (N15°W, N30°W) phases of tectonism, resulted in six cycles of deformations, classified into three left lateral (N35°E, N15°E and N–S) and three right lateral (N85°W, N45°W and N60°W). The basement relief map reveals a rough basement surface that varies in depth between 1 and 5.6 km. It outlines several offshore basins, separated from each other by ridges. The models show that the basement consists of tilted fault blocks, which vary greatly in depth and composition and slopes generally to the west. They indicate that the coastal plain is underlain by acidic basement blocks (continental crust) with no igneous activity while suggesting elevated basic materials (oceanic crust) below the rift axis. The study suggests that northern Red Sea forms an early stage of seafloor spreading or at least moved past the late stage of continental rifting.
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Integration of rock physics and seismic inversion for rock typing and flow unit analysis: A case study
Authors Benyamin Khadem, Mohammad Reza Saberi, Mohammad Eslahati and Bita ArbabABSTRACTRock typing and flow unit detection are more challenging in clastic reservoirs with a uniform pore system. An integrated workflow based on well logs, inverted seismic data and rock physics models is proposed and developed to address such challenges. The proposed workflow supplies a plausible reservoir model for further investigation and adds extra information. Then, this workflow has been implemented in order to define different rock types and flow units in an oilfield in the Persian Gulf, where some of these difficulties have been observed. Here, rock physics models have the leading role in our proposed workflow by providing a diagnostic framework in which we successfully differentiate three rock types with variant characteristics on the given wells. Furthermore, permeability and porosity are calculated using the available rock physics models to define several flow units. Then, we extend our investigation to the entire reservoir by means of simultaneous inversion and rock physics models. The outcomes of the study suggest that in sediments with homogeneous pore size distribution, other reservoir properties such as shale content and cementation (which have distinct effects on the elastic domain) can be used to identify rock types and flow units. These reservoir properties have more physical insights for modelling purposes and can be distinguished on seismic cube using proper rock physics models. The results illustrate that the studied reservoir mainly consists of rock type B, which is unconsolidated sands and has the characteristics of a reservoir for subsequent fluid flow unit analysis. In this regard, rock type B has been divided into six fluid units in which the first detected flow unit is considered as the cleanest unit and has the highest reservoir process speed about 4800 to 5000 mD. Here, reservoir quality decreases from flow unit 1 to flow unit 6.
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Bayesian inversion of joint SH seismic and seismoelectric data to infer glacier system properties
ABSTRACTIn glacial studies, properties such as glacier thickness and the basement permeability and porosity are key to understand the hydrological and mechanical behaviour of the system. The seismoelectric method could potentially be used to determine key properties of glacial environments. Here we analytically model the generation of seismic and seismoelectric signals by means of a shear horizontal seismic wave source on top of a glacier overlying a porous basement. Considering a one‐dimensional setting, we compute the seismic waves and the electrokinetically induced electric field. We then analyse the sensitivity of the seismic and electromagnetic data to relevant model parameters, namely depth of the glacier bottom, porosity, permeability, shear modulus and saturating water salinity of the glacier basement. Moreover, we study the possibility of inferring these key parameters from a set of very low noise synthetic data, adopting a Bayesian framework to pay particular attention to the uncertainty of the model parameters mentioned above. We tackle the resolution of the probabilistic inverse problem with two strategies: (1) we compute the marginal posterior distributions of each model parameter solving multidimensional integrals numerically and (2) we use a Markov chain Monte Carlo algorithm to retrieve a collection of model parameters that follows the posterior probability density function of the model parameters, given the synthetic data set. Both methodologies are able to obtain the marginal distributions of the parameters and estimate their mean and standard deviation. The Markov chain Monte Carlo algorithm performs better in terms of numerical stability and number of iterations needed to characterize the distributions. The inversion of seismic data alone is not able to constrain the values of porosity and permeability further than the prior distribution. In turn, the inversion of the electric data alone, and the joint inversion of seismic and electric data are useful to constrain these parameters as well as other glacial system properties. Furthermore, the joint inversion reduces the uncertainty of the model parameters estimates and provides more accurate results.
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An improved interpolation scheme at receiver positions for 2.5D frequency‐domain marine controlled‐source electromagnetic forward modelling
Authors Gang Li, Shuangmin Duan, Hongzhu Cai, Bo Han and Yixin YeABSTRACTAn improved interpolation scheme is presented for 2.5‐dimensional marine controlled‐source electromagnetic forward modelling. For the marine controlled‐source electromagnetic method, due to the resistivity contrast between the seawater and seafloor sedimentary layers, it is difficult to compute the electromagnetic fields accurately at receivers, which are usually located at the seafloor. In this study, the 2.5‐dimensional controlled‐source electromagnetic responses are simulated by the staggered finite‐difference method. The secondary‐field approach is used to avoid the source singularities, and the one‐dimensional layered background model is used for calculating the primary fields excited by the source quasi‐analytically. The interpolation of electromagnetic fields at the cell nodes for the whole computational domain to the receiver locations is discussed in detail. Numerical tests indicate that the improved interpolation developed is more accurate for simulating the electromagnetic responses at receivers located at the seafloor, compared with the linear or rigorous interpolation.
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Turning off low and high currents in a transmitter loop used in the transient electromagnetic method
Authors Nikolai O. Kozhevnikov, Maxim V. Sharlov and Sergei M. StefanenkoABSTRACTIn near‐surface transient electromagnetic studies, it is desirable to measure the transient response starting from the earliest possible time. This requires the current in the transmitter loop to be switched off quickly, which necessitates working with a low transmitter current. As for deep‐target transient electromagnetic studies, the transmitter current is as high as possible. The transmitter current's turn‐off waveform and total duration affect the transient voltage response, especially at early times, which is to be accounted for when interpreting transient electromagnetic data. This article discusses the difference in switching off low and high current in a horizontal loop used as the source of the primary magnetic field in the transient electromagnetic method. Low and high currents are turned off in fundamentally different ways. When the current to be switched off is low, the loop can be represented as a symmetric combination of two transmission lines grounded at the middle of the loop perimeter. Such a representation of a loop allows calculating the current turn‐off waveform at any point of the loop. The waveform and total duration of switching off a low current does not depend on its magnitude, but is determined by the period of natural oscillations of the current in the loop and the resistance of a shunting resistor. Switching off a low current in a loop can be represented as the sum of stepped current waves travelling along the loop wire. As a consequence, the current at different points of the loop perimeter is turned off at different times. In contrast to a low current, a high current is switched off linearly in time and synchronously at all points of the loop perimeter. The wave phenomena appear only at the very beginning of the current shutdown for a time interval that is much less than the total current turn‐off duration. Presentation of the loop using a simple lumped‐circuit model predicts the waveform and duration of the high current turn‐off that coincide with the measured ones. There are two reasons why the article may be of interest to those engaged in the theory and/or practice of electromagnetic geophysical methods. First, it contributes to a general understanding of how the current in the transmitter loop is turned off. Second, the article shows how the parameters of a transmitter loop determine the current turn‐off duration and thus the minimum depth of the transient electromagnetic sounding method.
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Enhancing imaging of the Sub‐Andean Foothills geology using magnetotellurics: A case history of a 3D survey in southern Bolivia
Authors Mauro Pezzoli, Evanz Lazaro, Nestor Cuevas and Sergio BurgaABSTRACTA magnetotelluric acquisition and modelling campaign, comprising 400 full tensor soundings, was carried out in the Sub‐Andean Foothills in the area located south of Santa Cruz de la Sierra, Bolivia. The objective of the survey was to improve the imaging of the Paleozoic section and provide insight to the overall structure of the Devonian Silurian complex. The acquired data were inverted assuming both two‐dimensional and three‐dimensional dimensionality of the subsurface structure, adopting a multi‐step iterative workflow, which began applying unconstrained inversion and introduced increasing constraints determined at each following step based on the interpretation of the latest inversion results. The geological interpretation of the final inversion results allowed to better image the expected complexity in the structures, particularly highlighting several different levels of detachment, mainly in the area of Kirusillas Formation. Furthermore, an oblique sense was identified of the main tendency of N‐S deformation, related to a dextral transgressional movement that generates discrete dislocations, at a length scale of the order of a few kilometres.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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