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- Volume 69, Issue 6, 2021
Geophysical Prospecting - Volume 69, Issue 6, 2021
Volume 69, Issue 6, 2021
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An aggregated template methodology: Novel automatic phase‐onset identification by template matching
Authors Laure Duboeuf, Volker Oye and Ben D. E. DandoABSTRACTThe precision of P‐ and S‐wave phase picking strongly determines the precision of earthquake locations, but such picking can be challenging in the case of emergent signals, large data sets or temporally varying seismic networks. To overcome these challenges, we have developed the concept of an aggregated template to perform automatic picking of the P‐ and S‐wave phases. An aggregated template is defined as a representative event for a small area, built by aggregating the best signal‐to‐noise‐ratio seismic traces from events with similar waveforms (i.e. multiplet events). A template matching procedure, based on the cross‐correlation between an aggregated template and an unpicked event, automatically determines the unpicked event P‐ and S‐wave phases. This method enables (1) consistent and accurate P‐ and S‐wave phase picking and (2) reduces processing time relative to traditional template matching by using a clustering method that finds the most representative templates for a region, and thus limiting the required number of templates. We established two parameters to weight the picking precision: (1) the cross‐correlation between the aggregated template and the unpicked event and (2) the number of P‐ and S‐wave picks determined per event. We tested this method on 2100 events recorded in the south‐west of Iceland. Nineteen aggregated templates have been defined and used to automatically pick ∼65% of the complete event catalogue with an accuracy within the range of the manual picking uncertainty. These automatically picked events can then be used for event location, even when characterized by low magnitude, low signal to noise ratios and with emergent P‐wave signals.
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Using microseismic events to improve the accuracy of sensor orientation for downhole microseismic monitoring
Authors Yuanhang Huo, Wei Zhang, Jie Zhang and Hui YangABSTRACTEvent locations are essential for microseismic monitoring endeavours to map hydraulic fractures. For downhole monitoring, the event back‐azimuthal angle, which is commonly evaluated as the average of the P‐wave polarization angle at each sensor, is necessary to project the 2D location in the vertical plane determined by the traveltime information to the 3D space. The accuracy of the P‐wave polarization angle at each sensor depends on the signal‐to‐noise ratio of the waveforms, as well as relies on the accuracy of the sensor orientation. The conventional approaches of the sensor orientation require an event with known back‐azimuth angle, such as surface orientation shots, perforation shots, ball‐drop events or teleseismic events. In terms of availability, the signals of the perforation shots or ball‐drop events are the most widely used information. But they are usually characterized by a low signal‐to‐noise ratio. We propose a new method to incorporate high‐signal‐to‐noise ratio microseismic events with unknown back‐azimuth angles into the sensor orientation process. It can significantly improve the accuracy of the relative orientation angles among all of the sensors, and also improve the accuracy of the absolute orientation angles by linking the relative orientation angles to the sensors with the high‐signal‐to‐noise ratio waveforms of the perforation shots or ball‐drop events. We use both synthetic and field data to test the feasibility and reliability of the new method. The results reveal that the new method can reduce the uncertainty of the orientation angle and improve the accuracy of the microseismic location, compared with the conventional approach when utilizing a perforation shot or ball‐drop event alone.
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First‐break automatic picking technology based on semantic segmentation
Authors Yinpo Xu, Cheng Yin, Yingjie Pan, Yudong Ni, Xuefeng Zou and Tianfu YangABSTRACTWith the wide application of the high‐density and high‐productivity acquisition technology in the complex areas of oil fields, the first‐break picking of massive low signal‐to‐noise data is a great challenging job. Conventional first‐break automatic picking methods (Akaike information criterion method, energy ratio method, correlation method and boundary detection method) require a lot of manual adjustments due to their poor anti‐noise performance. A lot of adjustments affect the accuracy and efficiency of picking. First‐break picking takes up about one‐third of the whole processing cycle, which restricts petroleum exploration and development progress severely. In order to overcome the above‐mentioned shortcoming, this paper proposes the first‐break automatic picking technology based on semantic segmentation. Firstly, design the time window for primary wave and pick a certain quantity of first breaks from newly acquired data in different zones of the exploration area using the commonly used Akaike information criterion method and interactive adjustments; and then perform pre‐processing on the data within the time window to extract multiple first‐break attribute features and perform feature enhancement, to obtain multi‐dimensional features data blocks, at the same time, label the first breaks. Secondly, u‐shaped architecture network‐like encoding and decoding network is used to implement end‐to‐end feature learning from primary wave attribute data to first‐break labels. The encoding and decoding process of the encoding and decoding network is used to fuse the extraction and feature positioning of primary wave attribute features. At the same time, normalize each layer and use the rectified linear unit function as a non‐linear factor to improve the generalization and sensitivity of network model to low signal‐to‐noise primary waves. Finally, an optimized deep network model is used to predict the first breaks of the data to improve the accuracy and efficiency of the first‐break picking. This method innovatively fuses the multiple conventional automatic picking methods (Akaike information criterion method, energy ratio method, correlation method and boundary detection method) to extract multiple attribute features of primary wave, and improves the accuracy of the training network model to the first‐break detection using the improved UNet‐like encoding and decoding network. The feasibility of the new method is proved by model data. A comparative test is conducted between the new method and the Akaike information criterion method with the actual data, which verifies that the method in this paper has a higher picking accuracy and stable first‐break processing capability for the data with low signal to noise, our method shows a significant advantage when applied to low signal‐to‐noise seismic records from high‐productivity acquisition and can meet the demands of the accuracy and efficiency for near‐surface model building and static calculation of massive data.
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Seismic velocity analysis in the presence of amplitude variations using local semblance
Authors M. Javad Khoshnavaz, Hamid Reza Siahkoohi and Amin Roshandel KahooABSTRACTObtaining accurate velocity models plays a crucial role in many routine seismic imaging algorithms. Seismic velocity models are normally made through seismic velocity analysis workflows. The routine workflows are not capable of dealing with polarity variations across moveout curves. We address this limitation by proposing a straight‐forward and robust semblance‐based workflow, which is a modified version of the conventional semblance function. The coherency function applies semblance analysis on separate clusters of receivers followed by averaging the corresponding coherency measures from all the clusters. The proposed approach is suitable for any case of amplitude variations including attenuation and any class of amplitude‐versus‐offset effects. The ability of the proposed workflow is demonstrated to two synthetic data as well as two field‐recorded common‐midpoint gathers. We perform accuracy analysis by comparing the results from the proposed approach with the results achieved from conventional velocity analysis, and another semblance‐based algorithm that is developed to address the polarity variation task. We also studied noise sensitivity analysis by computing and comparing mathematical expectations between theory and practice.
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Learning from unlabelled real seismic data: Fault detection based on transfer learning
Authors Ruoshui Zhou, Xingmiao Yao, Guangmin Hu and Fucai YuABSTRACTSignificant advances have been made towards fault detection using deep learning. However, the fault labelling of seismic data requires great human effort. The resulting small sample problem makes traditional deep learning methods difficult to achieve desired results. Existing research proposes to train a deep learning model with labelled synthetic seismic data to get good fault detection results. However, due to the complexity of the actual geological situation, there are inevitable differences between synthetic seismic data and real seismic data in many aspects such as seismic signal frequency, frequency of fault distribution and degree of noise disturbance, which lead to the fact that the deep learning model trained by synthetic seismic data is difficult to get good fault detection result in field data applications. We propose to use transfer learning to reduce the impact of data differences to solve this problem: part of the deep transfer learning model is used to learn fault‐related features. And the other part of the deep transfer learning model is used to mine common features between the real seismic data and the synthetic seismic data, which makes the deep transfer learning model more suitable for real seismic data. Compared with the latest research progress, our method can greatly improve the effect of fault detection without real data label, which can significantly save the cost of manual label processing.
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Observation and theoretical calibration of the fluid flow mechanism of artificial porous rocks with various size fractures
Authors Pinbo Ding, Jianxin Wei, Bangrang Di, Xiang‐Yang Li and Lianbo ZengABSTRACTFractures usually spread over various scales and strongly influence velocity and anisotropy. We investigate elastic velocity and anisotropy in rocks with fractures of different sizes. Based on synthetic rocks with controlled fracture geometries, we create a set of rocks with fracture diameter of 2, 3 and 4 mm and the fracture thickness is 0.06 mm. P‐ and S‐wave velocities are measured at 0.1 MHz, while the rocks are saturated with water and air. For a fixed measurement frequency (0.1 MHz), velocities are higher in rocks with larger fractures, while anisotropy is higher in rocks with smaller fractures, even for the same fracture density. These phenomena are associated with the wave‐induced fluid flow process. Some novel effective medium theories are adopted to calibrate with the laboratory data and analyse the anisotropy affected by the fluid flow mechanism and fracture size. The results of our study demonstrate the significant effects of fracture scale on wave responses by effective medium theories in different ways. We suggest that these scale effects should be of considerable concern in some disciplines (e.g. shear wave splitting in earth's crust and hydraulic fracture monitoring with microseismic data). Considering the scale effects of fractures, the accuracy during these investigations would be improved.
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Anisotropic poroviscoelastic medium: reflection of inhomogeneous waves at the plane boundary with restricted seepage of pore fluid
By M. D. SharmaABSTRACTAn attenuating wave field in an anisotropic poroviscoelastic medium is represented through the three‐dimensional inhomogeneous propagation of four bulk waves. Propagation of each wave is governed by a complex slowness vector, which specifies its phase direction, phase velocity and coefficients for homogeneous/inhomogeneous attenuation. Partially opened surface pores restrict the seepage of pore fluid at the boundary. A generalized reflection phenomenon is illustrated for incidence of inhomogeneous waves at this boundary. Horizontal slowness of this incidence derives the slowness vectors of four inhomogeneous waves reflected into the medium. Slowness vectors and polarizations of incident and reflected waves are used to calculate the amplitudes, phase shifts and energy fluxes of reflected waves in comparison to the incident wave. A particular example illustrates the numerical implementation of the derived mathematical model for anisotropic poroviscoelastic reflection.
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Analysis of the frequency dependence characteristics of wave attenuation and velocity dispersion using a poroelastic model with mesoscopic and microscopic heterogeneities
More LessABSTRACTSeismic waves passing through partially saturated porous rocks produce pressure gradients in the fluid phase, and, hence, the resulting fluid flow is accompanied by various length scales. The major mechanism responsible for seismic attenuation and dispersion is arguably known as wave‐induced fluid flow between inhomogeneities of microscopic, mesoscopic and macroscopic scales. Previous studies have revealed that differentiating the influence of heterogeneities at various scales on wave attenuation within seismic exploration and sonic frequencies, nevertheless, is very difficult. This is because wave attenuation mechanisms due to different heterogeneities are practically impossible to be unrelated. Therefore, it is important for a more quantitative interpretation of the relative contribution of inter‐dependent energy loss mechanisms through improved understanding of the combined influences associated to the microscopic squirt flow and mesoscopic fluid flow. We introduce a scaled poroelastic model to evaluate frequency‐dependent attenuation and velocity dispersion characteristics by considering the combined presence of microscopic and mesoscopic heterogeneities. To do so, the capillarity effects are incorporated into the poroelastic model with random distributions of the sizes of mesoscopic‐scale heterogeneities. A range of pertinent scenarios are calculated, and the acoustic properties indicate wave attenuation decreases whereas the phase velocity increases corresponding to additional capillary forces. Meanwhile, numerical results of the proposed model were compared with experimental measurements of a tight sandstone to examine its validity. Results of numerical simulations suggest that seismic reflections produce more complicated signatures in the presence of an interbedded structure of a reservoir exhibiting velocity dispersion. Therefore, the proposed procedure can help in assessing the sensitivities of frequency‐dependent seismic signatures to reservoir fluid mobility and patch heterogeneities.
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Use of rock‐physics analysis of well logs to determine compaction history of Cretaceous shales in the Rovuma basin, Offshore Mozambique
Authors Oscar J. Nhabanga, Philip S. Ringrose and Rune M. HoltABSTRACTCretaceous shales from the emerging gas province of the Rovuma basin are examined using well‐log data from two exploration wells. The P‐wave acoustic impedance (AIp) data were estimated, and then brittleness and ductility were assessed within the framework of the Reuss–Voigt limits for acoustic impedance. We then predicted the shale consolidation using a weighting function (Wc) which varies between 0 for the case of grains in suspension (the Reuss limit) and 1 for the case of consolidated rock or cemented shale (the Voigt limit). At the Reuss limit, the formation AIp is highly sensitive to pressure while at the Voigt limit the formation AIp is insensitive to pressure. For the wells in this study, most data plot close to the lower bound with Wc < 0.5, hence showing a significant sensitivity to pressure. Although the Cretaceous shales in the Rovuma basin are dominated by a mechanical compaction regime, some onset of chemical compaction is observed. The rock‐physics analysis showed that the same shale in these two exploration wells had very distinctive and contrasting elastic properties. The deeper well interval clearly shows a ductile shale while the shallower well shows a range of ductile to brittle behaviour matching with the onset of chemical diagenesis at temperatures >58°C. These differences in the development and rate of chemical diagenesis in the same formation are likely due to contrasts in the rate of burial and have important implications for future seismic exploration studies.
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Semi‐supervised deep autoencoder for seismic facies classification
Authors Xingye Liu, Bin Li, Jingye Li, Xiaohong Chen, Qingchun Li and Yangkang ChenABSTRACTFacies boundaries are critical for flow performance in a reservoir and are significant for lithofacies identification in well interpretation and reservoir prediction. Facies identification based on supervised machine learning methods usually requires a large amount of labelled data, which are sometimes difficult to obtain. Here, we introduce the deep autoencoder to learn the hidden features and conduct facies classification from elastic attributes. Both labelled and unlabelled data are involved in the training process. Then, we develop a semi‐supervised deep autoencoder by taking the mean of intra‐class and the whole population of facies into account to construct a classification regularization term, thereby improving the classification accuracy and reducing the uncertainty. The new method inherits the profits of deep autoencoder and absorbs the information provided by labelled data. The proposed method performs well and produces promising results when it is used to address problems of reservoir prediction and facies identification. The new method is evaluated on both well and seismic data and compared with the conventional deep autoencoder method, which demonstrates its feasibility and superiority with respect to classification accuracy.
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Estimation of shear‐wave velocities in unconventional shale reservoirs
Authors Sheyore John Omovie and John P. CastagnaABSTRACTAnalysis of compressional and shear‐wave sonic logs in seven organic‐rich shale reservoirs exhibiting a wide range of velocities, total organic content, thermal maturity, fluid compressibility and mineral composition indicates that compressional‐wave velocity is a strong predictor of shear‐wave velocity as is the case for conventional reservoirs. Excluding data with high water saturations, a simple linear relationship is found between the compressional and shear‐wave velocities in shale reservoirs in accordance with laboratory measurements. The relationship can be further refined for prediction purposes by local calibration or with a linear correction for total organic content. Correcting for compositional variation and fluid effects requires more complex treatment. An empirical rock physics model that explicitly incorporates these effects results in high accuracy (0.2% average mean error) in shear‐wave velocity estimation for the entire data set. The model also exhibits good precision with mean absolute error of 2%. This is significantly better than what is achieved by rock physics models that do not explicitly consider fluid properties. These results are obtained without local calibration and without requiring correction for thermal maturity, degree of lithification or the shape or distribution of the solid organic matter. Explicit consideration of mineral composition, fluid substitution effects and amount of solid organic matter was necessary in this formulation to achieve this high average prediction accuracy with near zero bias as well as good precision across all formations and without local calibration. The successful use of Gassmann's equations in this algorithm suggests that errors in shear‐wave velocity prediction in shale reservoirs due to violation of the assumptions underlying these equations are either small or self‐compensating.
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Seismic responses to an earthquake source in stratified transversely isotropic porous media
Authors Junhua Sun, Yongxin Gao, Ping Tong, Xiao He, Wei Wang and Shuwei GengABSTRACTIn this paper, we study seismic responses to an earthquake source in horizontally stratified transversely isotropic porous media with a vertical axis of symmetry that is, transversely isotropic porous media. We present a semi‐analytical method to simulate the three‐dimensional time–space–domain seismic wavefields. We obtain the solution in the frequency–wavenumber domain by the global matrix method and transform it to the time–space domain by the discrete wavenumber method and fast Fourier transform. Our method incorporates the moment tensor source so that it allows simulating seismic responses to an earthquake. We validate our method by degenerating the transversely isotropic porous media into the isotropic porous media and transversely isotropic solid media, respectively, and compare the results with the corresponding reference solutions. We then investigate the characteristics of seismic wavefields in the transversely isotropic porous media by considering an explosive source, a shear‐wave source and a double‐couple source. The results reveal interesting observations in the transversely isotropic porous media versus the isotropic porous media. For example, the wave can have a response to the pore pressure during its propagation in a transversely isotropic porous medium while the shear‐wave does not generate any disturbance of the pore pressure. Our method is semi‐analytical and computationally efficient and it can be used for rapid evaluation of the seismic responses in transversely isotropic porous media.
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Gravity inversion of 2D fault having variable density contrast using particle swarm optimization
Authors Arka Roy and Thatikonda Suresh KumarABSTRACTA Matlab‐based optimization algorithm is introduced for inverting fault structures from observed gravity anomalies. A convenient graphical user interface is also presented for incorporating the input parameters without any technical complexity to any users. The inversion code uses particle swarm optimization, and all control parameters are tuned initially for faster convergence. There is no requirement of prior choice of an initial model, that is the advantage of using global optimization. The optimization technique is versatile enough to handle any depth‐varying density distributions. The maximum number of iterations and stopping criterion is fixed initially for getting the best optimized solution. The inverted model's output in terms of fault structure, observed and inverted gravity anomalies and dip, and vertex location of fault plane can be viewed in the graphical user interface at the end of the optimization process. The optimization algorithm is applied to different synthetic models with fixed and depth‐varying density contrasts. All synthetic models are further contaminated with white Gaussian noise for sensitivity analysis, and detailed uncertainty appraisal was also performed for the reliability estimation. Finally, the optimization is implemented for fault structure inversion of the Aswaraopet boundary fault, India, and found that the optimized solution provides a good agreement with the previously published literature. Optimized results indicate that this novel optimization approach demonstrates a robust implementation of fault inversion for any depth‐varying density distributions.
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Full‐wave modelling of early‐time measurements in time‐domain electromagnetics: Consideration of coupling between Tx–Rx antennas and the ground
Authors Cécile Finco, Cyril Schamper, Fayçal Rejiba and Luis H. Cavalcante FragaABSTRACTEarly‐time measurements of time‐domain electromagnetics carry information of the near surface, and therefore, the interpretation of such measurements is important, particularly when utilizing small‐loop acquisition devices. However, these early‐time gates are often distorted by responses from the acquisition system itself, primarily caused by self‐transients associated with the transmission link composed of the transmitter and receiver loops and with the investigated medium electrical properties. Estimating all such potential couplings is crucial for interpreting early‐time measurements and gathering information on the near surface. In this study, a receptor loop made of 20 turns was built for this purpose. Field results and simulations were then compared, which shows how full‐wave finite‐difference method in the time‐domain simulations can be used to accurately calculate the impedance characteristics of time‐domain electromagnetics systems. All simulations were performed using finite‐difference time‐domain software based on a fully explicit three‐dimensional solver. First and second sets of simulations, respectively, were carried out in free space to compare the results to an alternate simulation method with equivalent lumped electrical circuits and on a homogeneous half‐space with varying electrical resistivities. The results were then used to improve the inversion of field data acquired on a test site in Garchy, France.
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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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