Azimuth Data Research Articles

Short-scale laterally varying SK(K)S shear wave splitting at BFO, Germany — implications for the determination of anisotropic structures

The splitting of SK(K)S phases is an important observational constraint to study past and present geodynamic processes in the Earth based on seismic anisotropy. The uniqueness of the derived models is unclear in most cases, because the azimuthal data coverage is often limited due to recordings from only a few backazimuthal directions. Here, we analyze an exceptional dataset from the permanent broadband seismological recording station Black Forest Observatory (BFO) in SW Germany with a very good backazimuthal coverage. This dataset well represents the potential teleseismic ray paths, which can be observed at Central European stations. Our results indicate that averaging splitting parameters over a wide or the whole backazimuthal range can blur both vertical and lateral variations of anisotropy. Within the narrow backazimuthal interval of 30–100°, we observe a complete flip of the fast polarization direction. Such a splitting pattern can be caused by two layers with about NW–SE (lower layer) and NE-SW (upper layer) fast polarization directions for shear wave propagation. However, the possible model parameters have quite a large scatter and represent only the structure to the northeast of BFO. In contrast, within the wide backazimuthal range 155–335°, we prevailingly determine null splits, hence, no signs for anisotropy. This null anomaly cannot be explained satisfactorily yet and is partly different to published regional anisotropy models. Our findings demonstrate that there is significant small-scale lateral variation of upper mantle anisotropy below SW Germany. Furthermore, even low-noise long-term recording over 25 years cannot properly resolve these anisotropic structural variations.

PP-wave reflection coefficient in stress-induced anisotropic media and amplitude variation with incident angle and azimuth inversion

The PP-wave reflection coefficient (i.e., the ratio of the amplitude of reflected P wave to the amplitude of incident P wave) has crucial practical applications in many field scenarios, such as seismic inversion and in-situ stress monitoring. However, the effect of biaxial stress, frequently encountered in the earth’s interior, on the PP-wave reflection coefficient is seldom studied. To fill this gap, we first formulate the P- and S-wave moduli and density in a biaxial-stress-induced anisotropic medium based on the acoustoelasticity theory. Then, the stress-dependent anisotropy parameters and normal/tangential rock weaknesses are proposed in the context of the stress-induced anisotropy model to obtain the corresponding effective elastic stiffness tensor and its spatial perturbation. Two stress-induced anisotropy factors (SIAF) are established to quantify the magnitude of anisotropy in a stress-induced anisotropic medium. The media with large SIAF tend to demonstrate strong anisotropy when stressed and tend to be more easily fractured from the perspective of engineering application. Furthermore, an approximate formula for the PP-wave reflection coefficient for biaxial-stress-induced anisotropic media is developed with the scattering theory. Numerical results illustrate how the reflection coefficients are affected by horizontal biaxial stress and demonstrate the feasibility and accuracy of our formula. Modeled and field inversion examples illustrate that SIAF can be reasonably inverted with our amplitude variation with incident angle and azimuth inversion method from azimuthal seismic data. Our results help to better understand the effect of biaxial stress on anisotropy and wave propagation in stress-induced anisotropic media.

Seismic Anisotropic Fluid Identification in Fractured Carbonate Reservoirs

Seismic fluid identification plays an important role in reservoir exploration and development. Natural vertical fractures are common in carbonate rocks, it is essential to consider fracture-induced anisotropy in the fluid identification of fractured carbonate reservoirs. We have developed a novel Bayesian elastic impedance variation with an angle and azimuth (EIVAZ) inversion approach for directly estimating the fracture fluid indicator (FFI), which can avoid cumulative errors produced in the indirect calculation process. Under the assumption of weak anisotropy and a small incident angle, we first derive a new approximate PP-wave coefficient for horizontal transverse isotropic (HTI) media. Analysis shows that the new approximation has reasonable accuracy at angles of incidence less than 30°. To estimate the FFI from observed azimuthal P-wave seismic reflection data, we further deduce the azimuthal EI equation and establish a two-step inversion workflow. Finally, the proposed approach is demonstrated by tests on a synthetic data example and a field data set of a fractured carbonate reservoir in the Sichuan Basin (China). Results show that the model parameters can be reasonably estimated even with moderate noise levels. The estimated FFI and quasi-normal fracture weakness show relatively high values at the location of reservoirs, which reliably indicate a fractured gas-bearing reservoir.

Fracture characterization based on improved seismic amplitude variation with azimuth inversion in tight gas sandstones, Ordos Basin, China

Fractures are essential for gas accumulation in tight sandstones, as they enhance reservoir permeability and provide migration paths for fluid transport. The presence of fractures facilitates the formation of fracture networks during hydraulic fracturing and, therefore, is important for effective development of tight gas sandstones. Accordingly, predicting fracture distribution could provide insight into the reservoir quality of tight gas sandstones. Seismic amplitude variation with azimuth can evaluate the properties of vertical fractures embedded in tight sandstone. This study proposes an improved inversion scheme based on azimuthal amplitude differences to compute anisotropic parameters for fracture characterization by incorporating rock physics relationships. Our proposed method improves the robustness of the anisotropic parameter estimates by removing the effect of the isotropic background using azimuthal amplitude difference data obtained after evaluating fracture strikes. Theoretical modeling and real data analysis illustrated the enhanced sensitivity of the azimuthal amplitude differences to the anisotropy related to fractures. Real data applications indicated that the predicted fracture strikes coincide with the results estimated by tectonic stress field analysis. The computed fracture density correlates with logging permeability, implying considerable influence from fractures on permeability improvement in tight sandstones. Further analysis indicated that the gas production capacity of tight sandstones is affected simultaneously by fractures and gas enrichment. Accordingly, an integrated indicator is proposed to evaluate reservoir quality, that simultaneously considers the degree of fracture development quantified by the fracture density, and gas enrichment represented by rock compressibility. The proposed indicator is correlated positively with comprehensive reservoir properties defined by the cumulative sand layer thickness, porosity, gas saturation, and permeability, providing a reliable and practical pointer to identify favorable areas in tight gas sandstones.

Estimation of in situ stresses from PP-wave azimuthal seismic data in fracture-induced anisotropic media

Horizontally transverse isotropy (HTI) induced by vertical or subvertical aligned fractures is common for unconventional fractured porous shale oil or gas reservoirs. Compared with the unfractured rocks, the seismic response characteristics of PP-wave azimuthal amplitudes are usually disturbed by the fractures and the in situ stresses. Knowledge of fracture properties, as well as in situ stresses, is required to optimize horizontal well planning and hydraulic fracturing during production, and the seismic inversion for in situ stresses from the PP-wave azimuthal amplitude data in fracture-induced anisotropic media is an essential step. Using the linear-slip theory and the effective stress law, we derive the fluid-saturated effective elastic stiffness tensors parameterized by background elastic moduli, the effective stress coefficient of isotropic host rocks, fluid modulus, porosity, and fracture parameters based on the anisotropic Gassmann’s fluid substitution equation. Combining the perturbations in saturated stiffness tensors and scattering theory, we formulate the reflection coefficient equation of PP-wave data as a function of background porosity-related stress parameter and two (i.e., normal and shear) fracture weakness parameters. Following Bayes’ rule, we estimate the porosity-related stress parameter and fracture weaknesses using the inversion method of azimuthal Fourier coefficients. Finally, we compute the effective horizontal and vertical in situ stresses using the estimated elastic moduli, porosity-related stress parameter, and fracture weaknesses. Synthetic and real data sets demonstrate that our proposed inversion approach based on the derived reflection equation provides us another way to obtain reasonable estimates of in situ stresses in a complex-fractured porous shale reservoir.

A practical approach to develop a proper anisotropic rock physics model for media comprising of multiple fracture sets in the absence of sophisticated laboratory/wireline data

For economical production from a fractured reservoir, a characteristic analysis of the fracture parameters like its density and orientation within the reservoir is essential to improve the fluid flow during extraction. This study deals with the development of a proper anisotropic rock physics model for a media with multiple fracture sets to study the spatial distribution of important fracture parameters i.e., fracture density and orientation in the absence of sophisticated laboratory/wireline and pre-stack seismic data. The crest of hydrocarbon producing fault-bounded Balkassar Anticline in Northern Potwar, Upper Indus Basin, Pakistan is selected as a case study representing a potential zone for development of fractures at reservoir level (Sakesar Limestone). The methodology consists of the interpretation of 3D post-stack seismic and conventional wireline log data to demarcate the reservoir containing fractures. The Ant-tracking discrete fracture network (DFN) attribute is applied on 3D post-stack seismic data to obtain an initial estimate about the presence of fracture corridors and their orientations. Based on this initial estimate, a proper rock physics model has been developed utilizing inverse Gassmann relations, T-matrix approximation, and Brown and Korringa relations. The output from the developed rock physics model has been displayed in the form of 13 effective independent elastic stiffness constants (monoclinic symmetry–representing media comprising of multiple fracture sets) as a function of fracture densities and azimuthal fracture orientations. A clear decreasing trend in effective elastic stiffness constants with increasing fracture densities can be observed. Similarly, a periodic trend of effective elastic stiffness constants with fracture orientations can be observed. These trends are more or less expected, but they would have been difficult to quantify without a proper rock physics model. The use of independent effective elastic constants for the generation of synthetic seismic amplitude versus angle and azimuth (AVAZ) data and its correlation with observed seismic AVAZ data in a geostatistical sense has been discussed.

USV Dynamic Accurate Obstacle Avoidance Based on Improved Velocity Obstacle Method

Unmanned surface vehicle (USV) path planning is a crucial technology for achieving USV autonomous navigation. Under global path planning, dynamic local obstacle avoidance has become the primary focus for safe USV navigation. In this study, a USV autonomous dynamic obstacle avoidance method based on the enhanced velocity obstacle method is proposed in order to achieve path replanning. Through further analysis of obstacles, the obstacle geometric model set in the conventional velocity obstacle method was redefined. A special triangular obstacle geometric model was proposed to reconstruct the velocity obstacle region. The collision time was predicted by fitting the previously gathered data to the detected obstacle’s distance, azimuth, and other relevant data. Then, it is combined with the collision risk to determine when obstacle avoidance should begin and end. In order to ensure safe driving between path points, the international maritime collision avoidance rules (COLREGs) are incorporated to ensure the accuracy of obstacle avoidance. Finally, through numerical simulations of various collision scenarios, it was determined that, under the assumption of ensuring a safe encounter distance, the maximum change rates of USV heading angle are optimized by 17.54%, 58.16%, and 28.63% when crossing, head-on, and overtaking, respectively. The results indicate that, by optimizing the heading angle, the enhanced velocity obstacle method can avoid the risk of ship rollover caused by an excessive heading angle during high-speed movement and achieve more accurate obstacle avoidance action in the event of a safety encounter.

Systematic shift in plume bending direction at Grotto Vent, Main Endeavour Field, Juan de Fuca Ridge implies changes in venting output along the Endeavour Segment

Analysis of the time-dependent behavior of the buoyant plume rising above Grotto Vent (Main Endeavour Field, Juan de Fuca Ridge) as imaged by the Cabled Observatory Vent Imaging Sonar (COVIS) between September 2010 and October of 2015 captures long term time-dependent changes in the direction of background bottom currents independent of broader oceanographic processes, indicating a systematic evolution in vent output along the Endeavour Segment of the Juan de Fuca Ridge. The behavior of buoyant plumes can be quantified by describing the volume, velocity, and orientation of the effluent relative to the seafloor, which are a convolved expression of hydrothermal flux from the seafloor and ocean bottom currents in the vicinity of the hydrothermal vent. We looked at the azimuth and inclination of the Grotto plume, which was captured in three-dimensional acoustic images by the COVIS system, at 3-h intervals during October 2010 and between October 2011 and December 2014. The distribution of plume azimuths shifts from bimodal NW and SW to SE in 2010, 2011, and 2012 to single mode NW in 2013 and 2014. Modeling of the distribution of azimuths for each year with a bimodal Gaussian indicates that the prominence of southward bottom currents decreased systematically between 2010 and 2014. Spectral analysis of the azimuthal data showed a strong semi-diurnal peak, a weak or missing diurnal peak, and some energy in the sub-inertial and weather bands. This suggests the dominant current generating processes are either not periodic (such as the entrainment fields generated by the hydrothermal plumes themselves) or are related to tidal processes. This prompted an investigation into the broader oceanographic current patterns. The surface wind patterns in buoy data at two sites in the Northeast Pacific and the incidence of sea-surface height changes related to mesoscale eddies show little systematic change over this time-period. The limited bottom current data for the Main Endeavour Field and other parts of the Endeavour Segment neither confirm nor refute our observation of a change in the bottom currents. We hypothesize that changes in venting either within the Main Endeavour Field or along the Endeavour Segment have resulted in the changes in background currents. Previous numerical simulations (Thomson et al., J. Geophys. Res., 2009, 114 (C9), C09020) showed that background bottom currents were more likely to be controlled by the local (segment-scale) venting than by outside ocean circulation or atmospheric patterns.

Bayesian amplitude variation with angle and azimuth inversion for direct estimates of a new brittleness indicator and fracture density

Estimates of natural fractures or fracture-prone brittleness sweet spots are of great importance for the seismic characterization of fracability and successful commercial production of a shale play. Young’s modulus is commonly considered as a measure of brittleness. However, Young’s modulus may be unsuitable to characterize the brittleness of the organic-rich shale gas reservoirs because it exhibits an inverse relationship to porosity, total organic carbon, and gas saturation. The [Formula: see text] attribute (the ratio of Young’s modulus E to the first Lamé constant [Formula: see text]) is more sensitive than the commonly used Young’s modulus to the organic-rich shale gas reservoirs, making it a sensitive brittleness indicator. We have proposed a novel inversion technique of amplitude variation with angle and azimuth (AVAZ) for direct estimates of the new brittleness indicator [Formula: see text] and fracture density in shale gas reservoirs. Considering a horizontally transverse isotropic model formed by a system of rotationally invariant vertical fractures embedded in an isotropic background, we first derive a linearized PP-wave reflection coefficient as a function of Young’s modulus, new brittleness indicator, density, and fracture density. The new linearized approximation allows us to estimate the brittleness and fracture properties of the reservoir in a more direct manner. Next, we present a Bayesian AVAZ inversion incorporating Cauchy-sparse and smoothing-model constraint regularization to directly estimate these model parameters from azimuthal seismic data. Finally, we apply the proposed inversion approach to a synthetic example and a real data set of a shale gas reservoir from the southern Sichuan Basin to illustrate its potential in the seismic characterization of brittleness and natural fractures.

Evaluation Of Measurement Data Across Eight GIS Basemaps Using Drumlins: Combining Basemaps Is Optimal

Choosing the best basemap for landform data collection and analysis is fundamental for accurate measurements and usability. Different types of basemaps may affect how we perceive relief through a map (Phillips et al., 1975); thereby, affecting the precision of data collected. This project collected length and azimuth data of 60 drumlins in Western, NY from four different, ArcGIS online (AGOL)-provided basemaps, as well as two parallel-to-strike and two perpendicular-to-strike hillshades (n=4). Testing the mean length (1,662m, ±529) and azimuth (171°, ±0.3) data for uniformity across basemaps determined if any basemap is more or less reliable for data collection. The Terrain map and 351° hillshade showed the lowest statistic values (t(59) = 1.84, p = .007, R2 = 0.4116). This was due to a poor direction of shading that caused visual loss of the drumlin tails. The least number of difference occurred between the USGS Topographic map and the USA Topographic map (t(59) = -0.01, p = .992, R2 = 0.9412), maps married in creation. The more traditional USGS/USA Topographic map is better for measurements of length and orientation due to the more established outline of landforms and less visual variability. Combining hillshade and topographic maps, however, can create an optimal representation of landforms for remote data collection. Choosing, or better yet, creating the correct basemap for an intended result can ensure readability and useability. It is essential that the creation of useful basemaps can keep up with the data being collected from advancing remote-sensing technologies.

Development of azimuthal resistivity survey data processing program with graphical user interface for anisotropic coefficient quantification

The physical properties of underground material are anisotropic. This applies to all physical properties including resistivity. Anisotropic property is quantified by the anisotropic coefficient. The anisotropy of physical properties can be shown by 2D and or 3D modeling, but it is not always economically efficient for small-scale studies, while 1D modeling is not capable of showing the anisotropy effect. One of the many geophysical methods developed to learn about anisotropic properties under the earth is the azimuthal resistivity survey (ARS) which is the improvement of the vertical electrical sounding (VES) method. Measurement is carried out in one dimension with various electrode spacing and azimuth of electrode lines. The result of this measurement is VES data for various azimuths. ARS data is then plotted with a polar plot to show the anisotropy of physical properties below the surface. The anisotropic coefficient calculation is carried out by performing the one-dimension inversion of the measurement data for every azimuth, for a user-defined number and constant thickness of the layer. The result is then used to calculate the variation of the anisotropic coefficient value against depth. The algorithm used to create the polar plot and VES data inversion are then combined into one program based on Python programming language. This program is equipped with a graphical user interface (GUI) to make the program more user-friendly. The program is tested with synthetic data and successfully imaged the anisotropic coefficient variation against depth for the synthetic data.

Optimization of Dynamic Speed-Homogeneous Space Using Location-Based Vehicle Data

Road extent compared with the number of automobiles registered in South Korea is the lowest among OECD (Organization for Economic Cooperation and Development) countries. Currently, traffic information is provided based on nodes and links using the centerlines of the roads. However, detailed speed information is unavailable. This study presents an optimization method for space to represent a dynamic speed-homogeneous space. Vehicle directions were separated by route using azimuth data based on global navigation satellite system (GNSS) location information, and the time mean speed and space mean speed were used to determine the homogeneous space of vehicle speed through the gradual separation of space. The processing speed of large-capacity data was increased by using Geohash, which applied a quaternary basis segmentation method that split data into two directions of longitude and latitude. The numbers of homogeneous speed spaces along the two directions of the Gyeongbu Expressway were determined to be 127 and 170, respectively, by the 12-step segmentation method. Spaces not included in the range of homogeneous speed spaces amounted to 5.19% and 7.51%, respectively, along the northbound and southbound roadways of the Gyeongbu Expressway. These non-convergence spaces were analyzed by the influence of vehicles entering or exiting slowly from the merging and diverging areas of the freeway junctions and interchanges. In other words, these were analyzed by the difference of speed by lane and not the driving direction.

The Offset Azimuth Prediction of Light Buoy Based on Markov Chain Optimization Multiplicative Seasonal Model

Aimed at the problem of the large error caused by uncertain factors in the fitting process of the traditional multiplicative seasonal model, the advantages of the Markov chain in this study are applied to the multiplicative seasonal model to optimize the prediction results. Based on the residual value between the theoretical and actual values, the values of different intervals are divided into states. The transition probability matrix is established through different probabilities; then, the weighted sum of different prediction probabilities is carried out to select the optimal prediction state. The real number of Meizhou Bay portlight buoys is used to verify the prediction effect of the model, and MAE, MAPE, RMSE, RRMSE, SSE, R2 are used to calculate the error between the predicted value and the actual value. The results show that compared to the traditional multiplicative seasonal model and other prediction models, the prediction MAE of the MC-SARIMA model is decreased by 2.19003794, the MAPE is decreased by 0.66%, the RMSE is decreased by 2.092671823, the RRMSE is decreased by 0.006221352, the SSE is decreased by 404.0231931, and the R2 is increased by 0.224686247. It shows that the multiplicative seasonal model optimized by the Markov chain can predict the azimuth data of the light buoy more effectively than the traditional multiplicative seasonal model and other prediction models.

Seismic Characterization of Decoupled Orthorhombic Fractures Based on Observed Surface Azimuthal Amplitude Data

Commonly-used seismic fracture characterization method assumes one single fracture set, and this article aims to relax the limitation and demonstrate the feasibility of seismic characterization for two sets of orthorhombic fractures. Elastic properties of orthorhombic fractures can be studied using a model of horizontal and vertical fractures as linear-slip interfaces embedded in an isotropic background. High fracture density in hydrocarbon reservoirs is the “sweet spot” of effective permeability for fluid flow, and seismic characterization of orthorhombic fractures using the variations in reflection amplitude versus offset and azimuth (AVOAz) helps to optimize the production of naturally fractured reservoirs. Based on an effective orthorhombic linear-slip model, we first construct the effective elastic stiffness matrix in terms of elastic moduli and decoupled horizontal and vertical fracture densities under the assumption of weak anisotropy (WA) or small fracture densities. Using the scattering function and first-order perturbation in effective elastic stiffness components, we derive a WA and linearized PP-wave reflection coefficient containing decoupled orthorhombic fracture densities to describe the characteristics of horizontal and vertical fracture sets. We then propose an efficient AVOAz inversion method to perform the seismic characterization of decoupled orthorhombic fracture densities based on observed surface azimuthal amplitude data in a Bayesian framework. We finally apply the inversion approach to synthetic and field datasets to demonstrate its feasibility and reasonability in orthorhombic fracture characterization.

Processing of Multichannel Sliding Spotlight SAR Data with Large Pulse Bandwidth and Azimuth Steering Angle

Azimuth multichannel synthetic aperture radar (SAR) working in the sliding spotlight mode can effectively achieve ultrahigh resolution and wide swath imaging. However, multichannel sliding spotlight SAR echoes with large pulse bandwidth and azimuth steering angle are difficult to reconstruct and process due to the large range frequency-dependent Doppler centroid varying frequency support. To resolve this problem, two azimuth preprocessing approaches both based on the range frequency-dependent deramping strategy are proposed in this article. In both approaches, echoes are handled by a range frequency-dependent azimuth deramping function at first, which assign the signal measurements to unambiguous Doppler frequencies. Afterward, in the first preprocessing approach, azimuth multichannel raw data can be reconstructed and combined before azimuth upsampling in the conventional two-step preprocessing technique. The other way to handle the deramped multichannel raw data is the modified full aperture azimuth multichannel sliding spotlight reconstruction approach, which embeds azimuth multichannel reconstruction into azimuth convolution between the deramped raw data and the selected range frequency-dependent azimuth deramping function. Furthermore, the azimuth data in the second approach should be resampled to obtain a uniform azimuth sampling interval. Computational complexity and effects on the following imaging algorithm of both approaches are analyzed and compared. Simulation results validate the proposed two azimuth multichannel preprocessing approaches.

Bayesian Seismic Azimuth-Difference Inversion of Horizontal Transversely Isotropic Media for Low-Frequency Component of Fracture Weaknesses in Laplace-Fourier Domain

Fracture weakness is one of the most important anisotropic parameters used to characterize the fractures and identify the fluids. The model of horizontal transversely isotropic (HTI) medium is usually utilized in seismic azimuthal inversion for the fracture weaknesses. Low-frequency component of fracture weaknesses plays a significant role in seismic fracture characterization and fluid identification due to the deficiency of low-frequency component in acquired seismic azimuthal data. The commonly used approaches to estimate low-frequency component include the smoothing model constraints and the damped wave field in complex-frequency domain. Following the Bayesian framework, we propose a novel approach used for compensating the low-frequency component of fracture weaknesses in Laplace-Fourier domain. Firstly, we reconstruct the seismic forward solver in Laplace-Fourier domain to obtain the low-frequency component of fracture weaknesses. Then, we propose a method of seismic azimuth-difference inversion for fracture weaknesses in Laplace-Fourier domain in a Bayesian framework. Finally, both synthetic and field data examples are used to demonstrate the superiority and stability of the proposed inversion approach. Compared with the conventional inversion approach, the proposed approach can reduce the dependence on the initial model of model parameters and weaken the effect of missing low-frequency components of fracture weaknesses in azimuthal seismic data, and it may help to improve the inversion accuracy of fracture weaknesses and reduce the uncertainty of inversion results.

Angular Superresolution of Real Aperture Radar With High-Dimensional Data: Normalized Projection Array Model and Adaptive Reconstruction

Angular resolution of real aperture radar (RAR) can be improved using deconvolution methods to achieve enhanced target information based on the convolution relationship between target scatterings and an antenna pattern. However, depending on the wide scanning scope and dense sampling angular interval, the computational complexity of the deconvolution methods will drastically increase as the dimension of azimuthal data increases. In this paper, to efficiently improve the angular resolution of RAR, a generalized adaptive asymptotic minimum variance (GAAMV) estimator that relies on a normalized projection array (NPA) model is proposed. On the one hand, the traditional convolution model of RAR is transformed into an NPA model to compress the data dimension. The proposed NPA model can normalize the signal model to make it independent of the sampling parameters. On the other hand, based on the NPA model, a GAAMV estimator is proposed to efficiently reconstruct the targets by adaptively updating each grid. Moreover, the penalty parameter is extended as a generalized case to improve its adaptability to different scenes. Based on the proposed model and method, the computational complexity can be decreased, especially for high-dimensional azimuthal data. Simulations and experimental data verify the proposed model and method.

Detection of Natural Tilted Fractures From Azimuthal Seismic Amplitude Data Based on Linear-Slip Theory

Tilted transverse isotropy (TTI) is common for naturally fractured reservoir when the fractures exist a tilted axis of symmetry. Seismic characterization of natural tilted fractures from observable azimuthal reflected amplitude data, however, is quite difficult. We focus on the detection of natural tilted fractures from azimuthal seismic amplitude data, especially for low- and high-angle tilted fractures. Using the linear-slip theory, we first express the effective elastic stiffness matrix of a TTI medium as a function of background elastic moduli, fracture density, and three angles, including the incident angle, the azimuth angle, and the tilt angle. Based on the scattering function and the first-order perturbations in stiffness components across a weak-contrast reflection interface separating two weak-anisotropy TTI media, we then derive a weak-anisotropy and linearized PP-wave reflection coefficient containing the tilt angle and fracture density. For low- and high-angle aligned fracture sets, we formulate the low- and high-angle approximate PP-wave reflection coefficient equations, respectively, and propose a Bayesian seismic inversion approach to estimate the tilt angle and the corresponding fracture densities using the azimuthal differences in seismic reflected amplitude data. Synthetic and real datasets are used to demonstrate the feasibility and reliability of the proposed inversion approach. Test results make us believe that our proposed inversion approach allow us to obtain the tilted fracture properties of hydrocarbon reservoirs in a more accurate manner than previous cases of vertically transverse isotropy (VTI) or horizontally transverse isotropy (HTI).

Fracture Detection With Azimuthal Seismic Amplitude Difference Inversion in Weakly Monoclinic Medium

The rock with a vertical group of aligned fractures in the tilted transverse isotropy (TTI) background medium is regarded as an effective monoclinic medium. Thomsen parameters and fracture weaknesses can reflect the development degree of TTI background anisotropy and vertical fractures, respectively. Thus, this paper first establishes the approximate stiffness matrix of the monocline medium through the Bond transformation from Thomsen weak anisotropy theory and effective medium theory. Moreover, considering the Born scattering theory, a linearized approximate formula of PP wave reflection coefficient is further deduced. This approximate formula contains the parameters of incident angle, azimuth, tilted angle, compressional and shear moduli, density, fracture weaknesses, and Thomsen parameters. Then, the contribution of each parameter on the reflection coefficient by numerical simulation is analyzed respectively. Finally, from the perspective of Bayesian inference, an azimuthal seismic amplitude difference inversion approach incorporating Cauchy prior information and low-frequency regularization of model parameters is proposed to realize the simultaneous estimation of fracture weaknesses and Thomsen parameters. The synthetic test demonstrates that the inversion approach proposed in this article can obtain information on vertical fractures and TTI background medium and can be applied to azimuthal seismic data with moderate noise well. Furthermore, the inversion results of the synthetic test and field data illustrate the stability and feasibility of the inversion approach and also confirm the fracture weaknesses and Thomsen parameters of the monoclinic medium can be reasonably estimated by the inversion equation derived in this paper.

Interactive web mapping tools and custom subsurface cross-sections for interdisciplinary geologic investigation

Using Python-based geospatial analytics, open-source web mapping technologies, geophysical data models, and subsurface stratigraphy models from the Regional Geology Geologic Framework Model database assembled by Los Alamos National Laboratory, we developed a suite of web-based geologic investigation tools to identify and understand subsurface structures and geophysical properties concerning salt and shale formations within the contiguous United States. Coupled with a web map interface, these tools allow for the interactive visualization of various geologic data and demonstrate the ability to quickly generate custom subsurface cross-sections, borehole charts, and diagrams for azimuthal orientation data. These capabilities were developed for stakeholder and researcher use to facilitate informed decision making for spent nuclear waste disposition. However, these capabilities provide a flexible model for a variety of subsurface investigation needs, and we have demonstrated this flexibility by adapting these tools to meet visualization needs for various subsurface models within a web-based platform.