- Home
- Conferences
- Conference Proceedings
- Conferences
The 20th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2016)
- Conference date: May 24, 2016
- Location: Chiba City, Japan
- Published: 24 May 2016
-
-
Fundamental study of seismic emission tomography in terms of fluid pressure fluctuations
Authors Akitomo Watanabe, Hitoshi Mikada and Junichi TakekawaRecent years, seismic emission tomography which utilizes seismic oscillation due to fluid flow inside fractures has drawn more attention. However, the relationship between observed seismic data and fluid behavior in a reservoir has not been revealed yet. In the present study, we conduct numerical experiments for understanding the mechanism of the induced microseismic emission in order to extract more information about fluid behavior from observed seismic data. We simulate fluid flow in a fracture using the lattice Boltzmann method (LBM). We adopt two numerical models, i) parallel plate model, and ii) pore throat model. We calculate stress changes at the fracture wall induced by unsteady flow and multi-phase flow fields. The unsteady flow is generated by cyclic pressure change at the inflow boundary. In this case, inner portion of the fracture is filled only water or oil. In the multi-phase flow, we consider migration of oil droplet in a fracture with a throat filled by water. In the parallel plate model, larger shear stress change can be observed in the case of oil. This stems from more rapid change in fluid velocity close to the fracture wall due to the high viscosity of oil. In the case of the multi-phase flow in the pore throat model, about 8 Pa of shear stress and 28 Pa of normal stress are observed at the fracture wall when an oil droplet whose diameter is 1 mm passes through the pore throat. We estimate where fluid flowing using seismic wave from that stress changes. Our results show that the induced microseismic emission by fluid flow is strongly dependent on the fluid viscosity, geometry of fracture network, etc., which influences the pattern and the flux of the flow.
-
-
-
Developing Initial Model for Seismic Full Waveform Inversion Using Conventional Data Processing Tools
Authors Ehsan Jamali Hondori, Hitoshi Mikada and Eiichi AsakawaWe believe Full waveform inversion (FWI) is capable to be used as a part of seismic data processing routines. In order to check the possibility of using FWI in conventional data processing sequences, we evaluated the effect of different initial models on inversion results. We developed new initial models for full waveform inversion using horizon-guided well interpolation and compared it against initial velocities converted from stacking velocities, with and without dip move-out (DMO) correction. Acoustic full waveform inversion results from Marmousi2 model showed that when the subsurface structure has strong dips, interval velocities which are converted from stacking velocity without dip corrections fail to initialize FWI properly. However, applying dip move-out correction on the seismic data will relax the dip complexities in the velocity analysis stage and a good initial model for FWI could be developed. Alternatively, horizon-guided well interpolation uses velocities derived from well logs and makes an interpolated velocity model along the picked horizon by a constrained post-stack inversion. This makes an initial velocity model for full waveform inversion which ensures the convergence to the correct solution.
-
-
-
Applicability of Phased Array Antenna to Ground Penetrating Radar
Authors Kaito Kikuchi, Hitoshi Mikada and Junichi TakekawaGround Penetrating Radar (GPR) has been widely used to detect objects in shallow subsurface. Measuring the reflected electromagnetic (EM) waves from the subsurface, mines, remains or buried pipes can be detected by GPR. However, there is one problem in the current practice of GPR surveys. No subsurface image could be obtained for subsurface beneath surface structures since no survey lines could be drawn above the structures. To overcome this problem, we propose the use of phased array antenna as a radar source.
We conducted some numerical simulations using a 3D-FDTD method and examined the performance of the phased array antenna in terms of signal-to-noise ratio of the generated EM wave field and the response to slanted EM wave field.
Our results indicate that the application of the phased array antenna to GPR could enhance the signal-to-noise ratio for reflectors located lateral to the survey line. Also, we conclude that our proposing GPR system has a potential to be used as an angular-scanner imaging tool that has not been attempted in the conventional GPR system.
-
-
-
A fast non-local transform-domain method for seismic random noise attenuation
Authors Sajjad Amani, Ali Gholami and Hamidreza Siah KouhiAll of the seismic data include different amounts of seismic random noises, even after doing a comprehensive seismic data processing. This results in lower signal to noise ratio (SNR) or in other word, lower quality of seismic data. Because of the time-consuming processes of methods for doing seismic random noise attenuation, data processing companies don’t perform additional processing for attenuating of random noises after doing conventional methods like stacking and applying some filters. BUT, what about a very fast method which increases SNR both in pre-stacked and post-stacked data, significantly? Here, in this study we introduce an algorithm which is called ‘Fast 3D Block Matching (F3DBM)’ which combines the advantages of non-local and transform-domain denoising methods. This method has superior capability for preserving discontinuities presented in seismic data both qualitatively and quantitatively. We compare the ability of F3DBM with that of the state-of-the-art fast curvelet-based seismic denoising method for random noise attenuation both in pre-stacked and post-stacked data.
Key words: random noise, block matching, curvelet-based denoising method, pre-stacked and post-stacked seismic data denoising.
-
-
-
Investigation of influence of dipping structures on microtremor exploration from case study research
Authors Kyosuke Okamoto and Seiji TsunoShallow S-wave velocity structures are estimated from dispersion curves of phase velocity, H/V spectral ratios, etc., using microtremor exploration technique. However the estimation is originally based on the assumption that layers of media are horizontally stratified. So, if layers of media incline or are discontinued, the estimated structures have errors to some extent. In this study, we tested a procedure of microtremor exploration for a dipping structure and examined the influence of the horizontal stratification assumption on the estimated structure. The followings are brief description of our procedure. At first, we determined S-wave velocity structures apart from the dipping area as references using the SPatial Auto Correlation (SPAC) method. The corresponding fundamental peak frequency of Rayleigh wave ellipticity was also obtained. Using the information from the reference structures, the dipping structure was determined by the H/V spectral ratios which were obtained along the dipping structure under the assumption of the horizontal stratification. We found that the fundamental peak frequency shifts toward lower frequency smoothly as the structure becomes deeper. Using the determined structure, we numerically calculated influence range of the dip on the wave filed. As a result, it was revealed that the structure within one wavelength likely gives effect on the wave field and disturbs the H/V spectral ratios.
-
-
-
Mechanism of complex fracture creation in hydraulic fracturing
Authors Masaya Nagaso, Hitoshi Mikada and Junichi TakekawaHydraulic fracturing is an essential technique for the development of unconventional oil reservoirs. If fracture network formation is evaluated before a real practice of hydraulic fracturing, the permeability of the rock could be optimized after the fracturing. Since a lot of factors are involved in fracture complexity, the mechanism of fracture network formation is not fully revealed. Although the strength heterogeneities of rock mass is known to be one of the factors, strength heterogeneities is rarely taken into consideration because of less understanding of the influence on complex fracture creation. We perform a series of numerical simulation using the discrete element method and investigate the mechanism of fracture network formation, focusing on the strength heterogeneities and brittleness, which is often used as an index of fracture network formation. In heterogeneous models, complex fracture is formed by micro cracks generated around the tip of main fracture and pores with specific shape. On the other hand, in a model with high brittleness, a lot of branches are created by shear failure with main fracture propagation. These results indicate that the mechanism of complex fracture formation due to strength heterogeneities is completely different from that due to brittleness, and that the effect of the strength heterogeneities of rock should be considered as a key factor of the complication of fracture networks.
-
-
-
Permeability variation due to sand particles in an infiltration flow using Smoothed Particle Hydrodynamics method
Authors Naoki Tanimoto, Hitoshi Mikada and Junichi TakekawaRecent years, sanding phenomenon makes some problems in resource engineering field. One of the characteristic problems is that small sand particles flow in accumulating layer and close pores of the layer. When the pore closed, sand particles prevent fluids from flowing the pore. Because of that, fluid velocity and permeability decrease. This is big problem for efficient produce of resource, so it is wanted to understand the mechanism for solve the problem. In this paper, we researched the mechanism using simulation by Smoothed Particle Hydrodynamics(SPH) method. As a result, we caught a phenomenon that sand particles close a big pore and fluid velocity decreased dynamically. We simulated in four models which has a difference of shape distribution of sand particles. We found that there is visible difference between permeability changing of these models and shape of sand particles is effective for permeability changing.
-
-
-
Waveform Disturbance by Transversely Isotropic Medium with a Tilted Axis of Symmetry around a Borehole
Authors Satoshi Fuse, Hitoshi Mikada and Junichi TakekawaSonic logging is an essential tool to understand the properties of reservoirs in detail. In particular, the more accurate method for understanding the anisotropy in the vicinity of well is demanded to develop the unconventional resources such as shale oil and gas. We use 3D Hamiltonian Particle Method (HPM) to simulate a seismic wave propagation including a borehole and transmit from dipole source to make use of the shear wave. We apply the cross-dipole orientation as the system of source and receivers. We set the Transversely Isotropic medium with a Horizontal axis of symmetry (HTI) as an anisotropic layer around a borehole. It is known that the shear wave splits into two polarized waves, i.e., as shear wave splitting. This is the distinctive feature to detect the azimuthal anisotropy. In addition, we conduct numerical simulation for Transversely Isotropic medium with a Tilted axis of symmetry (TTI). We use the conventional Slowness Time Coherence (STC) to detect the slowness of formation for both HTI and TTI. Our results clearly show that obvious difference appears as received waveform. We conclude that the Full Waveform Inversion (FWI) could apply to estimate anisotropic properties around a borehole with high accuracy and resolution.
-
-
-
Elastic imaging of subsurface Structure with Equivalent Offset Migration for multicomponent seismic data
Authors Tomoaki Tanaka, Hitoshi Mikada and Junichi TakekawaEquivalent Offset Migration (EOM) was proposed to have both advantages of the conventional post-stack processing and velocity analysis as an alternative method to partial prestack migration, and draw attention in exploration geophysics for its computational efficiency and imaging accuracy. In the conventional EOM, it is mainly to use the vertical component of received waveforms, not horizontal components. However, it is necessary to get S-wave velocity structure in order to establish the sub-surface model including petrophysical properties. Thus, we conduct numerical experiments to verify the possibility of extracting information about S-wave velocity structure using EOM with the horizontal components. Our numerical results show that EOM based on the horizontal components can increase the amount of information of S-wave velocity whereas some unique difficulties to the horizontal components should be addressed.
-
-
-
Numerical study for anisotropic influences on elastic wavefields near surface
Authors Rina Yoneki, Hitoshi Mikada and Junichi TakekawaWe think anisotropic velocity analysis, which is known important for understanding the behavior of hydraulically generated fractures and due to stress surrounded a borehole, would be key to understand the state consolidation of sediments near the surface. There are many studies on seismic wave propagation in transversely isotropic and orthorhombic media. In the most of those studies, the magnitude of anisotropy is assumed to be weak. In addition, there are few studies on seismic wavefields in quite strongly anisotropic media. Therefore, it may not be appropriate to apply their theories directly to strongly anisotropic subsurface media. It is necessary to understand the effects of the anisotropy on the behavior of seismic wave propagation in strongly anisotropic media in the seismic exploration. In this study, we investigate the influence of strong anisotropy on received seismic waveforms using three-dimensional numerical models, and verified capability of detecting subsurface anisotropy. Our numerical models contain an isotropic and an anisotropic (transversely isotropic) layer in an isotropic background subsurface. Since the difference between the two models is only the anisotropy in the vertical propagation velocity, we could observe the influence of anisotropy in the residual wavefield that is the difference in the observed wavefields of two models. The residual waveforms could be exploited to estimate both the order of anisotropy and the thickness of anisotropic layer in subsurface.
-
-
-
Optical Fiber Vertical Seismic Profile using DAS (Distributed Acoustic Sensing) Technology
Authors Tsunehisa Kimura, Gareth Lees and Arthur HartogDistributed optical fiber sensing technologies have been evolved over more than 30 years starting with DTS (Distributed Temperature Sensing). DTS is now commonly used for well monitoring in the oil and gas business. The subsequently developed DAS (Distributed Acoustic Sensing) technology was introduced more than 5 years ago to meet the demands of pipeline monitoring and intrusion detection. The latest optical fiber sensing technology now allows DAS to record borehole seismic signals including VSPs (Vertical Seismic Profiles). This system is called hDVS (heterodyne Distributed Vibration Sensing) to distinguish it from the pipeline monitoring system. Unlike conventional VSP recording tools, which usually use electromagnetic accelerometers or geophones, hDVS/DAS uses optical fiber as a vibration sensor. Because optical fiber can be deployed along the entire well depth as a sensor, either permanently (e.g., using control line) or temporarily (hybrid logging cable), the acquisition time required for hDVS VSP can be as short as a few minutes, which is essentially the time required for firing of the seismic source, compared with several hours to days for conventional VSPs, including the time required for multiple tool settings. The core part of the optical fiber is made of high-silica glass, so the high-temperature version of optical fiber is widely available rated to more than 200 °C and even above 500 °C in specialized cases. Hence, an optical fiber sensor can be deployed on a permanent basis in high-temperature environments, where conventional geophones cannot be used. This is an absolutely new way of acquiring borehole seismic data by using fiber-optic technology.
-
-
-
Preconditioning elastic full waveform inversion by scattering theory
Authors Keisuke Teranishi, Hitoshi Mikada and Junichi TakekawaThe waveform analysis is a powerful tool to investigate the physical properties in the areas of interest. Since the wave propagation is influenced by all elastic parameters, it is necessary to include these parameters in the inversion. On the other hand, multi-parameter FWI is a challenging problem because some elastic parameters increases the dimension of the solution space, in other words desensitization of each parameter occurs due to the difference of each radiation pattern and geometrical spreading. Some previous works used preconditioning gradient method using approximate Hessian that takes radiation pattern and geometrical spreading into account in order to compensate the desensitization. However, such methods solve many forward calculations to make the preconditioning operator so the computational cost becomes expensive. In this paper, we suggest new preconditioning gradient method that seeks preconditioning operator by scattering theory instead of many forward calculation to compensate the desensitization. We conduct numerical experiments to compare the results obtained by the new method with those from a conventional method. This method can estimate more reliable results than conventional one and restrain the rise of computational costs.
-
-
-
The role of physical and chemical processes of silica scale growth in geothermal wells
Authors Akihiro Mizushima, Hitoshi Mikada and Junichi TakekawaSilica scaling remains to be a major restriction for geothermal heat extraction. Our goal is to construct the model reproducing the real silica scaling. To meet this goal, we develop the multi-scale modeling of silica scale growth and compare the simulation result of the amount and the distribution of silica deposition and the data from a laboratory or a field experiment to verify our model. In meso-scale model, the adhesion of the colloidal silica is analyzed using Lagrangian method, while, in the macro-scale model, LB simulation is performed using the scale growth rate obtained at the meso-scale model. From our simulation result, the real phenomenon is reproduced quantitatively and quantitatively, which has not been reproduced in the reaction kinetics. It is, therefore, necessary to emphasize the adhesion of the colloidal silica should be taken into account for reproducing silica scaling.
-
-
-
Seismic scattering for a point scatterer -with a special interest to the application in full waveform inversion-
More LessWe revisit a seismic scattering as a key physical process to be considered in the processing of multiparameter seismic full waveform inversion (FWI). FWI implicitly uses seismic scattering theories based on the assumption that the contrast in seismic velocities is weak at each location of medium inhomogeneities since the fundamental equation of FWI comes from the perturbation theory. Since each perturbed location is considered as a point elastic scatterer, we try to relax the assumption of low contrast in elastic parameters at the scatterers by means of elastic scattering theories. In mathematics, a “point” does not have any volume so that it could not have any elastic parameters. We therefore start with a homogeneous spherical inclusion as a heterogeneous volume that has arbitrary contrast to the surrounding medium. The problem is nothing but a Rayleigh scattering of seismic waves that has exact solutions to incident compressional and shear waves without any assumption in the spherical inclusion case. After taking the smallest limit of the spherical inclusion, scattered waves are expressed by a polynomial of the finite number of terms with the Dirac’s delta function for incoming both compressional and shear waves to the point scatterer. Our results show that the scattered waves are similar to what has been obtained for a weak contract case but differs in terms of the combination of elastic parameters. Based on our results, we think our solution to the inhomogeneous wave equation could directly be applied to FWI for arbitrary contrast in elastic parameters like that from the conventional perturbation theory for weak contrast case.
-
-
-
Frequency-domain mesh-free finite-difference operator for visco-acoustic wave equation
Authors Junichi Takekawa and Hitoshi MikadaIn the present study, we apply a mesh-free finite difference method (MF-FDM) to frequency-domain visco-acoustic wave propagation. Although the full-waveform inversion (FWI) is a powerful tool to investigate the subsurface quantitatively, FWI requires a huge amount of computer resources. This poses an obstacle to apply FWI to practical studies especially in three-dimensional cases. MF-FDM has a potential to decrease the computational costs for making synthetic data sets in a simple manner. We demonstrate the effectiveness of MF-FDM using numerical experiments. Our results show that the proposed method can improve the numerical efficiency without sacrificing the numerical accuracy.
-
-
-
Seismic Velocity Monitoring Using Ambient Noise Observed by DONET Seismometers in the Nankai Trough, Japan
Authors Toshinori Kimura, Hitoshi Mikada, Eiichiro Araki and Yuya MachidaSubduction zones, where a tectonic plate subducting beneath the other plate, megathrust or interplate earthquakes could be generated repeatedly. Because of the nature of interplate earthquakes, the process of plate subduction governs the distribution, mechanics, and style of slip along the interplate fault. At the Nankai Trough subduction zone, located beneath the Pacific Ocean off the southeast coast of Japan, we have installed a seismic observation system, named DONET (Dense Oceanfloor Network system for Earthquake and Tsunamis), which is composed of twenty seafloor broadband seismometers and a borehole vertical seismic array to monitor the seismic activity and the process of earthquake generation including the stress accumulation. To elucidate earthquake generation and preparation process, it is necessary to investigate how the stress could be accumulated not only in deeper part but also in the shallow sediments, what the role of interstitial fluid could be in the stress accumulation processes, etc. There are some conventional methods to measure these physical properties, such as borehole strainmeter, borehole breakouts or borehole dynamic tests. However, these methods have some difficulties from the viewpoints of technical and/or cost. For example, borehole breakouts and dynamic tests can be conducted only while drilling and/or immediately after that. Therefore we need to have some other methods to see the state and variation of the stress in the subseafloor. In this study, we applied seismic interferometry technique to ambient noise records observed by horizontal components of DONET KMD13 seafloor seismometer to obtain time dependent S-wave velocity and its anisotropy as a proxy of stress state below the DONET observatory. We first calculated cross-diploe 4-C pseudo shot records from every 1 hour ambient noise records observed by horizontal components of the DONET seismometer for three years. In obtained 4-C shot records, clear phases, which should be caused by S-wave anisotropy, are visible. Alford rotation method was then applied to the 4-C shot records to obtain S-wave anisotropy parameters, directions of fast S-wave and time lag between fast and slow S-wave velocities below the DONET observatory. We expected that our method could be a simple tool to monitor stress state in the Nankai Trough seismogenic zone.
-