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The 4th International Symposium on Recent Advances in Exploration Geophysics (RAEG 1999)
- Conference date: 15 Dec 1999 - 15 Dec 1999
- Location: Kyoto, Japan
- Published: 15 December 1999
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Full-wave Inversion in Frequency-domain
By I. UesakaWe made a numerical experiment on full-wave inversion based on acoustic wave-equation in frequency-domain. The core of this method is that we can select frequencies to calculate the Frechét derivative of wave-field in terms of velocity at each iteration. Numerical experiments show that we don’t have to use all frequencies in the observed data at one iteration. In the early stage of iteration we calculate the Frechét derivative by using low frequency, and later by using high frequency. Consequently computing cost is lower than that by using all frequencies and we can get a high-resolution image.
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A Simple Method to Incorporate Correlation Information into the Inversion Process of Full-wave Tomography
By T. TsuchiyaInversion process in the Full-wave tomography (FWT) is delicate for noise. This is because the process cannot distinguish between signal and noise, and hence it works to fit even noise to model. A partial solution of this problem is to take care of the data uncertainty. While Tarantola's probabilistic theory, which is a basis of the standard waveform inversion, advises a formal methodology for this direction, his formula is not easy to realize in the practical calculation. A simplified expression, however, suggests that incorporation of correlation information of medium can be an approximation of the method. To seek a method to manipulate a noisy dataset, I did a numerical study based on the simplified expression. Since one of the efficacies of FWT is its high resolving power, I set up a discontinuous structure to see if it maintains the resolution at the discontinuity. Then I observed effects of random noise and tested a procedure for incorporation of correlation information. Consequently I found, 1) the effect of random noise appears as velocity fluctuation, 2) the incorporation of correlation information is effective, and 3) if appropriate correlation lines and reasonable candidates for correlation length are established, improvement of tomograms with maintaining the resolution can be achieved via an AIC-based method.
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Traveltime-based Raytracing for Reflection Waves
Authors E. Asakawa and N. ShimizuA new approach for the raytracing of reflected waves is proposed in this paper. The algorithm is based on the Fermat' s principle and reciprocity and it can be adapted with any type of traveltime calculation technique (Vidale (1988), Saito (1989), Moser (1991), Askawa and Kawanaka (1993)). The method is practical and efficient for complicated subsurface structures. ln accordance with Fermat's principle, raypaths with extremum traveltimes are traced, including the minimum traveltimes (first arrivals); multi-valued traveltimes can be handled with this method. First,using Linear Traveltime lnterpolation (Asakawa and Kawanaka (1993)) for traveltime calculations, the effectiveness of this approach is shown. Then we extend it for the calculations of traveltime/raypaths of normal incident rays. This approach can be also applied to compute traveltime/raypaths of P-S/S-P converted waves or multiple reflections.
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Viscoelastic Finite-difference Modeling with Application to Shallow Seismic Refraction Data
By K. HayashiI have developed a two-dimensional, viscoelastic, finite-difference modeling method for complex surface topography and subsurface geological structure. Realistic modeling of seismic wave propagation in the near surface is complicated by many factors, such as strong heterogeneity, topographic relief and large attenuation. In order to account for these complications, I use a velocity-stress staggered grid and employ an O(2,4) accurate viscoelastic finite-difference scheme. The implementation includes an irregular free surface condition for topographic relief. The algorithm is applied to 2-D modeling of the viscoelastic response of near surface structure beneath a 2-D refraction survey line. The P-velocity models were constructed by 2-D traveltime tomography, and S-velocity, density and Q were given empirically. Comparison of the observed waveform data with viscoelastic response clearly demonstrates the importance of inclusion of viscoelasticity. The character of the observed waveform data can be explained by velocities and Q distributions.
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Fracture Analysis in the Nojima Fault from FMI Image
Authors T. Kiguchi, H. Ito, Y. Kuwahara and T. OhtaniThe borehole was drilled by the Geological Survey of Japan to penetrate the Nojima fault. The coring, borehole logging and hydrophone VSP experiment were carried out. The fault zone is characterized by altered and deformed rocks from 426.1 m to 746 m, with coaxial zone of fault gouge at 623.3 m to 625.1 m. We detected a few thousands of fractures from FMI images. The trend of the fracture strike at the depth range where the conventional logging data showed extremely anomaly in the fault zone roughly parallel to the surface strike of the Nojima fault, although the strike of all fractures inside of the fault zone is widely dispersed. Seismic anomaly outside of the fault zone estimated from the DSI data is considered to be caused by the aligned fracture with NW - SE trend. It is noteworthy that almost all the resistive fractures are distributed in the shallower depth than the coaxial zone. The strike and dip of some fractures at coaxial zone are consistent with the surface strike of the Nojima fault and the dip estimated from the depth of coaxial zone. The fractures at coaxial zone are considered to be permeable from the results of hydrophone VSP analysis.
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Fracture Zone Imaging by Using Radar Yomography - Numerical Study by the Gradient Method
Authors Y. Sanada, T. Matsuoka, K. Yamamoto and Y. AshidaIn this paper we show the numerical study of the fracture zone imaging by the radar tomography. The sources are arranged in one bore-hole and the receivers are in another bore-hole. In order to image the conductive distribution between these bore-holes, the gradient method is adopted. It estimates from observed electric field data by using a gradient search for the minimum of error function, and does not require an integral equation or simultaneous equations. The advantage of this inversion scheme is to solve the revised subsurface model quite simply, if we have a forward modeling simulator. The modeling simulator using the Fourie transform based on the FDTD (Finite Difference Time Domain) method is adopted for this simulator since it has the accuracy of the computation results, relatively high speed calculations and small memory space. Numerical studies were carried out and we found that the shape of the fracture zone is recognized clearly after 30 iterations.
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Three-Dimensional Magnetotelluric Analysis for the Kakkonda Geothermal Field
More LessNEDO began the “Deep-Seated Geothermal Resources Survey" in Kakkonda (Iwate, Japan) from Fiscal year 1992 and the pilot survey well WD-1a reached 37,929m and over 500 degrees C in 1995. In1996, NEDO drilled a side-trucking well WD-1b and hit several high permeability zones. As the number of remaining shallow geothermal reservoirs diminishes through exploitation, the geothermal industry must turn its efforts towards the detection and delineation of deep-seated geothermal reservoirs. As part of NEDO project, an extensive tensor magnetotelluric (MT) survey was carried out over the Kakkonda area in 1998. 76 sites were arranged in four profiles to cross a major structural feature. To curtail spatial aliasing, the electric dipoles along each profile were deployed contiguously. The resistivity model within the area was derived by 3-D inversion analysis.
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