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International Symposium on Recent Advances in Exploration Geophysics (RAEG 1995)
 Conference date: 13 Nov 1995  14 Nov 1995
 Location: Kyoto, Japan
 Published: 13 November 1995


A Seismic Exploration over Kirishima Volcanoes with New Acquisition Systems
Authors T. Tsutsui, Y. Morita and T. KagiyamaA seismic exploration has been conducted over Kirishima Volcanoes, southern Kyushu, Japan in November to December 1994. The exploration is designed both to estimate velocity structure with reaction method and to detect magma pockets beneath the volcanoes with reflections. The survey lines consists of 163 stations with independent recording systems and total length is about 60 kilometers. The recording system in this exploration is newly developed for overcoming poor conditions over volcanic area with steep topography. Specifications of this recording system are as follows; light weight, high resolution, low power consumption, large capacity and precise time base synchronization with GPS. This recorder with lightweight and highperformance enables us to conduct seismic explorations over any volcanic area. An outline of the exploration and some results of analysis will be presented.



Parallel Computation in Reflection Seismology
Authors T. Sato, T. Matsuoka and T. TsuruSince the introduction of digital data processing in reflection seismology, new algorithms have been developed, in step with the progress of computer power. Massively parallel computers have appeared in the oil industry, and it has been recognized that parallel computation is very attractive in cost performance. In this paper we investigate algorithms in parallel computation for seismic modeling and prestack depth migration. The pseudospectral method which we adopted for the seismic modeling requires a 2D FFT. Each processor can contains only the part of data, therefore 2D FFT cannot be achieved in one pass. Since the data swapping is necessary, we developed here a parallel swapping algorithm and a parallel disk I/O method to reduce the computation time. There are many theoretical algorithms to apply prestack depth migration. For parallel computation we adopted the explicit extrapolation finite difference method in the frequency and space domain. Implementation was done using “ parallel frequency slice" computations. All algorithms developed here are easy to implement in workstation clusters since these algorithms are based on the message passing method.



Data Processing of Reflection Seismic Data by Use of Neural Network
By Y. AshidaThe present paper proposes an algorithm for data processing of reflection seismic data by use of neural network. The algorithm of neural network was applied to the reading of the first break, the recognition of waveform in seismic trace and the automatic picking of the result of the constant velocity scan among the various data processing techniques. A layered network with training by the error back propagation algorithm was used. The general procedure of processing of reflection seismic data by use of neural network is as follows. 1) Constitute the most suitable network for the target processing. 2) Set the weight values to all units in the layers and the teacher's signal. 3) Calculate the output signals from the output layer by activating the network. 4) Estimate the learning signal from the actual output signal and the teacher's signal. 5) Calculate the change of weight values so as to minimize the differences between the actual signal and the teacher's signal. 6) Repeat step 3) to 5) till the errors fall into the designated limitation or the designated leaning count is reached. As a result of model studies, it was clarified that the proposed algorithm performed the first break reading, the waveform detection and the automatic picking of velocity with good accuracy.



Seismic Investigations for the Final Disposal of Spent Nuclear Fuel in Finland
Authors C. Cosma and P. HeikkinenThe paper reviews the seismic data processing techniques based on a version of the Radon transform, developed for characterizing nonplanar and spatially limited structures of arbitrary orientations in crystalline rock. The examples are taken from the VSP surveys carried out in the borehole KR6, at the Olkiluoto site in SW Finland. Polarization analysis applied in the transform space allows the positions of the reflectors to be determined by estimating the direction of the arrival of the reflected waves. The approach permits the determination of both the 3Dposition and local orientation of the observed reflectors. Anisotropic effects are included and accounted for in the analysis.



Seismic Survey of Active FauIts in the Northwest Part of the Kanto Plain, Japan
Authors K. Yamaguchi, N. Kano, T. Yokota, T. Yokokura and T. KiguchiWe conducted seismic reflection surveys in the northwest marginal part of the Kanto plain, Japan. In the 12kilometers long seismic section, homoclinic reflectors dipping northeast are dominant between the Kushibiki fault and the Fukaya fault. They are traceable from the near surface down to 1.5 seconds in twoway time, but become unclear beyond there. Some of them are very continuous and probably correspond to acid tuff layers in the Neogene sedimentary rocks. A flexure continues vertically down to 1.5 seconds near the Fukaya fault. Reflectors are almost horizontal on the northeast of the Fukaya fault. Few events are seen on the southwest of the Kushibiki fault. Fault planes or gaps of reflectors are not recognized in the seismic section but the deep structure changes suddenly at the two faults.



Model Experiment on the Enhancement of Seismic Signal for Fracture Detection by Injecting Air Bubble
Authors T. Watanabe, Y. Takami and K. SassaThis paper presents the model experiment on the enhancement of seismic signals for fracture detection. To enhance seismic signals from fractures, we propose a repeated measurement and a differential analysis by changing the seismic properties of fractures actively. Air bubbles are injected in the fluidfilled fracture to change its seismic properties. We performed the bar model experiment and the block model experiment to investigate the effect of the injection of air bubbles on Pwave propagation. As the result, the amplitude of the transmitted Pwave is attenuated and that of the reflected Pwave is emphasized by injecting air bubbles in the fracture. The effect of air bubbles can be represented as the variation of the density, the velocity and the attenuation characteristics. We can estimate the area where air bubbles are distributed by monitoring the amplitude attenuation of the transmitted Pwave.



Three Dimensional Modeling of Magnetotelluric Data Using Finite Element Method
By T. MogiThe magnetotelluric response of a resistivity model is usually obtained by solving boundary value problem of the Helmholtz type equation concerning electric field， which introduced from the Maxwell's equation. In this case， electric field is not continuous at the boundary between different resistivity mediums. This condition raises error in the numerical modelling using the finite element method or the finite difference method. To overcome the condition, I formulated the finite element equation using secondary field components resulting from the anomalies. The primary field is calculated analytically for semiinfinite uniform or horizontally layered earth. I use the finite element method composed of 8 nodes isoparametric hexahedral elements to calculate the secondary field excited by electric charges appeared at the anomalies having deferent resistivity from the host medium. Current sources at the resistivity anomaly depends on the resistivity contrast and the primary electric field, which insert at the right hand side of the finite element equation. I use boundary condition which take account for asymptotic behavior of the secondary field far away from the resistivity boundary. I applied this modelling method to simple three dimensional prism model and compared the result with the responses of the integral equation modeling algorithm (Ting and Hohmann, 1981) and the hybrid FEM (Gupta，et al.1987).



A new Magnetotelluric Inversion Scheme Using Generalized RRI Method
Authors K. Yamane, S. Takasugi and K.H. LeeA new twodimensional (2D) magnetotelluric (MT) inversion scheme is proposed in this paper. This scheme is based on a locally twodimensional analysis in order to minimize computational time and computer memory. The MT governing equation is linearized in terms of the magnetic field and electrical conductivity for the perturbation analysis. The perturbed equation is then multiplied by a test function and integrated over the cross section. Integrating by parts and then substituting this test function with local magnetic fields, a new equation is obtained that is a 2D variational integral for the electrical conductivity. The new equation is general in the sense that it can explicitly include the horizontal derivative of the magnetic field. If the horizontal derivative term is eliminated, the new equation becomes identical to the Rapid Relaxation Inversion (RRI) scheme proposed by Smith and Booker (1991).



Development of the Vertical Electro Magnetic Profiling (VEMP) Method
Authors Y. Miura, K. Osato, S. Takasugi, H. Muraoka and K. YasukawaAs a part of the "DeepSeated Geothermal Resources Survey" project being undertaken by the NEDO, the Vertical ElectroMagnetic Profiling (VEMP) method is being developed to accurate1y obtain deep resistivity structure. The VEMP method takes multifrequency threecomponent magnetic field data in an open hole well using controlled sources (loop sources or groundedwire sources) emitted at the surface. Numerical simulation using EM3D (Wannamaker et al., 1984) demonstrated that phase data of the VEMP method is very sensitive to resistivity structure and the phase data will also indicate presence of deep anomalies. Forward modelling was also used to determine required transmitter moments for various groundedwire and loop sources for a field test using the WD1 well in the Kakkonda geothermal area (Miura et al., 1994). Field logging of the well was carried out in May 1994 and the processed field data matches well the simulated data.



Conceptual Design of the ElectroMagnetic (EM) Tomography System
Authors S. Takasugi, Y. Miura and E. AraiThe new EM tomography system was planned for the detailed survey of mineral exploration. EM tomography system consists of surface; transmitter (3component loop sources) and borehole; receiver (vertical component), and uses frequencies in the 8  8,192 Hz range. As the first step of the system development, we conducted model studies using SLDM (3D) developed by Druskin (1994). The phase data of EM tomography is very sensitive to the anomaly and 3component sources are useful especially for multidimensions structure case. Also, these simulation results were used for the system design such as necessary sensitivity of receiver, transmitting moment and so on.



Developments of Noncontact Data Acquisition Techniques in Electrical and Electromagnetic Explorations
Authors H. Shima, S. Sakashita and T. KobayashThis paper describes two noncontact electrical and electromagnetic exploration techniques. One is surface electrical exploration technique using a capacitive electrodes we recently developed. A capacitive electrode enables both injecting an electric current into the ground and measuring the ground's electric potential, which allow us to conduct noncontact surface electrical profile. The other method is borehole electromagnetic (EM) tomography, which transmits a magnetic field and measures the secondary magnetic field. It does not require direct contact with the ground. The authors developed prototype data acquisition systems for these two techniques and conducted two field experiments. The results of the first experiment show that it is possible to trail a multichannel capacitive electrode array having a speed of 3.6 km/h and explore to a depth of 6 m. The second experiment demonstrated that EM tomography can map the underground resistivity structure using dry holes or boreholes lined with plastic casing.



Depth Transform of Seismic Data by Use of Equitraveltime Planes
By Y. AshidaThe present paper proposes a new algorithm for the threedimensional depth transform of the surface reflection, the crosswell reflection and VSP data by use of equitraveltime planes. The equation of equitraveltime planes of waves which travel from source to receiver through reflection point on reflector is derived. The derived equation shows the equation of ellipsoid of which foci are source and receiver. This means that a reflection point on reflector is located on ellipsoid. Ellipsoids are drawn for each pair of source and receiver. Consequently, reflector is determined by drawing a common tangent plane to these ellipsoids. The procedure of depth transform is as follows. 1) The velocity distribution within survey area is assumed. 2) The ellipsoids for each sample time in seismograms for each pair of source and receiver are drawn. 3) Reflector is determined as the common tangent plane to these ellipsoids. Here, reflector is automatical1y drawn as follows. 1) Survey area is divided into cubic cells of appropriate size. 2) An ellipsoid is drawn from seismogram for each pair of source and receiver by changing with one sampling interval from the designated time after the travel time of first break. 3) The intersecting points of the ellipsoid with the vertical lines of cells are shifted to the nearest cell corners. 4) Amplitudes for the corresponding sample times on seismograms are stacked on these cell corners. The amplitudes on the cell corners where the common tangent plane of ellipsoids passes are amplified by summation with inphase. However, the amplitude on the other grid cell corners would be canceled because of the summation with outphase. Judging from the model studies, it was clarified that this algorithm reconstructed the structure with good accuracy, the short calculation time and the small requirement for core memory.
