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The 3rd International Symposium on Recent Advances in Exploration Geophysics (RAEG 1998)

  • Conference date: 14 Dec 1998 - 15 Dec 1998
  • Location: Kyoto, Japan
  • Published: 14 December 1998
8 results

    • Three Dimensional Velocity Images of the North-South Zone in China with LSQR Algorithm

      Authors Y. Liu and X. Chang
      https://doi.org/10.3997/2352-8265.20140015

      In this paper three dimensional velocity images of the crust and upper mantle beneath the North South Zone are reconstructed using 10277 P wave arrivals of regional earthquakes recorded at 157 stations in the study area with LSQR algorithm. The image results indicate that: 1. There is a significant lateral heterogeneity in the crust and upper mantle beneath the North South Zone persisting down to 450 km in depth. 2. The velocity images of the upper crust show the features closely related to the well known tectonic features on the surface. 3. The low velocity layer exists in a very wide range of the mid-crust. The lowest velocity value is near 5.60 km/s. 4. in a long narrow piece of area between 25°N-38°N and 100.0° E-I03.2°E, the low velocity anomaly about 7.49 km/s appears at the top of the upper mantle. The image result at a depth of 120 km also outlines dearly the welded boundaries between the different geological blocks. The statistical data show that seismic activity is related to the velocity structure of the North South Zone: almost all of the major earthquakes took place in the transition strips between high and low velocity zones in the crust above 20 km.

    • Fresnel Zone Wavepath Tomography - Theory

      https://doi.org/10.3997/2352-8265.20140017

      In this paper, a developing wavepath based on diffraction tomography in the space-time domain is presented. This wavepath is named Fresnel zone wavepath, derived from the imaginary part of Rhytov approximation. This wavepath is calculated using travel times from source to any point, receiver to any point and source to receiver. The Fresnel zone wavepath is a frequency dependent function. While the frequency is higher, the Fresnel zone wavepath oscillates more frequently than when the frequency is lower. In the high frequency, Fresnel zone wavepath becomes similar to the ray path. In the application of Fresnel zone wavepath to inversion tomography, a ray tracing procedure is not needed. A case of sparse data showed that the inversion result using Fresnel wavepath is better than one using the ray path. We also propose several techniques to enhance the inversion based on Fresnel zone wavepath. These include first Fresnel zone restriction, hybrid inversion and polychromatic wavepath. From the model studies, we show that the above mentioned techniques provide better inversion result.

    • Improvement of Data Quality in Seismic While Drilling

      Authors T. Tsuru and T. Yokota
      https://doi.org/10.3997/2352-8265.20140018

      Seismic while drilling (SWD) is a seismic survey technology using a working drill-bit as an energy source. We can specify bit location on the existing seismic time section and can predict a drilling hazard ahead of the bit, ‘while drilling’, by using SWD technology. Vertical Seismic Profiling (VSP) also can be used for the same purpose. In comparison with VSP, advantages of SWD are its survey cost and quick availability of the survey result, such as T-D (Time-Depth) information. However, a serious disadvantage exists in the data quality due to contamination by noise generated from the working rig. Furthermore, there is a limitation to applicability on the SWD technology currently in use, when drilling with a PDC (Polycrystal-line Diamond Compact) bit or drilling in unconsolidated rocks. In this paper, we proposed a data acquisition method, a data processing method and a new energy source system for improving the quality of the SWD data. Efficacy of those methods and the system were demonstrated by field experiments.

    • New Art for Crosshole Electromagnetic Tomography

      Authors C. Junxing, Z. Nie and J. Zhu
      https://doi.org/10.3997/2352-8265.20140019

      Crosshole Electromagnetic Tomography (CEMT) is a powerful geophysical exploration technique. Two new ray EM tomography strategies are proposed in this paper, one is named Average Multi-Frequency Absorption Tomography (AMFAT) and another DuaI-Frcquency Conductivity Tomography (DFCT). In AMFAT, the absorption tomography is done using the average of the data of different frequencies. The averaged data should be that measured at same configuration. In DFCT, the conductivity tomography is done using the following equation $$ E_1 - E_2 = 20 ¥log ( ¥sqrt{f_1 / f_2 } ) + 8.6859 ¥sqrt{¥pi ¥mu } ( ¥sqrt{f_1} - ¥sqrt{f_2} ) ¥int{ ¥hat{¥sigma} (r) dr} $$ where $E_1$, $E_2$ are electric field intensities (unit: dB) measured at same geometry configuration in far field, $f_1$, $f_2$ are frequencies, $¥hat{¥sigma} = ¥sqrt{¥sigma}$, $¥hat{¥sigma }$ is the conductivity of the rnedia detected. DFCT is suitable for the medium with high conductivity and AMFAT is suitable for the medium with low conductivity. AMFAT can increase the S/N ratio. DFCT can markedly eliminate the image distortion rising from the approximation of the directional factor of dipole antenna used by that of a half wave-length dipole antenna.

    • Magnetic Susceptibility Observation Across Himalaya-Karakoram Range, Northern Pakistan

      Authors M.N. Ahmad and M. Yoshida
      https://doi.org/10.3997/2352-8265.20140020

      Low field magnetic susceptibility was observed for the rocks exposed along Karakoram Highway, northern Pakistan, traversing from Hasan Abdal town, north western margin of Indo-Pakistan subcontinent to Ghunjerab river, southern part of the Karakoram block which is a part of Eurasian continent. The Kohistan block between these two terranes has relatively higher values of magnetic susceptibility values Koenigsberger ratio shows that gabbro-norite layering in the Kohistan block might has significant effect of natural remanent magnetization on geomagnetic anomalies. Most of granitic rocks of the block are correlated with ilmenite series showing low magnetic susceptibility while granotoids in northern part of the Kohistan block and Indo-Pakistan subcontinent belong to magnetite series. In the Kohistan block, pre-collisional granites are of magnetite series, while in the case of Indo-Pakistan subcontinent the granitic rocks yielding old age (e.g. 550 Ma) are of magnetite series and granitic rocks with younger radiometric age correspond to ilmenite series.

    • Three-Dimensional Magnetotelluric Inversion Using Generalized RRI Method

      https://doi.org/10.3997/2352-8265.20140021

      To handle the real geological environment, 3-D MT inversion studies has been developed by many researchers (Madden et al, 1989; Mackie et al, 1993). The method to calculate 3-D Frechte derivative takes enormous computational time and computer memory, while the RRI method (Smith and Booker, 1991), the approximate one, provides reasonable solution with less computing resource. But it may be liable to the static shift. The method discussed here is expanding the RRI method into the local 3-D area. We have developed a generalized method based on a locally three dimensional analysis to improve the computational efficiency and stability of inversion. The developed algorithm has been realized in the GRRI3D code, which has been tested on synthetic 3-D MT data. The case study includes interpretation of 3-D MT su1'vey conducted in the Minami-kayabe area located in the southern part of Hokkaido, Japan.

    • Reverse Time Migration for Ground Penetrating Radar Using Finite Difference Time Domain Method

      Authors Y. Sanada and Y. Ashida
      https://doi.org/10.3997/2352-8265.20140022

      The finite difference time domain (FDTD) method is calculated by stepping finite approximation scheme for two Maxwell's curl equations. Since it allows arbitrary electrical conductivity and permittivity variations within a model, the FDTD method has become one of the powerful forward modeling methods for electromagnetic (EM) phenomena. The reverse time migration, which is performed by inserting the recorded data as a boundary conditions at each recorder position in reverse time order, is one of the imaging algorithms. In this paper, the reverse time migration for the ground penetrating radar (GPR) data is formulated using FDTD scheme. In a lossless media case, the method is successfully demonstrated to synthetic data for two models: steeply dipping structure and point diffractors model. In a lossy media case, the forward scheme includes diffusion term, while the reverse time scheme includes divergence term. In such a case, when the EM wave velocity is regarded as constant, this methodology is applicable successively. We discussed the reverse time migration under the lossless media condition for the lossy media after the amplitude recovery.

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