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The 6th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2002)

  • Conference date: 28 Jan 2002 - 28 Jan 2002
  • Location: Kyoto, Japan
  • Published: 28 January 2002
4 results

    • 3-D Tomographic Images and Deep Ore-Control in Nothern Margin of the North China Plate

      Authors X. Chang and Y. Liu
      https://doi.org/10.3997/2352-8265.20140032

      3-D velocity images of the crust beneath the northern margin of the North China Plate have been constructed using P-wave travel time residuals of the latest earthquakes, with the data supplied by Chinese seismic networks. The seismic image results indicate that there is a lateral heterogeneity in the crust beneath the northern part of the North China block. The velocity images of the upper crust show features closely related to the tectonic features on the surface. It can be seen from these velocity images of the vertical sections, and from the horizontal slice images at depths of 11km and 16km that there exist East-West and North-East structures. The images indicate that the juncture zone of basin–and–range terrain is between the blue high-velocity block corresponding to the Yanshan mountain range that developed during the Yanshan period in northwest Beijing and the green low-velocity area corresponding to the North China basin in Southeast Beijing. The juncture zone between high-velocity and low-velocity and NE-EW fault zones have significant ore-control effects. From the chart of epicenters in the northern region of North China, we find that the epicenters of earthquakes are almost entirely distributed within the NE strip. Almost all major earthquakes took place in the transition strip between the high and low-velocity zones in the crust. The distribution of epicenters also reflects the strikes of known NE-faults. From the longitudinal image sections, we find that in the area between 114.0°E - 118.0°E, there is a blue high-velocity block standing upright from the Moho to the upper crust, from which can be deduced that some materials such as magma moved upward from the upper mantle during the history of its geological development.

    • A Feasibility Study of a Seismic Survey Along a Mountain Ridge Using Acoustic Wave Simulator

      https://doi.org/10.3997/2352-8265.20140033

      Oil and gas explorations in mountainous regions are becoming more　important to find unexplored possible reserves. Here, the seismic survey technique is a major exploration tool as well. However, difficulties often come from complicated topography change. For evaluating the topographic effects, we conducted the feasibility study for a 2D seismic survey in a particular mountainous area. In our study area, the locations where we can put sources or receivers are restricted, and it is possible to lay out dense sources and receivers on only one mountain road. Previously, reflection from mountainside or diffractions from topographic kink point were worried to damage reflection signals from subsurfaces. In this study, 3D acoustic wave simulation was conducted with the realistic topography model, and the calculated wavefield was examined. As the result, several different types of topography-related waves are identified. However, judging from this result and the other conditions, the seismic survey on a mountain ridge could be acceptable. And the acoustic simulator can be largely useful to the survey planning even though there were some limitations on the method.

    • 3-D Velocity Images in Near Surface

      Authors X. Chang and Y. Liu
      https://doi.org/10.3997/2352-8265.20140034

      A 3-D tomographic inversion approach for reconstructing 3-D near-surface model is presented in this paper. Direct, reflected and turning waves are used simultaneously to update the near-surface model. We analyze the characteristics of the first-break traveltime in complicated low-velocity layers. To improve the accuracy for the velocity model, the various first-break times from direct, reflected and refracted waves are considered for model inversion. A fractal algorithm is applied to pick first breaks, which overcomes the error caused by wavelet shape differences, and the leg-jump, of refractions. The method is capable of picking a large volume of first breaks automatically. The ray paths and traveltimes are calculated with a 3-D ray tracer which does not increase computation time for complicated geological models. Our method can determine the ray path associated with minimum traveltimes regardless of the wave mode (direct, refracted, or reflected waves). We use a least-squares approach in conjunction with QR decomposition to reconstruct a 3-D near-surface velocity model from the actual first-break times obtained from 3-D data.

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