地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
VSP法による地殻構造探査
栞原 保人伊藤 久男
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1995 年 104 巻 7 号 p. 1008-1018

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Subsurface exploration techniques using vertical seismic profile (VSP) are reviewed mainly on their principles and applications. VSP gives high-resolution and accurate images of seismic structure around a VSP well. The VSP measurement involves subsurface recording of seismic waves from a surface seismic source by means of geophones clamped in a drilled well. The VSP data set thus has the total seismic wavefields with downgoing waves (direct waves from the surface source and downgoing multiples) and upgoing waves (reflected waves toward the surface and upgoing multiples). The high-resolution images of subsurface reflectors produced by VSP data processing are particularly useful for reliable knowledge of fault positions around the VSP well. The depth accuracy of subsurface reflectors given by VSP is valuable in interpretation of surface-seismic data. The seismic tomography of velocity structure using only VSP data set gives a limited application because of its irregular and limited coverage of ray paths. The combination of VSP and surface-seismic data, however, provide a detailed image of reflector depths and velocity structure, based on an inversion formulation for seismic reflections. The VSP is also applied for delineating subsurface fracture systems and for evaluating fracture permeability. Measurements of shear-wave polarization anisotropy using downgoing shear-waves in VSP data gives clues to understand subsurface fractures. VSP observation of particular waves, say tube waves, generated by fluid-filled fractures is a promising method for a measurement of fracture permeability. Thus, the VSP technique is not only for imaging subsurface morphology but also for delineating rock properties including fractures.

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