Very shallow seismic reflection method using a portable vibrator:present status and future potential in application to characterization of an active fault

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The success of a trenching study is highly dependent on the selection of the trenching site. If there is a geophysical technique to image shallow subsurface structure without excavation, it can be a useful tool for site selection of the trenching point. Moreover, if the quality of the subsurface image obtained by the technique is high enough to estimate the characteristics of an active fault, it can be much more useful for the active fault study itself.<BR>From this point of view, we have starte d to apply the shallow seismic reflection method using a portable vibrator for imaging very shallow subsurface structure. The electro-dynamically driven portable vibrator is a high frequency seismic source which can generate seismic waves with more than 1 kHz. This paper describes the outline of the portable vibrator system and two feasibility studies using the system. The first feasibility study was conducted for the purpose of detection of pieces of styrene foam buried at known shallow depths. These targets were clearly imaged as the diffraction pattern at depths of 1.5m and 2.0m by using the system. The second one was conducted near the trenching site at Nashimoto on the Nojima earth quake fault for characterization of the very shallow part of the fault. The seismic reflection profile obtained at this site clearly showed displacements of the subsurface layers shallower than 10m, which might be caused by faulting. These two feasibility studies described here revealed that shallow seismic reflection method using the portable vibrator system was one of the potential methods for investigating an active fault characteristics.

Very shallow seismic reflection method using a portable vibrator:present status and future potential in application to characterization of an active fault

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