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| | Abstract | | We have developed a portable borehole seismic source (Controlled Rotational Borehole Vibrator: CRBV) for shallow subsurface survey, particularly for estimating a high-resolution and high-accuracy S-wave velocity structure using S-wave tomography. Eccentric rotators driven by two servo-motors operated independently generate seismic waves. Combination of clockwise and counter-clockwise rotation of the rotators realizes a linear vibration in any direction, resulting in generating S-waves propagating normal to the vibration. Repeated sweep rotation enhance the S/N ratio of the transfer function from the source to the receivers. Basic operation tests were conducted at a field with a borehole in December 2014. The signal was observed by the seismic station at 50 m from the source even in a single sweep. The signal was detected at the distance of 200 m after stacking. The field test shows a satisfactory result in terms of the basic performance as designed. However, it also revealed some severe problems that is need to be solved and ideas to improve the accuracy and operation depth. |
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