PROBLEM TO BE SOLVED: To realize a method for investigating the peripheral bed rock of a cavity with high accuracy. SOLUTION: As a measuring hole of representative cross-section in a bed rock cavity 1 using elastic wave tomography, an oblique boring hole 4 is arranged to surround the bed rock cavity 1 squarely as closely as possible thereto. All shaking points 6 and receiving points 5 provided in the oblique boring hole 4 are arranged at a constant interval with a shaking point 6 or receiving point 5 being located, without fail, at the corners of a rhombus defined by the oblique boring hole 4 arranged alternately with twenty seven shaking points 6 and twenty seven receiving points 5 surrounding the bed rock cavity 1. A receiving point 5 is located at a corner opposite to a corer located with a shaking point 6. Furthermore, receiving points and shaking points are arranged additionally at the intersection of a dislocation and the oblique boring hole 4.
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