To accurately measure a three-dimensional stress element acting on a ground constituting a crust. From the shape of a measurement cross section set in a direction perpendicular to the longitudinal direction of a boring core BC1 obtained by hollowing out the crust ground in the excavation direction of the well 1, the amount of distortion of the measurement cross section relative to the reference circle diameter is the largest. The length of the large diameter (Dmax) and the length of the diameter (Dmin) with the smallest amount of distortion with respect to the reference circle diameter are measured, and the ground is cut out from the inner surface of the well 1 in a direction different from the well 1. From the shape of the measurement cross section set in the direction orthogonal to the longitudinal direction of the sidewall core SC1 obtained in this way, the length of the measurement cross section in the vertical direction (d1 | φ = 0) and the horizontal diameter (d1 | φ = 90), the difference between the horizontal stress perpendicular to the longitudinal direction of the sidewall core SC1 and the vertical stress, and the equation expressing the differential stress between the maximum horizontal stress and the minimum horizontal stress acting on the ground From the stress expression, Obtaining a maximum horizontal stress and the minimum horizontal stress of. [Selection] Figure 4
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