For the practical measurement of crustal stresses at great depths more than 1,000m from the ground surface, we have been promoting research and development of borehole-jack fracturing technique. The principle of this technique is described with the relations between jack pressure and stress-state around the borehole. Specifically, when a borehole wall is loaded by a borehole-jack, a pair of new fractures will be induced oppositely in parallel to the borehole axis. After unloading, if the same place on the borehole wall is loaded again by a jack, the pair of fractures will be opened again. Two principal stresses and the orientation of crustal stress in the plane perpendicular to the borehole axis are determined by the re-opening pressures and the orientation of the fractures respectively. This technique is similar to hydraulic fracturing from the viewpoint of analyzing principle. Hence, there is no theoretical limit to the depth of measurement. The features of this technique are that it is possible to produce a pair of axial fractures in arbitrarily direction, possible to measure a displacement of the fracture opening, and, as a result, it is possible to determine the re-opening pressure accurately. On this field tests, the fractures induced with hydraulic fracturing method were re-opened by the borehole-jack in the range of pressure value similar to hydraulic fracturing. This paper describes our results of numerical analyses, laboratory experiments, and field tests.
In the earth science field, the distribution of the stress in the wide area at great depth more than 1,000m is required in order to determine the crustal stress around the epicentral area. On the other hand, the distribution of initial stress near by the structure shallower than 1,000mis required for civil engineering purpose. Both applications require high resolution measurement, of course.