Importance

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Abstract

A method and apparatus for measuring ambient stress states and material properties in underground media includes a borehole probe having a cylindrical tube formed of soft, elastic polymer material secured about a central mandrel. An upper end cap assembly removably secures the probe to a service module to provide high pressure hydraulic fluid and sensor connections. A distal end cap seals the tube to the mandrel, so that hydraulic pressure causes diametrical expansion of the tube. The end cap includes an annular seal formed of elastic polymer material and helical springs that are embedded therein in the circumferential direction. The interiors of the helical springs are filled with steel pins or balls to prevent deformation of the springs. High strength fibers are bonded in the outer surfaces of the annular seal and oriented longitudinally to permit radial expansion of the seal assembly without hydraulic leakage or extrusion of the soft polymer of the cylindrical tube. An inner laminar layer comprised of high strength fiber extending circumferentially about the tube defines a datum plane extending through the axis of the tube, so that the tube is expandable only in one diametrical direction. An outer laminar layer of braided steel wire mesh limits longitudinal expansion of the tube and provides a high friction outer surface for the tube. A plurality of LVDT sensors are aligned with the direction of diametrical expansion and spaced longitudinally. High pressure hydraulic fluid expands the outer tube, to drive the high friction outer surface into the borehole wall, consolidating the borehole boundary. The fracture pressures at various angles are recorded, and analyzed to yield the principal stress vectors and material properties of the underground media.

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First Claim