Hydraulic release system for use in a borehole apparatus

Apparatus for positioning a detector at a plurality of depth locations within a borehole includes an elongated body member having a pair of diametrically opposed hydraulic pad displcement members mounted thereon. A pad mounting member is pivotally attached to each pad displacement member and a pair of borehole wall contacting members are pivotally attached to each pad mounting member. A signal detector is mounted behind one or more of the pad members. At each test depth a hydraulic power system deploys the pad displacement members from the body to an extended position and extends the wall engaging pad members into contact with the borehole wall. A redundant hydraulic safety system is provided in case of electrical system failure for releasing the pad member. The safety system includes, in addition to an electrically operated release valve, a clock operated release valve and a tension actuated release valve, all able to release hydraulic pressure on the displacement members in case of the failure of another, allowing withdrawal of the apparatus from the borehole.