Housing on the exterior of a well casing for optical fiber sensors

At least one fibre optic detector is coupled to an external circumference of the lining and detects a stress applied to the lining along a detection section. A casing (9) fixed to the outside of the lining forms a closed cavity around the detection section. This casing encloses the fibre optic detector(s) to isolate the detection section from the external environment : The lining (1) is concentric about a central axis. The casing has a crack between the detector and the casing. The fibre optic detector is a reel (7) of fibre optic, defined by a pair of Bragg fibre nets. The reel is extended in a direction parallel to the central axis of the lining, or in a helix about the central axis, or wound round the upper circumference and is concentric with the central axis. The fibre optic detector is a Bragg fibre net. The casing is filled with a gas or a vacuum is established in the box. The system contains a number of fibre optic detectors which are multiplexed along a single optical trajectory. An independent claim is also included for a method of detecting stress in a lining, by isolating at least one detector which detects stress applied to the lining (1) along a detection section by fixing a casing (9) to the lining on opposing ends of the detection section. The casing forms a closed cavity with the lining and isolates the detection section from the external environment. The detector is enclosed in the cavity and a signal indicative of stress is detected. A further INDEPENDENT CLAIM is included for a method of measuring a microfracture inside a lining in a drill shaft, using at least one detector coupled to the outside of the lining, detecting acoustic waves crossing the lining and possessing a frequency continuity of at least 10kHz using the detector(s). The acoustic waves are analyzed to determining the presence of a microfracture acoustic wave and producing a signal indicating the microfracture in the lining. In addition, the location of the point generating the microfracture signal is determined.