Abstract: The application describes methods and apparatus for seismic monitoring using fiber optic distributed acoustic sensing (DAS). The method involves interrogating a first optical fiber (102) deployed in an area of interest to provide a distributed acoustic sensor comprising a plurality of longitudinal sensing portions of fiber and also monitoring at least one geophone (107) deployed in the area of interest. The signal from the at least one geophone is analyzed to detect an event of interest (105). If an event of interest is detected the data from the distributed acoustic sensor acquired during said event of interest is recorded. The geophone may be co-located with part of the sensing fiber and in some embodiments may be integrated (307) with the sensing fiber.

Abstract: A method for obtaining information about a hydraulic fracturing operation in a fracture zone in a well, comprises a) providing at least one acoustic sensor in the well and at least one acoustic source, b) injecting fracturing fluid into the well so as to cause fractures in a fracture zone in the surrounding formation, c) using the acoustic source to send an acoustic signal and using the acoustic receiver to receive the signal, d) repeating step c) at least once, and e) processing the received signals using a microprocessor so as to obtain information about the fractures. The source may be at the earth's surface or in a second well. Step e) may comprise measuring first-arriving acoustic waves or measuring reflected or diffracted acoustic waves. The information gained in step e) may be used to control the injection of fracturing fluid or detect out-of-zone water injection.

Abstract: The application describes methods and apparatus for seismic monitoring using fibre optic distributed acoustic sensing (DAS). The method involves interrogating a first optical fibre (102) deployed in an area of interest to provide a distributed acoustic sensor comprising a plurality of longitudinal sensing portions of fibre and also monitoring at least one geophone (107) deployed in the area of interest. The signal from the at least one geophone is analysed to detect an event of interest (105). If an event of interest is detected the data from the distributed acoustic sensor acquired during said event of interest is recorded. Th geophone may be co-located with part of the sensing fibre and in some embodiments may be integrated (307) with the sensing fibre.