Distributed acoustic sensing system (100), for coupling with an optical sensing fiber (110), which has a selectable fiber length and a down-and-up travel time for an interrogating light to travel down the fiber length and for backscattered light to travel up the fiber length, the down-and-up travel time being associated to the fiber length. The distributed acoustic sensing system (100) has: a coherent light source device (101) configured to repeatedly generate and send into an optical sensing fiber (110) a coherent interrogating light pattern (307, 507, 607, 707), which is out of a plurality of coherent interrogating light patterns and composed of coherent carrier light, during a measurement time duration, which is equal to or greater than one times the down-and-up travel time of the optical sensing fiber (110). The device (100) further has a detection device (114, 116) configured to detect over time light that is backscattered in the optical sensing fiber (110) in response to the repeatedly sent coherent interrogating light patterns and to generate and output a signal that is indicative of the detected backscattered light. The system (100) further has an evaluation device (118) configured to analyse the signal output from the detection device (114, 116). Each coherent interrogating light pattern (307, 507, 607, 707) out of the plurality of coherent interrogating light patterns has similar physical properties, wherein the similarity is in the sense that the light patterns cannot be distinguished on the basis of comparing the physical properties of the backscattered light generated from at least two successive interrogating light patterns (507, 607, 707) as such as detected. The coherent light source device (101) is capable to change the time period between successively sent the first coherent interrogating light patterns (507, 607, 707) after the passing of at least one measurement time duration.
DISTRIBUTED ACOUSTIC SENSING DEVICE USING DIFFERENT COHERENT INTERROGATING LIGHT PATTERNS, AND CORRESPONDING SENSING METHOD