A distributed optical fibre sensor for detecting changes in the environment of a sensing optical fibre , the sensor comprising: a probe light source arranged to generate probe light pulses; an optical detector arranged to receive probe light backscattered within the sensing optical fibre and to detect a coherent Rayleigh backscatter interference signal within the backscattered probe light; and an analyser arranged to determine, from the coherent Rayleigh backscatter interference signal, changes in the environment of the sensing optical fibre, wherein the determined changes in the environment are within an infrasonic frequency range, characterised in that : the determined changes in the environment comprise an inferred event, according to an event model available to or stored by the analyser, in the environment of the sensing optical fibre, and the analyser is arranged to make a comparison comprising matching one or more aspects of the detected coherent Rayleigh backscatter interference signal to the event model, based on a position of the inferred event relative to the sensing optical fibre, and to detect such an inferred event if the comparison is positive, wherein the event model is a model of a leak of fluid from a structure proximal to the sensing optical fibre , and the event model requires a characteristic of the matched coherent Rayleigh backscatter interference signal followed over time to reach a peak value wherein the peak value reached diminishes in magnitude with increasing distance from the inferred event. a probe light source arranged to generate probe light pulses; an optical detector arranged to receive probe light backscattered within the sensing optical fibre and to detect a coherent Rayleigh backscatter interference signal within the backscattered probe light; and an analyser arranged to determine, from the coherent Rayleigh backscatter interference signal, changes in the environment of the sensing optical fibre, wherein the determined changes in the environment are within an infrasonic frequency range, characterised in that : the determined changes in the environment comprise an inferred event, according to an event model available to or stored by the analyser, in the environment of the sensing optical fibre, and the analyser is arranged to make a comparison comprising matching one or more aspects of the detected coherent Rayleigh backscatter interference signal to the event model, based on a position of the inferred event relative to the sensing optical fibre, and to detect such an inferred event if the comparison is positive, wherein the event model is a model of a leak of fluid from a structure proximal to the sensing optical fibre , and the event model requires a characteristic of the matched coherent Rayleigh backscatter interference signal followed over time to reach a peak value wherein the peak value reached diminishes in magnitude with increasing distance from the inferred event. the determined changes in the environment comprise an inferred event, according to an event model available to or stored by the analyser, in the environment of the sensing optical fibre, and the analyser is arranged to make a comparison comprising matching one or more aspects of the detected coherent Rayleigh backscatter interference signal to the event model, based on a position of the inferred event relative to the sensing optical fibre, and to detect such an inferred event if the comparison is positive, wherein the event model is a model of a leak of fluid from a structure proximal to the sensing optical fibre , and the event model requires a characteristic of the matched coherent Rayleigh backscatter interference signal followed over time to reach a peak value wherein the peak value reached diminishes in magnitude with increasing distance from the inferred event. <EOS>