A distributed optical fibre sensor for determining a measure of acoustic vibration as a function of position along a sensing fibre from properties of probe light backscattered within the sensing fibre, the sensor comprising: a probe light source arranged to launch probe light pulses (22, 24) into the sensing fibre; a detector arranged to detect probe light backscattered in said sensing fibre; and an analyser arranged to determine said measure of acoustic vibration as a function of position along the sensing fibre from said detected backscattered probe light, characterised in that: the probe light pulses include successive groups each of two or more probe light pulses having different optical wavelengths to each other, the probe light source being arranged to launch the probe light pulses such that backscattered light from all the pulses of each group coexists within the sensing fibre; the detector is arranged to separately and simultaneously detect light of each of said different wavelengths; the probe light source is arranged to interleave launching of the probe light pulses (22, 24) of different wavelengths, and the analyser is arranged to construct, for a plurality of positions along the sensing fibre, a time series of said measure of acoustic vibration by interleaving data derived from the detected backscatter of probe light for each of said different wavelengths, the interleaving providing an enhancement of the acoustic frequency range of the sensor; and the detector comprises a plurality of photodetector elements, and the sensor is arranged such that each photodetector element detects light of a different one of the probe light pulses in each group. a probe light source arranged to launch probe light pulses (22, 24) into the sensing fibre; a detector arranged to detect probe light backscattered in said sensing fibre; and an analyser arranged to determine said measure of acoustic vibration as a function of position along the sensing fibre from said detected backscattered probe light, characterised in that: the probe light pulses include successive groups each of two or more probe light pulses having different optical wavelengths to each other, the probe light source being arranged to launch the probe light pulses such that backscattered light from all the pulses of each group coexists within the sensing fibre; the detector is arranged to separately and simultaneously detect light of each of said different wavelengths; the probe light source is arranged to interleave launching of the probe light pulses (22, 24) of different wavelengths, and the analyser is arranged to construct, for a plurality of positions along the sensing fibre, a time series of said measure of acoustic vibration by interleaving data derived from the detected backscatter of probe light for each of said different wavelengths, the interleaving providing an enhancement of the acoustic frequency range of the sensor; and the detector comprises a plurality of photodetector elements, and the sensor is arranged such that each photodetector element detects light of a different one of the probe light pulses in each group. the probe light pulses include successive groups each of two or more probe light pulses having different optical wavelengths to each other, the probe light source being arranged to launch the probe light pulses such that backscattered light from all the pulses of each group coexists within the sensing fibre; the detector is arranged to separately and simultaneously detect light of each of said different wavelengths; the probe light source is arranged to interleave launching of the probe light pulses (22, 24) of different wavelengths, and the analyser is arranged to construct, for a plurality of positions along the sensing fibre, a time series of said measure of acoustic vibration by interleaving data derived from the detected backscatter of probe light for each of said different wavelengths, the interleaving providing an enhancement of the acoustic frequency range of the sensor; and the detector comprises a plurality of photodetector elements, and the sensor is arranged such that each photodetector element detects light of a different one of the probe light pulses in each group.