A distributed acoustic sensor comprising: a first length of optical fibre, a source of electromagnetic radiation (112, 114), a detector apparatus and a processor; a second length of optical fibre running alongside the first length of optical fibre but separated therefrom in a first direction; wherein the source of electromagnetic radiation (112, 114) is configured to launch electromagnetic radiation into said first length optical fibre and said second length of optical fibre; the detector apparatus is for detecting electromagnetic radiation back-scattered from said first length of optical fibre and said second length of optical fibre; and the processor is configured to: process data corresponding to the detected back-scattered radiation to determine a measurement signal for each of a plurality of discrete longitudinal sensing portions of each of the first length of optical fibre and the second length of optical fibre; and analyse the measurement signal from said longitudinal sensing portions to identify signals corresponding to the same acoustic wave arriving at one or more longitudinal sensing portions of the first length of optical fibre and one or more longitudinal sensing portions of the second length of optical fibre; characterised in that the processor is further configured to determine, from the difference in time of arrival of said acoustic wave at said longitudinal sensing portions, the direction and/or distance of the origin of said acoustic wave from the optical fibre. a first length of optical fibre, a source of electromagnetic radiation (112, 114), a detector apparatus and a processor; a second length of optical fibre running alongside the first length of optical fibre but separated therefrom in a first direction; wherein the source of electromagnetic radiation (112, 114) is configured to launch electromagnetic radiation into said first length optical fibre and said second length of optical fibre; the detector apparatus is for detecting electromagnetic radiation back-scattered from said first length of optical fibre and said second length of optical fibre; and the processor is configured to: process data corresponding to the detected back-scattered radiation to determine a measurement signal for each of a plurality of discrete longitudinal sensing portions of each of the first length of optical fibre and the second length of optical fibre; and analyse the measurement signal from said longitudinal sensing portions to identify signals corresponding to the same acoustic wave arriving at one or more longitudinal sensing portions of the first length of optical fibre and one or more longitudinal sensing portions of the second length of optical fibre; process data corresponding to the detected back-scattered radiation to determine a measurement signal for each of a plurality of discrete longitudinal sensing portions of each of the first length of optical fibre and the second length of optical fibre; and analyse the measurement signal from said longitudinal sensing portions to identify signals corresponding to the same acoustic wave arriving at one or more longitudinal sensing portions of the first length of optical fibre and one or more longitudinal sensing portions of the second length of optical fibre;