An apparatus for detecting changes in a pressure distribution along an extended fluid path, the apparatus comprising: an optical fibre cable comprising an optical fibre ; a light source arranged to launch optical pulses into the optical fibre of the optical fibre cable; a polarisation detector for detecting polarisation states of light backscattered from within the optical fibre of the optical fibre cable; and an analyser which in combination with the polarisation detector is configured to determine linear retardance of the backscattered light and thereby determine a spatial variation in linear birefringence along the length of the fibre so as to detect said changes in the pressure distribution along the length of the optical fibre cable, the apparatus being characterised in that , the optical fibre is standard single mode optical fibre such as SMF-28, the optical fibre comprising a core and a cladding; and the optical fibre cable further comprising a buffer surrounding the optical fibre and having a radial thickness that varies with orthogonal symmetry, the buffer comprising one or more longitudinal regions (A) of a first thickness and one or more longitudinal regions (B) of a second thickness greater than the first thickness, the longitudinal regions of the first thickness are arranged to deform more than the regions of the second thickness when an isotropic pressure is exerted on the cable, thereby the buffer is adapted to deform asymmetrically under isotropic pressure such that the optical fibre experiences asymmetric strain, which changes the birefringence of the optical fibre in response to changes in the isotropic pressure, the buffer having an orthogonally asymmetric cross-section in a plane transverse to the fibre axis. an optical fibre cable comprising an optical fibre ; a light source arranged to launch optical pulses into the optical fibre of the optical fibre cable; a polarisation detector for detecting polarisation states of light backscattered from within the optical fibre of the optical fibre cable; and an analyser which in combination with the polarisation detector is configured to determine linear retardance of the backscattered light and thereby determine a spatial variation in linear birefringence along the length of the fibre so as to detect said changes in the pressure distribution along the length of the optical fibre cable, the apparatus being characterised in that , the optical fibre is standard single mode optical fibre such as SMF-28, the optical fibre comprising a core and a cladding; and the optical fibre cable further comprising a buffer surrounding the optical fibre and having a radial thickness that varies with orthogonal symmetry, the buffer comprising one or more longitudinal regions (A) of a first thickness and one or more longitudinal regions (B) of a second thickness greater than the first thickness, the longitudinal regions of the first thickness are arranged to deform more than the regions of the second thickness when an isotropic pressure is exerted on the cable, thereby the buffer is adapted to deform asymmetrically under isotropic pressure such that the optical fibre experiences asymmetric strain, which changes the birefringence of the optical fibre in response to changes in the isotropic pressure, the buffer having an orthogonally asymmetric cross-section in a plane transverse to the fibre axis. <EOS>