A distributed fibre optic sensor apparatus (100; 600) comprising: a first fibre optic cable structure comprising a first optical fibre; a second optical fibre deployed along the path of the first optical fibre; an interrogator configured to perform distributed acoustic sensing on the first optical fibre by repeatedly interrogating the first optical fibre with coherent electromagnetic radiation and monitoring radiation which is Rayleigh backscattered from within the first optical fibre to provide a measurement signal from at least one sensing portion of the first optical fibre, the interrogator being further configured to perform distributed acoustic sensing on the second optical fibre to provide a measurement signal from at least one sensing portion of the second optical fibre so that at least one sensing portion of the first optical fibre has a corresponding sensing portion of the second optical fibre at substantially the same location; and a processor; wherein the first fibre optic cable structure is configured such that, for said at least one sensing portion of the first optical fibre, a sensitivity of the sensing portion to an incident pressure stimulus, in terms of the variation in measurement signal from a sensing portion in response to a given incident pressure stimulus, depends on the ambient pressure acting on first fibre optic cable structure at the location of the respective sensing portion; characterized in that the second optical fibre is configured such that a sensitivity of the second optical fibre to an incident pressure stimulus has a relationship with ambient pressure that is different to that of the first sensing fibre, and wherein the processor is configured to process the measurement signal from a sensing portion of the first optical fibre together with the measurement signal from the corresponding sensing portion of the second sensing optical fibre in response to the same incident pressure stimulus based on a predetermined sensitivity profile of how the sensitivity of said at least one sensing portion varies with ambient pressure to determine an indication of the absolute value of the ambient pressure at the respective sensing portion. a first fibre optic cable structure comprising a first optical fibre; a second optical fibre deployed along the path of the first optical fibre; an interrogator configured to perform distributed acoustic sensing on the first optical fibre by repeatedly interrogating the first optical fibre with coherent electromagnetic radiation and monitoring radiation which is Rayleigh backscattered from within the first optical fibre to provide a measurement signal from at least one sensing portion of the first optical fibre, the interrogator being further configured to perform distributed acoustic sensing on the second optical fibre to provide a measurement signal from at least one sensing portion of the second optical fibre so that at least one sensing portion of the first optical fibre has a corresponding sensing portion of the second optical fibre at substantially the same location; and a processor; wherein the first fibre optic cable structure is configured such that, for said at least one sensing portion of the first optical fibre, a sensitivity of the sensing portion to an incident pressure stimulus, in terms of the variation in measurement signal from a sensing portion in response to a given incident pressure stimulus, depends on the ambient pressure acting on first fibre optic cable structure at the location of the respective sensing portion; characterized in that the second optical fibre is configured such that a sensitivity of the second optical fibre to an incident pressure stimulus has a relationship with ambient pressure that is different to that of the first sensing fibre, and wherein the processor is configured to process the measurement signal from a sensing portion of the first optical fibre together with the measurement signal from the corresponding sensing portion of the second sensing optical fibre in response to the same incident pressure stimulus based on a predetermined sensitivity profile of how the sensitivity of said at least one sensing portion varies with ambient pressure to determine an indication of the absolute value of the ambient pressure at the respective sensing portion.