A system for surveying a wellbore, the system comprising: a distributed optical fibre sensor having: a) a sensing optical fibre deployed along the wellbore, the wellbore having a plurality of respective acoustic propagation regions respectively corresponding to two or more of: an interior of an inner pipe; a wall of the inner pipe, an annulus surrounding the inner pipe, and/or a casing surrounding the annulus; b) a light source arranged to send optical pulses along the sensing optical fibre; and c) means for processing light reflected and/or backscattered from the optical pulses as they travel along the optical sensing fibre to measure acoustic signals incident along the length of the sensing optical fibre in order to provide distributed acoustic measurements from along the length of the sensing fibre; the system further comprising a processor arranged to: i) process the distributed acoustic measurements from along the length of the sensing fibre to obtain a plurality of distributed speed of sound measurements along the length of the sensing fibre respectively relating to the plurality of respective acoustic propagation regions; and ii) analyse variations in the plurality of distributed speed of sound measurements and determine respective acoustic modes corresponding to the plurality of respective acoustic propagation regions in dependence thereon to thereby derive information relating to any of a surrounding formation, fluid in the wellbore, and/or the condition of the wellbore in dependence on the determined respective acoustic modes, wherein the analysis comprises inverting a wellbore model against the plurality of distributed speed of sound measurements in order to determine a value of one or more unknown parameters in the wellbore model. a distributed optical fibre sensor having: a) a sensing optical fibre deployed along the wellbore, the wellbore having a plurality of respective acoustic propagation regions respectively corresponding to two or more of: an interior of an inner pipe; a wall of the inner pipe, an annulus surrounding the inner pipe, and/or a casing surrounding the annulus; b) a light source arranged to send optical pulses along the sensing optical fibre; and c) means for processing light reflected and/or backscattered from the optical pulses as they travel along the optical sensing fibre to measure acoustic signals incident along the length of the sensing optical fibre in order to provide distributed acoustic measurements from along the length of the sensing fibre; a) a sensing optical fibre deployed along the wellbore, the wellbore having a plurality of respective acoustic propagation regions respectively corresponding to two or more of: an interior of an inner pipe; a wall of the inner pipe, an annulus surrounding the inner pipe, and/or a casing surrounding the annulus; b) a light source arranged to send optical pulses along the sensing optical fibre; and c) means for processing light reflected and/or backscattered from the optical pulses as they travel along the optical sensing fibre to measure acoustic signals incident along the length of the sensing optical fibre in order to provide distributed acoustic measurements from along the length of the sensing fibre; the system further comprising a processor arranged to: i) process the distributed acoustic measurements from along the length of the sensing fibre to obtain a plurality of distributed speed of sound measurements along the length of the sensing fibre respectively relating to the plurality of respective acoustic propagation regions; and ii) analyse variations in the plurality of distributed speed of sound measurements and determine respective acoustic modes corresponding to the plurality of respective acoustic propagation regions in dependence thereon to thereby derive information relating to any of a surrounding formation, fluid in the wellbore, and/or the condition of the wellbore in dependence on the determined respective acoustic modes, wherein the analysis comprises inverting a wellbore model against the plurality of distributed speed of sound measurements in order to determine a value of one or more unknown parameters in the wellbore model. i) process the distributed acoustic measurements from along the length of the sensing fibre to obtain a plurality of distributed speed of sound measurements along the length of the sensing fibre respectively relating to the plurality of respective acoustic propagation regions; and ii) analyse variations in the plurality of distributed speed of sound measurements and determine respective acoustic modes corresponding to the plurality of respective acoustic propagation regions in dependence thereon to thereby derive information relating to any of a surrounding formation, fluid in the wellbore, and/or the condition of the wellbore in dependence on the determined respective acoustic modes, wherein the analysis comprises inverting a wellbore model against the plurality of distributed speed of sound measurements in order to determine a value of one or more unknown parameters in the wellbore model.