There is provided a system (100) for magnetic field detection, comprising a fibre optic interrogator (104) adapted to interrogate a first length of optical fibre (102) with interrogating radiation, detect radiation backscattered from said optical fibre and analyse said detected radiation to provide distributed sensing indicative of mechanical disturbances of said optical, wherein the optic fibre is mechanically coupled to a material whose dimensions vary dependant on applied magnetic field. Changes in dimensions of the optic fibre as can be detected by virtue of changes in back-scattering of light from said fibre using the principles of fibre optic distributed acoustic sensing.
