This thesis presents methods for optical frequency-domain sensing of the strain on a fibre for use in a range of geophysical purposes. The Long Period Fibre Grating is modelled in detail to ascertain its viability as a hydrophone instrument, requiring very high sensitivity. Uncertainty analysis of such gratings illustrates that this will be difficult with available fibres. Other configurations such as down-hole pressure sensors are possible for these gratings, and several other sensing mechanisms