Downhole fiber optic hydrophone

An apparatus for use in a borehole, comprising:

• an acoustic sensor including:
• a pressure transducer comprising a flexing member attached to an optical medium, the transducer configured to convert a low-amplitude, high-frequency acoustic pressure signal within a nominal borehole pressure incident on the transducer to an alternating strain on the optical medium along an axis of the medium via movement of the flexing member while suppressing conversion of low-frequency signals and constant pressure into strain of the optical medium, the acoustic pressure signal propagated via a downhole fluid coupled with the transducer, wherein the pressure transducer comprises an interface between reservoirs containing a reservoir fluid, the interface comprising the flexing member and at least one opening providing for communication of the reservoir fluid between the reservoirs;
• and
• a detector configured to generate acoustic measurement information responsive to received electromagnetic radiation transmitted through the medium along the axis, at least one optical property of the medium being responsive to the alternating strain on the medium such that the electromagnetic radiation received by the detector represents the acoustic signal.