Hydrophone performance in sediment

In situ measurements of acoustic signals in marine sediment are often performed using hydrophones which have been designed for use in water. Typically, these hydrophones are characterised for transmit or receive sensitivity in water. When the hydrophone is submerged in a medium other than water, the sensitivity (both amplitude and phase response) of the hydrophone, and its resonant characteristics, can be dramatically affected as a result of the differences in acoustic impedance of the medium and the different coupling to the medium. To investigate these changes, a series of measurements of electrical impedance and receive sensitivity were performed on a hydrophone in fine sand sediment using a novel method which does not require a priori knowledge of the absorption in the medium. The initial results of this investigation are presented in this paper demonstrating the change in the hydrophone characteristics when used in sediment at frequencies above 40 kHz, and the factors affecting hydrophone performance in sediment are discussed.