Absolute calibration of hydrophones immersed in sandy sediment

Abstract

An absolute calibration method has been developed based on the method of three-transducer spherical-wave reciprocity for the calibration of hydrophones when immersed in sandy sediment. The method enables the determination of the magnitude of the free-field voltage receive sensitivity of the hydrophone. Adoption of a co-linear configuration allows the acoustic attenuation within the sediment to be eliminated from the sensitivity calculation. Example calibrations have been performed on two hydrophones inserted into sandy sediment over the frequency range from 10 to 200 kHz. In general, a reduction in sensitivity was observed, with average reductions over the frequency range tested of 3.2 and 3.6 dB with respect to the equivalent water-based calibrations for the two hydrophones tested. Repeated measurements were undertaken to assess the robustness of the method to both the influence of the sediment disturbance associated with the hydrophone insertion and the presence of the central hydrophone. A simple finite element model, developed for one of the hydrophone designs, shows good qualitative agreement with the observed differences from water-based calibrations. The method described in this paper will be of interest to all those undertaking acoustic measurements with hydrophones immersed in sediment where the absolute sensitivity is important.

Innovation to Impact in a Time of Recession

“…141 These studies in turn led to new hydrophone calibration techniques (developed in collaboration with NPL). 142 The background work on methods to monitor oceanic bubble populations led to measurements of the interactions between atmosphere and oceans, particularly in the transfer of greenhouse gases between them; 143,144 and to sensors for use by the ceramics and neutron generation industries. [145][146][147] The study also led to a proposed explanation of how acoustic calls of humpback whales are used in feeding with bubble nets (proposed in 2004 114 and since developed, 115 and featured in TV Nature documentaries).…”

Modelling acoustic scattering, sound speed, and attenuation in gassy soft marine sediments

“…Accurate models of these effects are needed to predict the performance of the systems that might, in situ, involve propagating waves through gassy sediments (e.g. for sending data to and from buried transponders, for sub-bottom surveying [11] and for in situ calibration [12]). Accurate models are also needed if the measured attenuation and sound speed are to be inverted to estimate the gas bubble population present in the sediment.…”