Revisiting experimental methods for studies of acidity-dependent ocean sound absorption

Abstract

The practical usefulness of long-range acoustic measurements of ocean acidity-linked sound absorption is analyzed. There are two applications: Determining spatially-averaged pH via absorption measurement and verifying absorption effects in an area of known pH. The method is a differential-attenuation technique, with the difference taken across frequency. Measurement performance versus mean frequency and range is examined. It is found that frequencies below 500 Hz are optimal. These are lower than the frequency where the measurement would be most sensitive in the absence of noise and signal fluctuation ͑scintillation͒. However, attenuation serves to reduce signal-to-noise ratio with increasing distance and frequency, improving performance potential at lower frequencies. Use of low frequency allows longer paths to be used, with potentially better spatial averaging. Averaging intervals required for detection of fluctuations or trends with the required precision are computed.

Depth-profiling ambient noise in the deep ocean.

“…Using the surface generated winds as a constant source of ambient noise, a relationship between depth, spectral slope over the wind noise band and the frequency variation of the sound absorption is developed. Sound absorption between 1 -10 kHz is driven by viscous and chemical relaxation (Francois and Garrison, 1982a;b) and is a strong function of pH (Mellen et al, 1983;Duda, 2009). A simple best-fit inversion of the measured frequency variation of sound absorption over the wind driven noise band was used to estimate the mean water column pH at 7.9, in reasonable agreement with insitu measurements in the Philippine Sea (Chen and Huang, 1996) II.…”

“…Active acoustic measurements of absorption have had a renewed interest because of the connection between the acidity of the ocean and the absorption of sound below 10 kHz (Hester et al, 2008;Duda, 2009). Hydroxide OHand carbonate CO 3 2ions are involved in pressure-sensitive reactions, which modify the phase lag between pressure and velocity in sound waves and cause the relationship between ion concentrations and energy loss (absorption).…”