The effects of pH on acoustic transmission loss in an estuary

Abstract

Increasing atmospheric CO2 will cause the ocean to become more acidic with pH values predicted to be more than 0.3 units lower over the next 100 years. These lower pH values have the potential to reduce the absorption component of transmission loss associated with dissolved boron. Transmission loss effects have been well studied for deep water where pH is relatively stable over time-scales of many years. However, estuarine and coastal pH can vary daily or seasonally by about 1 pH unit and cause fluctuations in one-way acoustic transmission loss of 2 dB over a range of 10 km at frequencies of 1 kHz or higher. These absorption changes can affect the sound pressure levels received by animals due to identifiable sources such as impact pile driving. In addition, passive and active sonar performance in these estuarine and coastal waters can be affected by these pH fluctuations. Absorption changes in these shallow water environments offer a potential laboratory to study their effect on ambient noise due to distributed sources such as shipping and wind. We introduce an inversion technique based on perturbation methods to estimate the depth-dependent pH profile from measurements of normal mode attenuation. [Miller and Potty supported by ONR 322OA.]