The feasibility of measuring ocean pH by long‐range acoustics

Abstract

Low frequency (<1000 Hz) sound absorption in the ocean depends exponentially on ocean pH. While the absolute amplitudes measured in long‐range (order of 1000 km) acoustic transmission experiments depend in a complex manner on ocean structure and are difficult to interpret without ambiguity, measurements of differential absorption as a function of frequency contain significant information. For surface‐reflected rays (RSR) there is the additional factor of frequency‐dependent reflectivity. In principle, a comparison of transmitted and received acoustic spectra can be inverted to obtain ocean pH and surface roughness. Calculations performed with preliminary data from a 750‐km path in the North Pacific give qualitative agreement with theory (higher frequencies are attenuated more than lower frequencies) but are inadequate for quantitative comparisons (the experiment was not optimized for differential attenuation measurements). Our conclusion is that it is possible, but not easy, to obtain quantitative information on ocean pH and surface roughness from measurements of differential attenuation with frequency along resolved ray paths. To obtain an accuracy of 0.05 in pH with a 750‐km transmission at 550±100 Hz would require about 100 independent samples for the North Pacific (pH ≈ 7.7) and 60 independent samples for the Atlantic (pH ≈ 8.0) in order to achieve adequate averaging of amplitude fluctuations induced by internal waves and ambient acoustic noise.