Acoustic tomography in a deep stratified lake; toward reliable estimates of large-scale currents by differential travel time

Lakes worldwide are under increasing stress. For example climate change tends to strengthen stratification, promoting hypoxia and other nefarious effects. Targeting scientific investigation of such changes, and operational monitoring, we have been developing the lacustrine application of Coastal Acoustic Tomography (“CAT”) at Lake Biwa. The largest lake in Japan, Biwa supplies water to 14 million, and has a mean depth in its main basin of 41 m. Since November 2016, we have used M-sequences (5 kHz carrier) to execute the first successful tests of CAT at multikilometer ranges in any lake, with multiweek deployment of 3 (5) stations in Nov 2017 (2018). In 2021, we confirmed reciprocal transmission in spring through early summer between two stations at 7 km. In autumn, we often observed two distinct groups of arrivals. Ray-tracing simulations, based on sound-speed distributions from realistic hydrodynamic simulations, revealed a warm “surface channel” and cold “deep channel” whose predicted arrival times corresponded well with the observations. In continuing work to be reported, we are targeting reliable measurement of raypath-averaged currents by differential travel time, as already achieved by the second author’s team for the stronger tidal currents in the Seto Inland Sea at ranges of 3 km. Preliminary processing has revealed continuous evolution of phase difference between reciprocal receptions. By exploiting such phase differences we hope to achieve a useful level of accuracy in differential travel time despite the weak currents.