Acoustic Communications for Bottom-toBottom Ocean Sensor Networks

A bottom-to-bottom acoustic communications network is feasible in areas of the ocean where the combination of the depth and near-surface sound speed support direct-path refracted rays which connect multiple sensor nodes located on or near the seafloor. In this paper the initial results of modeling and testing the physical layer of a communications system that will support a bottom-based sensor network in the Mentawai Basin west of Padang, Sumatra in Indonesia are reported based on an experiment done in 2015. The acoustic communications system utilizes a carrier frequency between 3 and 4 kHz, and achieves data rates from 60-700 bps, depending on the range. The combination of a bottom depth of 1750 m and a deep and seasonally persistent thermocline provide the necessary conditions for direct-path propagation via surface refracting rays. The results that are presented include signal-to-noise ratio (SNR), adaptive equalizer mean-square-error and sample multipath measurements from real-time acoustic modems (the WHOI Micro-Modem) at ranges of 20 and 27 km. Raw acoustic data was also recorded in parallel for post-processing and additional analysis. Refracted rays provided direct-path ranges to 33 km, so the results shown here can be extended through use of lower frequencies to increase the range.