Internal waves on a steep shelf and their influence on coherent underwater acoustic communications

Abstract

<p>The results of observations of internal waves in the coastal zone of the northeastern shelf of the Black Sea during the summer and autumn seasons in 2018–2021 are presented. Each dataset consists of long-term (more than ten days) observations of vertical temperature variability, speed of sound, currents, and backscatter signal. The study site is distinguished by a sharp drop in depths, which determines the development of features in the hydrodynamics of the observed coastal processes, in particular, internal waves. Near-inertial internal waves are usually registered and come close to the coast due to the features of the bottom topography. Often approaching near-inertial internal waves to the coast generated short-period internal waves on their crests. Trains of short-period internal waves related to the first mode and individual manifestations of internal waves of the second mode were recorded. Groups of internal waves of the first mode were most often observed, including 3–4 (less often up to 8–10) waves. Short-period internal waves with a 5–10 min period and a height of 1–2 m were recorded in the summer season. In autumn, their parameters were as follows: periods 2–8 min, heights 0.5–4 m, lengths from 30 to 100 m.</p><p>In some cases, solitary waves ("solitons") were identified, their periods and amplitudes exceeding the average values of the waves observed in the groups. Such internal "solitons" features are due to a high degree of nonlinearity. According to the data of autumn observations in 2019, short-period internal waves were detected regularly approaching the coast from the southeast. In addition, waves propagating from the shore (reflected waves) were recorded. Short internal waves reflected from the coast are probably the main distinguishing feature of the internal-wave field of a narrow shelf compared to internal waves of a regular or wide shelf.</p><p>We used the thermistor data obtained in field experiments for simulations of medium-range coherent underwater acoustic communications (UAC) in a number of frequency bands between 6 kHz and 17 kHz to assess the magnitude of uncertainties introduced into UAC performance by internal-wave-induced perturbations of the sound speed profile. Both near-inertial and short-period internal waves were considered. The channel impulse response was modeled as a baseline response calculated using ray tracing techniques with added random fluctuations of ray arrival amplitudes and phases. The simulations show that for the same UAC parameters, the level of inter-symbol interference (ISI) can vary by more than 8 dB during a passage of a near-inertial wave and by 4 dB in the case of short-period waves. We analyze the time-frequency patterns of the ISI levels and propose an ad hoc solution for the problem of the optimal frequency band selection during passages of intense short-period internal waves. </p><p>The research was conducted by the assignment of the Ministry of Science and Higher Education of Russian Federation No. 0128-2021-0010; data processing and analysis were carried out with the support of a grant of the President of the Russian Federation МК-5064.2021.1.5.</p>