Proceedings of the Institute of Acoustics

Ocean current observation with mirror-type underwater acoustic tomography sensing network

Guangming Li, National Innovation Institute of Defense Technology, Beijing, China
Dexin Zhao, National Innovation Institute of Defense Technology, Beijing, China
Shijie Xu, Zhejiang University, Zhoushan, China
Pan Xu, National University of Defense Technology, Changsha, China
Luwen Meng, National Innovation Institute of Defense Technology, Beijing, China
Ge Niu, National Innovation Institute of Defense Technology, Beijing, China

1 INTRODUCTION

Sound wave is a basic tool for information transmission and communication in marine environment, which can also be used in ocean dynamic process observation. Ocean acoustic tomography (OAT) is an advanced technique for observing marine environment via signal travel time delay variations, it was first proposed by Munk [1]. Woods Hole Oceanographic Institution successfully observed three-dimensional (3D) structure of mesoscale eddies at the first OAT experiments in Bermuda Sea [2]. Coastal acoustic tomography (CAT) was developed by Kaneko, Hiroshima University based on OAT for shallow water observations at coastal region [3]. OAT reconstructs flow and temperature fields using long-distance acoustic signal transmission via ocean acoustic channel [4-8]. Compared with deep sea environment, the acoustic field in shallow sea area is more complex and variable. Different from fix-point measurement using Acoustic Doppler Current Profiler (ADCP), Conductivity Temperature Depth system (CTD), PIES, etc., acoustic tomography is a more efficient way to reconstruct 2D or 3D results with acoustic stations [9-10].

In recent years, new CAT observations and methods were developed. Park et .al proposed a new method for vertical temperature profile estimation using sea surface temperature, near-bottom temperature and water depth based on CAT. They successfully obtained monthly temperature variations in the southern coastal region of Korea with mean root-mean-squared difference for February, May, August, and November were 0.23, 0.30, 0.50, and 0.24℃ , respectively [11]. Minmo Chen et. al reconstructed two-dimensional flow fields using a coast-fitting inversion model of five- station reciprocal transmission and obtain the M2 and M4 tidal currents and the residual current [12]. Ze-Nan Zhu, et. al successfully observes two complete flood processes of the Yangtze River via two CAT systems with synchronized transmission [13].

Mirror-transponder costal acoustic tomography (MCAT) is a further development of traditional CAT, its key point is to obtain the sound wave reciprocal transmission information by signal mirror transferring [3]. It is an innovative method for underwater signal receiving by solving the travel time between station pairs through the mirror signal after networking transmission. Thus, the underwater signal transmissions can be obtained with one land station, which simplifies experimental setting and forms the networking array observation model. MCAT was initially applied for two-station transmission experiments. Chen et. al first introduced the MCAT system, and performed a feasibility experiment by the minimum unit (one land and one key station) in the Nekoseto Strait of the Seto Inland Sea, Japan. The experiment prove performance of MCAT via the root-mean-square error (RSME) of the hourly mean range-average currents calculated by mirror travel time is significantly smaller than the variation ranges of the semidiurnal tidal current [14]. Syamsudin et. al successfully observed the subsurface structures of internal solitary waves via two MCAT over a path length of 18.286 km in the Lombok Strait, Indonesia, and reconstructed temperature profiles by four layer-averaged which temperature peaks is synchronized with the diurnal tides [15].

The station types in MCAT experiment are divided into land station, key station and normal station. In general CAT experiments, acoustic stations are deployed at same depth and fixed by a anchored boat. However, in MACT experiments, key and normal stations are designed as shallow markers, which are deployed at deep sea. The land station is placed in the shallow water near shore, key station is placed between land station and normal station for signal transferring. The non-fully depth region derived from land to ocean is the most suitable observation area for MCAT with the most significant environmental variations. But the area is not applicable for CAT due to the fluctuated terrain and complex shallow acoustic channel. Thus, mirror signal transmissions have advantages in shallow sea.

2 MIRROR RECIPROCAL TRANSMISSION MODE

The mirror reciprocal transmission mode of MCAT is combined with a land station, a key station and several normal stations. The core of this model is the travel time obtained through mirror-type signals transmission via key and normal stations. In a stranded multi-station remote sensing system, different station has its role: land station is deployed near shore and can obtain mirror data, key station is the offshore autonomous underwater station which is used for mirror reciprocal transmission with normal stations and transferring received data to land station, normal station is also the offshore autonomous underwater station which is used to reciprocal transmitting between normal and key stations. During the mirror transmission, three signal sending and receiving exist between station-pairs, which named original signal, mirror 1st signal and mirror 2nd signal.

Figure 1: Time chart for reconstructing travel times between land (M0), key (M1) and normal (M2-M3) stations. Red solid and dashed rectangular boxes are transmit pulses and receiving windows in original transmission, respectively. Blue dashed rectangular boxes are extra record time. Yellow solid and dashed rectangular boxes are mirror transmit pulses and receiving windows in mirror 1st transmission. Green solid and dashed rectangular boxes are mirror transmit pulses and receiving windows in mirror 2nd transmission. Gray dashed arrows are the direction of sound transmission that begins at the start of transmit pulses and ends at the start of data recording. Black dashed, dotted, and solid peaks are the arrival peaks of coming signals. The lower right figure shows the sending and receiving status of each station.

Figure 1 shows four-station mirror reciprocal transmission mode and signal sending and receiving procedure. Three sound signal transmissions are introduced as follow (M0 denotes land station, M1 denotes key station, M2 and M3 denotes normal station). Before sound transmission, an extra record time t_ex is set at all acoustic stations and the receive starting time is calculated by referring the reference travel time by GPS. At the original transmission, M0 only send signal to M1; signal reciprocal occurs between M1, M2 and M3; each station i starts record received signal t_s-i , t_s-i  equals receive starting time minus extra record time. At the mirror 1st transmission, M0 only records received signals; M1 sends the record results at original transmission to M0; sound reciprocal transmission occurs between M1, M2 and M2; M0, M1 and M3 start record signals after sending signals ( t_w-i = t_Ms-i ). At the mirror 2nd transmission, M2 and M3 have finished jobs and gone to sleep; M0 records received signals that send by M1 of record results at mirror 2nd. Thus, at the standard transmission circle, three times signal transmissions and the reciprocal travel time between different station pairs can be obtained via the record data from land station. This is a much simpler and more efficient mode than single simultaneous reciprocal transmission mode.

CAT is enhanced with mirror-transponder functionality to allow the transfer of subsurface environmental data shoreward from the offshore station. The environmental data are specifically a mirror image or a duplicate of the acoustic signal received by a subsea MCAT unit. Therefore, the core observation area of MCAT experiment is among the key stations and normal stations. MCAT system design is an extension of CAT [14-15].

To solve the time synchronization problem, land station uses precise GPS clock signals, whose system and power are on land base and transceiver is underwater. Key and normal stations are the integration of all modules as the shallow marker, where chip-scale atomic clock (CSAC) is operated as clock signals. CSAC of all stations are synchronized to GPS clocks before system deployment. In this way, high precision time synchronization is achieved.

Different from the shipboard moorings of CAT, MCAT is more convenient and applicable in a wider area, more suitable for deep-sea areas. It can be used to obtain long-range observations of underwater environment. The inversion method that used in CAT research can also be applied. Layer-averaged or grid-averaged temperature and flow fields can be solved.

3 FIELD WORK

The MCAT feasibility experiment was conducted between the Liuheng Island and Xiazhi Island, Southwest of Zhoushan, China. The southwest area of Zhoushan is formed by island chains, which is also at the entrance of East China Sea. The observation area was set in the middle of the island with large quays on both sides. During tide rising period, water enters in the southeast entrance (Blue arrow), and during tide falling time, it flows out from northwest direction (Green arrow). Due to shipping channel control and weather conditions, the experiment was carried out at period with no tide.

A feasibility experiment for MCAT was carried out at June 26th, 2022 with four sound stations. Experiment settings of four stations are shown in Figure 2. M20 was the land station, its system controller and power is stored in a plastic container and was put on the land base. The transceiver was fixed between a underwater floating ball and a weight, and connected to the system controller by a cable. M21 was the key station and M22-M23 were the normal stations, both of them were shallow marker mode and fixed between a underwater floating ball and a weight. A buoy was used for labelling and recoving nearby each station. M20 was deployed at shallow water area, and M21- M22 were deployed at deep water area. The terrain between acoustic station pairs were obtained by depth gauge scanning.

9th-order M sequence pseudorandom modulated 5-kHz carrier frequency sound wav were transmitted in this experiment for multi-station networking. The unique signal with different sequence code were assigned for each station during the experiment. The time resolution of multi-arrival signals was 0.6ms (one-digit length of the M-sequence). The cycle-per-digit value of the modulated transmission signal (Q-value) was set to 3 so that one-digit length was 0.3072s (1535 bits). Considering the time elapsed in system control and data recording, four stations for reciprocal transmission at 10-min interval, the original transmission, mirror 1st transmission and the mirror 2nd transmission were also performed simultaneously at 1-min during 10-min. The signal-to-noise ratio (SNR) of received signals in this experiment are increased  via cross correlation of the received signal with the M-sequence replica used in the transmission. The more detailed descriptions of mirror system design and configuration were introduced in previous research [38]. Different from the two-station signal reciprocal transmission, this paper is the first feasibility experiment to focus on four-station mirror signal transmissions in of standard mode communication.

Figure 2: Experimental settings of standard four-station transmission. Black lines denote the original signal, arrows denote the direction of sound transmission. Red lines denote the mirror 1st signal and green lines denote the mirror 2nd signal. Top of shallow marker is transceiver and bottom of shallow maker is releaser.

Ray simulations are obtained via bottom depth (by depth gauge) and sound speed profile (by CTD), then, the matched sound rays are selected by multi-peak identifications (arrival peak travel time). Figure 10 shows the results of ray simulations and the relationships between launch angle with travel time. More than two sound rays are matched at each station pair. The process of sound ray matching is introduced in previous study, it is not be expanded here.

Due to the terrain variations in experiment environment, the acoustic signals exist reflections between sea surface and seafloor in transmission. The signal reflections reduce the SNR and increase difficulty in matching sound ray (Figure 3 (a-f)). This suggests that future experiment need to avoid setting up terrain drop areas to ensure the signal transmission process.

Figure 3: Ray simulations and launch angles. (a-f) are the ray simulations of M21-M20, M22-M20, M23- M20, M21-M22, M21-M23 and M22-M23 via BELLHOP, respectively. (g) is the relationship between launch angle and travel time. (h) is the sound speed profile of observation area. The colors used here are same as Figure 4.

4 MULTI-PATH ARRIVAL SIGNAL

The stack diagrams of multi-peak identification results obtained in original transmission between station pairs are shown in Figure 4. From the reciprocal signal transmission between each station pair, the SNR of each arrival peak is about 100, which can be significantly extracted. Two or three peaks basically existed between each station pair. The data of land station (M20) can be directly read in the experiment, while data of key station (M21) and normal stations (M22, M23) are obtained from the system after experiment.

From Figure 4, the travel time existed a drift of (a), (c), (d), (e) and (f) around 12:20 during the experiment. The reason of this is the station drift at M21 and M23. Due to two stations were arranged near the main boat channel, variations of flow field is big. The existing of station drift can be almost ignored in flow field solutions of this paper. Overall, original signal transmissions verify the communication networking between four stations. This provides the basis for mirror transferring. In other words, the 1st mirror transmission is the sending and receiving of received signal in original transmission. Besides, only Figure 4 (a) is received of original transmission in the standard model. Table II shows the reference travel time (RTT) of direct ray in each station pair.

Figure 4: Stack diagram of the correlation results of original transmissions. Dots with different colors are assigned for different travel time of arrival peak. (a)-1 and (a)-2 are reciprocal transmission of M20- M21 and M21-M20, respectively; similarly: (b)-1 M20-M22; (b)-2 M22-M20; (c)-1 M20-M23; (c)-2 M23-M20; (d)-1 M21-M22; (d)-2 M22-M21; (e)-1 M21-M23; (e)-2 M23-M21; (f)-1 M22-M23; (f)-2 M23-M22. The abscissa axis is the travel times of signals; ordinate axis is the time of sending signals; the peak height is the SNR value.

5 FLOW CURRENT OBSERVATION

The 2D horizontal inversion flow field of Figure 5 directly shows variations between key station and normal stations. Actually, the horizontal current is a projected plane of the oblique plane combined by stations at different depth. From the inversion flow field at different moments, the current velocity is significant. The observation area is in the center of island chain and the ship channel, which leads to the eddy flow and rapid tidal flow.

Figure 5: 2D horizontal inversion flow field between M21 to M22 to M23. (a-f) show horizontal inversion flow field at six different moments. The blue arrows and red arrows are the inversion current velocity by original transmission and mirror transmission, respectively.

According to the preliminary investigation of observation area, the flow field is composed of the downward flow from northwest to southeast along Liuheng island and the upward mixed flow from southeast to northwest among island chain such as Xiazhi island. Also, the complex terrain and the distribution of island leads to rapid and significant flow variations, which is correspond to the flow field at 2D horizontal inversion flow field. Unfortunately, ADCP is not used for measuring the current in observation area due to navigation control restrictions. The 2D flow field reconstructed demonstrates the mixing current and the hedging effect of flow over observation area. The experiment introduced in this paper is an application of MCAT in shallow ocean. Although the sound transmission is obtained completely, there are still some problems need to solved in land station. The 2D flow field inversion reveals the hedging and mixing current in the observation area, which means the strong flow filed exists. In addition, the differences between two different transmissions are discussed, and it needs further study.

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