ObjectiveHydroacoustic detection of multi-type targets is of great significance in marine safety, maritime rescue, marine ecological protection and fishery development. In recent years, the phase-sensitive optical time domain reflectometer (
Φ-OTDR) has received extensive attention in hydroacoustic detection due to its large-scale networking capability, flexible array reconstruction, and uniform wet-end structure. The working principle of
Φ-OTDR is based on the interference effect of intra-pulse Rayleigh backscattering (RBS), which can quantitatively demodulate the acoustic information at each fiber position and thus provide a good solution for obtaining the target voiceprint information and orientation. The current research has realized the information detection of a certain type of ships to varying degrees. However, in practical applications, hydroacoustic targets are usually multi-type, such as ships, divers, and organisms. It is still an important challenge to obtain and master the hydroacoustic characteristics of multi-type targets. To this end, this paper detects and analyzes the hydroacoustic signals of multi- type targets in a non-anechoic pool through frequency-diversity
Φ-OTDR and suspended sensitized cables. In addition, the
Φ-OTDR system uses a phase demodulation method based on orthogonal I/Q or Hilbert transform, and the obtained phase is wrapped in the range of -π to π, so it is necessary to recover the actual phase signal by phase unwrapping. However, due to the value of the maximum detectable phase of
Φ-OTDR is π, when there is a phase spike or strong background environment interference, the lack of dynamic range will cause the additional phase jumps and results in further distortion. For this, this paper constructs a time-space two-dimensional Kalman algorithm model to unwrap the phase to achieve high-fidelity phase detection.
MethodsBased on the two-dimensional continuity of the time domain and spatial domain of detection data of
Φ-OTDR system, the time-space two-dimensional Kalman algorithm for unwrapping phase is constructed. In the non-anechoic pool, the
Φ-OTDR system based on frequency diversity is combined with the suspended sensitized cable to detect and analyze the hydroacoustic signals of multi-type targets (
Fig.5). The spectral characteristics of multi-type targets such as remote operated vehicle (ROV), bionic fish and divers simulation are analyzed by the proposed algorithm (
Fig.6).
Results and DiscussionsThe time-space two-dimensional Kalman algorithm is proposed to break through the limitation of phase wrapping characteristics on dynamic range and repetition frequency of
Φ-OTDR, and ensure the high-fidelity detection of multi-type targets information. To verify the effectiveness of the algorithm, the experiment of multi-type hydroacoustic targets detection in non-anechoic pool is carried out. The phase unwrapping results of ROV show that the proposed algorithm could improve the dynamic range of
Φ-OTDR system and restore the phase information accurately. Furthermore, the spectral characteristics of ROV, bionic fish and divers simulation are analyzed. The results show that the ROV is a continuous signal, the frequency range is 200-
1700 Hz, and there is a line spectrum with equal frequency interval (~24 Hz) in the spectrum (
Fig.9). The signal interval of the bionic fish is 0.5 s, corresponding to the tail swing period, and the frequency range is
1500-1700 Hz (
Fig.10). The signal interval of divers simulation is 0.25 s, corresponding to the bubble ejection period of the oxygen cylinder, and the frequency range is 150-400 Hz (
Fig.11).
ConclusionsThis work make full use of the two-dimensional continuity of time and space domains of the detection data of
Φ-OTDR, and innovatively introduce the time-space two-dimensional Kalman algorithm. The proposed algorithm can effectively break through the limitation of phase wrapping characteristics on dynamic range and repetition frequency. In order to verify the feasibility of this method in the field of actual hydroacoustic sensing, the
Φ-OTDR system with frequency diversity is combined with the suspended sensitized cable to detect and analyze the hydroacoustic signals of multi-type targets. The results show that the time-space two-dimensional Kalman algorithm can restore the spectral characteristics of ROV, bionic fish and divers simulation with high fidelity. This work is based on the hydroacoustic characteristics of multi-type targets, which can further promote the development of marine target classification and recognition.