Abstract
As one of the most commonly used acoustic systems in seabed surveys, the altitude of the side scan sonar from the seafloor is always difficult to determine, especially when raw signal levels and gain information are unavailable. The inaccurate sonar altitudes would limit the applications of sonar image geocoding, target detection, and sediment classification. The sonar altitude can be obtained by using bottom tracking methods, but traditional methods often require manual thresholds or complex post-processing procedures, which cannot ensure accurate and real-time bottom tracking. In this paper, a real-time bottom tracking method of side scan data is proposed based on a one-dimensional convolution neural network. First, according to the characteristics of side scan backscatter strength sequences, positive (bottom sequences) and negative (water column and seabed sequences) samples are extracted to establish the sample sets. Second, a one-dimensional convolution neural network is carefully designed and trained by using the sample set to recognize the bottom sequences. Third, a complete processing procedure of the real-time bottom tracking method is established by traversing each side scan ping data and recognizing the bottom sequences. The auxiliary methods for improving real-time performance and sample data augmentation are also explained in detail. The proposed method is implemented on the measured side scan data from the marine area in Meizhou Bay. The trained network model achieves a 100% recognition of the initial sample set as well as 100% bottom tracking accuracy of the training survey line. The average bottom tracking accuracy of the testing survey lines excluding missed pings reaches 99.2%. By comparison with multi-beam bathymetric data and the statistical analysis of real-time performance, the experimental results prove the validity and accuracy of the proposed real-time bottom tracking method.
Highlights
A side scan sonar can rapidly obtain large-area seabed images, has been widely used in seabed investigation, and plays an important role in seabed target detection [1,2,3,4] and investigation as well as research of the seabed ecological environment [5,6] due to its low cost and simple installation
These results prove the validity showed that the bottom tracking accuracy can reach 100% on the training survey line
When the sample size was 40, the training and validation accuracies were further improved, and the bottom tracking accuracy increased to 100%, which suggests that the samples can accurately reflect the variation characteristics of backscatter strengths
Summary
A side scan sonar can rapidly obtain large-area seabed images, has been widely used in seabed investigation, and plays an important role in seabed target detection [1,2,3,4] and investigation as well as research of the seabed ecological environment [5,6] due to its low cost and simple installation. A side scan sonar is usually dragged by a towing line to get close to the bottom of the sea to obtain high-resolution seabed images. The depth of the side scan sonar can be obtained by using depth sensors, the Remote Sens. 2020, 12, 37 height of the sonar from the seabed cannot be accurately obtained [7]. The bottom tracking of side scan data can accurately obtain the sonar height from the seabed by finding the first echo that reaches the seabed. Real-time bottom tracking can quickly detect changes in sonar height and seabed terrain, and enhance the safety of sonar equipment and ship navigation
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