**2. Theory and Method**

This chapter introduces the proposed real-time bottom tracking method using the 1D-CNN model. The basic theories of side scan operation and data characteristics, the recognition of bottom data sequences, and bottom tracking using the trained model will be explained successively.

### *2.1. Side Scan Sonar Operation and Data Characteristics*

The operation of a side scan sonar is shown in Figure 1. The side scan sonar, which is usually a towfish, is towed by the survey vessel using a tow cable to get close to the seabed. The side scan sonar transducer projects a single wide sound beam (e.g., 50◦, as shown in Figure 1) at the port and starboard sides. After the sound is projected from the side scan transducer, the transducer receives and records the backscatter strengths in the time sequence at the port and starboard sides, respectively, and these strengths are used to construct side scan sonar images. During the sound propagation, the backscatter strengths received from the water column are usually much lower than those received from the seabed, as shown in Figure 1. The special variations in signal levels (or backscatter strengths) when the sound arrives at the seabed serve as the basis for the bottom tracking of side scan sonar data. However, given the effects of sonar self-noise, suspended objects in the water column, and other instrumental and environmental factors, the many uncertainties in the sonar data introduce difficulties in bottom tracking.

**Figure 1.** Operation of the side scan sonar and one-ping backscatter strength sequence.

As shown in Figure 1, the acoustic backscatter strengths of a ping have the following characteristics.

