# Cause Analysis and Preventive Measures against False Bottoms in Echosounder Data
## 2 Materials and Methods
### The Influence of False Bottoms on Echosounder Detection and Its Causes
#### 2.3.2 The First Ping Echo Signal Is Reflected Twice to Form a False Bottom on the Second Ping  
A false bottom can also be formed by the first ping echo signal having two reflections, resulting in the formation of a false bottom during the second ping. This principle is illustrated in Figure 1b. The formation process occurs when the receiver partially receives the true-bottom echo after the first reflection, resulting in the presence of a genuine bottom echo in the echosounder image. Subsequently, the remaining echo signals are reflected at the water surface and then once more at the bottom before being received by the receiver in the subsequent cycle. Mistakenly, these echoes are perceived as signals that were emitted  
Figure 2: Target intensity (dB) echogram. (**a**) First type. (**b**) Second type. (**c**) Third type. by the echosounder in the next cycle, consequently creating a false bottom. In this case, the transmission interval \(t_{v}\) between the two ping signals is as follows:  
\[t_{v}=t_{m}+t_{i} \tag{6}\]  
From Figure 1b, it is evident that the false bottom occurs upon the second ping signal. \[2d_{f2}=vt_{f2}=v(2t_{r2}-t_{v}) \tag{7}\]  
\[d_{f2}=2d_{r2}-d_{m}-\frac{vt_{i}}{2} \tag{8}\]  
where \(d_{f2}\) is the false-bottom depth of the second type, \(d_{r2}\) is the seabed depth of the second type, and \(t_{f2}\) is the time between the start of the second ping and the arrival of the second bottom echo from the previous ping. In this case, the true-bottom echo will appear in the current ping echosounder image, so the following equation holds:  
\[d_{m}>d_{r2} \tag{9}\]  
The depth of the false bottom (\(d_{f2}\)) is greater than zero, and the interval (\(t_{i}\)) between the adjacent ping-receiving cycles is also greater than zero. Using Formula (8), we can conclude that this false-bottom scenario occurs when the actual depth (\(d_{r2}\)) of the seabed is greater than half of the maximum detection depth (\(d_{m}\)) and less than the maximum detection depth. The true echo signal of the seabed will appear in the first ping, and the secondary echo signal of the seabed will be received by the receiver in the next cycle, which is then considered to be the signal echo of the subsequent cycle. Consequently, a false-bottom image is formed at the corresponding depth (\(d_{f2}\)) in the second signal cycle. The specific presentation of this phenomenon in echosounder images is illustrated in Figure 2b. Compared with the real-bottom echo situation, the false-bottom signal has two more reflections (one seabed reflection and one water-surface reflection), and travels farther in the water, resulting in greater loss of acoustic signal and a weaker received signal. However, this false-bottom signal will be randomly distributed above the real seabed, and in the detection of fishery resources, the depth range in which the target object appears is approximately the same. If the influence of the false bottom is ignored, the false-bottom signal will be mistaken for the target object, resulting in detection errors. Therefore, it is necessary to avoid the occurrence of such false bottoms. 3.3 The Bottom Echo Signal Outside the Detection Range of the Previous Ping Forms a False Bottom in the Next Ping  
In this case (Figure 1c), the true-bottom depth exceeds the maximum detection depth. However, in the actual detection process, the echosounder's reflection still appears in the echosounder image under specific circumstances. Using Formula (6), the transmission interval between two consecutive ping signals (\(t_{v}\)) can be calculated. When the echosounder reflects off the true bottom at a depth greater than the maximum detection range, the echo's reflection time extends beyond one complete reception cycle. If a false bottom is present, the echo signal will be received by the receiver in the subsequent cycle.