JP7591316B2 - Underwater detection methods - Google Patents

Description

The present invention relates to an underwater detection method that is applied to a quantitative fish finder that uses ultrasonic waves.

Quantitative fish finders that use acoustic techniques are known. In these quantitative fish finders, the ratio of the sound pressure level of the ultrasonic waves transmitted from a transducer to the sound pressure level of the echo reflected by a fish (Target Strength (TS): reflection strength) is calculated, and the fish's body length L is calculated from the reflection strength TS. The following relationship holds between the reflection strength TS and the fish's body length L:
TS=20logL+20logA
Here, A is a coefficient determined by the signal frequency and the fish species.

For example, Patent Document 1 describes a technique for accurately measuring the value of reflection intensity TS, which employs a split beam method to calculate the echo arrival angle θ, and corrects the echo reflection intensity using the directional characteristics of the transducer according to the calculated arrival angle θ to obtain an accurate reflection intensity TS. Furthermore, Patent Document 1 describes the generation of a transmission signal with a time width T1 consisting of an FM (frequency modulation) signal.

Patent Document 2 also describes a fish finder that has a first judgment unit, a second judgment unit, and a third judgment unit, and that judges that the peak is due to an echo of a single fish when the first judgment, the second judgment, and the third judgment are satisfied. The first judgment unit detects the peak position on the time axis where the amplitude of the echo signal peaks and the amplitude value of the peak position, detects a first range that includes the peak position in the echo signal and has an amplitude greater than the first judgment reference value, and judges whether the echo is due to a single fish based on the time width of the first range. The second judgment unit sets a second range that includes the peak position in the echo signal and has an amplitude greater than the second judgment reference value, and judges whether the echo is due to a single fish based on the phase difference of the echo signals in multiple directions within the second range. The third judgment unit judges whether the echo is due to a single fish based on the comparison result of the amplitude of the echo signals of the channels at the peak position.

Furthermore, the distribution density (hereinafter simply referred to as density) can be obtained by counting the number of single echoes and dividing by the observed volume. Patent Document 3 describes how to obtain the density of a school of fish. It describes how the output signal of the received signal is integrated to generate a school estimation signal GES containing the average volume scattering strength (Sv) per cubic meter of the school of fish, and how the distribution density (n) of the school of fish is calculated by dividing the average volume scattering strength (Sv) by the average target strength (Ts).

When the pulse width of the transmission signal is wide, in addition to the basic components due to reflection from individual fish, interference components due to other fish appear, and both the basic components and the interference components change significantly (they become very active), and the interference components cannot be ignored, making it impossible to obtain accurate individual fish data (TS, etc.). Therefore, a short pulse width of the transmission pulse (transmission signal TX) is advantageous for obtaining individual fish data. On the other hand, when the distribution of fish schools is dense, they can be considered uniformly random and the interference components can be ignored, so a long pulse width of the transmission pulse is advantageous for obtaining school echo data. In Patent Document 3, a transmission signal with a predetermined short pulse width and a transmission signal with a long pulse width are transmitted alternately.

JP 2005-249398 A JP 2015-210142 A JP 2000-147118 A

Underwater detection devices are often installed on research vessels. In the conventional detection of individual fish described in Patent Documents 1 and 2, no consideration was given to the speed of the research vessel, resulting in problems with reduced accuracy in detecting individual fish and reduced accuracy in detecting the number of fish tails.

Furthermore, while information on fish school density is important to fishermen, conventional technologies such as those described in Patent Document 3 were unable to obtain accurate density information. The first problem is that at deep depths, the echo level becomes small due to distance attenuation, making it impossible to detect fish according to the directional characteristics. The second problem is that conventional techniques did not require consideration of movement of the observation point when calculating volume. The third problem is that at shallow depths, the volume of the cone is small, leading to overestimation of density.

Therefore, the object of the present invention is to provide an underwater detection method that can, for example, count the number of fish tails and accurately measure the density of fish schools.

In the present invention, as a result of detecting a fish , information on time, depth, TS, and fish length is given to each detected fish ,
Counting the number of fish tails detected for each unit time and unit depth and calculating an average TS using the TS of the fish ;
Calculate the minimum echo level at which fish can be detected using the average TS value and the sonar equation, and calculate the directivity angle at the calculated minimum echo level.
This is an underwater detection method in which the volume at each depth is calculated from the calculated directivity angle, and the density at each depth is calculated by dividing the number of fish by the volume.

According to the present invention, it is possible to accurately detect individual fish. Furthermore, according to the present invention, it is possible to improve the accuracy of density calculation. Note that the effects described here are not necessarily limited, and may be any of the effects described in this specification.

FIG. 1 is a flow chart for explaining a conventional single fish detection method. FIG. 2 is a schematic diagram for explaining the echo connection process. FIG. 3 is a block diagram showing a system configuration according to an embodiment of the present invention. FIG. 4 is a flowchart for explaining the process of the single fish detection unit in one embodiment of the present invention. FIG. 5 is a schematic diagram for explaining the angle of incidence when an ultrasonic wave is incident on a single fish. FIG. 6 is a graph showing an example of the incidence angle and the reflectance. FIG. 7 is a schematic diagram for explaining a case where a plurality of fish are swimming in a straight line at the same depth. Figure 8A is a graph showing the change in echo amplitude for successive pings, and Figure 8B is a graph showing the change in depth for successive pings. FIG. 9 is a schematic diagram for explaining the calculation of the volume of a cone from the directivity angle θ of a transducer. FIG. 10 is a schematic diagram for explaining a first method of density calculation according to the present invention. FIG. 11 is a graph showing an example of the calculation result of the directivity angle θ′. FIG. 12 is a schematic diagram for explaining the second problem of density calculation. 13A and 13B are schematic diagrams for explaining a third problem in density calculation and a third method for solving this problem. FIG. 14 is a flow chart for explaining density calculation according to one embodiment of the present invention.

The following describes embodiments of the present invention. Note that the embodiments described below are preferred examples of the present invention, and include various technically preferable limitations. However, the scope of the present invention is not limited to these embodiments unless otherwise specified in the following description that the present invention is limited to these embodiments.

Prior to explaining the present invention, a conventional method for detecting a single fish will be described with reference to the flowchart in FIG. 1. A transmission signal is sent at a predetermined time interval, and the echo is received and measured. In the initial state, the count is set to 0 (step S1).

The correlation value is found by correlation analysis of the transmitted signal and the received signal (step S2). The correlation value is calculated for each depth. Local peaks are detected from the correlation value, and the distance is estimated (step S3). Note that, even if the correlation value is not used, the received signal may be used as is if the transmitted signal is short.

The estimated distance is compared with the distance of the local peak for the previous transmission (step S4). Since the local peak may disappear due to the influence of noise, etc., it is also possible to compare up to two transmissions ago. In step S5, it is determined whether the difference distance is within a threshold. If it is determined in step S5 that the difference distance is within the threshold, the signals are linked, and Count is incremented by 1 in step S6.

If it is determined in step S5 that the difference distance is not within the threshold, the linking is terminated and it is determined whether the Count number is within a certain threshold (step S7). If it is determined in step S7 that the Count number is within a certain threshold, it is detected as a fish (step S8).

The processing performed in steps S4 and S5 will be described with reference to FIG. 2. FIG. 2 shows an example of an echo waveform, with the vertical axis indicating time (s) and the horizontal axis indicating distance (m). For example, 20 transmissions are made per second, so the time interval (ping interval) is 0.05 seconds. If there is a peak at the same distance as the previous one, they are linked as one fish, as shown by the dashed line. In other words, if the number of consecutive transmissions that can capture consecutive peaks reaches a certain threshold, it is detected as one fish. This processing can remove noise. That is, in the case of echoes from fish, echoes are measured continuously, but in the case of noise, there is a high possibility that an echo is present only once.

In the conventional technology described above, the number of connections (Count) is calculated when the connection is completed, and if the number is within a range (threshold), it is detected as a fish. However, the speed (ship speed) of the research vessel on which the underwater detection device is installed was not taken into consideration. When the ship speed is fast, the time during which echoes can be measured is short, so there was a risk that the accuracy of fish detection would decrease if a fixed threshold was used.

Figure 3 shows the system configuration of one embodiment of the present invention. Echoes from an echo signal receiving unit 11, which includes an ultrasonic transmitter/receiver, are supplied to a calculation unit 12, which is made up of an individual fish detection unit 13 and a density calculation unit 14. The results obtained in the calculation unit 12 are displayed on a display unit 15. The calculation unit 12 is realized by a data processing device that processes digital data and associated software. The display unit 15 provides a display screen that displays the received individual echoes and school echoes, TS distribution, density, etc. on a nautical chart.

The processing of the single fish detection unit 13 in one embodiment of the present invention will be described with reference to the flowchart in Figure 4. The processing from step S11 to step S16 is the same as that of the conventional single fish detection method described above. First, a transmission signal is emitted at a predetermined time interval, and an echo is received and measured. In the initial state, the count is set to 0 (step S11). A correlation value is found by correlation analysis between the transmission signal and the reception signal (step S12), and the correlation value is calculated for each depth. A local peak is detected from the correlation value, and the distance is estimated (step S13). Note that even if it is not a correlation value, if the transmission signal is short, the reception signal may be used as is, or a value obtained by removing noise from the reception signal using a band pass filter or the like may be used.

The estimated distance is compared with the distance of the local peak for the previous transmission (step S14), and it is determined whether the differential distance is within the threshold (step S15). If it is determined that the differential distance is within the threshold, concatenation is performed, and the Count indicating the number of concatenations is incremented by 1 (step S16). If it is determined that the differential distance is not within the threshold, concatenation is terminated, and it is determined whether the Count value (number of concatenations) is within a certain range (within a range determined by an upper limit value and a lower limit value) (step S17). If it is determined in step S17 that the number of concatenations is not within the range, the process returns to step S12 (process of calculating the correlation value).

In the judgment of step S17, the range of the number of connections (upper and lower limits) is changed based on the speed of the observation point, e.g., boat speed, and depth (step S18). If the boat speed is fast, the number of connections may be set to a small number. If the depth is shallow, the area in which fish can be detected from the directional characteristics is narrow, so the lower and upper limits of the number of connections may be set to a small number. Alternatively, the lower and upper limits of the number of connections may be set according to the depth.

If it is determined in step S17 that the number of connections is within the range, then in step S19, TS is calculated from the connected echo levels. Conventionally, TS is calculated using the maximum value, but in one embodiment, TS is also calculated from the average of multiple pings including the maximum value (the averaging method is not only the normal average, but also the harmonic average). This makes it less susceptible to the effects of noise, interference, etc.

In other words, in the case of the linked maximum value, if multiple echoes overlap, there is a possibility of overestimating. By calculating not only the maximum value but also the harmonic average, it is possible to calculate the echo level with less influence of outliers and calculate the fish length. For example, if the echo levels of four pings are A1, A2, A3, and A4, the formula for the harmonic average is as follows. The number of harmonic averages can also be set according to the depth.
A=(1/A1)+(1/A2)+(1/A3)+(1/A4)

The harmonic mean is a calculation method that is less affected by a single outlier (a value that is inflated due to noise). Mathematically, it has been shown that the harmonic mean is greater than the arithmetic mean (the normal average) and converges to a small value.

In step S20, it is determined whether the TS calculated from the representative value of the linked maximum, average, or harmonic mean is within the range of the upper and lower limits. A check is made to see whether this TS is within the TS range of the specified fish.

If it is determined in step S20 that the TS is within the range, the next step S21 is performed. In step S21, the fish-likeness is evaluated based on the amplitude change or depth change of the echoes of each linked ping. In step S22, it is determined whether the fish-likeness is within a certain criterion. If the fish-likeness is within the criterion, it is detected as a fish (step S23).

As shown in Figure 5, when ultrasonic waves are incident on a single fish from the transducer 21, the ultrasonic waves are reflected mainly by the swim bladder of the single fish. The reflectivity in this case depends on the angle of incidence of the ultrasonic waves from the transducer 21 on the single fish. Figure 6 shows an example of the incidence angle and reflectivity. In the example of Figure 6, for example, the reflectivity is maximum when the incidence angle is around -17 degrees. The successive echo levels change depending on the position and attitude angle.

The criterion for evaluating fish-likeness is whether the change in echo amplitude between successive pings is within a certain reference range. As mentioned above, the main reflection from a fish is the swim bladder, and TS changes depending on the angle of incidence of the ultrasound on the swim bladder. Therefore, the change in successive echo levels depends on the position and attitude angle. The echo amplitude in successive pings changes according to a certain law. For example, in the case of a fish whose main reflection is the swim bladder, the pattern will be one of an increase, decrease, or maximum in successive pings. Also, in the case of a swim bladder and a fish eye, the pattern will be one with two amplitude peaks in successive pings. For example, fish-likeness can be evaluated based on whether the change in echo level in successive pings is close to a pattern of uniform increase, decrease, and peak.

As shown in FIG. 7, when multiple fish are swimming in a straight line (in the direction of the arrow) at the same depth, the regularity of the depth change is used to detect fish-likeness. In FIG. 8A, the horizontal axis shows successive pings, and the vertical axis shows echo amplitude. In FIG. 8B, the horizontal axis shows successive pings similar to those in FIG. 8A, and the vertical axis shows depth. Depth changes in successive pings that are within a certain reference range are evaluated as fish-likeness. When a fish is swimming in a straight line, the depth changes in successive pings will be on a hyperbola. Therefore, whether the depth change is close to a hyperbola is evaluated and used as an index of fish-likeness. When using depth changes as an index for determining whether an object is a fish, the depth after sway correction may be used.

Next, the density calculation of the present invention will be described. Prior to the description of the present invention, a conventional density calculation will be described. Density is calculated by dividing the value obtained by the echo integration method by the volume. As shown in FIG. 9, the volume of a cone calculated from the directivity angle θ of the transducer is calculated, for example, for every 1 m. The volume is calculated by (base area S1 x depth D x (1/3)). The volume at a position 1 m deep is calculated by (base area S2 x depth (D+1) x (1/3)). The difference between the two is the volume v. The base area is calculated from the radius determined by the directivity angle θ.

The echo integration method is a method for calculating distribution density by dividing the reflection intensity from a school of fish per unit depth by the average TS (reflection intensity of an individual fish). Even when detecting individual fish, density is calculated by dividing the number of individual fish detected per unit depth by the volume. A cone shape is used when the transducer is circular, and a square pyramid is used when the transducer is square. The volume is calculated from the directivity angle represented by the transducer shape.

Conventional density calculation methods had the problem that at deep depths, the echo level becomes smaller due to distance attenuation, making it impossible to detect fish according to their directional characteristics. A second problem is that when surveying using a moving object such as a boat, if the boat speed is fast compared to the movement of the fish, it is necessary to convert the volume for that speed. However, conventional methods did not take this into account. A third problem is that at shallow depths, the cone-shaped volume is small, so density is overestimated. Most echoes from fish come from the swim bladder (e.g. 90%), but reflections from the body surface and other areas also contribute. Conventionally, the position of the fish was calculated as a point reflector, and the size of the fish was not taken into account.

The present invention solves these problems. With reference to FIG. 10, a first method of the present invention that can solve the first problem will be described. The first method uses the echo level, noise level, and distance attenuation to calculate the directional angle at which fish can be detected for each depth, and calculates the volume and density based on the directional angle, thereby improving accuracy.

In the conventional method, the volume of a cone is calculated, for example, every 1 m, based on the directivity angle θ of the transducer (FIG. 10A). The echo level EL is expressed by the following sonar equation:
EL=SL-2TL+TS
When the detection threshold is DT, the noise level is NL, and the directional characteristic is DI, detection is possible when the following formula is satisfied.
DT≦SL-2TL+TS-(NL-DI)
EL: Echo level DT: Detection threshold SL: Transmission level TL: Underwater propagation loss (attenuation depending on distance)
TS: Target strength of the target DI: Directivity characteristic → Approximated by Bessel function if circular NL: Noise level The minimum echo level EL0 at which a fish can be detected is calculated using the following formula.
EL0=NL-DI+DT

From the above formula, if the echo level EL (=SL-2TL+TS) is greater than the minimum echo level EL0 at which a fish can be detected, it can be detected as a fish. However, at deep depths, if the echo level EL is less than EL0 due to the propagation loss TL, directional characteristic DI, and noise level NL, it cannot be detected. The volume is determined by the TS of the fish present at that depth, the propagation loss TL in the sea, and the noise level NL. The first method of the present invention calculates the value of the directional characteristic DI at each depth at which a fish can be detected based on the TS of the fish at that depth, the propagation loss TL in the sea, the noise level NL, and the detection threshold DT, and calculates the directivity angle θ'. As shown in Figure 10B, the density is calculated based on the volume calculated by the directivity angle θ' for each depth.

Figure 11 is an example of a directivity angle θ'. The vertical axis indicates depth, and the horizontal axis indicates directivity angle θ'. The directivity angle θ' at a depth of 0 (i.e., the position of the observation point (transducer)) is set to 5 degrees, for example, and the directivity angle θ' becomes smaller as the depth becomes deeper.

Figure 12 is a diagram to explain the second problem. As the survey vessel moves, the survey area changes over time. In particular, if the vessel speed is fast compared to the movement of the target organisms, overestimation may occur if the volume conversion does not include the distance traveled. For example, consider the case where density is calculated from the number of fish detected per unit time. Here, if the unit time is 1 second and the vessel speed is 10 knots (5.14 m/s), then the fish moved 5.14 m per unit time. Therefore, density is calculated based on the volume of a cone shape plus the volume of a cube calculated from the vessel speed.

Figures 13A and 13B are diagrams for explaining the third problem and a third method for solving this problem. In the conventional method shown in Figure 13A, when calculating the volume of a cone calculated from the directional angle θ of the transducer, for example, every 1 m, the position of the fish is calculated as a point reflector, and the size of the fish is not taken into consideration. In reality, reflection from the body surface etc. also contributes. Therefore, when the depth is shallow and the volume of the cone is small, the problem of overestimating the density occurs. In the third method of the present invention, as shown in Figure 13B, the volume is increased taking into account the size of the fish, and the density is calculated based on the corrected volume. That is, the radius at each depth is added by the fish length L, the volume is calculated, and the number of detected fish is divided by the volume to calculate the density. The third method can improve accuracy.

The first, second and third methods of the present invention described above may be combined. For example, the first and third methods may be combined, or the second and third methods may be combined.

The density calculation unit 14 finds density, for example, by the first method. The processing of the density calculation unit 14 will be described with reference to the flowchart in FIG. 14. Through the above-described processing of the individual fish detection unit 13, information on time, depth, TS, and fish length is assigned to each individual fish detected (step S31). In the next step S32, the number of fish tails detected per unit time and unit depth is counted. In addition, the average TS is calculated using the TS of each fish. The unit time is, for example, 10 seconds, and the unit depth is, for example, 1 m. These values may be changed as appropriate.

In step S33, the directivity angle (volume) is evaluated for each depth range. Then, in step S34, the density is calculated from the number of fish and the directivity angle for each depth range. That is, as described above, the minimum echo level EL at which fish can be detected is calculated from the noise level NL, the value of the directivity characteristic DI at each depth is calculated, and the directivity angle θ' is calculated.

In step S34, the volume at each depth is calculated using the calculated directional angle θ', and the density at each depth is calculated by dividing the number of tails by the volume.

To implement the second method of the present invention described above, the volume is increased by the amount of movement of the observation point. To implement the third method of the present invention, the volume is increased by the size of the fish.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-mentioned embodiments, and various modifications based on the technical ideas of the present invention are possible. For example, the configurations, methods, steps, shapes, materials, and values given in the above-mentioned embodiments are merely examples, and different configurations, methods, steps, shapes, materials, and values may be used as necessary. For example, the present invention can be applied to the detection and density calculation of objects to be detected in water, such as microplastics, in addition to fish.

11: echo signal receiving unit, 12: calculation unit, 13: single fish detection unit,
14... Density calculation section, 15... Display section

Claims (3)

When a fish is detected, the time, depth, TS, and fish length are assigned to each individual fish .
Counting the number of fish tails detected for each unit time and unit depth and calculating an average TS using the TS of the fish ;
Calculating a minimum echo level at which a fish can be detected using the average TS value and a sonar equation, and calculating a directivity angle at the calculated minimum echo level.
The underwater detection method calculates the volume at each depth from the calculated directivity angle, and calculates the density at each depth by dividing the number of fish by the volume. The underwater detection method of claim 1, in which density is calculated based on the volume of a cone plus the volume of a cube calculated from the ship's speed. The underwater detection method of claim 1, in which the radius at each depth is added by the length of the fish, the volume is calculated, and the number of detected fish is divided by the volume to calculate the density.

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