NEW SOFTWARE FUNCTIONS OF MULTI-PURPOSE SPLIT BEAM ECHOSOUNDER FOR STOCK ASSESSMENT

In 2018, the LCC Vector Marine Electronics Design Bureau developed a portable multipurpose research split beam echo sounder (MIEL), designed to study aquatic biological resources. In the same year, MIEL was tested in natural conditions together with VNIRO specialists in the water area of the test site with a known ichthyocenosis [1].

The need to create a domestic echo sounder for stock assessment in inland shallow waters is due to the fact that foreign scientific echo sounders have narrow beam width, which does not allow them to be used for vertical sounding at depths of less than 3 meters due to small volumes of sounding water and, consequently, low data representativeness.

After the introduction of sanctions by Western countries, many foreign manufacturers stopped selling high-tech products to our country, and those that can be supplied are practically devoid of technical support, since many representative offices in Russia were closed, and the development of any high-tech products implies close cooperation between the developer and the user.

In 2019-2020, the MIEL software was revised: the user interface was improved and translated into English, new functions were added, including those that have no analogues in foreign echo sounders of the same type. Tests of MIEL with updated software for mobile and stationary placement were carried out in natural conditions.

New functions of MIEL software:

1. adaptive threshold processing of the echo signal;
2. determination of the hardness of the bottom surface;
3. detection and counting of tracks of single fish, determination of the relative direction of movement of single fish along their tracks: "downstream", "upstream".

In 2020, after the new MIEL software functions were finalized, it was tested in the waters of the Taganrog Bay of the Azov Sea and the estuary of the Mius River flowing into it.

In Taganrog Bay, a new MIEL software algorithm for detecting and counting the tracks of single fish with the determination of the relative direction of their movement was tested. The MIEL transducer was placed stationary (on a pontoon) at a depth of 1.5 m, the beam axis was oriented horizontally parallel to the bottom and surface of the reservoir, which depth was 3 m at the site of the experiment.

The algorithm for detecting and counting tracks of single fish is designed to combine the related marks of detected single targets in adjacent pings. Traditionally, in scientific echo sounders, the calculation of a histogram of the target strength (length) of single fish is carried out according to the detected single targets, that is, according to individual echo contacts with fish in adjacent pings. This leads to some “blurring” of the histogram maximum.

The newly developed MIEL “track” algorithm “collects” the marks of the echo trace of a single fish into a single track, determines the strength of the fish target based on the totality of echo contacts, determines the relative direction of fish movement “downstream” or “upstream” (for stationary placement) and calculates it as tracks, and in certain directions. The latter is important for counting spawning fish and for studying fish migration. The histogram is calculated based on the number and target strength of the detected tracks, which increases the “resolution” of the histogram in terms of target strength.

This "real-time" algorithm has no analogues in the software of foreign echo sounders, since the track analysis is traditionally performed in the post-processing software.

The tracking algorithm was tested using an artificial target (a steel sphere) moved across the beam in known directions. At the same time, single fish also fell into the beam. Tracks of single targets were confidently detected, counted, and the direction of their movement was determined. Figure 1 shows an echogram of the Sp signal ("raw" signal), Figure 2 shows an echogram of the detected tracks of single targets.

Figure 1 – Sp echogram

Figure 2 – Track echogram

Figure 3 shows the Track information panel, which displays the total number of detected tracks, as well as the number of tracks in certain directions.

Figure 3 – Track information panel

In the water area of the Miuss estuary, at a pasture aquaculture enterprise, the adaptive threshold processing of echo signals MIEL software algorithm was tested. The operation of MIEL was tested in super-shallow water conditions with traverse sounding (side scan) over a relatively extended range of 50 m in the presence of bottom reverberation interference. MIEL was used mobile and was located on the left side of the boat "Progress" at a depth of 0.4 m with the transducer axis tilted 10 degrees from the horizon towards the bottom. The depth of the reservoir is 1.6 m, the bottom is silty. A hydroacoustic survey was carried out at a speed of the boat of 4 knots during the period of feeding the fish in a given area of the reservoir.

Figure 4 shows an echogram of a survey with a 256 μs pulse duration. The vessel (the origin of the distance reading) is on the right side, MIEL sounds to the left. The length of the “clear water zone” (before the arrival of the bottom reverberation echo) is about 2 m, after which useful echo signals are received against the background of the “underlying” bottom reverberation noise. The contour of the first arrival of the bottom echo signal shows the effect of rather strong rolling on the vessel (wind speed of about 5 m/s).

The settings of the adaptive threshold processing algorithm during the survey were adjusted by the operator, therefore, the echogram (Figure 4) shows wide horizontal stripes with a degree of interference “flare”. The length of the echogram based on the distance traveled by the boat is estimated to be about 40 m, the marked rectangular area is 5x5 m. The “shadows” are clearly visible behind the “highly exposed” sections of the echo traces of single fish. The “shadow” appears as an “unlit” (white) strip running parallel to the “bright” mark of the fish echo trace itself. These are echo tracks of carp feeding from the bottom with previously scattered food: the bottom fish “screens” the bottom reverberation directly behind its body.

Figure 4 – Side scan echogram.
Vessel is on right sounding to portside 0-50 m

In addition to the algorithms described above, an algorithm for determining the hardness of the bottom surface has been added to the MIEL software.

Determination of the parameters of the seabed surface is widely used for various purposes, in particular, both for mapping the seabed itself and the bottom "flora" and "fauna". The hardness of the bottom surface is determined qualitatively in the range from "Soft" to "Hard". To determine the hardness, the bottom echo strength is calculated [2].

The scattering strength of the bottom surface is calculated for the current (last) ping. The calculated value is indicated in the "Bottom hardness" window in two ways (Figure 5):

1. as a numerical value in dB above the strip of the color code of the bottom hardness;
2. as a narrow vertical bar of black color on the strip of the color code of the bottom hardness; the position of the bar on the palette corresponds to the display area of the color code of the scattering strength value, calculated similarly to the color code of the echogram palette.

Figure 5 – Bottom hardness window

Figure 5 shows the Bottom Hardness window containing a color palette that encodes the value of the scattering strength of the bottom surface. The set of colors corresponds to the palette chosen by the operator to display the echogram.

It is also possible to display the “Bottom hardness” on the echogram as a line repeating the bottom surface profile. The display of the “Bottom hardness” line is carried out by adding to the vertical line of the color code of the echo signal amplitude a certain number of pixels located below the line of the detected echo signal from the bottom surface (by analogy with displaying the “White line”). Thus, the information contained in the echo below the bottom line is not overwritten, but displaced downward. The color of the pixels of the "Bottom Hardness" line corresponds to the color of the palette in Figure 5 at the location of the vertical segment of a bar. The display of the Bottom Hardness line is shown in Figure 6.

Figure 6 – Bottom hardness line

The choice of the line to be displayed ("Bottom line", "White line", "Bottom hardness") is made in the "Lines" dialog box of the user interface menu.

The bottom hardness value is calculated only if an echo from the bottom surface is detected. If the bottom echo signal is absent within the specified range of ranges or if it is lost, the bottom hardness value is not calculated, a dash is displayed in the numeric field of the “Bottom hardness” window, on the echogram the color of the bottom hardness line is replaced with white.

In addition, the MIEL simulator is currently being developed, which will not only train the operator to work with the basic functions of the echo sounder for stock assessment, but also teach the procedure for calibrating the scientific echo sounder. This is a new feature. It is not present in any foreign hydroacoustic simulator today.

MIEL will find wide application in scientific research of fish stocks and in industrial fishing:

• counting the number of fish in real time with horizontal and vertical sounding in traditional modes of areal echo surveys;
• counting the number of fish spawning with a stationary installation of the MIEL transducer;
• provident industrial fishing (taking into account the obtained fish size range) on small-tonnage fishing vessels in coastal sea zones and in inland waters.

References

1. Goncharov S.M., Popov S.B., Dolgov A.N., Kutsenko A.N., Raskita M.A. Test results of a new domestic scientific split-beam echo sounder designed for stock assessment in inland waters. VNIRO Proceedings, v. 177, 2019, p. 167-179.
2. Henry M. Manik. Seabed Identification and Characterization Using Sonar. - Hindawi Publishing Corporation Advances in Acoustics and Vibration Volume 2012, Article ID 532458, 5 pages.