#side-scan sonars
SKNARYA Anatoly Vasilyevich
TRUSILOV Vladimir Tarasovich
SEDOV Maxim Vyacheslavovich
USE OF SIDE-SCAN SONARS TO SOLVE NAVIGATION SAFETY AND ENVIRONMENTAL MONITORING PROBLEMS
The increase in human economic activity in recent decades has led to active involvement of inland waters and the sea shelf in this area of activity, which required the implementation of measures aimed at ensuring safety during work on water areas and environmental monitoring of the consequences of human activity.
There is a need to develop both effective methods and hardware that would allow such activities to be carried out in the shortest possible time and with high quality.
The latter include side-scan sonar (SSS).
The main advantage of SSS is the ability to obtain high-quality acoustic images of the bottom in real time over a large area at virtually any depth and in any body of water.
The acoustic image of the bottom contains enough information that can be used to solve many problems, including such as ensuring the safety of navigation on rivers and canals, analyzing the impact of human activity on the environment, etc.
The quality of acoustic images — this is both better detail and background contrast — has improved significantly in recent years.
This is primarily due to the widespread use of modern digital methods of signal generation and processing in GBO, and the use of a modern element base in them has made it possible to significantly reduce the weight and size characteristics of sonars and make them smart and easy to operate.
GBOs are active hydroacoustic systems.
Acoustic imaging in sonars is based on the fact that the acoustic signal emitted by the transmitting antenna is reflected back to the receiving antenna from any inhomogeneity in the water column along its propagation path. The bottom is also such an inhomogeneity. And the more the acoustic characteristics of water and inhomogeneity differ from each other, the stronger the reflected signal will be.
For example, the acoustic signal is very well reflected from the water-air, water-stone boundary and is very poorly reflected from the water-silt boundary. Based on this, the brightness in the acoustic image will be different and can be used in its analysis to solve a particular problem.
The use of acoustic signals in gas and wind equipment makes them indispensable when carrying out work on large areas, since it allows to significantly reduce the time for their inspection.
The fact is that all other types of vibrations (light and radio waves) have significantly greater attenuation in water and allow to carry out detailed inspections in water at distances of several meters.
In recent years, the task of monitoring the condition of oil and gas pipelines (given the long service life of some of them) across rivers, including prompt detection of leaks from them, has become very relevant.
To solve this problem, the use of gas equipment is very relevant, since it will allow contactless, real-time detection of leak locations using an acoustic image.
Another equally urgent task is to study the degree of pollution of water areas. The latter includes both pollution of the surface of water areas (garbage and oil spills, first of all), and pollution, heavier, which is deposited on the bottom. And if the former are easily detected even purely visually, then the latter are not so easy to detect.
In this case, the use of a sonar can also be very useful, since it will allow you to quickly survey a large area of the bottom and identify individual sections of the bottom for further, more detailed research.
As an example, Fig. 1 shows an acoustic image of a section of the Black Sea bottom at the mouth of a river, obtained during tests of the GBO «Gidra» in the summer of 2002.
This sonar was specially developed for operation from small boats and motorboats and is intended for operation in water bodies with a depth of up to 50 meters. The operating frequency of the sonar was chosen to be 240 kHz, which made it possible to make a small-sized antenna system on the one hand and to obtain a high range resolution of 4 cm on the other hand.
Such a high range resolution, together with the use of digital methods of processing and signal generation in the sonar, made it possible to obtain high-quality acoustic images of the bottom, and the use of a linear frequency-modulated probing signal increased its range.
If you look at the acoustic image, the vessel moved from bottom to top. The line of the vessel's movement coincides with the vertical scale in the center of the image, which shows the distance traveled by the vessel in conventional units of length.
The acoustic image is divided into two parts, to the left of the vertical scale is shown the acoustic image of the bottom from the left side, and to the right — from the right side. Above, above the acoustic images of the bottom of the left and right sides, two scales are shown — these scales show the slant range, respectively, for the left and right images, in meters. The viewing strip for the right image is more than 90 meters, and for the left — more than 110 meters.
The dark strip to the left and right of the vertical scale is the layer of water between the sonar receiving and transmitting antenna and the bottom, and the boundary of this dark strip can be used to judge the depth under the vessel. The depth varied from 6 meters at the top to 10 meters at the bottom.
In this place the sea bottom is rocky, the acoustic image shows mountain folds as bright areas, and in the center of the image is a muddy river bed on the sea bottom.
Silt inclusions are very clearly visible in the form of dark spots in Fig. 2.
Fig. 1
Fig. 2
A special class of problems that can be successfully solved with the help of gas-canceling equipment is ensuring the safety of navigation. The capabilities of gas-canceling equipment for solving this problem are illustrated in the following two figures.
Figure 3 shows an acoustic image of the bottom section of the southern part of the Astrakhan Canal, also obtained during the tests of the Hydra sonar.
This figure clearly shows the area and boundary of the dredging equipment (marked with number 1). The depth under the vessel was 7 meters.
Numbers 2 and 3 show the water layer and the bottom line under the vessel (depth), respectively, and number 4 shows a small hole.
Fig. 4 shows an acoustic image of a section of the bottom, also obtained in the southern part of the Astrakhan channel.
The vessel was moving from bottom to top, the line of the vessel's movement is indicated in this figure by the number 2. To the left of this line is shown an acoustic image obtained from the left side of the vessel, and to the right — from the right side of the vessel.
In the center of the figure is shown the conventional scale of the distance traveled by the vessel, and above is the slant range in meters, on the left — for the left side, on the right — for the right side. The number 5 shows the layer of water between the antenna and the bottom, and the number 3 — the bottom line of the left and right sides (the first reflection from the bottom).
The peculiarity of this tack is the following. At the very beginning (the very bottom of the image) the vessel was moving away from the main channel, in this place, according to the upper scale, the depth was about 3 meters.
At the same time, on the right (number 4) you can see the main channel as a dark strip of irregular shape.
It is clear that it is quite narrow, its width is slightly more than 10 meters. Then the vessel entered the shipping part of the channel, the depth immediately increased to 8 meters (according to the upper scale).
At the same time, on the left in the acoustic image (1) you can see the results of the dredging equipment and, according to the upper scale, estimate the boundaries of its work in meters. The jaggedness of its edge is clearly visible.
Fig. 3
Fig. 4
The high resolution of sonars allows them to be used as a search system when clearing fairways to ensure the safety of navigation. Thus, in Fig. 3, the number 6 indicates small objects (the smallest of which is less than 1 meter in size) lying on the bottom, and the number 5 indicates a trace left on the bottom by a ship's anchor.
Conducting search operations at sea is a task in which side-scan sonars play, if not a primary, then a very important role. And this is due, first of all, to the high productivity of these systems and the high detail of the acoustic image of the bottom area being studied.
Thanks to these characteristics, it is possible to explore large areas of the bottom in a fairly short time and identify only individual areas for further, more detailed research.
Then more detailed research is carried out using video or photo equipment both from unmanned and manned vehicles, up to research using divers, if the latter is possible.
An example of this technology is the search for and inspection of the submarine Kursk, which tragically perished in the Barents Sea.
The use of side-scan sonars is especially relevant for searching for objects at great depths.
In this case, along with near-surface SSS, bottom SSS can also be used (this depends on the task being solved).
The former allow for a survey of a large surface area of the bottom, while the latter allow for a more detailed search in individual areas of the bottom.
To conduct an even more detailed search, it is possible to use underwater vehicles that are launched to known areas of the bottom.
In the mid-80s, such technology was used, for example, to search for and outline iron-manganese nodules (IMN) on the bottom of the Indian and Pacific Oceans.
At first, using a near-surface side-scan sonar (these were the Okean SSS, which still exists in the Yuzhmorgeologiya association, and the Sonac-LF SSS, which was installed on the Akademik Yoffe vessel of the P. P. Shirshov Institute of Oceanology of the USSR Academy of Sciences), a survey of the surface of the ocean floor at depths of 5,000 — 6,000 meters was carried out.
These sonars allow obtaining an acoustic image of the bottom in a strip of up to 30 km on both sides.
Another feature of these sonars was that they were the first in the world to use linear frequency-modulated probing signals, and they were formed digitally and processed using a computer.
It was thanks to the use of these signals that it was possible to obtain such a large viewing band with a peak radiation power of only a few hundred watts.
The resulting acoustic image of the bottom was analyzed and, based on the results of the analysis, an unmanned vehicle with video and photo equipment was lowered to the bottom and towed at a small height from it, and then soil samples were taken using samplers.
This technology proved to be very effective and was used for several years both in the USSR Academy of Sciences and in the USSR Ministry of Geology. It remains relevant today.
Another example of using SSS to search for objects at great depths is the search for sunken submarines, among which we can name the submarine «Komsomolets», which sank at a depth of more than 1,500 meters.
SSS allows you to detect objects on the seabed that are much smaller in size — a few meters.
An example of this is the acoustic image shown in Fig. 5 – this is an acoustic image of a military aircraft from the Second World War, which lies at a depth of about 36 meters on the bottom of the Black Sea.
And Fig. 6 illustrates the possibility of conducting search operations using the example of a larger object – the steamship “Admiral Nakhimov” (these acoustic images were obtained during tests of the “Gidra” gas-sensing equipment).
Fig. 5
Fig. 6
In addition to solving the problem of searching for sunken objects both at great and at shallow depths, sonars help to solve another problem – the problem of ensuring safe navigation in difficult or unfamiliar conditions. For this, sector-scan sonars are used. For example, sonars from Interphase (one of the latest models is the PC 180). Furuno and Simrad also produce a whole range of sector-scan sonars.
These sonars allow you to search for, detect dangerous objects or dangerous bottom elevations along the vessel's course.
The solution to this problem is also relevant when moving in difficult ice conditions, when the dimensions of the underwater part of the glacier can only be detected and determined using these sonars.
Currently, underwater space is scanned by scanning a single beam, usually mechanically, less often electronically. It takes a long time to scan a large sector in this way, which imposes restrictions on the maximum speed of the vessel. For high-speed vessels, it is necessary to solve the problem of scanning the space ahead in a different way — by forming several beams at once.
An acoustic image alone is not enough to ensure the safety of movement on water.
A depth map, a bathymetric map, is also required. This problem is traditionally solved using single-beam survey echo sounders, an example of which is the well-proven series of domestic echo sounders PL, in particular the echo sounder PL-5.
However, their use for solving problems of constructing bottom relief over large areas has a number of significant disadvantages — this is a long shooting time and insufficient accuracy of constructing bottom relief. To overcome these disadvantages, multi-beam echo sounders were developed, which have recently found wide application.
However, to obtain reliable results of surveying sections of the bottom of reservoirs when solving the problem of ensuring the safety of movement along them, it is desirable to combine the acoustic image of the bottom section and its relief.
This is due to the fact that only an acoustic image can reveal such hazards as free-standing piles or other objects protruding from the bottom.
One of the signs of identifying such hazards is the presence of their shadow, which is clearly visible on the acoustic image.
As an example, consider the acoustic image of the bottom shown in Fig. 7.
This acoustic image was obtained during tests of the Hydra-2 sonar while passing along the coast near Novorossiysk.
On the left side of the image, mountain ranges are visible in the form of parallel lines going from bottom to top.
These ranges protrude from the bottom, which is visible by the shadows after each of them (dark areas to the left of the light stripes), and the more the range protrudes, the longer the shadow.
Fig. 7
Multi-beam echo sounders do not fully provide a solution to this problem due to the lack of high-quality acoustic imaging; it can be successfully fully solved using another class of sonars – interferometric side-scan sonars, a representative of which is the hydrographic complex for areal bottom survey – “Hydra-2”, as well as the “Hydra” SSS, developed by NPF “Ekran”.
As an illustration of the use of bottom relief data in solving the problem of ensuring the safety of movement on rivers, we can use Fig. 8, which shows a bathymetric map of the bottom section of the Astrakhan channel, obtained during the tests of the Hydra-2 sonar. This map was obtained after processing the data for only one tack and is constructed in a rectangular coordinate system with a depth step of 0.25 meters.
This figure shows that in the middle part there is a rather steep slope, and it coincides with the boundary of the shipping channel.
Fig. 8
In conclusion, it should be said that sonars are a necessary and important tool in solving the problems described above, and their role and importance in solving these problems, as well as others, will only increase in the future.