세종대학교

Technological Trends and Development Directions in Predicting Mine Burial Rates

Graduate School of Sejong University

Professor Lee Geun-hwa, Department of Ocean Systems Engineering 

Mine: Cheap asymmetrical underwater power

A girwe, also known as an underwater mine, is a bomb installed in the ocean to destroy vessels. It has the same function as a landmine.  While the word for mine(jirwe) has ji, which means the earth or ground, girwe is not associated with the ocean. Actually, girwe is the abbreviation of gigyesurwe. Considering a case that in the book The Theory of Military Power (Hungiksinjogungidoseo) written during the regime of Emperor Gojong at the end of Joseon Dynasty, surwe is believed to refer to a bomb exploding in the water. The word girwe used in modern times is thought to have been coined by the Japanese Rangakusya until recently. For reference, eorwe is the abbreviation of eohyeongsurwe, and in Japan, both girwe and eorwe are called surwe. However in Korea, the term does not seem to be exactly defined. Girwe in its early stages was installed as a floating or mooring type with positive buoyancy relative to itself, but nowadays, it is mostly a bottom mine type that is spread on the sea-bed with negative buoyancy relative to girwe.

Figure 1. Types of Girwe (Szturomski, 2015). For reference, the operation methods of girwe (underwater mine) are classified into direct impact, magnetic mine, electric mine, pressure mine, acoustic mine and acoustic trigger.

Figure 2. (a) Example of underwater mine, (b) Mooring mine (Source: Google Image)

The reason that underwater mines are so formidable is their cost-effectiveness. In principle, an underwater mine is simply a bomb that explodes upon activation, so there should be no reason that it be expensive. In general, However, in general, a single mine can cost millions of won, and the latest models equipped with self-propulsion and compound sensing can cost hundreds of millions of won. However, the prices of naval vessels that can be damaged by underwater mines are at least hundreds of billions of won, and Jeongjo the Great, which is the latest AEGIS of the Korean navy, costs over 1.2 trillion won. If an enemy imposes a blockade of major naval ports using underwater mines, or key vessels get damaged by mines, the damage could be astronomical. In fact, during the Korean War in 1950, North Korea delayed the Wonsan landing operations of coalition naval forces for a while to install mines at the port. In the recent Russia-Ukraine war, the two countries respectively scattered thousands of mines at major naval ports around the Black Sea. Therefore, import-export routes were closed, and damage to freighters and naval vessels of nearby countries occurred in surprising amounts.

Hunt when you can, sweep when you must

There are to methods to remove mines; minehunting and minesweeping. Minehunting refers to the process of searching, detecting and removing mines in the water. In the minesweeping, the procedures of searching and detecting are omitted. It is to remove mines by scanning suspected areas with sweeping equipment. In the USA, a vessel that performs the former is called a minehunter, and that of the latter is minesweeper. Of course, a minehunter is bigger because it has to load a lot of cutting-edge equipment. Minesweepers of Korea generally perform both minehunting and minesweeping. 

Which method to remove underwater mines completely depends on the commander’s judgment. In general, the minesweeping method is adopted if searching mines is quite difficult. Often the ocean is too rough and there is no light at a few meters of depth. Minesweeping is also utilized when it takes too much time to search for mines due to a vast number of them. The minesweeping method is quicker than minehunting, but it has uncertainties as well. Minesweeping is done only on mines that react to electromagnetism, but what if there were mines that react to acoustic elements? Mechanical sweep gear was used on entire areas, but what if a few mines escaped the weep gear? The minesweeping method is fast, but always comes with risks. That’s why, the US Navy always recalls ‘Hunt as you can, sweep as you must’ when engaging in mine warfare.

Searching mines: Sound volume detection using the SONAR and its limitation  

SONAR (sound navigation and ranging) is used to search underwater mines. It is hard for light to penetrate water due to high turbidity, and electromagnetic waves that are useful on land have a high decay rate in water and cannot be used. The equipment mostly used is side scan sonar. Side scan sonar is a type of underwater imaging system that uses a sensor and signal conditioning device attached to the side of a tow-fish that is towed by a command ship. It scans the sea-bed with sound according to the directions of the tow-fish. The principle of the side scan sonar is to generate a sound beam using a piezoelectric element to irradiate the beam on the sea-bed and to make images using the signal intensity of sound waves. Recently, the resolution of sound images is as high as 4cm with the wide application of the multi-beam side scan technology and the Synthetic Aperture Sonar, but when compared with the resolution of optics image under 0.1mm pixel size, there is no sensation with naked eye. In addition, sound waves have no color information, so putting realistic colors with a three-dimensional effect on sound images is another task to be solved. The most difficult part in searching mines with sound images is when searching bottom mines on the sea-bed. The sea-bed is not as hard as concrete or steel. Mostly, it is covered with sediment like fragile sand or mud that originated from the land or the air by volcanic eruption. When bottom mines are dropped from the air or water, they are stuck in the sediment by fall shock. Partially stuck bottom mines are called partially buried mines, and completely stuck mines are called buried mines. As these buried mines are shown crushed or not shown in the SONAR, it is impossible to search with a simple side scan sonar. 

Figure 3. Image gained by side scan sonar (Source: www.edgetech.com). The white line in the middle is the trajectory of the tow-fish.

Figure 4. A dummy mine (Source: www.whoi.edu/oceanus/fature)

Prediction of the percentage of mines buried: Measuring and physical modeling 

The mine burial rate is the quantitative measure of the degree to which mines are buried in sea-floor sediments. It is defined as the ratio of the buried volume of mines to the total volume of mines. While it may be difficult to locate buried mines through searching, their management in major harbors and sea routes is crucial. As mentioned earlier, the completeness of mine burial depends on the marine environment and the manner in which the mine is dropped. Therefore, predicting the percentage of buried mines (m) in major seas can provide valuable information for mine countermeasures and sweeping operations. Additionally, prior knowledge of mine burial can aid in sound detection and enable more precise observation of images. 

The most direct method to predict the mine burial rate is to drop dummy mines into each sea. The problem is the cost. The cost to set an experiment ship afloat on the ocean is generally tens of millions of won. Searching the entirety of the seas adjacent to the Korean Peninsula comes at a tremendous cost. Even if the budget is secured, time is also a limiting factor. While dropping mines may be easy, measuring the rate of mine burial and collecting dropped mines is a time-consuming task. Therefore, conducting measurements in the ocean should be done sparingly, taking into account both time and space constraints.  Actually, this is the problem not only for marine researchers but also for those who conduct research in the environment of space. 

Figure 5. Model dropping experiment by each dropping angle conducted by Sejong University Underwater Sound Lab

Another method to predict the mine burial rate is to use a physical model. To lay mines, they drop mines in the water from helicopters, submarines and warships. From a  mechanical perspective, laying mines can be considered as a falling body of motion. A mine is a rigid body that has a finite volume and is either in the shape of a cylinder or a broken cone. It is necessary to consider the six degrees of freedom that include rotation.  There are three mediums that can pass through mines: air, water and sub-marine sediment. Therefore, when establishing an equation of motion, it is important to reflect the dynamic relations between the external medium and the motion of the mine, taking into account the three different phases of the medium (air, water, and sub-marine sediment). The results show that mines exhibited various behaviors depend on their center of gravity and initial conditions when they were dropped, indicating that similar simulations are possible using physical models.   Calculating the mine burial rate with a physical model is significantly more favorable with respect to time and money compared to direct measurement. Nevertheless, marine environment data such as sub-marine sediment, particle size and depth of water are necessary to operate a physical model, and to enhance the accuracy of physical model, the range of parameters should be limited and updated through comparison with actual measurement data. 

Figure 6. Comparison of result of model fall test and prediction of physical model

Latter mine burial rate: The sea is always changing. 

The mine burial rate explained above refers specifically to the early mine burial rate, which is the rate at which mines are buried as they fall. This initial burial rate is known as the early mine burial rate.  Mines do not work as soon as they are laid. They need time until missions are completed. Lost mines can remain for months or years deposited under the sea-bed. Unfortunately, the underwater environment is not static as there are waves on the surface of the sea and sub-layer currents on the sea-bed. Currents keep transporting sub-marine sediment. Thus, some mines were laid on the sea-bed at first, but were buried later, and vice versa. The mine burial rate that changes over time is known as the latter mine burial rate. The main physical phenomena related to the latter mine burial rate include scour, bedform migration, and liquefaction of sub-marine sediment. Scouring occurs when the sub-marine sediment around the mine is washed away by bottom layer currents, causing furrows in the surrounding area. This phenomenon is commonly observed in bridges and fixed marine structures. Bedform migration refers to the macroscopic migration of sub-marine sediment hills or curves along the direction of the current. It is similar to the movement of sand dunes in the desert. The liquefaction of sub-marine sediment is a phenomenon in which the inside of the sediment particles has increased the pore water pressure by repetitive sharing force or strong impact, and loses sharing strength. Therefore, it behaves like liquid. Liquefaction occurs in a lot in sand sediment with insufficient adhesive strength and can cause a mine to subside or change its position.  

Figure 7. Measurement of currents around an object laid on the seabed (Acoustic Doppler Current Profiler)

While it is theoretically possible to predict the latter mine burial rate using a physical model, it is not currently practical because it needs large scale numerical analysis on marine fluid and a significant amount of marine environment data. It might be possible in the distant future when quantum computers are available for personal computers. Therefore, the latter mine burial rate is predicted mostly with an empirical formula or analytical expression acquired in a simple environment. The latter mine burial rate is expressed by values that change over time, unlike the early mine burial rate. The rate of change is almost proportional to the timescale of marine environmental changes that affect the mine burial rate. Figure 8 below shows the change in the mine burial rate due to layer structure movement using geometrical methods.

Figure 8. An example burial depth change by the movement of dune in time

Development direction of prediction of mine burial rate: Improving the accuracy of measurement and physical model 

Predicting mine burial rates accurately is essentially similar to forecasting the weather. When the accuracy of actual measurement data is enhanced along with that of physical model, the accuracy of the prediction is enhanced when the two data are assimilated. Traditionally, there were two methods to measure mine burial rates in actual sea. One is manual measurement by diver after a dummy mine is dropped. Another way to estimate the latter mine burial rate is to indirectly assume it based on the properties of the sea-bed sediment obtained through marine research. However, this method can be costly and may not be very accurate. With the above methods, measuring the latter mine burial rate is hardly impractical as it may take weeks or months of observation. One method to solve this is to apply the latest sensing technology to dummy mines. It can measure mine burial rate directly after acquiring geometrical information about burial state by attaching an optical or sound sensor to the surface of dummy mine. 

Another method for estimating the latter mine burial rate is through sound detection using sonar technology. By analyzing sound images acquired from a side-scan sonar, the burial rate of partially buried mines can be estimated. However, sonar images have lower resolution and are often noisy, so the application of artificial intelligence technology is necessary to improve accuracy. In theory, low-frequency sonar with synthetic aperture technology uses low frequency waves that can penetrate strata, making it possible to detect buried mines.  When this data is visualized, the posture and location information of buried mine can be indirectly acquired. Making low frequency sensors and securing signal generation technology are very difficult. Measurements with the use of the sonar can be done in unmanned undersea vehicles or unmanned surface vehicles, which is more cost effective. 

As the accuracy of measurement improves, it is necessary to advance the physical model to enhance its accuracy. One of the main challenges encountered when using physical models to predict mine burial rates is ensuring the validity of assumptions used in the model, as well as deciding on kinetic parameters such as additional mass coefficient and drag coefficient. In particular, deciding on hydrodynamic coefficients is critical for kinetosomes, such as mines, which change position over time. Therefore, it is necessary to obtain hydrodynamic coefficients through numerical experiments or model/actual ship experiments under various conditions. Ultimately, the accuracy of the physical model's prediction can be enhanced through data assimilation, which combines measured values from the actual sea with estimated values from the physical model.

Conclusion 

In this study, mine burial rate prediction models are introduced along with their direction of future development. If we consider mine warfare from a broader perspective, we can see that the recent trend is towards unmanned systems. In mine warfare, combatants are always directly exposed to mines, and even with thorough preparation, accidents can still occur. The best way to minimize these risks is to make mine warfare unmanned. Instead of investing in expensive minesweepers, it is much better to operate several low-cost unmanned surface vehicles and undersea vehicles in order to minimize the risk and acquire information more effectively. In order to realize the full potential of unmanned warfare, we need to improve the level of intelligence of the targeted systems. The future of mine warfare will involve operating unmanned ships cooperatively and automatically classifying, identifying, and removing mines using collected sound and optical information.  Sejong University also conducts various key studies related to unmanned technology of future weapon systems and artificial intelligence technology in collaboration with each other. National defense R&D is difficult for students at the undergraduate level due to the complex nature of the convergence studies, so most work is done at the graduate level. The graduate school also plays a vital role in providing manpower and research human resources. It is our hope that many outstanding students from Sejong University will take an interest in national defense R&D for each branch of the military, with a special focus on naval arms systems. As I finish this paper, I would like to express my gratitude to Professors Chu Yeong-min and Hong Woo-yeong of the Sejong University Underwater Sound/Signal Process Group, Professor Kim Se-won of the Autonomous Navigation Lab, and Professor Kim Jin-hwang of the Department of Defense System Engineering. 

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