Seafloor acoustic detection - clearing the fog under the deep sea

Produced by: Science Popularization China

Author: Guo Zheng (East China Sea Research Station, Institute of Acoustics, Chinese Academy of Sciences)

Producer: China Science Expo

On the Internet, the Earth is often referred to as the "Blue Planet" because the Earth's surface is "70% water and 30% land", that is, the land area accounts for about 30% of the Earth's surface, and the ocean area accounts for about 70% of the Earth's surface. So in the universe, looking around, the Earth looks like a blue planet.

The ocean not only adds blue to the earth, but also contains a large number of valuable resources, such as oil, natural gas, combustible ice, polymetallic crusts, etc. Therefore, if we can explore the mysteries in the ocean, it will bring convenience to the development and utilization of resources. In addition, understanding the geological structure and topography of the seabed is also of great significance for studying the movement of the earth's plates and the changes in the earth's strata.

How can we explore the mysterious and vast ocean? Acoustic detection is an effective means of seabed detection and is of great research value.

Figure 1: Test mining of combustible ice in the South China Sea and seabed polymetallic crusts

(Image source: 1. Sohu https://m.sohu.com/a/249639518_100194551/?pvid=000115_3w_a

2. Shandong Ocean https://mp.weixin.qq.com/s/oNCFBM-STiQCpaCmxzFm4Q）

The challenges of seafloor acoustic detection

We all know the saying, "The sky is high, birds fly freely; the sea is wide, fish leap freely." But have you ever thought about why we say "the sea is wide" instead of "the sky is high"?

This is because as the sea gets deeper, the pressure of the water environment will also increase. Except for some organisms that have adapted to the high pressure in the deep sea, other organisms will inevitably not be able to survive in such a high pressure environment. In addition, the lack of light and low temperature make the deep sea a paradise for extremely prosperous life. (In fact, birds cannot fly freely even if the sky is high. Due to a similar principle, the air is thin at high altitudes, so it is difficult for birds to survive at high altitudes.)

The depth of the sea has naturally become one of the difficulties in our acoustic detection of the seabed. It is difficult for ordinary organisms to survive such high pressure on the seabed, but fortunately we have a variety of detection instruments.

Figure 2 The deep sea floor and its various forms of deep sea creatures: seafloor communities, anglerfish, "Dumbo", sun jellyfish

(Image source: Sohu https://www.sohu.com/picture/240705294)

Seabed acoustic detection instruments and simple principles

As the saying goes, "If you want to do your work well, you must first sharpen your tools." Carrying out seabed acoustic detection research certainly requires a series of technical and equipment preparations. We need some equipment that acts as our "eyes and ears" underwater, such as multi-beam sonar, shallow layer profiler, side-scan sonar, underwater robot (Remote Operated Vehicle, ROV), etc. Most of these devices use acoustic methods to conduct underwater detection activities.

Figure 3 Multibeam sonar, subsurface profiler, side-scan sonar, underwater robot (ROV)

(Image source: 1. Qingdao Water Tools http://www.watertools.cn/index.php/product/info/311?one_id=248&two_id=323&parent_id=289

2. Qingdao Water Tools http://www.watertools.cn/index.php/product/info/303?one_id=248&two_id=294&parent_id=289

3. Sohu https://www.sohu.com/a/349110652_743632

4. Sohu http://it.sohu.com/a/442729792_100275934)

So how do these devices work?

The working principles of these underwater detection sonar devices have something in common with the echolocation of animals, that is, they also emit sound waves to the seabed, receive echoes, and then analyze and process the echoes to get the results we need.

Why can these sonar devices determine different terrain and seabed composition by emitting sound waves to the seabed and analyzing and processing the echoes?

Similar to the reflection of light that we see in our daily lives, sound waves will also be reflected when they propagate to the surface of a medium. After the sonar equipment transmits sound waves to the seabed, the hard and convex seabed will have a stronger echo; the soft and concave seabed will have a weaker echo; and the obscured seabed will not produce an echo. At the same time, the closer the distance, the stronger the echo; the farther the distance, the weaker the echo. After comprehensive analysis and processing of this information, we can obtain information such as the topography and composition of the seabed.

Figure 4 Seabed plan

(Photo source: Blue Ocean http://lcsonar.com/cesaocase/98.html)

Some application directions of seabed acoustic detection

As a universal technology, seabed acoustic detection is widely used in many fields.

Before the construction of large facilities such as cross-sea bridges and offshore oil platforms, the seabed topography and bottom texture are usually surveyed. This is when equipment such as side-scan sonar, multi-beam sonar and shallow stratigraphic profilers come in handy. When we need to detect whether the seabed contains mineral deposits or to find out the reserves of minerals, seabed acoustic detection methods and equipment will also be used. When the Jiaolong and Fendouzhe manned submersibles dive deep, they also carry seabed acoustic detection equipment to conduct seabed detection work.

In short, in the fields of engineering, production and scientific research, seabed acoustic detection technology is widely used and will have even more extensive applications in the future.

Editor: Guo Yaxin