Underwater Archaeology: A Whisper Across Time

Produced by: Science Popularization China

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

Producer: China Science Expo

The study of "From Fish to Man" explains the long-standing debate about the origin of mankind. For a long time, mankind has always been moving forward in the process of exploring the vast and mysterious origin of life - the ocean. However, the seemingly calm and fascinating sea surface may also bring turbulent waves and take human lives at any time. So, facing the unpredictable ocean, how will humans conduct archaeological research in the ocean?

From ancient times to the present, what is the purpose of underwater archaeology?

In the pre-Qin period, the inhabitants of Lingnan began to engage in commercial activities in the South China Sea and the Pacific coast. After the development of the Qin and Han, Sui and Tang, Song and Yuan dynasties, by the time Zheng He sailed to the West in the Ming Dynasty, the "Maritime Silk Road" had developed to its peak.

Maritime Silk Road

Image source: The Paper https://m.thepaper.cn/newsDetail_forward_14617327

However, as we all know, strong winds and huge waves at sea can bring disaster to sailing ships. Modern ships need to take shelter when encountering wind and waves, not to mention ancient ships. Therefore, after thousands of years, countless ships have sunk on this route.

Most of these shipwrecks are merchant ships, and the goods they carried, such as porcelain, usually represent a high level of craftsmanship at the time. One of the important reasons for conducting underwater archaeology is to bring these exquisite crafts that were once submerged under the sea back to light. In addition, underwater archaeology can also enhance our understanding of ancient shipbuilding, navigation and other technologies, as well as maritime transportation and trade activities.

Sunken Ship

Image source: Sohu https://www.sohu.com/a/196614541_100027108

If you want to do your work well, you must first sharpen your tools: Acoustic methods help underwater archaeology

As the saying goes, "If you want to do your work well, you must first sharpen your tools." Conducting underwater archaeology certainly requires a series of technical and equipment preparations.

In addition to the training of divers and the development and production of diving equipment, how to understand the underwater situation and accurately find archaeological sites is also an extremely important part of underwater archaeological work. In fact, these devices that act as our "eyes and ears" underwater, such as multi-beam sonar, shallow layer profiler, side-scan sonar, underwater robot (Remote Operated Vehicle, ROV), etc., mostly use acoustic methods to conduct underwater detection activities.

Multi-beam sonar, subsurface profiler, side-scan sonar, underwater robot (ROV)

Image source: 1. Qingdao Watertools 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

Here you may have a question - why must we use acoustic methods to detect? Would it be more intuitive to use other methods, such as putting a camera underwater and watching the video or taking pictures directly? Speaking of this question, it involves a phenomenon, that is, the attenuation of electromagnetic waves in water is quite large, which explains multiple problems at the same time:

1. The essence of light is electromagnetic waves within a certain wavelength range. Since sunlight cannot penetrate deep underwater, the seabed is usually quite dark, and visibility is very low below a few dozen meters. For this reason, even if lighting equipment is turned on and videos or images of the seabed are taken, the coverage area is very small. Therefore, underwater archaeological work is usually carried out after a large-scale detection is completed, and after the basic underwater conditions have been understood through acoustic methods and a relatively accurate archaeological site has been located, close-range, small-scale video or photography operations are carried out.

2. Communication methods that rely on electromagnetic waves do not perform well in underwater archaeology, so underwater communication and data transmission generally use wired methods or rely on sound waves for communication and data transmission.

Usually, underwater robots are equipped with camera equipment and can perform underwater photography.

From animal echolocation to how underwater archaeology works

In the above article, we mentioned equipment such as multi-beam sonar, shallow layer profiler and side-scan sonar. So, how do these devices work in underwater environments?

Let me give you an example. We all know that animals such as bats and dolphins have the ability to echolocate. They can emit sound waves, which pass through the air or water to the surface of obstacles and reflect. After these animals receive and analyze the reflected sound waves, they can get information such as the distance, size, and shape of the obstacles in front.

The working principles of these sonar devices used in underwater archaeology have certain similarities with the echolocation of animals, that is, they also emit sound waves to the bottom of the water, receive echoes, and then analyze and process the echoes to get the results we need.

However, the analysis processes and results of different equipment are different. For example, the shallow-sediment profiler obtains geological information of the seabed strata, while the side-scan sonar obtains topographic information of the seabed.

Processing results of multi-beam sonar, side scan sonar and shallow layer profiler

Image source: 1. Blue Ocean http://lcsonar.com/cesaocase/100.html

2. China Surveying and Mapping Network https://www.chinaunsv.com/mall/858.html

3. Beijing Taifukun http://www.time-f.com/product-31395-52569-158001.html

The booming underwater archaeology industry

With the help of these acoustic devices, my country's underwater archaeology has flourished in recent years.

Abroad, the East China Sea Research Station of the Institute of Acoustics, Chinese Academy of Sciences cooperated with Sri Lanka, carried out multiple marine surveys and shipwreck detections in Sri Lankan waters, and discovered a shipwreck; in China, in June 2022, the pre-processing phase of the overall relocation of the "Yangtze River Estuary No. 2" ancient ship was completed, and the boundaries and burial depth of the ancient ship were accurately located, and scattered cultural relics such as the rudder (rudder stock and tiller handle), anchor, cables, building materials, and purple clay teapots were extracted.

I believe that in the future, relying on the unremitting efforts of scientific researchers and with the help of various advanced equipment, my country's underwater archaeology will continue to open up new areas and achieve new breakthroughs.

Editor: Ying Yike