How big a fuss is needed to carry out an archaeological "Haidilao"?

Review expert: Cai Dawei, Professor of the School of Archaeology, Jilin University

Whether in cultural works or in real scientific expeditions, ancient shipwrecks hidden under the water are full of mystery. However, unlike the plots in cultural films and television works where the protagonists explore the deep sea as if nothing were there, in real-world archaeological work, it is extremely difficult to uncover the secrets behind the shipwrecks hidden deep under the water.

In 2015, during an underwater survey in the Hengsha waters of Chongming, Yangtze River Estuary, the Shanghai Municipal Cultural Relics Bureau accidentally discovered a wooden shipwreck and named it "Yangtze River Estuary No. 2". In 2022, Yangtze River Estuary No. 2 was finally salvaged in its entirety. This ancient ship is the largest wooden ancient ship discovered in China, setting many world records.

The main operation vessel of the salvage project, "Da Li"

Source: Shangguan News

For such precious cultural relics, the salvage work is naturally carried out with great care, and various underwater archaeological techniques have come to the fore in this salvage project. This article will give you a brief introduction to underwater archaeology and the techniques involved.

Finding secrets underwater: underwater archaeology

Underwater archaeology is a branch of archaeology . As the name suggests, it is a discipline that studies underwater human cultural relics. Underwater archaeology must follow the principles of archaeology and be conducted in accordance with relevant archaeological theories and technical specifications. In addition, comprehensive historical and archaeological research is required for underwater sites.

In the 1980s, underwater archaeology was introduced to China. Thanks to the efforts of a group of underwater archaeologists, my country has achieved important results in the investigation, excavation and protection of many underwater sites or water-related relics, such as the "Nanhai No. 1" and "Bajiao No. 1" Song Dynasty shipwreck sites, coastal sand dunes and shell mounds.

Although underwater archaeology can be considered to be in its infancy in the archaeological system, it still has great potential in the study of ancient and modern Sino-foreign trade, water transportation, ship history, etc.

Underwater shipwreck source: pexels

Although underwater archaeology is relatively young, we have summarized various archaeological methods and technologies through continuous exploration. In addition to underwater photography, ocean scanning, ocean measurement and other technologies, there is also the original technology used in the "Yangtze River Estuary No. 2" project - the arc beam non-contact overall transfer of cultural relics.

Ocean Scanning Sonar Technology

The first thing to introduce is the ocean scanning sonar. As a convenient and easy-to-use "artifact" in ocean exploration, sonar naturally has a very high status in ocean exploration. In underwater archaeology, sonar detection also plays a pivotal role.

Ocean scanning sonar technology is mainly used to scan the seabed over a large area. Among the various sonar technologies, three types are the most widely used: side scan sonar, multi-beam sonar, and synthetic aperture sonar . Since these three types of sonar have different working principles and application technologies, they also have different advantages and applicable scenarios.

Sonar system

Source: China Youth Daily

Side-scan sonar is towed by a mother ship to navigate in the sea. Compared with other sonar technologies, although side-scan sonar may have problems with unstable speed or jitter during operation, it can obtain higher-definition images and clearer terrain display. In addition, the on-site operation of side-scan sonar is simple, and the detection results are intuitive. It can read and recognize images on site, and realize on-site judgment of abnormal conditions of underwater detection defects.

Multi-beam sonar uses wide-coverage, high-resolution multi-beam bathymetric technology, roll stabilization technology and other technologies to achieve the unification of the two indicators of bathymetric depth and coverage width and compensate for data errors caused by roll.

Composite image of multi-beam sonar scanning of the ancient ship "Yangtze River Estuary No. 2"

Source: News Photo

Multi-pulse technology increases the amount of information in a single detection and improves the refresh rate by emitting pulse sound signals of different frequencies in multiple directions. In addition, there is synthetic aperture sonar technology, which has the characteristics of high resolution, high processing efficiency for large amounts of data, and the heading resolution is independent of the effective distance and signal frequency.

The significance of underwater filming

Underwater photography and other work are indispensable in underwater archaeology. Due to the special underwater shooting environment, when diving and shooting, divers are required to master the shooting equipment and be familiar with the characteristics of the ocean, so as to take underwater photos with research significance.

During the salvage of the ancient ship at the Yangtze River Estuary, a more advanced intelligent underwater camera system was used. This system was developed to cope with the complex underwater archaeological environment of the Yangtze River. This system made extensive use of intelligent identification and operation, including unmanned boats and automatic identification systems, and made great contributions to the underwater archaeology of the Yangtze River Estuary.

Although there is an intelligent system, the photos of the ruins sent back are still blurry. At this time, Li Xiaomao, an engineer from Shanghai University, bravely took on the task and enhanced the images through professional means to obtain clear underwater images.

Original "arc-beam non-contact overall transfer technology of cultural relics"

In 2007, the "Nanhai No. 1" was salvaged, creating a new salvage solution - the salvage of ancient ships as a whole . 15 years later, this salvage method was applied to the salvage of the "Changjiangkou No. 2". This salvage also opened up a number of new technologies, the most special of which is the "arc beam non-contact overall transfer technology of cultural relics", which is the first in the world.

Due to the high sediment content and turbulent water flow in the Yangtze River Estuary, the "Yangtze River Estuary No. 1" cannot be salvaged using segmented caissons. Fortunately, after research by relevant institutions, the "arc-beam non-contact overall transfer technology for cultural relics" was created. Its principle is to use an arc-beam to form a huge caisson at the bottom of the sunken ship, salvage the sunken ship together with the nearby sediment, and then transport it to the destination by an engineering ship.

Before salvaging, workers used water to flush the silt around the caisson to reduce the adsorption of sea mud on the caisson.

Source: Shangguan News

During the salvage, in order to stably support the ancient ship, 22 sets of curved beams and 46 sets of steel strands were used to fix it. After the huge curved beam No. 22 held the ancient ship without contact, it was lifted at a speed of 2 meters per hour. After more than two hours, the ancient ship was successfully lifted off the bottom. Then, at a speed of 3-4 meters per hour, it took more than two hours of ascent to embed the ancient ship into the engineering ship "Fenli". In this process, more than a dozen advanced technologies such as hydraulic synchronous lifting technology and integrated monitoring system were integrated.

Curved beam through beam scene Source｜Shanghai Municipal Administration of Culture and Tourism

Although the Yangtze River estuary is a muddy water area and salvage is very difficult, the staff were not discouraged by the difficulties. During this process, Peng Yan, director of the Shanghai University Unmanned Boat Engineering Research Institute, led the team to meet the challenges and solve many technical problems. For example, they innovatively solved the problem of collecting ultra-low frequency energy, which can realize the real-time collection of seabed energy and monitor underwater cultural relics for a long time.

The unmanned boats developed by Peng Yan's team also played an irreplaceable role in the research process. For example, the "Jinghai No. 6" unmanned exploration boat was responsible for collecting regional data.

The source of the Yangtze River Estuary No. 2 ancient ship｜Xinhua News Agency

Underwater sites are beautiful and wonderful. Even after hundreds or thousands of years, we can still read their stories. This is the significance of underwater archaeology. Under the vast water, there are still many stories about human civilization waiting for us to explore and read.