China's 1,400-year-old pagoda gets an AI-aided checkup

XI'AN, May 25 (Xinhua) -- Until now, no one knew the building health and composition of the Giant Wild Goose Pagoda, a 64-meter-tall ancient Buddhist landmark located in Xi'an, the capital of Shaanxi Province in northwestern China. Was the central structure solid? Were there hidden cracks? Would it survive the next big earthquake?

Now, scientists using electromagnetic waves, satellite radar, and artificial intelligence (AI), have gotten to the core of these questions without breaking a single brick.

The UNESCO World Heritage site -- one of China's oldest surviving brick structures dating back nearly 1400 years -- underwent its first full-body checkup which began in May last year, carried out by a team of researchers.

Employing cutting-edge electromagnetic resonance technology, they found the tower is stable, but faces invisible threats.

PEELING BACK THE LAYERS

Until this project, most studies focused only on surface damage of the pagoda, built in 652 during the Tang Dynasty (618-907), such as visible cracks and weathering.

"What was missing was a complete picture, from internal structure to hidden flaws," said Li Dongbo, deputy dean of the College of Science at Xi'an University of Architecture and Technology (XAUAT), an academic director of the project.

The team spent nearly a year building what they call a "space-air-ground" detection system. Satellites mapped the area around the pagoda, including surrounding buildings. Drones created millimeter-accurate 3D models. But use of electromagnetic resonance testing answered the key questions researchers had posed.

"Every material has its own unique electromagnetic resonance frequency, just like a fingerprint," said Miao Yuanyao, the project leader.

By sending specific waves into the pagoda's walls, the team could map its internal anatomy without drilling or sampling.

For the first time, they identified a multi-layer structure: a rammed earth core, Tang Dynasty brickwork, later Ming Dynasty (1368-1644) reinforcements, and an internal wooden pillar system. They also found a 50-millimeter gap between the Tang and Ming layers, a structural separation that had never been documented before.

WATER, SALT AND VIBRATION

But knowing the structure is only half the battle. The team also had to identify the reasons for the pagoda's slow deterioration.

"Water is the number one factor," the project report stated. It seeps from the damp foundation and moves upward through the brick walls, carrying dissolved salts. As the water evaporates, salt crystals form and expand, chipping away at the bricks.

Fu Qiang, a professor of XAUAT's College of Civil Engineering, tested heavily-weathered areas and found salt concentrations dozens of times higher than what is typically found in healthy bricks. In winter, freezing water makes it worse.

"Freeze-thaw cycles and salt crystallization work together, creating a combined effect greater than the sum of its parts," Fu said.

The pagoda also tilts slightly about 0.9 degrees to the northwest. That is not enough to cause immediate collapse, but it amplifies earthquake stress on upper sections by more than 40 percent.

"Environmental vibration is a chronic disease, like high blood pressure. It needs long-term monitoring," Zhang Ruifu said, a researcher from the Shanghai-based Tongji University. "Earthquakes are acute emergencies with huge instantaneous energy that can cause catastrophic damage."

While current vibrations from traffic and tourist footsteps are close to safety limits, the team found that this does not pose a public danger yet. They propose limiting tourist foot traffic during peak hours to reduce vibration stress and that the pagoda be monitored.

AI WRITES PRESCRIPTION

To track surface damage, the team trained an AI system to automatically identify cracks, flaking bricks, and eroded mortar from photographs. "What used to take manual labor several days, the AI system can complete in a few hours with over 90 percent accuracy," team member, Dong Zhenping said.

The team recommends building a high-precision "digital twin" of the pagoda to test different protection strategies before applying them to the real tower, while enabling real-time visual assessment and dynamic early warning of the tower's structural safety status.

"The next phase of research will systematically establish a standardized digital archive of the pagoda's structural safety and a full-life-cycle damage database," Miao said.

"Based on this, we will conduct multi-scenario virtual simulation tests to provide technical support for developing optimal protection strategies for the pagoda."