After nearly a year of systematic surveys, the Great Wild Goose Pagoda in Xi'an in northwest China received its first-ever "full-body health check report." This square, pavilion-style brick pagoda dating from the Tang Dynasty (618-907) has stood for over 1,300 years. For the first time, its internal structure has been clearly revealed through a 3D model.
The report reveals the pagoda's multi-layered, composite structure and systematically assesses risks such as weathering, water damage, and vibration. It also proposes scientifically grounded conservation recommendations.
An ancient pagoda under CT scan
The project to survey the structure of the Great Wild Goose Pagoda and assess its stability was launched in July 2025. The project team established an integrated detection system. Satellite remote sensing revealed an average subsidence rate of merely 1.76 millimeters within a 100-meter radius of the pagoda over the past three years, indicating overall stability. Drone-based oblique photogrammetry and dense matching algorithms were employed to reconstruct a 3D model with millimeter-level resolution, making every brick crack and instance of mortar crumbling clearly discernible.
"Every material has a unique electromagnetic resonance frequency, much like a human fingerprint," explained project leader Miao Yuanyao. By emitting electromagnetic waves at specific frequencies, the technology can clearly identify material composition and layered structures, as well as detect hidden defects, such as voids and cracks, without causing any damage to the structure. The pagoda's complex structural composition was uncovered for the first time: a rammed-earth core, Tang Dynasty brick masonry, Ming Dynasty (1368-1644) brickwork repairs, and a timber column support system.
Deciphering the 'medical history' hidden in brick joints
Identifying visible signs of damage was the first step. Understanding the underlying causes is equally crucial. Professor Fu Qiang from Xi'an University of Architecture and Technology revealed the microscopic mechanisms behind this deterioration.
Samples taken from the areas where the pagoda's structure was crumbling showed that salt concentrations in the severely weathered zones were dozens of times higher than in unweathered bricks. The soluble salts migrate within the bricks alongside moisture, repeatedly crystallizing and dissolving. This process acts like a microscopic chisel, gradually disintegrating the brick structure.
"Soluble salts are the direct trigger for surface deterioration," Professor Fu said.
Winter freeze-thaw cycles and salt erosion create a synergistic effect as well. Laboratory simulations demonstrated that the combined pressure from expanding ice crystals and salt crystallization produces cumulative damage greater than the sum of its parts. Using non-destructive microwave testing, the team mapped the pagoda's moisture distribution.
"Moisture is the primary factor driving the deterioration of ancient brick pagodas," the report stated. Sun Qi, deputy director of the Great Wild Goose Pagoda Preservation Office, added, "In recent years, precipitation has been the main factor destroying the pagoda."
Combating invisible enemies
In addition to weathering and moisture-related issues, the pagoda is threatened by two invisible enemies: long-term, low-frequency environmental vibrations and sudden, transient seismic forces.
To address these risks, the team has made the following recommendations:
`Establishing a three-tiered health monitoring system that covers the global, local, and micro scales;
`Implementing visitor management strategies that combine flow limits, time-slot scheduling, and off-peak access; and
`Conducting specialized maintenance, such as targeted desalination, weathering prevention, and crack repair.
This "health check report," which embodies multidisciplinary expertise, provides a scientific foundation for the preventive conservation of the Great Wild Goose Pagoda and establishes a new paradigm for the systematic study of ancient masonry pagodas in China.