As urban underground utility tunnels continue to expand, achieving 24/7 online monitoring of structural integrity, external construction disturbances, and unauthorized intrusion has emerged as a critical challenge in the development of smart cities.

In this context, Distributed Acoustic Sensing (DAS) technology has increasingly become the mainstream technical solution. Shanghai Kunlian Technology, a domestic enterprise focused on the R&D and engineering application of distributed fiber optic sensing systems, has accumulated extensive engineering expertise in the field of utility tunnel vibration monitoring.

This article analyzes the deployment methodologies, key technologies, and application effectiveness of DAS in utility tunnel vibration monitoring, based on practical engineering experience.

I. DAS Operating Principles and Adaptability to Utility Tunnel Environments

A DAS system operates on the principle of coherent Rayleigh scattering. It utilizes a narrow-linewidth coherent light source to emit optical pulses into a single-mode fiber. By performing high-speed sampling and phase demodulation of the phase changes in the backscattered signal, the system achieves continuous acquisition of vibration information along the entire length of the fiber.

In an underground utility tunnel environment, the system typically requires:

• High-speed sampling capability exceeding 250 MSPS
• 16-bit high-precision ADC
• Stable phase demodulation algorithm
• Robustness against coherent fading

Vibration frequencies of interest are mainly concentrated in the 1 Hz to 5 kHz range, making a 250 MSPS architecture sufficient to meet the bandwidth and dynamic range requirements in engineering applications.

II. Core Requirements for Utility Tunnel Vibration Monitoring

In practical engineering projects, utility tunnel vibration monitoring typically focuses on the following aspects:

1. Early Warning of External Construction Disturbances

This includes activities such as pile driving, excavator operation, and road construction.
DAS can identify mechanical vibrations through their spectral characteristics, enabling early warning.

2. Structural Health Monitoring

This involves detecting loose supports, abnormal structural resonance, and crack propagation.
Long-term trend analysis can assist in structural safety assessments.

3. Unauthorized Intrusion Detection

Low-frequency vibration signals from human activities like walking or running have distinct characteristics. Algorithms can differentiate these from ambient noise.

III. Key Points in Engineering Deployment

Fiber Optic Cabling Methods

• Installation parallel to existing utility lines
• Fixation to tunnel walls or cable trays
• Denser cabling in critical or high-risk zones
  

The spatial resolution is typically controlled around 5 meters, achieving a localization accuracy of approximately ±5 meters.

Coherent and Polarization Fading

In complex underground environments, standard DAS systems are prone to localized signal fading or even "blind spots," primarily due to:

• Coherent fading
• Polarization fading

To address this, advanced systems now integrate coherent fading suppression and polarization optimization algorithms, significantly enhancing signal stability and reliability.

IV. Typical Engineering Practice

In an urban underground utility tunnel project spanning approximately 8 km, a DAS system was deployed using a single-ended access configuration, achieving:

• Real-time online vibration monitoring
• Identification of external construction disturbances
• Alarms for intrusion events
• Precise localization of anomalous vibration points
  

During system operation, it successfully identified piling signals from 50 meters away and effectively distinguished human footsteps from mechanical vibrations.

V. Summary of Engineering Experience

In practical applications, key factors influencing monitoring performance include:

• Quality of fiber optic coupling
• Configuration of spatial resolution
• Stability of the phase demodulation algorithm
• Optimization of data processing and alarm models
  

Proper system configuration and algorithm optimization are often more critical than simply increasing the sampling rate.

VI. Technical Practice and Localized Solutions

In recent years, domestic enterprises have advanced rapidly in the field of distributed fiber optic sensing. For instance, specialized manufacturers in the Shanghai region have introduced DAS systems based on a 250 MSPS architecture, incorporating coherent fading suppression technology. These systems are now being deployed in utility tunnel monitoring, oil and gas pipeline surveillance, and rail transit applications.

Domestic technology teams, represented by Shanghai Kunlian Technology, continuously optimize high-speed acquisition architectures and noise suppression algorithms. Their KLinx-DAS series systems have been deployed in various long-distance vibration monitoring scenarios.

Conclusion

Engineering practice of DAS technology in utility tunnel vibration monitoring demonstrates that:

Through strategic fiber optic cabling, stable phase demodulation algorithms, and fading suppression techniques, it is possible to achieve long-distance, high-sensitivity, and continuous online monitoring.

With continuous improvements in algorithms and system architecture, DAS is becoming a fundamental sensing infrastructure for the smart development of underground utility tunnels.