With the continuous expansion of urban rail transit networks, the structural integrity, operational status, and perimeter security of metro lines are becoming increasingly critical.

Distributed Acoustic Sensing (DAS) systems, owing to their advantages such as long-range coverage, continuous spatial resolution, immunity to electromagnetic interference, and covert deployment, have become a pivotal technological approach for metro safety monitoring.

However, in practical projects, clients frequently inquire:

For metro monitoring, is a DAS system with a 250 MSPS sampling rate truly sufficient?

As a manufacturer with extensive experience in Distributed Optical Fiber Sensing system R&D, Shanghai Kunlian Technology provides a definitive technical conclusion based on substantial engineering practice.

1. First, Clarify a Core Question: What Does Metro DAS Truly Need to Monitor?

The primary monitoring targets for DAS in metro scenarios include:

• Train-induced vibrations
  
• Rail structure anomalies
  
• Trackbed settlement
  
• External construction intrusion
  
• Personnel intrusion
  
• Perimeter breach
  
• Equipment abnormal noise
  

The common characteristic of these signals is:

👉 Effective frequency spectrum concentrated within the 0–5 kHz range

👉 Rarely exceeding 10 kHz

This is crucial.

Metro monitoring is not ultrasonic imaging, nor is it a GHz radar system; it is a typical low-frequency mechanical vibration coupled with acoustic wave signals.

2. What is the Real Role of 250 MSPS?

A common misconception:

“A higher sampling rate always equates to better DAS performance.”

This is not necessarily true.

In DAS systems, sampling rate primarily affects:

The main influences of sampling rate are:

• Spatial resolution
  
• Phase recovery accuracy
  
• Coherent demodulation stability
  

It does not directly determine the "ability to detect metro vibrations."

Theoretical spatial resolution calculation:

Speed of light (in fiber) is approximately:

c = 2×10⁸ m/s (in optical fiber)

250 MSPS corresponds to a sampling period of:

T = 4 ns

Spatial length per sampling point:

Δz = c × T / 2 ≈ 0.4 m

This means:

👉 250 MSPS enables sub-meter level spatial resolution capability

In metro engineering:

• Sleeper spacing: 0.6 m
  
• Effective coupling bandwidth limit of cable installation: ≈1–2 m
  

Therefore:

250 MSPS already significantly exceeds the practical physical limits on-site.

Further increasing the sampling rate would only result in:

• Exponential growth in data volume
  
• Reduced real-time performance
  
• Increased FPGA load
  
• Higher system cost
  

Without delivering perceivable engineering benefits.