Distributed Acoustic/Vibration Sensing (DAS/DVS)

 

Distributed Acoustic/Vibrational Sensing systems detect vibrations and capture acoustic energy along optical fibers. Existing fiber optic networks along the asset are utilized and turned into a distributed acoustic sensor, capturing real-time data and helping operators take right actions. Classification algorithms are used to detect and locate events such as leaks, cable faults, intrusion activities, or other abnormal sounds.

Various DAS/DVS technologies are used in the market; the most common is based on Coherent Optical Time Domain Reflectometry (C-OTDR).
C-OTDR utilizes Rayleigh back-scattering, allowing acoustic frequency signals to be detected over longdistances. The interrogator sends a coherent laser pulse along an optical fiber (sensor cable). Scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length like the pulse length (e.g. 10 meters).

Acoustic disturbance on a fiber generates microscopic elongation or compression of the fiber (micro-strain), which causes a change in the phase relation and/or amplitude.

Before the next laser pulse can be transmitted, the previous pulse must have had time to travel the full length of the fiber and for its reflections to return. Hence the maximum pulse rate is determined by the length of the fiber. Therefore, acoustic signals can be measured that vary at frequencies up to the Nyquist frequency, which is typically half of the pulse rate. As higher frequencies are attenuated very quickly, most of the relevant ones to detect and classify events are in the lower of the 2 kHz range. 

In the very low frequency range, phase-based C-OTDR systems enable the highly sensitive measurement of transient temperatures due to the elongation/compression of the fiber with temperature changes. This measurement mode is called Distributed Temperature Gradient Sensing (DTGS) and is well-known in the oil & gas industry.

 

With the combination of phase and amplitude measurements, AP Sensing's proprietary 2P squared technology offers low sensitivity to the fading signal commonly observed in other C-OTDR systems.

AP Sensing's DAS system provides constant performance over the entire fiber resulting in a high measurement quality. 

 

Key Features of AP Sensing’s 2P Squared DAS technology:

  • Accurate measurement and location of the amplitude, frequency and phase of the incident acoustic field
  • True linearity over distance, time and acoustic intensity
  • Signal quality is achieved with leading signal to noise ratio
  • Advanced “2P Squared” optical techniques improve signal quality over long measurement ranges
  • World-leading measurement range of 70 km

 

Benefits:

  • An acoustic event applied at the beginning and at the end of the fiber is captured with the same sensitivity and “fingerprint”
  • Excellent raw data is the basis for pattern recognition and machine learning algorithms