AP Sensing

DAS / DVS (Distributed Acoustic / Vibration Sensing)


These systems detect vibrations and capture acoustic energy along the optical fiber. The fiber functions as if there were thousands of microphones installed. Classification algorithms are used to detect and locate, for example, intrusion activities, leaks or other abnormal sounds.

Various DAS / DVS technologies are used in the market, but the most common is based on Coherent Optic Time Domain Reflectometry (C-OTDR). C-OTDR utilizes Rayleigh back-scattering, allowing acoustic frequency signals to be detected over large distances. The interrogator sends a coherent laser pulse along an optical fiber (sensor cable) and 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 to the far end of the fiber and for the reflections from there 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 frequencies to detect and classify events are in the lower of the 2kHz 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 DTGS (Distributed Temperature Gradient Sensing) 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.