Distributed acoustic sensing (DAS) is the measure of vibrations, caused by acoustic disturbances, along the length of a fiber optic cable, which acts as a continuous sensor.
Fiber optic cable is made of glass or plastic strands. These strands are highly efficient at transmitting light. DAS systems use an interrogator unit to pulse light down the fiber using a laser. This light pulse travels down the entire path of the fiber until it reaches the end, and then comes back. Along the fiber path, the light may encounter an acoustic disturbance that causes a microscopic elongation or compression of the fiber—strain on a micro level. This strain causes a change in the phase relation/amplitude. When the pulsed light is received back at the interrogator unit, we read in the acoustic disturbances and use them to tell what is happening along the fiber path. This method of using a coherent laser pulse and measuring strain along fiber is called coherent optic time domain reflectometry (CODTR) and utilizes the principles of Rayleigh back-scattering.
Once a pulse has been sent along the fiber and received back at the interrogator unit, another pulse is sent down the fiber. This process repeats as pulses are sent out and received over and over again. The pulse rate is determined by the length of the fiber: the shorter the run of fiber, the less distance the light has to travel before another pulse can be sent.
What frequencies can be detected?
Acoustic signals can be measured at frequencies up to the Nyquist frequency, which is half of the pulse rate. Humans perceive frequency as pitch, and the human-audible range is generally considered to be between 20 Hz and 20 kHz. Terra Sound can detect frequencies from 5 Hz to 2.5 kHz—reaching down into frequencies that a human cannot hear.
As an example, footsteps on the ground are typically between 20–90 Hz, although this can vary depending on the surface. Footsteps also carry a certain force as they make contact with the ground. This force is the decibel loudness, and it affects the amplitude of the disturbances we measure.
The spatially continuous sensing made possible with fiber optic cable presents a key advantage over traditional discrete sensors, which can only measure data at set points. DAS systems generate a stream of real-time measurements along the entire distance of the installed fiber. A Terra Sound DAS system is cable of reporting measurements accurate to within +/- 5m every 2–10 seconds along distances up to 100km for a single interrogator unit, and can cover much longer distances when multiple interrogator units are installed.