Distributed Acoustic Sensing (DAS) is the measure of vibrations, caused by acoustic disturbances, along the length of a fiber optic cable. The fiber optic cable 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 acoustic disturbance which 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, which we can use 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) utilizing 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 repeats as pulses are sent out and received over and over again. The pulse rate is determined by the length of the fiber. Simply put, 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. Frequency is perceived by humans as pitch, the audible range of which we can hear is generally considered to be between 20 Hz - 20 kHz. Whereas decibels is considered the measure of loudness. Terra Sound can detect frequencies from 5 Hz - 2.5 kHz. As an example, footsteps on the ground are typically between 20-90 Hz (although this can vary depending on the surface). These footsteps will also carry a certain force as they make contact with the ground. This force is the decibel loudness and affects the amplitude of the disturbances we measure.
Above: Walking is detected in Terra Sound's software
Continuous sensing presents a key advantage over a traditional discreet sensor which can only measure data at set points. DAS systems generate continuous, real-time measurements along the entire distance of the installed fiber. The Terra Sound DAS system is cable of reporting measurements accurate within +/- 5m every 2-10 seconds along distances up to 100km for a single interrogator installation and distances much longer when multiple interrogator units are installed.