In DAS, the optical fiber cable becomes the sensing element and measurements are made, and in part processed, using an attached optoelectronic device.
[1] Since the light must make a double pass along each section of fiber, this means each 1 km causes a total loss of 0.4 dB.
To improve the maximum range, it would be desirable to use a longer pulse length to increase the reflected light level but this leads to a smaller spatial resolution.
It is possible to obtain samples at separations less than the spatial resolution and although this produces signals that are not independent of each other, such an approach does offer advantages in some applications.
Distributed acoustic sensing relies on light which is Rayleigh backscattered from small variations in the refractive index of the fiber.
As with Brillouin scattering, both Stokes and anti-Stokes components are produced and these are shifted from the wavelength of the incident light by several tens of nanometers.
Phase-sensitive coherent optical time-domain reflectometry (Φ-OTDR) is a technique that can provide sufficient sensitivity and resolution for these distributed acoustic sensing systems.
This can yield a sum of backscattered intensities from each scattering center, which allows monitoring splices and breaks in fiber optic cables.
The sensitivity and speed of Rayleigh-based sensing allows distributed monitoring of acoustic signals over distances of more than 100 km[3] from each laser source.
The ability of the optic fiber to operate in harsh environments makes the technology especially well suited for scenarios in which typical sensing systems are unusable or impractical due to environmental conditions.
One cable can provide a continuous line of regional seismic activity monitoring, and also detect earthquakes thousands of kilometers away.