The use of multi-frequency acquisition to significantly improve the quality of fibre-optic distributed vibration sensing

Abstract

Unlike conventional sensors that measure the passage of seismic waves at a single position, distributed vibration sensing (DVS) systems, also known as distributed acoustic sensing (DAS) systems, detect the passage of seismic waves by averaging a measurement of strain over a section of fibre optic cable. DVS systems work by transmitting pulses of light down the fibre and measuring the phase of the Rayleigh backscatter. At random positions along the fibre, however, fading occurs; this is where the amplitude of the backscattered signal is very small due to cancellation of the scattered electric fields, resulting in anomalously noisy traces. The appearance of fading is dependent on the frequency of the laser pulse used to interrogate the fibre; thus, by transmitting a number of different frequencies, we can avoid its effects as well as reduce the overall noise of the system. Once the noisy traces are removed, the results of our experimentation and modelling suggest that the improvement is approximately vn where n is the number of frequencies being stacked.