Intelligent Distributed Acoustic Sensor (IDAS)

When a pulse of light travels down an optical fibre, a small amount of the light is naturally scattered (through Rayleigh, Brilliouin and Raman scattering) and returns to the sensor unit. By recording the returning signal against time, a measurement of the light generated all along the fibre can be determined.

A common instrument that uses the intensity of the backscattered Rayleigh light to determine the optical loss along the fibre is known as an Optical Time Domain Reflectometer (OTDR). Rayleigh backscatter light is also used for coarse event/vibration sensing. Raman light is used by a Distributed Temperature Sensor (DTS) to measure temperature, achieving a temperature resolution of <0.01°C and ranges of 30km+. However the response time of distributed temperature sensors is typically a few seconds to several minutes. Distributed Brillouin based sensors have been used to measure strain and temperature and can achieve faster measurement times of 0.1 second to a few seconds with a resolution of around 10microstrain and 0.5°C.

Silixa’s IDAS uses a completely new optoelectronics architecture to accurately and rapidly measure this backscattered signal with a precision and speed that allows acoustic measurements. The IDAS is so sensitive that it allows digital recording of acoustic fields at every location along an optical fibre with a frequency of up to 50kHz . This breakthrough is technically many steps ahead of what has been achieved before.

In addition, by using digital signal processing, the acoustic response along the fibre can be combined to enhance the detection sensitivity by two-orders of magnitude, thereby exceeding the sensitivity of point sensors as well achieving high directional information.

With the IDAS, the fibre acts as an acoustic antenna whose sensitivity and frequency response can be adjusted electronically by using different sensing configurations. For example, the fibre can be deployed in linear, directional or multi-dimensional array configurations. In addition, the precision that the DAS can achieve uniquely allows the speed of sound in the material surrounding the fibre to be accurately determined. This allows the DAS to detect, for example, the presence of gas in oil (a necessary step towards for multiphase flow measurement).