intelligent Distributed Acoustic Sensor (iDAS™)
Silixa’s intelligent Distributed Acoustic Sensor (iDAS) is an optoelectronic system which records the true acoustic signal continuously along the path of sensing fibre 10s of kilometres long.
When a pulse of light travels down an optical fibre, a small amount of the light is naturally backscattered (through Rayleigh, Brillouin and Raman scattering) and returns to the sensor unit. The nature of this scattered light is affected by tiny strain events within the optical fibre structure which themselves are determined by the localised acoustic or seismic environment. By recording the returning signal against time, a measurement of the acoustic field all along the fibre can be determined. The iDAS has a frequency range from millihertz to hundreds of kilohertz.
It also offers the flexibility to operate on singlemode or multimode fibre without the introduction of any external or additional apparatus, with no loss of signal quality and while preserving the true acoustic nature of the measurement. This unique feature makes it possible to access legacy DTS installations for new acoustic surveys, for example borehole seismic or flow measurements.
The world’s finest distributed acoustic sensor, the iDAS, has a novel optoelectronics architecture that allows for digital recording of acoustic fields at every location along a singlemode or multimode optical fibre. With a frequency range of less than 1mHz to over 100kHz and a spatial resolution down to 1 metre. Amplitude, frequency and phase fidelity allows for numerous advanced applications.
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/disturbance 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 >100kHz . This breakthrough is technically many steps ahead of what has been achieved before.
The importance of collecting the true acoustic signal – amplitude, frequency and phase – cannot be underestimated as this opens the door to a wide range of array processing techniques to be used to extract the maximum value from the data. For example, this capability uniquely allows the iDAS to determine the speed of sound in the material surrounding the sensing cable, enabling the iDAS to quantify, for example, the proportion of oil and water in a flowing pipe.
In addition, as all sensing points are phase-matched, the acoustic response along the fibre can be combined to enhance the detection sensitivity by two-orders of magnitude, which would enable to step beyond the performance of current point sensors and as well as, if desired, achieving highly directional information. These capabilities can be further enhanced by forming the sensing cable into an acoustic antenna whose sensitivity and frequency response can be adjusted dynamically.
iDAS collects the true acoustic signal – amplitude, frequency and phase – at all points along an optical fibre. This breakthrough capability uniquely allows the iDAS to determine the speed of sound in the material surrounding the sensing cable; hence opening the door to a wide range of applications, such as flow quantification.