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Distributed Acoustic Sensing Technique that Simultaneously Implements both Intensity and Phase Based Measurements

IP.com Disclosure Number: IPCOM000237589D
Publication Date: 2014-Jun-25
Document File: 8 page(s) / 200K

Publishing Venue

The IP.com Prior Art Database

Abstract

This paper introduces a novel concept related to Distributed Acoustic Sensing wherein both intensity and phase based measurements from a sensor fiber are made simultaneously. This technique contributes to enhanced spatial resolution, reduced data interpretation errors, and increased application flexibility, particularly when related to oil and gas applications.

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Distributed Acoustic Sensing Technique that Simultaneously Implements both Intensity and Phase Based Measurements

Abstract

This paper introduces a novel concept related to Distributed Acoustic Sensing wherein both intensity and phase based measurements from a sensor fiber are made simultaneously. This technique contributes to enhanced spatial resolution, reduced data interpretation errors, and increased application flexibility, particularly when related to oil and gas applications.

Background of DAS

The fiber optic Distributed Acoustic Sensing (DAS) concept and associated technology was originally conceived for physical security applicationsi and has generated very recent interestii in oil and gas applications.

The basis of this technology is intrinsically simple and is based on Optical Time Domain Reflectometry (OTDR). An optical pulse made by a multiple longitudinal linewidth laser (of relatively low coherence) is launched into the fiber and minute portions of the light traveling outward are (Rayleigh) scattered backwards in a distributed manner such that this scattered light returning may be measured temporally with a fast receiver to produce a record or trace of distributed loss of the fiber out to distances of 10's of kilometers (see black trace in Figure 1).

If the optical pulses are made instead with a narrow line width (single longitudinal linewidth), high coherence laser, then these scattered return light signals will interfere with each-other which causes signal ripples which superimpose the loss trace. This is exemplified in Figure 1. These ripples, because they are related to the fixed Rayleigh scatter sites within a fiber are static as long as the fiber is static. If the fiber experiences any perturbations, or external forces, which strain the fiber, the positions of the scatter sites change. This in turn changes the interferometric induced ripples. Dynamic sensitivities down to the few angstrom levels are achievable. The result is a highly sensitive distributed sensing mechanism, known as Coherent Optical Time Domain Reflectometry (COTDR), and more recently known as Distributed Acoustic Sensing (DAS).

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Pulsed Light

Self interfering
scattered light Rayleigh
effect causes distributed back scattering of light, which self interferes

distributed fiber sensor

Laser

Optical pulse

OTDR / COTDR Key Elements

SMF

few ns - 100 ns

Optical receiver

  COTDR Trace made with high coherence laser

  OTDR Trace made with low coherence laser

Signal Level

Distance

Figure 1. OTDR configuration shows pulsed light injected into distributed fiber sensor. The resulting trace resulting from the Rayleigh backscatter in the fiber is shown as the black curve. If the laser is highly coherent, the trace develops "ripples" (red curve) resulting from interference of the scattered light.

DAS Interrogation

Generally, there are two different time domain interrogation approaches for DAS. For convenience, we have coined names for these which may...