Distributed Acoustic Sensing (DAS) Interrogator

One fibre, thousands of virtual acoustic sensors

The Distributed Acoustic Sensor (DAS) interrogator converts a standard single-mode optical fibre into a continuous array of virtual acoustic sensors, enabling real-time acquisition of acoustic and vibration signals at any location along the cable without any downhole electronics. Because sensing and signal transmission are performed by the same fibre, a single cable is functionally equivalent to thousands of discrete point sensors. The ORCA - DAS interrogator is a true-phase demodulation instrument that outputs unwrapped phase data, in the ProdML DAS Schema 2.0 HDF5 data format.

01ORCA - DAS interrogator — photoelectric detection module and processing unit in a 4U+2 rack enclosure.

Specification

Coherent Rayleigh backscattering with differential coherent detection

The DAS interrogator operates on the principle of coherent Rayleigh backscattering. A highly coherent laser pulse is launched into the single-mode sensing fibre, and minute, naturally occurring inhomogeneities distributed along the fibre core continuously backscatter a small fraction of the incident light. External acoustic or vibrational disturbances perturb the local strain field of the fibre, modulating the phase of the backscattered light at the corresponding location.

A differential coherent-detection architecture demodulates this phase information quantitatively, yielding a substantially higher signal-to-noise ratio than direct-detection designs, while allowing spatial resolution and other acquisition parameters to be reconfigured entirely in the digital domain. This approach effectively suppresses the signal-fading artefacts inherent to coherent OTDR systems and, combined with an ultra-low-frequency (ULF) demodulation capability, extends the usable frequency range to support high-precision measurement in oil and gas applications.

The delivered system comprises a complete photoelectric interrogation unit, data-acquisition module, and demodulation/software suite built on a common architecture.

Key features

• Every point along the sensing fibre functions as a discrete vibration sensor, equivalent to thousands of point sensors on a single strand
• GPS/GNSS precision timing option — for VSP and other seismic-monitoring applications requiring precise multi-channel/multi-well time synchronization, the DAS interrogator can be configured with an integrated GPS/GNSS timing module providing nanosecond-level timestamp accuracy, together with a dedicated GPS/GNSS antenna, ensuring that acquired phase data is precisely time-referenced for accurate seismic-event correlation, first-break picking, and integration with surface seismic recording systems
• Differential coherent detection enabling quantitative phase demodulation with improved SNR relative to direct-detection designs
• Digitally reconfigurable spatial resolution and acquisition parameters
• Effective suppression of fading artefacts, with dedicated ultra-low-frequency (ULF) demodulation capability
• Complete turnkey system: photoelectric interrogation unit, data-acquisition module, and demodulation software suite
• ProdML DAS Schema 2.0 HDF5 output — the file structure (group hierarchy, dataset names, and attribute keys) is fully aligned with the ProdML 2.0 specification

Typical applications

• Downhole acoustic-wave detection, energy-conversion analysis, and stress measurement in oil and gas wells
• Production-profile monitoring for oil, gas, and water wells
• Wellbore integrity monitoring
• Hydraulic-fracturing monitoring using the ULF configuration, including same-well flow monitoring and offset-well (adjacent-well) fracture-hit monitoring
• Conveyor belt monitoring in coal mines, power plants, and large-scale port/terminal facilities — continuous monitoring of belt condition, idler-bearing overheating, belt misalignment, and material-flow irregularities to enable early fault detection and reduce unplanned downtime
• Coal mine geotechnical monitoring — roof and rib (slope wall) stability monitoring, coal-seam stress-change monitoring, shaft (vertical well) stability monitoring, and tailings-facility monitoring, providing continuous, wide-area deformation and stress-change data to support ground-control safety management and early warning of instability
• Long-distance pipeline monitoring — leak detection, third-party intrusion (e.g. excavation) detection, and ground-movement/geohazard monitoring along the full pipeline route
• Urban sensitive-area security monitoring — perimeter security for critical infrastructure, underground utility corridors, and other high-consequence urban zones requiring continuous, covert, real-time threat detection
• Vertical Seismic Profiling (VSP) for seismic exploration and reservoir monitoring — using the sensing fibre as a continuous, densely sampled receiver array along the wellbore to acquire high-fidelity seismic data for well-to-seismic tie, subsurface imaging, and time-lapse (4D) reservoir monitoring, offering wider array coverage and lower deployment cost than conventional geophone-based VSP tool strings