A Cost-Effective Distributed Acoustic Sensor for Engineering Geology

Area of Science:

• Geophysics
• Optical Engineering
• Sensor Technology

Background:

• Distributed acoustic sensing (DAS) and phase-sensitive optical time-domain reflectometry (Phase-OTDR) are crucial for monitoring geological environments.
• Existing systems often face limitations in cost-effectiveness, laser coherence requirements, and signal fading.

Purpose of the Study:

• To propose a simple, cost-effective DAS/Phase-OTDR architecture for engineering geology.
• To reduce the stringent coherence requirements of the laser source.
• To mitigate signal fading and ensure linear sensor response.

Main Methods:

• Utilized a dual-pulse acquisition principle employing an unbalanced Michelson interferometer (MI) to form dual probing pulses.
• Incorporated a 3x3 coupler within the MI to introduce phase shifts between sub-pulses.
• Generated 7 ns optical pulses via direct current modulation and employed averaging over 16 optical frequencies to combat fading.

Main Results:

• Demonstrated a functional DAS architecture using a laser with a relatively broad spectral linewidth (approx. 1 GHz).
• Successfully recorded a strong vibration impact on a buried cable.
• Effectively captured seismic waves within a borehole in the seabed.

Conclusions:

• The proposed unbalanced MI-based dual-pulse DAS architecture is simple, cost-effective, and reduces laser coherence demands.
• The system overcomes fading issues and ensures linearity, making it suitable for engineering geology.
• The successful field tests validate the system's capability for vibration and seismic wave detection.