Quantifying the Surface Strain Field Induced by Active Sources with Distributed Acoustic Sensing: Theory and Practice

Area of Science:

• Geophysics
• Geotechnical Engineering

Background:

• Quantitative dynamic strain measurements are crucial for engineering applications, including natural hazard monitoring and soil-structure interaction studies.
• Distributed acoustic sensing (DAS) is a promising technology, but its accuracy for ground strain measurement requires further investigation regarding directional sensitivity, spatial positioning, and amplitude response.

Purpose of the Study:

• To validate the physical measurements of ground strain obtained using DAS against theoretical models and geophone measurements.
• To assess the consistency of DAS measurements with theoretical transfer functions and reception patterns.

Main Methods:

• Comparison of DAS strain measurements with geophone data under various broadband, high-amplitude, and complex wavefield conditions.
• Development of a theoretical transfer function for DAS arrays, considering gauge length, pulse shape, and cable design.
• 3D elastic finite-difference simulations to establish the theoretical relationship between DAS-measured and pointwise strain for active sources.

Main Results:

• DAS and geophone measurements show consistent phase and amplitude for broadband, high-amplitude, and complex wavefields when proper alignment, cable coupling, ground coupling, and laser stability are ensured.
• A transfer function for DAS arrays is presented, and a theoretical relationship between DAS and pointwise strain is derived.
• A method for precise spatial alignment of DAS channels with geophone locations is proposed, achieving tolerances below the DAS spatial resolution.

Conclusions:

• DAS technology is validated as a reliable tool for quantitative dynamic ground strain measurements in engineering applications.
• The study provides a framework for understanding and improving DAS measurement accuracy, including methods for quality estimation and spatial alignment.
• Findings support the broader application of DAS in geotechnical engineering, natural hazard monitoring, and site investigations.