Ground-to-Cable Strain Transfer in Unburied DAS on Earth and the Moon

Abstract

Distributed Acoustic Sensing (DAS) measures dynamic strain along a fiber-optic cable, offering a robust, densely-sampled alternative to traditional seismic sensors. To ensure good ground-to-cable coupling, cables are typically buried in a shallow trench. Unburied surface deployments are attractive for rapid-response terrestrial applications as well as extraterrestrial missions, such as on the Moon, where burial is impractical. However, unburied DAS often suffers from severely degraded signal quality, due to poor strain transfer from ground to cable. The physical mechanism responsible remains unknown. Here, we identify bending stress relief as a mechanism that can explain this loss: suspended cable segments accommodate ground strain by bending rather than by stretching or compressing, reducing the measurable axial strain that reaches the fiber. We develop the first analytical and numerical model of unburied DAS coupling, representing the draped cable as a series of suspended segments between discrete ground contact points, to explain and quantify the bending stress relief mechanism. Our analysis reveals a dimensionless parameter, Theta, set by the ratio of the cable's initial gravity-induced sag to its radius, which governs the strain transfer efficiency. Once a segment's sag exceeds a quarter of the cable's radius, ground displacement starts to be absorbed by bending rather than being transferred as measurable axial strain. This framework predicts how mechanical properties, cable dimensions, pretension, and gravity affect strain transfer efficiency and provides quantitative guidelines for optimizing cable design and deployment strategies on both Earth and the Moon.