Non-Contact Dual-Laser Doppler Vibrometry for Synchronous Sensing of Vibration Response Evolution and Collapse Precursors of Unstable Rock Mass Under Repeated Aftershock Impacts

After a mainshock, unstable rock masses often collapse with a time lag under repeated aftershocks, while conventional contact sensors struggle to capture high-frequency micro-amplitude pre-collapse responses on inaccessible high-steep slopes, and existing single-point laser Doppler vibrometry (LDV) cannot distinguish external excitation from structural degradation. This study proposes a non-contact dual-LDV synchronous sensing scheme with a bedrock reference channel and an unstable-rock measurement channel. Ten collapse tests were conducted on a cement-block model with an ice-bonded interface using repeated ball-drop impacts to simulate aftershocks, and time-domain, frequency-domain, and cross-channel coupling indicators were extracted from 114 effective segments. The results show that all specimens failed after 6–21 impacts; the zero-lag correlation deficit increases first in the progressive stage, and normalized RMS jumps sharply before collapse; the dominant frequency remains stable at 10–11 Hz and is insensitive to interface damage; and only six specimens show persistent 3σ RMS precursors, while the multi-indicator fusion framework effectively suppresses false and missed alarms from single thresholds. The proposed scheme provides an experimental basis and a multi-indicator analytical framework for non-contact early warning of aftershock-driven unstable rock collapse.