Abstract
<title>Abstract</title> <p>The Space-Air-Ground-Sea Integrated Network demands cross-medium underwater sensing, yet severe electromagnetic attenuation and acoustic total internal reflection at the sea-air interface form a fundamental barrier. The acoustic-induced surface micromotion effect offers an alternative non-contact mechanism, but extracting such signals under real sea conditions remains challenging: strong non-stationary sea clutter degrades the signal-to-noise ratio, and prior work, limited to anechoic tanks, millimeter-wave frequencies, and communication link construction, lacks system-level verification under actual sea states. To address these gaps, this paper proposes a non-contact cross-interface sensing paradigm grounded in surface micromotion physics, establishing a full-chain framework that integrates time-varying sea-surface scattering modeling, a phase-stable terahertz sensing system with a wavelet-packet-transform-based dynamic threshold algorithm for sub-micrometer micromotion extraction, and hierarchical field validation across nearshore fixed and airborne platforms. To our knowledge, this study presents the first successful field experiments under sea state 2, achieving detection of underwater acoustic sources, extraction of micrometer-scale micromotion signals, and effective separation of acoustic-induced signals from strong sea clutter. This work constitutes a paradigm shift from cross-medium communication to non-contact subsurface sensing via surface micromotion physics, filling the critical gap in real-sea-state verification for complex marine environments.</p>