Sound-Speed Inversion and LFM Layer-Drift Discrimination in Thin Sediments Based on Interference-Phase Self-Consistency

Pulse compression in broadband linear-frequency-modulated (LFM) sub-bottom profiling suffers from coherent sidelobe coupling, causing peak-position displacement (layer drift) and pseudo-layers in thin sediments. For a low-impedance soft-mud upper layer, the interface echo also undergoes a π phase reversal, distorting envelope-based interpretation. We propose a thin-sediment sound-speed inversion and LFM layer-drift discrimination method based on interference-phase self-consistency. Using a self-developed high-frequency sub-bottom profiling sonar (HF-SSBP; 110 kHz center frequency, 30 kHz bandwidth), stepped-frequency measurements were performed from 95 to 125 kHz at 1 kHz intervals, and the interference amplitude in the overlap region of upper- and lower-interface echoes was extracted. The oscillation period and peak–trough phase encode layer thickness and sound speed and, unlike the LFM envelope, are unaffected by pulse-compression sidelobes. With thickness constrained by geometry, the LFM sound speed served as an initial reference for inversion, and LFM layer drift was diagnosed via a half-wavelength phase-error criterion. Water-tank tests on soft-mud–sand bilayers showed good agreement for Sample 2 (interference 1167 m/s; LFM 1152 m/s), confirming the two-layer interpretation, whereas Sample 3 gave 1281.69 m/s (LFM) versus 1098 m/s (interference), revealing substantial LFM drift. The π phase reversal must be explicitly modeled, with peak–trough inversion directly indicating a low-impedance zone. The method enables accurate sound-speed inversion and robust LFM layer-drift diagnosis in thin sediments.