Acoustic Characterization of Fluid Seepage Controlled by Tectonic Structures Offshore Southwestern Taiwan

This study quantifies spatial variations in acoustic seepage intensity offshore southwestern Taiwan and assesses whether margin setting or conduit continuity better explains the observed differences. Seepage variability was characterized using volume backscattering strength (Sv), plume geometry, and subsurface structural features. A total of 21 plumes from 14 seep sites were characterized based on Sv and geometry derived from Simrad EK60/EK80 echosounder data. After applying transmission-loss correction, seepage sites on the passive margin (e.g., Horseshoe Ridge, Pointer Ridge, and Formosa Ridge) exhibit higher Sv and taller plumes than those on the active margin. Integration with multichannel seismic profiles and sediment-core records reveals extensive free gas beneath bottom-simulating reflectors (BSRs) and gas chimneys, indicating sustained fluid migration through persistent conduits. In contrast, relatively weak Sv at the mixed-origin G96 site suggests partial conduit infilling. These observations indicate that although tectonic deformation establishes the first-order structural framework for fluid migration, the continuity and evolutionary state of seep conduits exert the dominant control on seepage intensity. Potential tidal modulation was further evaluated by comparing Sv with the rate of tidal pressure change, but only weak correlations were observed, suggesting that tidal forcing plays a secondary role in controlling seepage variability.

Keywords: seepage intensity, margin setting, conduit continuity, volume backscattering strength (Sv), offshore southwestern Taiwan