Fig. 1

Location of the profile HXN01. The red five-pointed star is the position of OBS0106, the red circle is OBS0103, and the yellow circle is the OBS with problem and missing data, the blue circle is the OBS with intact data, the gray dashed line is the boundary of rift and uplift zone, the blue pecked line is the boundary of sag. Bathymetric data are from GEBECO_2014 (http://www. geobco.net)"

Fig. 2

Comprehensive seismic recording profile of OBS0103 (a) and OBS0106 (b). Reduced velocity is 6.0 km·s-1. Ps indicates sedimentary refractive phase, Pg indicates crustal refractive phases, PcP indicates reflective phases from intra-crustal interface, PmP indicates reflective phases from Moho"

Fig. 3

The primary Ps and secondary Ps phases picked up at the trace No.5415 of OBS0106 station (a) and waveform comparison (b). The red waveform is the secondary phase waveform"

Fig. 4

The particle trajectories of the primary phase (a ~ d) and the secondary Ps phase (e ~ h) recorded by OBS0106 station. Time windows are 1.6 ~ 2.0 s and 3.26 ~ 3.66 s; the red solid line represents the diameter component; the black solid line represents the tangential component; the black dashed line represents the vertical component"

Fig. 5

Ray tracing (a, c, e) and travel time simulation (b, d, f) of first-arrival phase and secondary phase. Orange represents Ps1 phase, light pink represents secondary Ps1 phase, grass green marks Ps2 phase, light blue marks secondary Ps2 phase, the black solid line represents the theoretical travel time, the short vertical line represents the observation travel time, and its length represents the uncertainty in the travel time picking-up. Figures a, b; figures c, d and figures e, f present the results of ray tracing with seawater-upper sediment interface, lower sediment, and seawater-lower sediments as the second reflecting layer"

Fig. 6

Recovery result of sedimentary interface inversion. (a) Initial model; (b) Inversion result using first-arrival phase data; (c) combined inversion result using first-arrival phase data and the secondary Ps phase. The black solid line represents the sedimentary basement"

Fig. 7

Distribution of inversion errors. The red dots are the interface depth recovery errors obtained from the reverse evolution of the first-arrival phase, the blue dots are the interface depth recovery errors obtained by the joint inversion of the first-arrival phase and the secondary Ps phase. Between the black dotted lines is the recovery error within ± 0.5 km"

Fig. 8

Ray density distribution of the OBS0106 station. (a) The ray density distribution of first arrivals; (b) The ray density distribution after the addition of secondary Ps phases. The black solid dashed line represents the velocity discontinuity"