# Travel-Time Inversion Method of Converted Shear Waves Using RayInvr Algorithm
## 3 Types and Applications of Converted S-Waves Phases
### PPS Phases and Its Role in Constraining Sediments Vp/Vs Ratios  
The sediments' Vp/Vs ratios were constrained well by PPS phases, which were generated by P-waves converted to S-waves, at the interface, on the way up, and across the sediment as S-waves (Figures 5 and 6). The geometry of the S-wave model was kept consistent with the P-wave model, and the sediment interface depths were fixed and only the Poisson's ratio (\(\sigma\)) of different blocks was changed. The RayInvr software [17] was used to test the various Poisson's ratios within sedimentary layers until the best agreement was obtained between the calculated and observed PPS phase travel-times. The sediments S-wave velocity is calculated by the Vp/Vs ratios (the equation of Vp/Vs and Poisson's ratios is \(\frac{\
u_{\rm P}}{\
u_{\rm s}}=\sqrt{1+\frac{1}{1-2\sigma}}\)) and the P-wave velocity.  
The sedimentary layer in the P-wave model of the profile NS5 is divided into three layers (Figure 7), in which sedimentary layers 1 and 2 are Cenozoic sedimentary layers, and sedimentary layer 3 is Mesozoic strata. Sedimentary layer 1 is characterized by the thin thickness (0.16-0.5 km) and low Vp (1.7-2.2 km/s), hence the bottom of sedimentary layer 1 is a P- to S-wave conversion interface (e.g., PSSuc in Figure 6). Similarly, P- to S-wave conversion can also occur at the bottom of sedimentary layer 2, due to differences in petrophysical properties between sedimentary layer 2 and sedimentary layer 3 (e.g., PPSs2, PPSs3s2 in Figure 6). Refracted phases of Mesozoic sediments (Ps3) were converted to S-waves on the way up, at the bottom of sedimentary layer 2, and formed the PPSs3s2 phase. The PPSb phase was generated by the crustal refracted phases (Pg) converting to S-waves at the sedimentary basement on the way up, whereas the PPSs2 phase is from Pg phase converted at the bottom of sedimentary layer 2. The sediments' Vp/Vs ratios beneath the OBS were defined by the PPS phases. The S-wave velocity model of the sediment (Figure 7c) was obtained from dividing the previous P-wave velocity model (Figure 7a) by the Vp/Vs model (Figure 7b), which was calculated by the interpolation of the adjacent OBS.  
Figure 5: Cartoon illustrating the ray paths of converted S-wave phases. The PPS phases (PPSb) constrain the sediments S-wave velocity, while the PSS phases (PSSuc/PSSlc) constrain the crustal S-wave velocity.  
Figure 6: Seismic record section, S-wave phase identification, and ray tracing of OBS 02. (**a**) Seismic profile of radial component. (**b**) The identification and picking of S-wave phases. The coloured lines represent picked travel times and the thin black lines represent calculated travel times. (**c,d**) Show the ray tracing of PPS and PSS conversion modes, respectively. Solid lines represent P-wave paths, dotted lines represent S-wave paths, and the different coloured lines indicate different phases.  
Our results show that sedimentary layer 1 has a high Vp/Vs of 3.05-7.14, representing unconsolidated sediments. The Vp/Vs of sedimentary layer 2 is also relatively high, with an average of 1.90 on the landward and 5.10 on the oceanward. The Mesozoic strata are characterised by high velocities and low Vp/Vs ratios of 1.71-1.76, due to the effects of long-term tectonism, sedimentary compactions, and later magmatism. The Vp/Vs of the sedimentary layers reveal two trends: one is that the Vp/Vs gradually decrease with the increased depth, which resulted from the confining pressure with depth; another is a seaward increase in Vp/Vs ratios, which may be related to composition and consolidation.