# Travel-Time Inversion Method of Converted Shear Waves Using RayInvr Algorithm
## 1 Introduction  
The Ocean Bottom Seismometer (OBS) survey is a kind of seismic observation technology whose geophones are directly deployed on the seafloor, which has a wide range of use in the research of deep seismic structures [1, 2]. In contrast to Multichannel Seismic Reflection (MCS) survey, the OBS survey has wider detection coverage, both in depth (\(>\)25 km), and width (\(>\)100 km) [3]. Another advantage is the ability to record converted S-waves [4, 5], because OBSs, including 4-component geophones, are placed on the seafloor. The joint studies of P- and S-wave velocities derived Vp/Vs ratios of the crustal structures, which provide direct evidence for constraining the lithology and geophysical properties [6, 7]. Therefore, the S-wave research has made a huge contribution to distinguishing crystalline basement compositions [4, 8, 9, 10] and mantle serpentinization [11], as well as estimating the saturation of gas hydrates or fluids [12]. However, the RayInvr forward approach, one of the common modelling methods of the converted shear wave, has complicated steps and leads to difficulties in ensuring the optimal solution, which is urgently needed to improve the existing methods.  
The detailed studies of converted S-waves remain a weakness. One reason for this is that the converted S-waves are hard to observe due to the complicated submarine environment (the coupling between the instruments and seabed) and the specific conversion interface for P- to S-wave. Previous studies have mainly been limited to modelling P-wave velocity structure, and only three OBS profiles (Figure 1) have revealed the S-wave velocity structure in the north-eastern continental margin of the South China Sea (SCS) [4, 13, 14].  
Additionally, the research of converted S-waves has a critical defect because of the deficient modeling method. For example, only the Vp/Vs ratio of the entire layer in the model was obtained by using 2-D ray tracing along the profile on the Lofoten continental margin, northern Norway [15]. The same cases were seen by [PERSON] and [PERSON] in 1994 [8], and [PERSON] et al. in 1996 [16], too. Almost all similar studies used the RayInvr forward modelling [17] after 2003 [9], when the forward approach was first applied to modelling the observed converted S-waves data [4; 10; 11; 18]. In the RayInvr model, the depth and velocity nodes of layers defined variable-sized trapezoidal blocks. In that case, only Poisson's ratio (\(\sigma\)) of different blocks was adjusted to acquire the forward S-wave model. The forward modelling method is often affected by human subjectivity (e.g., if the grid size of blocks is set too small, it will result in a huge workload; otherwise the model resolution will be insufficient), so a more objective inversion method is needed.  
In May-July 2014, a deep seismic refraction profile (NS5) was carried out in the north-eastern SCS (Figure 1). A large number of converted S-wave phases through the crust were recorded by the OBSs, which provided good data for crustal S-wave velocity inversion. In this study, we introduced the improvements in the inversion method to acquire the crustal S-wave velocity model using converted S-waves phases.