Fig. 1

Map for designed drilling wells of IODP expedition Legs 367-368 in the South China Sea. The black dashed and solid lines show water depth contours of 1000 and 3000 m, respectively. The magnetic anomalies in orange and purple colors are referenced by Briais et al (1993) and Li et al (2014), respectively. Red and white pentagrams represent designed and alternative wells (Sun et al, 2015, 2016), respectively. Lines 1530 and 1555 are two multi-channel seismic profiles across the designed drilling wells"

Fig. 2

Multi-channel seismic profile of Line 1555 across the designed drilling wells of IODP expedition Legs 367-368. (a) the original profile of Line 1555, with the blue frame showing the domain for building the initial models in Fig. 3; (b) the interpreted profile of Line 1555; (c) the predicted geological model. The increment of CDP is 12.5m in Fig. 2a and Fig. 2b. The magnetic anomalies (C10r, C11r) are referenced by Li et al (2014). The locations of designed drilling wells (41A, 1A, 8B, 9B) (Sun et al, 2016) are seen in Fig. 1. The box in Fig. 2a shows the domain of models in Fig. 3"

Fig. 3

Three original velocity models based on three geological types of the COT. (a), (b) and (c) refer to the exhumed lower crust, the oldest oceanic crust and the exhumed continental upper mantle, respectively. The black box shows the domain of obscure zone in Fig. 2c. The drilling wells (41A, 1A, 8B, 9B) are designed in IODP367-368 (Sun et al, 2013) in Fig. 1"

Fig. 4

Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 1 (Fig. 3a). (a) and (b) are theoretical travel-time calculation and ray density and distribution for the 10 km OBS interval, respectively; (c) and (d) are for the 7 km OBS interval, respectively. Pdw in red lines represent the direct water wave arrivals, Ps in green lines represent the sediment refractive arrivals, Pg in blue lines represent the crustal refractive arrivals, and PmP and Pn in pink and orange lines represent the wide-angle reflective arrivals and the refractive arrivals from Moho, respectively. The reduced travel time = (absolute travel time - distance/reduced velocity) and the reduced velocity is 6.0 km•s-1"

Fig. 5

Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 2 (Fig. 3b). Other symbols are the same as those in Fig. 4"

Fig. 6

Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 3 (Fig. 3c). Other symbols are the same as those in Fig. 4"

Fig. 7

Check-board test results of the 20-km-in-size velocity anomaly body. (a) and (b) are the input and output models with 10-km OBS interval; and (c) and (d) are the input and output models with 7-km OBS interval, respectively. The grid size is 15 km×5 km"