To better understand deep reflectors in the multi-channel seismic profile D80 in the northern South China Sea, a coincident ocean bottom seismic survey line (OBS2015-1) with 15 OBSs was inversed. This OBS line extends ESE 300 km, from 800 m in the upper slope to 3760 m in the abyssal plain. OBS data processing includes position and time corrections, phase identification using Obstool software, and velocity inversion with FAST tomography software. The resultant velocity shows that Cenozoic strata has a velocity of 1.6-3.5 km·s-1 and a depth of 2 km. Mesozoic velocity is 3.5-5.5 km·s-1 with an average depth of 3 km. In the oceanic and continental transition zone (OCT), the sediment basement is greatly affected by Late Cenozoic magmatic activity. Obvious high velocity anomaly and isoline uplift (5 km) were detected in the upper crust of the upper slope, corresponding to chaotic reflection anticline structure in D80; and the overlying Late Cenozoic strata also presents as synchronous deformation, which may result from the Late Cenozoic magmatic intrusion. The crustal thickness decreases gradually from 23-20 km beneath the continental slope to 8 km in the oceanic basin. A high velocity (7.0-7.6 km·s-1) layer was seen in the lower crust, which changes from 5 km in the upper slope to 2 km in the ocean basin. Because of the Late Cenozoic magmatic activity in the continental slope and OCT, we interpret that the high crustal velocity layer originated from magmatic intrusion after the cession of seafloor spreading.%为深入理解南海北部多道地震测线D80显示的深反射信息,沿此测线布置OBS(15台)地震测线(OBS2015-1),测线长300km,方向NNW—SSE,从水深800m陆坡延伸至3760m深海平原.文章利用Obstool软件进行预处理(位置校正和时间校正等)、震相识别,利用FAST tomography软件进行速度层析成像.速度结果表明,新生代沉积层速度1.6～3.5km·s-1,厚度约2km;中生界速度3.5～5.5km·s-1,平均厚度约3km.在洋陆过渡带处,沉积基底受新生代岩浆活动影响,有较大起伏.在上陆坡处,上地壳存在向上凸起约5km的高速异常,在多道地震剖面中表现为杂乱反射的背斜构造,上覆晚新生代地层也同步形变,推测可能是新生代晚期岩浆侵入造成.地壳厚度由陆坡的23～20km减薄至洋盆的8km.地壳下部存在7.0～7.6km·s-1的高速层,高速层由陆坡的5km左右逐渐递减至海盆的2km左右,因上陆坡和洋陆过渡带晚新生代岩浆活动活跃,作者认为地壳下部高速层是由海底扩张停止后岩浆侵入形成.
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