南海马尼拉海沟中部俯冲增生楔中发现似海底反射

BSRs in the subduction accretion wedge in the central Manila Trench,South China Sea

通过多道地震数据处理,首次在马尼拉海沟中部俯冲增生楔的地震剖面上发现似海底反射(Bottom Simulating Reflectors,BSR),其与海底面反射近似平行,振幅极性相反.通过对多个地震道集的速度谱提取地震波层速度,结果显示,BSR上覆地层速度为2267~2553 m·s^(-1),下伏地层为1977~2132 m·s^(-1),两者差异明显.BSR与海底面之间地层即水合物稳定带的时间厚度为440~519 ms.BSR特征明显的区域位于中段,宽度为38 km,水深为2850~3130 m,靠近海沟的西段以及与吕宋海槽相邻的东段间断有BSR分布,特征不是很明显.综合以上特征,推测BSR上覆地层是含水合物层,下伏地层疑似含气.基于水合物的形成与保存条件以及逆冲断层的发育程度,认为马尼拉海沟中部俯冲带,增生楔的中段是水合物分布有利区,赋存水合物的可能性比较大.

According to the processing of multi-channel seismic data, Bottom Simulating Reflectors(BSR) is first found in the seismic profile of subduction accretion wedge in the middle of Manila trench. The BSR was approximately parallel to the seabed reflection and the amplitude polarity was opposite. The interval velocities of seismic wave are extracted from the velocity spectrum of multiple CDP. The results show that the velocities of BSR overburden layer are 2267~2553 m·s^(-1), and the velocities of underlying layer are 1977~2132 m·s^(-1), with obvious difference between the two sets of velocities. The thickness of the formation between the BSR and the seabed, namely the hydrate stability zone, is between 440 m and 519 m.The region with obvious BSR characteristics is located in the middle section, with a width of 38 km and a depth of 2,850~3,130 m. Distribution of BSRs is not obvious in the western section near the Manila trench and the eastern section adjacent to the Luzon Trough. Based on the above characteristics, it is inferred that the overburden layer of BSRs is hydrate bearing layer, and the underlying layer is suspected to contain gas. Based on the formation and preservation conditions of hydrates and thrust faults, it is considered that the subduction zone in the middle of the Manila Trench and the middle section of the accretion wedge are favorable areas for the distribution of hydrates, and the possibility of hydrates occurrence is relatively large.