南海沉积物中天然气水合物饱和度与声学特性的关系

中国南海蕴藏丰富的天然气水合物（气水合物）资源，研究其赋存区沉积物中气水合物的声学响应特征，对南海气水合物的调查和资源估算具有重要意义。利用新型弯曲元探测技术和套管型时域反射技术（TDR），采用模拟实验手段研究了南海神狐海域沉积物中气水合物形成的声学响应特征，发现在气水合物饱和度（S_h）为0~14% 时，声波信号随气水合物形成而逐渐减弱，而当S_h>14% 后，声波信号渐渐增强；气水合物形成过程中，纵、横波速度（v_p、v_s）随S_h的增加而增大，且v_s在S_h>14% 后增长速度加快。这些现象表明，气水合物在南海沉积物中可能先在孔隙流体中以微粒子的状态生成，对声波造成了较大的散射衰减，导致声波信号变弱，但对剪切模量的贡献不明显；当S_h高于14% 后，微粒子数量的增多使气水合物聚集，并与沉积物颗粒接触，从而加快了v_s的增长。

Relationship between gas hydrate saturation and acoustic properties of sediments in the South China Sea

Gas hydrate has been recognized to be a potential energy resource in the South China Sea (SCS) and the acoustic response of gas hydrate formations in the SCS sediments is of significance in the regional hydrate investigation and quantification. We conducted a simulation experiment to investigate the acoustic response of hydrate formations in the Shenhu area of the SCS using a new bender-element detection technique combined with the casing time-domain reflectometry (TDR). The results showed that the acoustic signal becomes weaker with the occurrence of hydrates as the saturation of hydrates (S_h) ranges from 0 to 14%; then it strengthens gradually as S_h is higher than 14%. The acoustic velocities (v_p, v_s) of sediments increase with the increase of S_h during hydrate formation and v_s increases relatively faster as S_h becomes higher than 14%. These phenomena indicated that hydrates may form firstly in pore fluids in the form of tiny particles, which results in a weaker acoustic signal due to the considerable scattering attenuation of acoustic wave, but has little effect on shear modulus. As time lapses, when S_h exceeds 14%, numerous tiny particles of hydrates coalesce together and contact with sediment particles. As a result, v_s has a sharp increase.