浅层地震技术在陆域天然气水合物勘探中存在的问题及对策

Existent problems and countermeasures in application of shallow seismic technology to exploring natural gas hydrate in land area

在青藏高原永冻土区利用浅层地震技术探测天然气水合物是一种尝试。当可控震源遇到局部起伏不平的地表时,地表面不能与震源激震板有效地耦合,激发效果变差。在每个物理点数据采集前实施一次空振,有助于提高地表与震源激震板的耦合度。另一方面,夏季施工,浅地表低速融化层与下伏的高速冻土层之间的界面为强反射界面,该反射界面产生所谓的能量屏蔽。冬季施工虽可避开能量屏蔽问题,但浅表层高速冻土层常使可控震源激发出现谐振,地震记录上的谐波干扰严重降低了地震采集质量。在地表接近"半刚度"的半结冻条件下,可控震源激发既避开了表层融化低速层形成的能量屏蔽,又避开了冻结高速层引起的谐波干扰。采用相应的去噪技术能够有效地压制地震记录上的各种干扰,提高地震记录的信噪比。

It is an attempt to detect natural gas hydrate by using shallow seismic technology in permafrost regions of the Tibetan plateau. When the vibrator encounters the locally undulating surface ,the ground surface cannot be effectively coupled with the vibrator＇s excita- tion plate, and the excitation effect becomes worse. Conducting an empty vibration before the data acquisition of each physical point is helpful to improving the coupling degree between the surface and the seismic vibrator.On the other hand, the interface between the shal- low surface low velocity melting layer in summer and the lower high velocity layer is a strong reflection interface.The reflective interface produces so-called energy shielding problem. In winter, the problem of energy shielding can be avoided, but the shallow surface high speed permafrost often causes the vibrator excitation resonance.The harmonic interference in seismic records seriously reduces the quali- ty of seismic acquisition.Under the condition that the surface is close to the ＂half rigidity＂, the source excitation not only avoids the en- ergy shield formed by the melting of the low velocity layer on the surface but also avoids the harmonic interference caused by the high velocity layer.The corresponding denoising technique can effectively suppress the various kinds of interference on seismic record and in- crease signal-to-noise ratio of seismic record.