过汶川地震主断裂科学钻WFSD-1反射地震采集方法研究

Acquisition method study of 2D reflection seismic short profile through WFSD-1 in main faults of Wenchuan earthquake

横穿龙门山地震反射剖面系列探测是WFSD工程中的一项重要研究内容.通过WFSD-1先导孔区二维反射地震的观测方法研究和试验,实现了预期目标:(1)在震后恶劣的天气、陡峭地形和地表严重破裂、破碎、滑塌等困难条件下,获得了WFSD工程首条分辨率较高的二维反射地震剖面;(2)从地学研究目的出发,结合龙门山地区的复杂地震地质条件,提出了以浅、中、深部复杂反射体探测为目标的高密度时间、空间采样的数据采集方法,为后续剖面的实施提供了方法依据;(3)发现了映秀—北川断裂带地表出露位置以西5 km范围内垂向上的多层次地质体的反射;1.2 s以下的中深部反射体明显具有层状沉积岩或变质岩的特征;推断认为测线上彭灌杂岩最大厚度约3 km;(4)灌县—安县断裂带呈明显的双层结构,山前隐蔽带沉积体系完整;(5)在孔区附近800m范围内,进行了20～30 m激发点距、10 m道距的密集观测,为WFSD-1孔完钻深度以浅反射体的高精度标定提供了可靠的原始数据.

The serial reflection seismic profiles investigation across the Longmen Mountain is an important project in Wenchuan Earthquake Fault Zone Scientific Drilling （WFSD）. The 2D artificial reflection seismic experiment in Scientific Drilling Site of WFSD-1 Pilot hole completedthe prescribed task, including： （1） Successfully obtained the first 2D reflection profile of WFSD in extreme adverse environments with steep terrain, developing ruptures, fracture, slumped bodies and bad weather, right after the 5 - 12 Wenchuan earthquake; （2） Based on the study of the complex structure condition in Middle Longmenshan Fore-mountain area, seismic data acquisition methods featuring high spatial and time sampling density were proposed for detecting the complex geological reflective bodies in the shallow, middle and deep part of the area, providing the basic idea and acquisition parameters for the next observations; （3） It is the first of such studies to find multi-level geological body reflectors in the western part （5 km） to the exposed location of Yingxiu-Beichuang faults. The middle and deep reflection body below 1.2 s show obvious layer formation of sediment or metamorphosed rocks. It is estimated that the maximum thickness of Penguan Complex is about 3 kin. （4） The Guanxian-Anxian faults area shows clear double-layer reflection structure. The concealed piedmont zone has complete sediment system. （5） Employing dense observation method with 20-30 m shot and 10m receiver spacing within 800 m of WFSD-1, the study acquired reliable raw data for highly-accurate calibration to reflectors within the depth of WFSD-1.