准南缘山前带精细近地表建模技术研究

Research on fine near-surface modeling in south foothill belt of Junggar basin

准南缘米泉区块位于乌鲁木齐市东北部,博格达山西北缘,海拔高程在500~3500 m之间,地形高差较大.工区北部覆盖有较厚的黄土砾石,地震波吸收衰减严重,工区南部地层倾角大,并有灰岩出露.前期二维地震调查所获的成像剖面信噪比低,一个很重要的原因是小折射、微测井等常规近地表调查方法难以建立准确的山前带近地表模型,进而造成激发井深设计不准,并影响了后续的近地表静校正效果.为了建立山前带精细近地表模型,对工区进行了详细踏勘,利用地表露头信息明确了地表岩性分布规律;开展了试井及岩性录井工作,了解了近地表层岩性及厚度变化;利用微测井及超深微测井调查,得到了测点位置处黄土与砾石层的速度与厚度;实施了浅层二维层析,查清了沿线的近地表结构,并以浅层层析反演结果为依据,对老的二维地震测线开展了不同偏移距资料层析反演.综合利用这些资料,建立了全区精细的近地表模型.分不同近地表结构开展点试验,发现了激发井深与激发岩性及激发速度的关系,逐点确定了最佳激发井深,同时为后续近地表校正处理奠定了基础.

The Miquan Block on the south margin of Junggar basin is located in the northeast of ürümqi City and the northwest of the Bogda Mountain, with an elevation ranging from 500 to 3500 m. The north of this work area is covered by loess and gravel, which causes serious absorption and attenuation of seismic wave. In the south of the work area, there are steep strata and exposed limestone. In earlier 2 D seismic surveys, the obtained seismic imagings have poor signal to noise ratios. One of the most important reasons is that conventional methods such as micro-refraction and micro-logging are difficult to establish an accurate near surface model, which leads to inaccuracy of excitation well depth design and near-surface correction. In order to establish a fine near-surface model for the piedmont zone, a detailed survey was carried out, and surface lithologic distribution was clarified through collecting surface outcrop information;well testing and lithologic logging were carried out to understand the changes in near-surface lithology and thickness;micro-logging and ultra-deep micro-logging were used to estimate velocity and thickness of loess and gravel;shallow 2 D tomography was performed, and the near-surface structure along the line was ascertained, which is the basis of tomography on earlier 2 D seismic survey lines with different offset data. Using obtained data comprehensively, a fine near-surface model of the entire block was established. Point tests of surface structure were carried out to determine the optimum excitation mode, and laid a foundation for subsequent near-surface correction.