勘探地震中横波分裂研究四十年回顾

四十年前,地壳中的横波分裂现象首次在天然地震中得到证实,并引入到勘探地震领域。回顾了横波分裂研究的发展历史,重点阐述横波分裂的由来、原理及其在裂缝性油气藏描述中的应用。横波在裂缝介质中传播时,必然会分裂为一个快波和一个慢波,该现象被称为横波分裂。通常快波的偏振方向平行于裂缝走向而慢波的偏振方向垂直于裂缝走向,快、慢横波时差与裂缝密度(或强度)成正比。记录和分析裂缝油气藏中存在的横波分裂现象可以更好地描述裂缝的走向和密度。另外,通过多分量地震资料分析得到的相关横波分裂频变特性可以为研究裂缝尺度、流体类型及分布等提供重要信息。多年研究表明,地壳中广泛存在横波分裂现象,并且近地表(地下1200m内)比深部地层存在更强的横波分裂现象。由此并结合横波资料的品质等问题,使横波分裂在勘探地震中的应用仍然受到限制。主要表现为:应用对象选择上必须慎重,即近地表要相对简单;裂缝储层相对较厚,同时需要采集横波VSP资料进行标定。总之,数据品质、近地表和储层条件是影响横波分裂实际应用效果的3个关键因素。只有当横波资料的品质(包括转换横波)和采集成本与纵波可比时,横波分裂的应用潜力才能充分发挥。

This paper reviews the use of shear-wave splitting in exploration seismology,how it began and how it has been used to characterize fractured hydrocarbon reservoirs in the past forty years since it was first confirmed in earthquake seismology and subsequently introduced to the hydrocarbon industry.Shear-wave splitting (or birefringence) refers to the phenomenon that when a shear-wave enters a fractured medium,it necessarily splits into a fast and slow wave.The fast wave is usually polarized along the fracture strike,the slow-wave perpendicular to the strike,and the time-delay between the fast and slow wave is proportional to the fracture density.Therefore,recording and analysis of the resultant shear-wave splitting in fractured hydrocarbon reservoirs can give us better definition of fracture orientation and density.Furthermore,frequency-related changes in shear-wave splitting derived from multicomponent seismic data can provide us with vital information about fracture scale-length,fluid type and distribution.Current surveys and various research works have revealed that shear-wave splitting is widely present in the Earths crust,and more profound splitting occurs in the near-surface (the first 1200m of the Crust) than the deeper subsurface.This and some other shear-wave data quality issues restricted the application of shear-wave splitting in seismic exploration to certain areas with favourable conditions such as a relatively simple near surface and relatively thick and heavily fractured reservoirs,together with detailed calibrations of shear-wave splitting from shear-wave VSPs.Overall:data quality,near surface and reservoir conditions are the three key factors which affect the applications of shear-wave splitting.Only when shear-wave data quality and the cost-effectiveness in shear-wave data acquisition become comparable with that of the P-wave,can the full potential of shear-wave splitting be realized.

基金项目：国家自然科学基金项目( 41074080,U1262208,41681230562)部分资助。