气枪震源激发条件对走时变化观测结果的影响

Influence of different triggering conditions of airgun source on travel time changes

基于4支单枪容量2 000 in3的气枪组成的阵列,2017年2月在宾川地震信号发射台大银甸水库内开展了不同激发气枪压力、沉放深度和水平位移情况下的对比试验。利用发射台周边9个流动宽频带数字地震仪与震源附近3个参考台的数据,使用互相关时延检测的方法计算了不同激发条件下的气枪信号的走时变化,并结合激发条件进行了分析。研究表明:(1)不同气枪压力或沉放深度下,激发的气枪信号的波形和互相关系数存在明显差异;(2)气枪压力从9 MPa升为15 MPa,走时变化可达0.06 s;(3)走时变化的可能原因是主频段(3~5Hz)中的相对高频成分发生变化,相对低频成分未变化,二者耦合在一起形成了伪走时变化;(4)由于气枪信号不同时窗走时变化不尽相同,技术手段难以消除压力的影响,所以建议未来气枪激发时固定激发条件;利用已有资料做走时变化相关研究时选用震幅相近的数据。

Airgun source is a newtype of artificial source,and it performs very well in travel-time changes research because of its high repeatability. To get the effect of exciting condition,we carried out the time changes analysis with excitement in different pressure,depth and location. The results showed that：（1） Shapes and cross correlation coefficients between waveforms in different exciting conditions showclear diversities.（2） Travel time changes reach 0. 06 s if air-gun＇s pressure increased from 9 MPa to 15 MPa.（3） High-frequency component of the received data has changed and low-frequency component has not,so the reason of the time changes which associated with triggering condition change is the coupling of the two components. These changes do not real travel-time-changes.（4） Because the tendency of different time windowof received data is not similar,it is difficult to remove influence of triggering condition. We suggest keeping triggering condition unchanged in later experiment if possible. If the already produced data are used,it＇s better to select data which have the same amplitude.