高密度面波勘探和小型台阵地脉动观测在西部地区地震防灾中的应用

Application of Surface Wave Surveys and Array Microtremor Observations for Earthquake Disaster Mitigation in Western China

摘要:

鉴于我国西部地区地震防灾需要一种简便快速、低成本的地下构造调查方法,本文利用对甘肃岷县漳县地震灾害科考机会,对高密度面波勘探和小型台阵地脉动观测在西部山区地下构造调查中的有效性进行了试验.试验选取房屋倒塌和滑坡等不同灾害类型以及不同地质条件的场地进行;2种方法的数据采集都使用普通小型便携式面波仪和4 Hz地震检波器,其中,小型台阵采用底边长16m的正三角形台阵,观测时间约30分钟.试验结果显示,高密度面波勘探能够探测地下约30～40m以内的剪切波速度构造,小型台阵探测深度稍深,约为40～50m,但是浅层分辩率较低.试验还显示,两种勘探方法使用同一台普通面波仪和地震检波器,设备小、成本低、便于携带,非常适合西部地区地震防灾之地下构造调查.

Abstract:

Since a simple,fast,and low-cost method for investigating subsurface structures to prevent seismic disasters is needed in the western part of China,the authors have taken the opportunity to study the Gansu Mingxian Zhangxian MS6.6 earthquake and have conducted field tests for the effectiveness of surface wave surveys （SWS） and small size array microtremor observations.Sites having different geological conditions and having undergone different types of earthquake disasters,such as collapse of houses and landslides,were selected for the test.A portable seismic recorder and 4 Hz vertical component geophones were used for the acquisition of data for both surface wave surveys and small size array microtremor observations.Surface wave survey was done using a 24-receiver spread with a 2 m receiver interval for 5 m,10 m,15 m,and 20 m source offsets.Small size array microtremor observations were done using a triangular array of 16 m for observation duration of 30 minutes.The results have shown that the SWS can become an effective method for the geological investigation of the low western area covered by loess and the low groundwater level.The thickness of the loess is large and the inner structure is simple,which provides favorable conditions for surface wave surveys.The depth of the survey is 30 m more due to the seismic energy sources from artificial hammering.High-frequency microtremors of more than 2 Hz can also be used for subsurface structure surveys in the mountainous rural areas for rare mechanical vibrations.Small size array microtremor observations need a small site and euvironmental requirements are not strict.The depth of survey is 30 m more using small size array microtremor observation,which is seen to be highly effective.The same types of equipments were employed for surface wave surveys and small size array microtremor observations,which improved the usage efficiency of the equipments.Small size array microtremor observations were done using a triangular array of 16 m and for observation duration of 30 minutes.The results have shown that surface wave surveys can estimate the shear wave velocity for structures down to a depth of 30～40 m,while the small size array microtremor observations can estimate down to a depth of 40～50 m.Small size array microtremor observations can explore slightly deeper but the resolution of shallow layers is lower.As the instrument is potable,the method is both simple and lowcost; hence,surface wave surveys and small size array microtremor observations are very suitable for the prevention of seismic disasters in the western region of China.