时移跨孔地震全波形反演方法在水泥搅拌桩检测中的应用研究

Application of time-lapse cross-hole seismic full waveform inversion method in cement mixing pile detection

水泥搅拌桩是处理沿海地区软土地基的常用手段,然而在实际工程中存在偷工减料、成桩质量不稳定等问题.目前的搅拌桩检测手段成本高,抽检比例低且可能对桩身完整性造成永久破坏,亟需无损、准确的检测方法.本文将跨孔地震、全波形反演及时移地震勘探相结合,提出了三种时移跨孔地震全波形反演方法(分别反演、连续反演和双差法反演),并将其应用于新加坡樟宜机场扩建项目中,分析比较了各种方法的检测效果.试验结果表明:跨孔波速测试易受环境干扰,难以反映出桩基的实际深度.背景场对全波形反演分辨率有一定影响,早先注浆失败残留的浆液会导致反演结果中搅拌桩的整体轮廓相对模糊.连续反演可以有效凸显桩基的位置和范围,双差法反演可以在连续反演的基础上进一步弱化背景场的影响,从而提高成像精度.研究成果可为水泥搅拌桩的无损检测提供重要指导.

Cement mixing piles are common method of treating soft soil foundation in coastal areas.However,there are some problems in practical engineering,such as cutting corners and unstable pile quality.Current methods for detecting mixed piles are high cost,have a low sampling-ratio and can permanently damage the integrity of the pile body.Non-destructive and accurate detection methods are urgently needed.In this paper,three methods of time-lapse cross-hole full waveform inversion(separate inversion,continuous inversion and double difference inversion)are proposed by combining cross-hole seismic,full waveform inversion and time-lapse seismic exploration.These methods are applied to the expansion project of Singapore Changi Airport,and the detection effects of various methods are analyzed and compared.The test results show that the cross-hole wave velocity test is easily disturbed by the environment and it is difficult to reflect the actual range of the pile foundation.The background field has a certain influence on the resolution of full waveform inversion.Continuous inversion can effectively highlight the location and scope of the pile foundation and double difference inversion can further weaken the influence of background field on the basis of continuous inversion,so as to improve the imaging accuracy.