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| Research on MSFM-based microseismic source location of rock mass with complex velocities |
| GUO Liang1，DAI Feng1，XU Nuwen1，FAN Yilin2，LI Biao1 |
(1. State Key Laboratory of Hydraulics and Mountain River Engineering，Sichuan University，Chengdu，Sichuan 610065，China；
2. China Three Gorges Corporation，Beijing 100038，China) |
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Abstract A rapid and accurate microseismic(MS) source location method is the foundation of MS monitoring to play a better forecasting and early-warning role for rock mass stability. The accurate travel time calculation of first arrival wave is the key to improve the location accuracy of MS source. Aiming at rock mass with complex velocities in actual engineering，like voids filled with air and partitioned velocities，the multi-stencils fast marching methods(MSFM) was introduced to calculate the travel time of first arrival wave，which is fast and with high precision and strong applicability for complex model. In this method，a new calculation stencil is generated by means of coordinate rotation to make the diagonal adjacent points of the mesh calculated，and the precision of the fast marching method(FMM) is improved in diagonal direction. Firstly，the accuracy and efficiency of the first or second order FMM and MSFM were compared and analyzed，showing that the second order MSFM has higher accuracy. Secondly，for rock mass with partitioned velocities model and one with a void，both single velocity model and the second order MSFM were used to calculate the travel time. Through comparing them to analytical solution，the average absolute errors of the second order MSFM were reduced by 97.65% and 95.18% compared with the single velocity model. Then，the rock mass model with partitioned velocities model and tunnels was built，and it was proved that the second order MSFM has strong applicability for complex model. Finally，the MSFM-based MS source location algorithm of rock mass with complex velocities was proposed，and applied to Baihetan hydropower station left bank slope of MS location. By trial location of four blasting events collected in fields，the relative optimal layer velocities combination was obtained，which had the minimum mean value(9.6 m) of location errors and was used to establish the void-considered velocity model. The reliability of this velocity model was validated by the travel time forward calculation of a blasting event. The 128 MS events in May 2015 were located by both the single velocity model and the proposed location algorithm，by comparing the location results，the located events by the latter were distributed as belts more obviously near the main damage zone(LS331 and LS337) than those by the former. The study concludes that the MSFM has very valuable application as a travel time calculation method，the proposed MSFM-based MS source location method can improve the source location precision effectively.
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2017, Vol. 36 
