Test And Calibration Techniques For The Frequency Characteristics Of The Tunnel Geological Prediction Geophones

There are a lot of cataclastic zone, fault, karst, rich water rock structure in regions of complex terrain conditions and poor geological structure,and tunnel excavation is prone to cause subsidence, landslides, mud or water gushing, cracking and other conditions, which makes it difficult and dangerous for construction. In order to ensure the tunnel engineering construction's quality, punctuality and safety, the tunnel geological prediction has become an important step before tunnel engineering. And the method of seismic wave is the most important means of tunnel geological prediction. The geophones(the vibration sensor), which is the key equipment of the wide application in the field of seismic physical exploration, is also the key equipment of the tunnel earthquake prediction. How to accurately capture the seismic reflection wave in the surrounding rock is the most important step to determine whether the prediction is accurate or not.This paper briefly introduced the classification of seismic geophone, analysised the working principle and characteristics of moving-coil geophone and piezoelectric geophone for tunnel seismic prediction.Studied geophone testing method, constructed the geophone amplitude frequency characteristics measuring platform, and researched the effect of fastening method of geophone testing. Then the shaking table and sine frequency sweep excitation method of geophone were tested. Test tells that the frequency characteristics of the moving coil geophone in high frequency section differs from the theory,and the piezoelectric geophone has better performance in both low frequency and high frequency.Several sensor dynamic compensation method is discussed in this paper. After the analysis of the motheods,designing low-frequency expansion of the moving coil geophone by pole zero compensation method. After making the PCB circuit board, the test results basically meet the design requirements on the shaking table. Through the compensation circuit, the natural frequency of geophone from 100 Hz dropped to 10 Hz, damping ratio is close to the optimal damping, and the sensitivity was improved greatly.This paper analysised several geophones coupling mode, studied the butter coupling and perfect coupling condition under the rules of seismic wave propagation by finite difference numerical simulation, in order to prove that the butter coupling would have a certain impact on the quality of seismic data. These effects may affect the accuracy of the detection results.Finally, the piezoelectric acceleration-geophones are applied to Jinyun Mountain tunnel's prediction to get the geological information in front of tunnel face, and provide effective suggestions for the construction of the tunnel. The spectrum analysis of tunnel seismic wave tells that the seismic wave basicly are in 0~1000Hz, and lots of vibration below 100 Hz, 1000 Hz signal is very weak, indicating that the low frequency extension of moving-coil geophone's bandwidth is ideally suited to tunnel seismic prediction.