The Experimental Research On Attenuation Characteristics Of Near-Surface Low-Velocity Layer

The high absorption and attenuation characteristics of the low velocity layer in the near surface seriously affect the high resolution of seismic data.Although the seismic processing method and existing software can reduce the attenuation effect of near surface,most of the processing results only partly eliminate the attenuation effect,which is mainly due to the lack of a more comprehensive understanding of the characteristics of near surface.The seismic physical modeling can provide high quality seismic physical simulation data for the comprehensive,accurate and quantitative research on the attenuation characteristics of the low velocity layer in near-surface using its high controllability,strong authenticity,high signal to noise ratio.Whether in laboratory research or field exploration,the accurate Q value is the basis of attenuation characteristic research.Especially,when the Q value of the low velocity layer in the near-surface is estimated,it will be influenced by the near field effect,which leads to the great error on Q obtain.It is also affected by the near field effect when the Q value is measured in the laboratory.The diffraction effect caused by the near field effect of the transducer seriously affects the measurement and estimation of the Q value.According to the diffraction effect on the ultrasonic attenuation measurement,through numerical simulation and experimental measurement method,the diffraction effect on ultrasonic attenuation measurement in different measurement conditions are analyzed.Meanwhile some experimental methods to reduce the diffraction effect are given.The results show that the diffraction effects on ultrasonic attenuation measurement can be reduced by increasing the transducer's center frequency,transducer radius,the distance between transducers(sample length),and reducing the velocity difference between reference sample and test sample.Under the same test conditions,the diffraction effect on high Q value samples estimation is much greater than that of low one.There are many measurement methods to measure the Q value in the laboratory,but there is a large error in all kinds of methods.Taking standard samples and physical model samples as the research object,the various influencing factors are analyzed and the accuracy and stability are studied by using different method.An improved method is proposed to solve the problem that the theoretical correction method is not accurate when the diffraction effect is strong.The results show that it can improve the accuracy and stability of the Q values in laboratory through the improved measurement method and using the theoretical correction method.At the same time,according to the time-window influence problem when Q value is obtained by frequency domain method,an ultrasonic attenuation calculation method based on S transform is proposed.The experimental results show that the ultrasonic attenuation calculation method based on S transform improves the stability and accuracy of ultrasonic attenuation measurement.The different near-surface attenuation materials are studied based on the seismic physial modeling techniques.The attenuating characteristics of the attenuated material are qantificationally studied.The Q-V relationship of the attenuation material is compared with the field empirical Q-V relationship.The quantitative simulation experiment of seismic attenuation is studied based on the mathematical relationship of the attenuation material.The experimental results show that the attenuating materials have high controllability and have high similarity with the characteristics of the field attenuation.At the same time,the method of seismic physical modeling for quantitative simulating on formation attenuation has high accuracy.Finally,the method is applied to the physical modeling of the low velocity layer in the near surface.The first seismic physical model with the low velocity layer in the near surface was designed and produced combined with actual simulation area.Then two and three-dimensional seismic physical simulation data were collected and processed.The experimental results show that the attenuation characteristics of the low velocity layer in the near surface can be simulated effectively and accurately by the method of the formation attenuation quantitative simulation.The simulated seismic records and spectral characteristics are very similar to the field situation.The acquired seismic data can be used for further research on the attenuation characteristics of the near surface.