Research On The Vibrator-Ground Coupling Vibration And Bandwidth Extension

As an efficient,environment-friendly and safe exploration equipment,seismic source has been widely used in oil and gas exploration.With the exploration moving towards deep,ultra-deep and unconventional oil and gas resources,higher requirements are placed on the excitation accuracy and bandwidth of seismic sources.However,for existing seismic sources,the low-frequency component of the excitation signal is insufficient and the output ground force of high-frequency drops dramatically,resulting in weak signal-energy and low signal-to-noise ratio.So,the excitation accuracy and bandwidth of the seismic vibrator must be further improved.This research mainly focuses on the vibrator-ground coupling vibration and bandwidth limitation of the seismic vibrator.Based on a combination of theoretical analysis and field tests,a model for vibrator-ground coupling vibration is established,and the research on vibrator-ground coupling vibration and bandwidth expansion is carried out.This research provide theoretical basis for understanding the vibration of the vibrator-ground and expanding the bandwidth of the vibrator,which mean a great significance for improving the performance of the vibrator,reducing the signal distortion and enhancing the exploration ability of the seismic source.The main research works and achievements of this dissertation are as follows:(1)Development status and trends of integrated seismic source technology and vibrator-ground coupling vibration at home and abroad are studied.Based on elastic half-space theory,a model for baseplate-ground coupling vibration is established.The pressure distribution on the bottom surface of the baseplate is parabolic distribution according to finite element analysis of vibrator-ground coupling vibration.Dynamic damping and dynamic stiffness are used to describe the interaction between the baseplate and ground,with the coefficients of the equation of motion are considered as functions of frequency.Then the model for the vibrator-ground coupling vibration is established,which provides the foundation for subsequent research works.(2)The dynamic response of the vibrator-ground coupling vibration was studied,including the dynamic stiffness,dynamic damping,modal analysis,structural response,and energy transfer of the coupled-vibration system under the sweep frequency.The dynamic behaviors of the dynamic stiffness and the dynamic damping analysis shows that the interaction between the baseplate and the ground decreases as the frequency increases.The modal analysis of the vibrator-ground coupling system is carried out,and the resonance frequencies and mode shapes of the structure are as follows:the first-order vibration mode of the structure is the synthetic resonance of the reaction mass and the baseplate,and the response of the structure is mainly dominated by the reaction mass;the second-order vibration mode of the structure is the inverse resonance of the reaction mass and the baseplate,and the response of the baseplate occupies a dominant position.Newmark method is employed to analyze the dynamic response of the structure.The response(displacement,acceleration,and velocity),the rule of the ground force,and energy transfer law of the structure under sweep signal are obtained.At low frequencies,the response of the reaction mass is much greater than the response of the baseplate.As the frequency increases,the response of the baseplate rapidly increases and exceeds the response of the reaction mass.The relationship between the ground force and sweep frequency can be divided into three stages,and different factors dominate different frequency bands.A method for calculating the energy transfer is proposed.The analysis shows that energy is mainly for moving the reaction mass during the low-frequency band,and the energy of the high-frequency band is mainly converted into the kinetic energy of the baseplate.The proportion of effective work(transferring into the earth)done by the system is very small,and most of the energy is consumed by the kinetic energy of the reaction mass and the baseplates,and damping.(3)The coupling vibration tests are studied and compared with the theoretical research from single frequency sweeps and linear sweeps.The research shows that as the sweep frequency increases,the frequency component of the reaction mass acceleration signal becomes closer to the sweep signal,the displacement of the reaction mass is smaller,and the control of reaction mass gets worse under the inertial force,and the acceleration phase difference of the reaction mass also increases.Comparing the response results obtained by three different methods shows that the weighted sum method overestimates the ground force and shows a significant phase difference at high frequencies.The measured ground force obtained by the model is in accordance with the load cell data during a full sweep,which indicates that the vibrator-ground coupling vibration model has good accuracy and reliability.(4)The analysis of the influence rules of the coupling system parameters on the vibration response are carried out.The dynamic stiffness,dynamic damping,natural frequency,phase,ground force,and energy transfer are analyzed in these five aspects.Each aspect is investigated from the mass of the baseplate,the mass of the reaction mass,the area of the baseplate surface,the elastic modulus of the earth,and the density of the soil with proportional change.The influence of coupling system parameters on the dynamic response of the vibrator-ground coupling vibration is divided into two stages.In the low frequency band,the mass of the reaction mass plays a decisive role in the dynamic response.In the middle-high frequency band,the mass of the baseplate,the area of the baseplate surface,the elastic modulus of the earth,and the density of the soil work together,with baseplate mass and elastic modulus of the soil having the greatest impact.The response laws of the system under different parameters and operating conditions are obtained,providing reference for structural design and field operation of the vibrator.(5)It is found that the stroke and pump displacement are the main factors limiting the low frequency expansion of the vibrator,and the servo valve and the high frequency output ground force limit the high frequency expansion.The objectives and direction of the vibrator optimization are obtained based on the study of the performances of the main seismic sources.The Monte Carlo simulation analysis of the frequency of vibrator-ground resonance shows that the first-order and second-order resonance bandwidths approximately obey the normal distribution.The first-order resonance frequency is mainly dominated by the reaction mass,and the second-order resonance is determined by multiple factors.The analysis of the second-order resonance bandwidth indicates that high-frequency output ground force inequality will inevitably occur when the vibrator expands to a high frequency.The particle swarm multiobjective optimization algorithm is used to improve the bandwidth of the vibrator.After the optimization,the low-frequency load capacity is further improved.It increases from 19.38%to 21.96%,and the ground force at low frequency also increases;the ground force at high frequency band is significantly improved,especially at 120Hz with an increase of 34.07%,but the output in the mid-band decreases slightly.