Research On Coupling And Output Characteristics Of Electromagnetic Vibroseis

In seismic exploration,artificial seismic sources are the key equipment for exciting seismic waves.According to the different characteristics of the excitation signals,there are two types of artificial seismic sources: impact-type seismic sources and vibroseis.Explosive and other impact-type seismic sources excite sharp pulse signals,which face many limitations in seismic exploration.The vibroseis generates sinusoidal sweep signals through continuous vibration.The sinusoidal sweep signals can be precisely controlled both in energy and frequency band.The vibroseis plays an important role in seismic exploration because of the advantages of safety,environmental protection,efficiency,and controllability.Electromagnetic vibroseis is a type of wide band vibroseis driven by electromagnetic force,which converts electrical energy into mechanical energy through internal electromagnetic-mechanical coupling.With the development of technology,electromagnetic vibroseis has broad application prospects in urban underground space exploration,land mineral resource exploration,and marine oil and gas exploration.In land exploration,electromagnetic vibroseis outputs ground forces in the form of vibration plate-ground coupling;In marine exploration,electromagnetic vibroseis excites dynamic pressure in the form of vibration plate-seawater coupling.The output characteristics of electromagnetic vibroseis are closely related to system coupling and directly affect the effectiveness of seismic exploration.Due to factors such as its own structure and exploration environment,electromagnetic vibroseis has complex coupling.This paper focuses on the coupling and output characteristics of electromagnetic vibroseis,and conducts relevant research from three aspects: internal electromagnetic-mechanical coupling,vibration plate-ground coupling,and vibration plate-seawater coupling,and analyzes the output characteristics of electromagnetic vibroseis in land and marine environments.The main research content is as follows:(1)A study was conducted on the measurement,analysis,and optimization of electromagnetic-mechanical coupling within electromagnetic vibroseis.Aiming at the problem of electromagnetic-mechanical coupling occurring inside the device and difficult to measure directly,an indirect measurement method based on the law of electromagnetic induction is proposed.The method converts electromagnetic-mechanical coupling measurement into measurement of the relative magnetic field motion velocity of the driving coils and the induced electromotive force generated by the driving coils.The experimental results show that the proposed method can achieve fast and accurate measurement of electromagnetic-mechanical coupling.Then,the influencing factors of electromagnetic-mechanical coupling were analyzed.Based on the finite element analysis method,the effects of static air gap magnetic field(magnet,air gap),alternating air gap magnetic field,and driving coils position on the nonlinearity of electromagnetic-mechanical coupling were analyzed.By fitting the electromagnetic-mechanical coupling nonlinear curve with a Gaussian function and iteratively solving the electromagnetic-mechanical coupling motion equation,the influence of electromagnetic-mechanical coupling nonlinearity on vibration signals was analyzed.The results indicate that for portable electromagnetic vibroseis,the static air gap magnetic field is the main factor affecting the nonlinearity of electromagnetic-mechanical coupling,which will cause distortion of low-frequency vibration signals,manifested as the superposition of fundamental and third harmonics.Finally,an air gap magnetic field structure consisting of an axial magnet and a radial magnetic ring is proposed.The radial magnetic ring compensates for the radial magnetic induction intensity in the air gap region between the magnet and the outer yoke,thereby suppressing electromagnetic-mechanical coupling nonlinearity.The results show that the amplitude ratio of the third harmonic of the reaction mass acceleration signal to the fundamental wave decreased from 0.4244 to 0.0466 after optimizing the magnetic field structure.(2)A study was conducted on the vibration plate-ground coupling and the characteristics of ground force based on field test experiments.Mathematical coupling models are assumptions and approximations of real physical systems,making it difficult to accurately characterize the vibration plate-ground coupling,as well as the ground force.Field testing method to study the vibration plate-ground coupling,as well as the characteristics of ground force,can overcome the shortcomings of existing mathematical coupling models method.This paper designs field measurement experiments from two dimensions: surface environment and driving level.Two typical surface environments were selected: hard asphalt road and soft soil.Ground force and acceleration signals were collected under different driving levels.Based on the pure phase shift filtering method,the fundamental and multiple harmonics in the ground force and acceleration signals were separated.Firstly,study the vibration plate-ground coupling by analyzing the ground impedance.Based on the real ground force and the velocity of the vibration plate,the ground impedance was calculated.The viscosity and stiffness decay exponentially with the vibration velocity and displacement,gradually decreasing and eventually stabilizing with the increase of vibration frequency and driving level.Increasing the driving level will slow down the attenuation trend of viscosity and stiffness with vibration velocity and displacement.Then,the interference of multiple harmonics on the fundamental wave was analyzed.In both surface environments,the proportion of ground force fundamental wave energy is greater than 98%,and the distortion is mainly second to third harmonics.The interference frequency band of different order harmonics on ground force fundamental waves has significant differences and is closely related to the surface environment.In some frequency bands,increasing the driving level leads to different trends in the interference of the second harmonic on the fundamental wave.But increasing the driving level can reduce the interference of the third harmonic on the fundamental wave and phase distortion.Then,a frequency domain estimation method was proposed to calculate the ground force response delay.The results indicate that there is a millisecond level delay between the ground force and the sweep signal,and the response delay is greater in soft soil.In hard asphalt roads and soft soil environments,the effect of driving level on response delay shows an opposite trend.Finally,the estimation errors of weighted-sum ground force were analyzed.The analysis results indicate that there are not only amplitude errors but also phase errors in weighted-sum ground force.In a soft soil environment,there is a significant arrival time error in the cross-correlation wavelet,when weighted-sum ground force is used as the cross-correlation reference signal.The above analysis enriches the study of ground force characteristics and provides support for the study of the coupling mechanism and modeling of vibration plate and the earth.A study was conducted on the vibration plate-seawater coupling and dynamic pressure characteristics based on finite element analysis method.Marine electromagnetic vibroseis transmit dynamic pressure to the water through the vibration plate-seawater coupling.The radiation theory of vibration plate in infinite plane is not applicable to describe the radiation characteristics of marine electromagnetic vibroseis.This paper conducts research through finite element analysis methods.By establishing a 3D coupling model to study the vortex phenomenon in the coupling process between vibration plates and seawater,the effects of planar,conical,and spherical vibration plates on vortex were analyzed.Vortex phenomenon in seawater mainly occur during the stage when the vibration plate moves to its maximum or minimum displacement.The design of conical vibration plate improves the performance of electromagnetic vibroseis while reducing vortex in water.The solution time for the 3D coupled model is too long.To improve the efficiency of vibration plate seawater coupling analysis,a2 D axisymmetric model of vibration plate-seawater coupling based on dynamic grid technology and user defined function method was established.On the same hardware system,the solution time for 2D axisymmetric coupled models is only 5% of that for3 D model.Based on the 2D axisymmetric coupling model,the radiation theory errors of vibration plate in infinite plane,the influence of barriers and vibration differences between left and right vibration plates on the dynamic pressure of electromagnetic vibroseis were studied.The error proportion of the theory exceeds 25%,gradually decreases and tends to stabilize with the increase of vibration frequency,and fluctuates and rises with the increase of vibration velocity.The simulation results of different sized baffles indicate that adding baffles will help increase the dynamic pressure excited by the marine electromagnetic vibroseis and reduce theoretical errors.When the size of the baffle increases from 0.05 m to 0.4 m,the error proportion of the theory decreases from 26.75% to 20%.The vibration difference between the left and right vibration plates will reduce the dynamic pressure of marine electromagnetic vibroseis.When the vibration plates on both sides vibrate in the same phase,the dynamic pressure decreases by about 64.84%.The study on the internal electromagnetic-mechanical coupling,vibration plate-ground coupling,and vibration plate-seawater coupling not only enriches the research content of electromagnetic vibroseis,but also provides support for the optimization of electromagnetic vibroseis,coupling mechanism analysis and coupling modeling.