Study On The Source Characteristics And Influencing Factors Of Land Air Gun By Numerical Simulation

Monitoring subsurface structures and media changes is one of the tasks of seismologist,and a suitable signal source is a key factor to monitoring results.As an ideal source for regional geophysical exploration,compared with the traditional artificial sources,air guns are highly repeatable,rich in low frequency and evironment friendly,and greatly expand our tools to exploring the earth.There is a significant difference between the airgun stimulated in terrestrial water and in the marine.The land airgun usually stimulated in different shapes of limited small water body.The excitation and propagation of the land air gun signal are inevitably affected by the solid-liquid interface.When use the air guns to monitor high precision medium properties,we need to comprehend the characteristics of the air gun source to remove the effect of source on the measurement results.In order to study the effect of the excitation environment on the air-gun excitation signal,and investigate the energy propagation process of the air-gun signal,we use the data of some experiments of the land air gun applied in regional scale geophysical exploration,with numerical simulation method,to investigate the influence of different water bodies on the characteristics of air gun excitation signal in this paper.The mechanism of the influence of water shape on the excitation signal of air gun is analyzed.Combined with the actual observation data and numerical simulation results,a simple calculation method is proposed to quickly evaluate the energy propagation of air gun after excitation.In this paper,we analyze the data of airgun seismic singal emitting station in Binchuan,Hutubi and Zhangye,Gansu Province and found that the application of large-capacity air guns to regional geophysical surveys in land areas is operable and has the very important scientific significance.It is effective to study the propagation of seismic waves by numerical simulation.In the domain of active source detection,numerial simulation can reveal the propagation of seismic waves in complex media.It can provide theoretical support for the analysis and interpretation of active data and the inversion of regional media structure,which can help to use the active source to carry out underground study of the characters of the media.The experiment situation is very complicated in the air gun active source experiment.Such as the reflection and refraction of the solid-liquid interface,the change of the water level of the water body,the deviation of the position of the air gun in the water body and so on,are likely to change the wave field characteristics.It is very difficult to isolate these influencing factors from the observed waveforms to obtain the source characteristics.Therefore,the forward modeling of the numerical simulation of the air gun source will help us to obtain more accurate characteristics of the source itself.The environment of airgun stimulated in terrestrial water and in the ocean is different.We analyze the influence of the shape of the water body on the excitated signal of the large-capacity air gun from the aspects of absolute amplitude,relative amplitude,frequency and shape parameters by analyzing the data of the actual air gun excitation experiment and the numerical simulation experiment in this paper.The results show that the shape of different water bodies has influence on the signals,and the different shapes of water bodies has different effects on different frequency signals.Within the water scale of tens of meters,the land limited water body has a strong influence on the high frequency signal,but has little effect on the low frequency signal.The land air gun signal to form a complete bubble pulse need enough water volume,and in the case of water volume is large enough,the steeper water body boundary,air gun energy into low-frequency energy efficiency is higher,the more exciting signal waveform is sharper.The classic bubble oscillation theory is the basic theory to explain the excitation mechanism of marine airgun.It divides the gun air signal into two parts: the pressure pulse signal and the bubble pulse signal.However,there is no accurate theoretical model on the study of land air gun excitation signal with solid-liquid interface.Although signal equation of the air gun emitted in the semi infinite space water can be given by the classical bubble oscillation theory,it is difficult or impossible to find the analytical solution of the air gun excitation signal in the land limited water due to the complexity of the environmental boundary conditions.This makes it necessary for us to find a way to study the source characteristics of land-based air guns.According to the basic principle of the back projection method,we improve the algorithm to make it suitable for land air gun.Through five sets of numerical simulation experiments,we find that the energy of the air gun signal can be effectively monitored by using the signal received by the hydrophone arranged in the water body.The method is easy to set up,the calculation is quick,and the result is reliable.One can effectively and timely assessment of the effect of air gun source,and provide helpful guidance to make full use of air gun source using our method.We use land air gun to detect the velocity changes,but we can not locate the velocity variations using traditional methods due to the data from a single source and single side of the receiving stations in air gun application.Numerical simulation seems to be an alternative way to provide some constrains on the position of the velocity change.Firstly,we use detailed velocity structure information to establish test models of different velocity anomalies.Then we use numerical method to calculate the theoretical waveform signal for each model.By comparing the calculated waveform and observed waveform,we can select the optimal model to estimate the position and magnitude of the velocity change.