Study On Air-gun Source Bubble Characteristics And The Simulation Of It’s Signature

The competitive exploration and development of marine mineral resources(gas hydrate,iron-manganese nodules,sand mines and petroleum)has made great progress in marine seismic exploration technology in recent decades.The most useful exploration technique for submarine resources is seismic methods.The large-capacity air-gun source and gun arrays are undoubtedly the most practical and most popular source excitation methods for seismic exploration.Because the bubble pulsation process excited by the air gun is very complicated,the research on the pressure wavelet generated by it is very difficult.The actual excitation test experiment requires a lot of manpower and material resources.Therefore,many related aspects of the research use numerical simulation.At present,the domestic application of air guns is mainly based on imports.The simulation calculations of wavelets mostly use is that imported foreign software such as Nucleus and Gundalf,which greatly limits our research on seismic theory and the seismic exploration.And the current industrial air-gun source simulation principle is limited to the spherical bubble theory,without considering the influence of the deformation,jet,migration and other factors of the air-gun bubble in actual conditions.In order to carry out relevant theoretical research and develop mobile and miniaturized land water gas gun sources and corresponding practical data processing software,the Fujian Provincial Seismological Bureau conducted a large-capacity airgun source excitation test in several reservoirs in the province in recent years.In this paper,the data of the excitation test of different reservoirs using the mobile reservoir airgun source in 2014-2018 were used(including the street reservoir in Youxi County,Sanming City,the Longmentan Reservoir in Quanzhou City,Fujian Province,and Quanzhou City).Shanmei Reservoir),the experimental data of actual working conditions of airgun bubbles carried out in the Taiwan Strait,the main work carried out is as follows:Combining the theory of spherical bubble(taking Rayleigh-Plessent bubble motion equation as an example)and the non-spherical theory of boundary element method to study the bubble characteristics excited by airgun source,especially the characteristics of bubble shape,jet and migration,and further understand the motion behavior of air-gun bubble.And compare with the actual measured pressure wavelet.This paper shows that the air-gun bubble will deform when it collapses,forming a liquid jet in the direction of motion of the bubble centroid.These deformations have an effect on the radial and vertical motion of the bubble,increasing the oscillation frequency,decreasing the signal damping rise rate,and the like.Summarized several classical models in the spherical bubble theory(Schulze-Gattermann model,Safar model,Johnson model,Ziolkowski model),based onthe most industrialized Ziolkowski model,The bubble oscillation equation and the pressure wavelet equation for the large-capacity air-gun source are established.The influencing factors of the air-gun wavelet characteristics of the land reservoir are analyzed with the actual measurement data,especially the excitation depth,the reservoir solid-liquid interface,the number of guns,and the virtual reflection.The influence of other factors.Because of the established bubble oscillation equation and the pressure wavelet model as a simplified model,there are still many deficiencies,such as the instantaneous release of all gases and the assumption that the spherical shape does not match the actual situation,without considering the existence of the gun body and between the bubble and the water.Heat exchange,etc.,causes the predicted oscillation result to decay slowly,last for a long time,and the amplitude of the main pulse is too large,and the actual measurement wavelet record shows that a typical air-gun bubble will be exhausted after the first few oscillations.Based on this,we analyzed some factors neglected in the air-gun source wavelet simulation,and improved the original model by introducing additional influence factors.Correction factors that have important influences on the simulation results,including opening-based thermodynamic models,muzzle throttling,evaporation and condensation,gun body effects,gas release rates,etc.,are used to improve the traditional method of calculating single-gun pressure wavelets.And the results of the comparison of the measured wavelets are used to analyze the influence degree of each correction factor on the wavelet simulation,and further calculate the pressure wavelet of the large-capacity gun array through the improved single-shot signal,and the wavelet waveform and measured by the improved simulation method are predicted.The comparison of the signals shows that the improved model can better simulate the source wavelets of single guns and gun arrays than the original model,and analyze the causes of the simulation errors of the model.Finally,the numerical parameters of the planar array of the coherent gun and the tuned gun are analyzed in the deep detection of the land.