| Air-gun source, which is widely used in offshore oil exploration, has many advantages, such as stable performance, low cost, easy to operate, repeatable, high precision, no pollution and so on. In order to explore the the application prospect of mobile air-gun source in a reservoir in depth exploration of regional scale,Earthquake Administration of Fujian Province carried out a comprehensive experiment of mobile type air-gun vibroseis system at Jiemian Reservoir in Youxi County, Sanming City, Fujian Province.This paper is based on the records of near field observation system, which is composed of hydrophone, OBS, seismograph and macroseismograph around the reservoir area of floating platform,studies the air-gun's near field characteristics and its influencing factors,simulates air-gun wavelet based on open thermodynamic system and compares with the near field experimental observations, and studies air-gun signal propagation characteristics of far field seismic stations records surrounding and in Fujian.The study result shows that the air-gun signal which has the characteristics of deep penetration, far propagation and abundant deep crust information,can be used in deep exploration of regional scale.The main work is as follows:Near field characteristics of air-gun source. We studies time-frequency characteristic, energy characteristics, environmental impact and repeatability of air-gun signal based on near-field experimental records,and analyzes wavelet parameters influenced by gun depth, work pressure,gun number, array size, water depth combined with the hydrophone and OBS records.The results show that: (1) The air-gun wavelet is composed of high frequency main pulse and low frequency bubble pulse.Main pulse which is short duration and wide frequency band,can be used in high precision shallow exploration. Bubble pulse which is rich in low frequency energy below 10Hz,can be used in deep exploration. (2) Air-gun is a small equivalent source,excitation energy of single air-gun equals to M0.23 earthquake,and excitation energy of four gun array equals to M0.48 earthquake. (3) Air-gun source is environmental protection. When four gun array excites,the peak acceleration of the vibration of dam is 0.45gal, which does not destroy the dam and surrounding environment. (4) The repeatability of air-gun signal is high. The peak cross correlation coefficient bigger than 0.95 of the nearest station records occupies 93.26%. (5) The deeper the gun depth is, the larger the bubble oscillation energy is, and the smaller the bubble period is.Deep exploration need select suitable gun depth to consider both low frequancy and energy. (6) Bubble oscillation energy and bubble period increase with the increase of working pressure. (7) Excitation energy of gun array increases with the increase of gun number, and excitation frequency is affected by single gun volume.(8) When the gun array size is greater than 5 times bubble radius, the interaction between bubbles is very small,the air-gun array wavelet is approximate linear superposition of every single gun,with little influence by the change of array size. (9) When gun depth is half of water depth, bubble oscillation energy and bubble period are large, and the excitation effect is good.Air-gun wavelet simulation based on open thermodynamic system. (1) We introduce bubble oscillation mechanism, simulate and compare Rayleigh (1917) bubble model of zero order incompressible fluid, Ziolkowski (1970) bubble model of two order nonlinear compressible fluid,Johnson (1994) bubble model and Ziolkowski (1998) bubble model of first order linear compressible fluid. The results show that Rayleigh (1917) model does not consider acoustic radiation and has no energy loss,which is a harmonic oscillation.The simulation results of the other three models are basically same,which are damped oscillation with gradually decreased period and amplitude.Ziolkowski (1970) model with two order accuracy is irreplaceable in the study of shock wave and high pressure pulse propagation process, but simulating wavelet has problem that initial amplitude is too large and attenuation velocity is too slow compared with measured wavelet. (2) Based on Ziolkowski (1970) model, we introduce open thermodynamic system with mass and energy exchange with outside world,consider variety influence factors on bubble oscillation,such as air release rate, the volume of gunbody, muzzle throttling, heat conduction, water evaporation and condensation,viscosity, establish a new single gun wavelet model. The simulation results show that the volume of gun body, muzzle throttling, heat transfer,water evaporation and condensation are main factors affecting the attenuation of air-gun wavelet.The volume of the gun body and muzzle throttling affect the amplitude of main pulse. Heat transfer, water evaporation and condensation affect the attenuation of bubble oscillation. The actual gas volume is equal to bubble volume minus the volume of the gun body,thus increasing wavelet amplitude. Muzzle throttling reduces the gas release velocity and decreases the amplitude of the main pulse. Heat transfer, water evaporation and condensation accelerate the attenuation of bubble oscillation, and the bubble pulse amplitude is greatly reduced. (3) Based on single gun wavelet simulation,we conside the interaction between bubbles varing with the distance of air-gun, further simulate the wavelet of air-gun array. Comparing with measured wavelet, we find when the distance between air-guns is greater than 5 times bubble radius, the interaction between bubbles is very small, and the wavelet of air-gun array is approximate linear stack of every air-gun wavelet. (4) Based MATLAB platform we independently develop an air-gun wavelet simulation program. The consistency between measured wavelet and simulated wavelet shows single air-gun wavelet model and air-gun array wavelet model that we establish have good accuracy and feasibility, which can be applied to design air-gun working condition, detect air-gun source performance, provide air-gun wavelet and so on.Technology and application of air-gun signal stack. (1) We introduce linear stack and phase weighted stack, and propose frequency division phase weighted stack based on phase weighted stack.We simulate and analyse the relationship between signal-to-noise ratio(SNR), correlation and stack taimes, and compare the stack effect of three kinds of stack method. The results show that phase weighted stack further uses phase information based on linear stack. Its efficiency of improving SNR is higher than that of linear stack, but causes waveform distortion.In the case of low SNR and small stack number, the effect of frequency division phase weighted stack improves SNR and reduce waveform distortion is best (2) We develop a method of using the average SNR offar field stations records to evaluate the optimal working conditionThe larger the average SNR is, the farther the signal propagates. (3) We use frequency division phase weighted stack method processing formal working condition data with 1024 times stack,propose the seismic profile,and determine the propagation distance of the air-gun signal.The longest propagation distance of single shot is about 150km through tracking in-phase axis. The longest propagation distance is 450km after 1024 times stack. In reduced profile we can see rich Pg, Pm, Pn, Sm and other phases.It explains the penetration depth can reach the lower crust and upper mantle, fully verifing the wide application prospect of deep exploration by using large volume air-gun. |
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