Study On Characteristic Of Binchuan Air-gun Signal And The Calculation Of Wave Velocity Change In Binchuan Area Using Air-gun Signal

Large volume air-gun source which is used to explore the earth's internal structure in recent years is a new type of artificial source.It has characteristics of environmental friendly,high repeatability,far-propagation-distance and so on.Binchuan active transmitting seismic station in Yunnan province is the first fixed air-gun signal transmitting station.Since it was built in 2011,a large amount of data has been accumulated and a wealth of research results has been produced.The excitation condition of Binchuan active transmitting seismic station is depth with 10 meters and pressure with 15 MPa.Different air-gun signals from different excitation conditions usually have different wavelets.To get the wavelet parameter of different excitation,this article uses the experiment data in February,2017 of Earthquake Administration of Yunnan Province to analysis the air-gun wavelet of different excitation conditions.We find that different excitation conditions whether in well or in reservoir have different wavelet parameters and the law of wavelet change are basically the same as the theoretical formula of each parameter.Meanwhile,the phase also will change when the excitation condition changes.We analysis the signal's characteristic of the waveform and frequency from same epicenter distance but different azimuths.We find that the difference of waveform and frequency characteristics among the signals which spread through more water and less water is obviously.The signals which spread through more water have easier phases and their high-frequency part attenuate faster.In contrast,the signals which spread through less water have more complicated phase and their high-frequency part attenuate more slowly.The air-gun array single excitation's energy of Binchuan active transmitting seismic station at depth with 10 meters and pressure with 15 MPa is similar to a M_L0.7natural earthquake by calculating.To contrast the characteristics between the two types of signals,we choose a M_L0.7 natural earthquake which occurred in Binchuan.Through analysis and comparison,we find that the amplitude of air-gun signal is smaller than natural earthquake,and the air-gun signal's energy mainly concentrate in the low-frequency part and the natural earthquake signal's energy mainly concentrate in the high-frequency part.And the air-gun signal can be recorded on the station at 151kilometers while the natural earthquake signal can be recorded at only 60 kilometers,and the phase of air-gun signal is more abundant.To get the survey distance of the air-gun signal,we stack the same signal by linear stacking and phase-weighted stacking.The signal can be recorded respectively at 225kilometers and 350 kilometers.The two stacking methods both have their own advantage and disadvantage,so we should choose the right type according to the actual situation.Air-gun signal is highly repeatable.The change of the signal recorded by the station can be regarded as the change of medium.So we can calculate the change of the air-gun signal to get the change of the subsurface medium.This article use the cross correlation delay technical and choose the station recording signal with higher signal to noise ratio to calculate the travel time change and get the travel time change about 10^-510^-22 second.By analyzing,we find that travel time change is related to the change of reservoir water-level and the atmospheric pressure.Travel time change is negative correlation with the water-level change and the atmospheric pressure.And we can also find that the travel time gradually increases before earthquake and gradually recover after earthquake.Because the earthquake of Binchuan is small,the conclusion of the relationship between earthquake and time travel remains to be further studied.