Study On The Acoustic Characteristics Of Multi-Electrode Plasma Discharge In Water And The Three-dimensional Seismic Source

People mainly use high-resolution seismic source devices to explore energy of seabed.The plasma seismic source could generate high stable and repeatable wavelet,and has gradually become the main equipment for marine seismic exploration.The wavelet characteristics of the plasma seismic source depend on the design of the structure of electrode arrangement in array,however scholars have little basic research on the electroacoustic characteristics of the multielectrode structure,and cannot provide a theoretical basis for the design of the multi-electrode.This paper relying on the 30 J seismic source device and ten-way delay device studies the changes of electroacoustic characteristics by changing the number of electrodes,electrode spacing,and delay time.The experimental conclusions guide the design of the three-dimensional plasma seismic source discharge circuit and the array.The main contents and conclusions are as follows:(1)The electrical characteristics of synchronous discharge are studied.When the number of electrodes increases from 1 to 4,fhe load voltage decreases,and there is a residual voltage of0.18 kV,the current,power,energy of load gradually increases;The discharge time decreases from 50 p,s to 35 μs,and the energy injection rate gradually increases.As the electrode spacing increases from 6 mm to 30 mm,the peak voltage,energy,discharge time of load decrease slightly;the load current and load power gradually increases.(2)The acoustic characteristics of synchronous discharge are studied.With the increase of the number of electrodes,the sound pressure of the expansion pulse and the sound source level increase first and then decrease,reaching the maximum at three electrodes.The acoustic energy and electroacoustic conversion efficiency are largest at the double and three electrodes.The acoustic pulse energy,electro-acoustic conversion efficiency and period decrease with the increase of the number of electrodes,the main frequency increases;The initial bubble ratio is the largest at four electrodes and stable at 1.1.When the discharge voltage is-4.76 kV,the change of the electrode spacing has little effect on the peak sound pressure of the direct wave and the sound pulse period.The initial bubble ratio is larger when the electrode spacing is 12 mm～24 mm,and the best is 18 mm(3)The acoustic characteristics of delayed discharge are studied.When the discharge voltage is-4.97 kV,the expansion pulse half width increases with the increase of the delay time and the number of delay way.In the 25 μs delay,the number of delay way increases from 2 to 4,the expansion pulse energy increases from 65.6 mJ to 126.6 mJ,and the half width increases from 69.5 μs to 118.3 μs;The electroacoustic conversion efficiency is in the three-channel delay Reach the maximum.(4)The experiment conclusions have guided the design and development of the threedimensional plasma seismic source device,and conducted marine tests.The plasma seismic source device has been tested to work stably,and the multi-electrode array generate the sound source level of expansion pulse up to 230.1 dB at 4.7 m below the sea surface.Spectrum analysis shows that the main frequency of the wavelet is between 162 Hz and 372 Hz,which has a wide frequency band,the initial bubble ratio could reach up to 5.0.