Electro-acoustic Characteristics Of Plasma Sparker And Deep-towed Development

Exploration of marine oil and gas resources especially incluing gas hydrates is an important way to relieve the energy crisis in the 21th century,it is necessary to develop the technologies and equipments for high-resolution oceanic seismic exploration.Sparker is a controllable and simple seismic source with rapid repeated shots.However,the sparker based on pulsed arc discharge has some disadvantages such as high voltge level,short electrode lifetime and poor repeatability.Hence,a new sparker namely plasma sparker based on pulsed corona discharge has been developed,it has a low voltage level,long electrode lifetime and perfect repeatability.However,little systematic study on the electro-acoustic characterictics of plasma sparker can be found in the literature.In this dissertation,based on the plasma sparker module with a 30 J output,the electro-acoustic characteristics and bubble dynamics have been investigated systematically.Meanwhile,to meet the requirement of deep-sea high resolution seismic exploration,the electro-acoustic characteristics under different water depths have been studied as well,the main contents and conclusions are summarized as follow:1.Under the same discharge energy,the higher charging voltage,the greater load power peak.The larger charging capacitance,the larger load power width.The electrode diameter has little influence on the load electric characteristics,when the electrode number,electrode distance,conductivity and temperature increase,the load power peaks increase whereas the power widths and load energies decrease.The experimental results indicate that the pre-discharge time is several?s and pre-discharge energy is hundreds of mJ,so the pre-discharge is negligible.The temperatures of the discharge plasma range from 9987 K to 11920 K with the discharge energy range of 5 J?30 J.The intensity and width of the luminescence flash is much weaker than the discharge plasma,and the inception time of the luminescence flash is slightly before the bubble collapse point.Moreover,the temperatures of the luminescence flash are 6673 K and 6728 K under 25 J and 30 J,respectively.2.In terms of the single-bubble dynamics,the experimental results show that the bubble period increases with the increasing of maximum radius linearly,in accordance with the Rayleigh model.Meanwhile,according to the Rayleigh model and the assumption of a constant bubble energy efficiency under different ambient pressures,a simple model is developed to predicte the bubble behavior and fundamental frequency under different ambient pressures.In terms of the multi-bubble dynamics,four results under different dimensionless distances and phase differences can be observed,including coalescence,jetting towards,jetting away and catapult.3.The acoustic emissions induced by pulsed corona discharge have several acoustic pulses,including an expansion pulse and several bubble pulses,the fundamental frequency is around hundreds of Hz.In terms of the expansion pulse width of the single-electrode discharge and multi-electrode discharge,the expansion pulse widths increase with the load power widths following a power function.The expansion pulse peaks increase with both the load power peaks and energy dissaption rates following a power function under single-electrode discharge as well.In terms of the multi-electrode discharge,the power relationships can also be observed under different electrode numbers and distances.Due to the collapse pulses are reduced under the multi-electrode discharge,only single-electrode discharge is studied and the collapse pulse peaks increase with the bubble energies following a power function.However,the collapse peaks decrease when the water temperatures are larger than 20 ? and the luminescence flashes are more intense.4.The electro-acoustic characteristics have been inverstigated,the results show that the ambient pressure has little effects on the electric characteristics,whereas the expansion pulse peaks and corresponding source levels decrase with the increae of ambient pressure.The source level loss is less than the transsion loss until the diacharge energy larger than 20 J at 2000 m.However,the FFT results show that the dominant energy is around 20 kHz for all source depths.The simulation result shows that the fundamental frequency will reach 30 kHz under 30 J and 2000 m,indicating a direct deep-towed source is not suitable and needs a further optimization.5.Two optimizations are proposed including multi-output with delay time and pressure-resisting and acoustic transmission array.The expaerimental results show that independent acoustic pulses with time-delay obey the principle of linear superposition at both atmospheric and high ambent pressure,the longer time-delay induces a smaller expansion pulse peak and longer width.Moreover,a new micro-hole discharge is found and generates a much more intense acoustic pulse.A homemade acoustic transmission array can generate a maximum expansion pulse peak at 40 mS/cm.The carbon fiber is a perfect pressure-resisting and acoustic transmission material.The calculation results show that the thickness of carbon fiber cylinder reaching 10 mm can work at 20 MPa with a length and outside diameter of 1 m and 10 cm,respectively.Meanwhile,the ideal calculation results show that little transmission loss of acoustic emissions under 10 kHz will be induced by the carbon fiber cylinder with a thickness of 10 mm.