| The vast sea area in China is rich in mineral resources,such as natural gas,oil and flammable ice.The resources buried in the seabed are not only difficult to exploit,but the exploration of these resources is also a big problem in engineering.Nowadays,the most widely used is the method of seismic exploration in view of that seismic waves decay slow in the seawater.In the exploration,seismic waves emitted from the artificial seismic sources and will be reflected at the seabed.Through the acquisition of reflection signal at sea surface,we then can get the general information of the inner structure of seabed for different submarine structures have different reflection characteristics.For the deep-sea resource exploration,seismic waves are not only needed to penetrate the sea water with a thickness of up to thousands of meters,but also to penetrate the inner structure of the seabed with a certain depth.Thus,the seismic waves generated by the seismic sources must have the characteristics of high energy,low frequency and enough broadband.Air gun is the most widely used low frequency artificial seismic source.The study of this paper is mainly focused on the actual problems encountered in engineering.The first is the rapid optimization of air gun array.With the exploration of marine resources gradually from the shallow into the deep sea,people put forward higher requirements on the use of seismic sources.How to make the far field pressure wavelet spread farther distance and have more energy to penetrate the inner structure of the seabed is a big problem.The second is the use of cluster guns in engineering.As the design of the air gun array,with a better arrangement of the cluster guns,bubble pulse of the pressure wavelet can be effectively suppressed.The study of the best distance between cluster guns is of great significance for the increase of the resolution of submarine resource detection.The third is the design of the key parameters of the air gun body,including the size,the position and the form of the air gun opening etc.,which may have a big impact on the signal from the air gun.The design of these parameters is also a major problem.The final is the study of the electric spark source.The spark source is the earliest non-explosive source used for marine exploration.For the pulsation of the spark bubble is complex,it is not enough to study the spark bubble just by means of numerical and theoretical methods.The pulsation of the air gun bubble is often accompanied by the formation of turbulence and vortex,adding a lot of difficulties to the study.A variety of methods is used in this paper to study the pulsation of the air gun bubble and the characteristics of the surrounding liquid,including the spherical bubble theory,the boundary integral method,the finite volume method and the spark bubble experiment.Each method has its advantages and disadvantages,and different methods can be chosen to different engineering problems.At present,most used in engineering is spherical bubble theory.With a series of assumptions,pressure wavelet can be quickly obtained.In combination with the particle swarm algorithm,a method of the air gun array design and optimization is established in this paper.But this method ignores the effect non-spherical bubbles,which is not suitable for the study of the near-field pressure.With BIM,the problem of the cluster air guns and the interaction between bubble and air gun body are studied.The boundary integral method(BIM)has more advantages in the simulation of nonspherical air gun bubble,but the stability of computation for strong nonlinear problems such as jet impact and tear of bubble etc.needs to be further improved.The introduction of the finite volume method(FVM)is mainly used to study the effect of the air gun body.With FVM,the computational divergence caused by the bubble tearing and fusion is successfully avoided.As compared to the boundary integral method,FVM has lower computational efficiency and is not suitable to the modelling of the large air gun array bubble,but in the design of the key parameters of the air gun body FVM is quite effective.Limited to the scale of the computational domain,FVM can't be directly used to calculate the pressure at the far field measure point.Spark bubble is a good auxiliary way to study the mechanism of the bubble oscillation in the laboratory environment,which is often used to the study of the explosion bubble before.It overcomes the major disadvantages of the explosion bubbles,such as pollution,environment,high cost and low transparency.With the spark bubbles,the dynamic characteristics of the bubble oscillation under different boundary is studied.Although the shape of air gun bubble is different from the electric spark bubble,there is still a lot of reference for the study of air gun bubble.A numerical method for improving the quality of the far-field signal generated by a marine seismic air-gun array is presented.Two sets of objective functions,including the primary-tobubble ratio and the variance of the normalized amplitude spectrum,are built to screen the array.It is found that the far-field signal can be largely improved just through the simple adjustment of the chamber volume,the firing depth,the horizontal location or the firing time of each individual gun in the array.To simulate the motion of the non-spherical bubble generated by air gun,boundary integral method is used in conjunction with the basic theory of the air gun.For the clustered air gun bubbles,the primary-to-bubble pulse ratio will reach a maximum when the standoff distance d is about 1.6 times maximum radius of a single bubble.With the finite volume method,it is found that the opening position,both the opening height and the opening form of the air gun have a certain influence on the far field pressure wavelet,and their value is very critical in the design of the gun body.Experiments on the pulsation of the high-voltage electrical-spark bubbles near different boundaries are conducted by means of high-speed photography.Some intriguing details are observed clearly,such as the formation of the jet,and bubble splitting etc. |
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