【作者】 张帅；

【导师】 张阿漫；

【作者基本信息】 哈尔滨工程大学 ， 船舶与海洋结构物设计制造， 2018， 博士

【摘要】 我国海域辽阔,海底蕴藏着丰富的矿产资源、如天然气、石油、可燃冰等,这些深埋海底的资源不仅给开采带来极大难度,探测资源在海底的分布情况也是工程中的一大难题。鉴于地震波在海水中衰减较慢的特性,海底资源探测中采用最多的即为地震勘探法,利用人工地震源向海底发射地震波,然后采集海底反射回来的地震波信号,最后通过不同海底结构的不同反射特性,反向解析出真实的海底结构,即达到了海底资源勘探的目的。对于深海资源勘探来说,探测波不仅要穿透上千米深的海水,还要穿透一定深度的海床结构,综合各方面因素考虑,这种声波必须具备低频、高能、宽带的特性,才能胜任深海勘探任务,而气枪就是目前使用最为广泛的低频人工震源。本文的研究主要着眼于工程中遇到的实际问题。1)工程中气枪阵列的快速优化设计问题,随着海洋资源的探索逐渐由浅海走入深海,人们对海上震源的使用提出了更高的要求,如何使气枪产生的远场压力子波能传播更远的距离,且具有足够的能量穿透海底的地质结构,一直是人们亟待解决的问题。2)相干枪在工程中的使用问题,同阵列设计问题一样,相干气枪应该如何排布才能起到更好的气泡脉冲压制效果,而又不会对主脉冲的幅值造成很大影响,对于增加海底资源探测的分辨率十分有意义。3)是气枪结构设计中关键参数设定问题,我们知道气枪开口尺寸、开口形状、开口位置、枪体长度和厚度等均会对气枪发射的探测压力波产生影响,如何设计这些参数才能使单枪震源激发具有更大的初始能量,以及足够的低频成分,满足更深、更远探测要求,实现海底资源的高精度探测。4)是电火花震源的研究,电火花震源也常被用作海底资源探测,是最早用于海底资源勘探的非炸药震源,但气泡的生成及脉动机理都相当地复杂,理论方法和数值模拟所能求解的情况比较有限。气枪气泡的脉动过程往往较为复杂,常常伴随着湍流和漩涡形成,为气枪气泡脉动中基础力学问题的研究增加了很多困难,为进一步加强人们对气枪气泡的了解,本文采用了多种方法对气枪气泡脉动及流场压力特性进行了研究,包括球形气泡脉动理论、边界积分法、有限体积法以及电火花气泡实验法,这些方法各有其优势和不足,工程中可根据气枪使用的不同要求,选择合适的计算模型和方法。目前工程中使用最多的即为基于球形气泡理论对气枪气泡脉动方程直接求解,采取一定的近似手段来快速获取远场震源子波信号的大致形态,考虑到该方法在计算速度上的优势,本文将其与粒子群算法相结合建立了气枪阵列快速设计及优化的方法,但该方法在计算过程中忽略了气泡形态对震源信号的影响,不利于气枪气泡近场压力的计算,以及气枪气泡脉动过程中一些复杂机理的解释。边界积分法在模拟气枪气泡非球形变化过程中具有较大优势,本文基于边界积分法系统地研究了气泡非球形变化对远场压力子波影响,以及多枪气泡相互耦合问题,对最佳相干距离等问题进行了深入研究,但边界积分法对于气枪气泡与不同边界的强耦合问题的计算上仍差强人意,如气枪气泡与枪体的耦合、气泡射流融合撕裂等,另外,边界积分法在计算速度和稳定性上也远不如球形气泡脉动模型。有限体积法的引入主要是用来帮助加深气枪气泡工作原理的了解,基于有限体积法可以很好的解决边界积分法中由气泡撕裂融合等引起的计算发散,大大提高了计算的稳定性,有限体积法可以很好的再现气枪与枪体耦合情况,对于解决气枪设计过程中开口尺寸、枪体直径等关键参数的确定具有重要意义,但相比于边界积分法,有限体积法的计算效率更低,并不适用于工程中大量气枪气泡同时激发的情况,而且受限于计算域的尺度,并不能直接算出距离气泡足够远处测点的压力。电火花气泡的研究和使用可以追溯到上世纪中期,电火花气泡同激光气泡等空化气泡一样,之前多被认为是爆炸气泡的研究手段,它很好地克服了爆炸气泡污染环境、费用高和透明度低等主要缺点,是人们实验室研究气泡脉动机理的较好的辅助手段,借助电火花气泡实验本文研究了气泡与不同边界的相互作用,虽然同气枪气泡形态具有一定的差异,但二者同属于高压气团驱使下的气泡弹性震荡,电火花气泡对于气枪气泡的研究仍有很多借鉴意义。本文建立了带约束的气枪阵列优化算法,考虑了空间位置、震源容量等实际施工限制,发现在给出合适的目标函数后,仅通过对气枪发射时间、空间分布等阵列布置参数的简单调整,即可实现阵列远场压力子波的优化。通过对不同间距下相干气枪远场压力子波主脉冲、初泡比和气泡周期的无量纲分析,发现随着气泡无量纲距离的增大,初泡比呈现先增大后减小的趋势,在无量纲距离约为1.6的时候(多气泡即将融合),初泡比存在一个极大值。基于有限体积法的非球形气枪气泡研究,发现气枪开口位置、开口高度、开口形式等均对远场压力子波具有一定影响,在枪体设计中它们的取值极为关键。通过高电压电火花气泡放电实验研究,发现电火花气泡形态、周期、射流、迁移与距离参数和边界种类关系密切,并对其变化规律进行了总结。更多还原

【Abstract】 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.更多还原