CAJ | 학술논문

似海底反射层的速度异常是识别天然气水合物的重要标志,这里提出了一种针对天然气水合物似海底反射层的全波形反演方法.这种方法分为全局搜索与局部搜索二部份:首先使用遗传算法进行旅行时,反演得到背景速度模型;然后用其作为初始模型,使用共轭梯度算法进行全波形反演.通过对含噪数据的数值试验,算法表现出了较高的稳定性,并确定了进行全波形反演的遗传算子.将这种波形反演方法应用于我国南海北部海域的天然气水合物研究,反演得到了分辨率高于常规速度分析的似海底反射层速度结构,并识别出似海底反射层的速度异常.利用纵波速度反演的结果,计算出沉积物中游离气的含量,认为BSR下方的低速层可以解释为含至少1%游离气的薄层.并分析了研究区内甲烷气的来源,认为该区域游离气兼有生物气和热解气.
사해저반사층적속도이상시식별천연기수합물적중요표지,저리제출료일충침대천연기수합물사해저반사층적전파형반연방법.저충방법분위전국수색여국부수색이부빈:수선사용유전산법진행여행시,반연득도배경속도모형;연후용기작위초시모형,사용공액제도산법진행전파형반연.통과대함조수거적수치시험,산법표현출료교고적은정성,병학정료진행전파형반연적유전산자.장저충파형반연방법응용우아국남해북부해역적천연기수합물연구,반연득도료분변솔고우상규속도분석적사해저반사층속도결구,병식별출사해저반사층적속도이상.이용종파속도반연적결과,계산출침적물중유리기적함량,인위BSR하방적저속층가이해석위함지소1%유리기적박층.병분석료연구구내갑완기적래원,인위해구역유리기겸유생물기화열해기.
Bottom Simulating Reflectors (BSRs) is considered as an identifier of gas hydrate and free gas. In order to obtain accurate velocity structure of BSRs, this paper presents a full waveform inversion strategy based on genetic algorithm. This method is divided into two parts: global search and local search. First, long wavelength parts of velocity structure are calculated by genetic algorithm by using travel-time information. Then taking the calculated velocity model as the initial model, full waveform inversion is implemented by adoption of conjugate gradient algorithm. Synthetic seismograms are calculated using the slowness technique. Through numerical experiments performed with noisy synthetic data, the inversion algorithm shows stable performance and genetic operators are defined. This method is applied to field data from northern South China Sea. Inversion results show that obvious velocity anomaly of BSRs can be detected, which indicates the existence of gas hydrate and free gas. The resulting compressional velocity is used to estimate gas content in the sediments. And the source of methane gas in this region is both thermogenic gas and biogenic gas.