海洋天然气水合物地震识别标志

杨木壮

摘要　天然气水合物的地震识别标志对海洋天然气水合物勘探和研究具有十分重要的意义。本文根据国外探测和研究成果，详细分析了似海底反射波（BSR）、振幅空白、负极性和异常高速带等海洋天然气水合d物的地震识别标志及其特征。

关键词　天然气水合物　识别标志　似海底反射波

1　前言

天然气水合物由于其储量巨大和分布广泛，已引起人们的极大兴趣和关注，被普遍认为是地球上一种比常规石油、天然气和煤等更有潜力的巨大能源矿产。天然气水合物是一种形成于特定低温－高压环境的冰状混合物，由水分子与气体分子（主要为甲烷）络合而成。天然气水合物遍布全球，但由于压力－温度条件和气体含量的限制，主要分布于陆地冻土层和深水海洋，尤其广泛分布于水深大于300～500m的陆架外缘陆坡和陆隆沉积物中[1]。据统计，世界上已有52处海域直接或间接发现了天然气水合物，其中16处见到含天然气水合物的岩心[2]。有关天然气水合物的调查和研究，美、俄、加、德、日等国已开展了大量工作，我国在这方面的工作近几年才刚刚起步，并很快掀起了对天然气水合物的探索和研究热潮。有关部门1999年底进行了海上天然气水合物试验性地震调查，经资料处理和解释分析，初步证实我国海域存在天然气水合物这一潜在能源矿产。姚伯初（1998）通过对已有多道地震剖面和声纳浮标资料的仔细分析，首次指出南海北部可能存在天然气水合物[3]；杨木壮等（1999）认为南海具有形成天然气水合物的有利条件，尤其在南北宽阔的深水陆坡和陆隆很可能蕴藏着丰富的天然气水合物[4]；张光学等（2000）通过对笔架南盆地等地的地震资料分析，发现该区具有天然气水合物存在的地震特征[5]。那么，海洋中天然气水合物的存在标志是什么？怎样利用地震调查资料寻找天然气水合物？这是天然气水合物调查和研究首先面临的重要问题。为此，笔者根据国外探测和研究成果，详细分析了海洋中天然气水合物的地震识别标志及其特征，如似海底反射波（Bottom Simulating Reflector，即BSR）、振幅空白、极性反转和高速带等，希望能为我国海洋天然气水合物调查与研究提供有益的参考。

2　似海底反射波（Bottom Simulating Reflector）—BSR

含天然气水合物的地层在地震反射剖面上常常会出现一强振幅的连续反射波，大致与海底反射波平行，故称似海底反射波（Bottom Simulating Reflector，即BSR），它大致代表水合物稳定域的底界。国外有关研究成果表明，水合物稳定域底界代表的是一特定的压力和温度面。由于海底下地层压力变化不大，而地温变化却很大（存在地温梯度），因此海底的起伏变化将造成地层中等温面的起伏变化，从而形成水合物稳定域的底界。因此，BSR大致与海底地形平行，而与地层层面斜交（当地层层面与海底斜交时）或平行（当地层层面与海底平行时）。

关于BSR的形成和演化，Kvenvolden（1993）认为有两种模式。第一种模式：在水合物稳定域内有机质经微生物作用生成甲烷（Claypool和Kaplan,1974），水合物形成与沉积作用同时发生，当甲烷水合物带变厚和变深时，其底界最终沉入造成水合物不稳定的温度区间，在此区间内可生成游离气，如果有合适的运移通道，这些气体将会运移回到上覆水合物稳定区（Kvenvolden和Barnard,1983a）。这一模式的结果是水合物将在整个水合物稳定域内生成，而在BSR下方可有或可无游离气存在。第二种模式：下伏孔隙流体中微生物生成的甲烷向上运移进入水合物稳定域而形成水合物（Hyndman和Davis,1992）[9]。这一模式的结果是水合物聚集在BSR附近的稳定区域底部，BSR之下不存在游离气。

虽然BSR在形态上简单地平行于海底，但其振幅、连续性等往往具有多变性，在地震剖面上呈现各式各样的BSR反射。根据反射波振幅强弱和连续性，可将BSR分为三类：S-BSR（强BSR）、W-BSR（弱BSR）和I-BSR（推测BSR）（Tucholke等，1977；Kvenvolden,1993）[7]。

S-BSR具有强振幅，在地震剖面上容易识别。大多数S-BSR为强振幅谷－峰组合（双峰，成对出现），而不是孤立的波峰和波谷。双峰波形是高阻层内具低阻抗薄层的典型地震响应。W-BSR以弱振幅的波谷－波峰为特征，由于振幅低，除非它毗邻S-BSR，一般在地震剖面上难以辨认，然而，W-BSR的存在却相当广泛。

图1为美国东南近海布莱克海岭的一条6道叠加浅层地震剖面，图中B与C之间显示了S-BSR，这是发生在深海沉积物中的典型BSR（Shipley等，1979；Dillon和Paul,1983）[8]。图中A与B之间为W-BSR，毗邻S-BSR，以弱振幅的波谷－波峰为特征，波谷的视反射系数小于－005。

推测BSR（I-BSR）是一个非连续的反射界面，位于水合物稳定带的理论底界附近，通常为空白带的底界。图2同样是来自美国东南近海布莱克海岭的地震剖面，表示推测的BSR（I-BSR），即D和E之间的连线。因I-BSR的视反射系数通常小于-005，故直接识别I-BSR较困难。主要根据I-BSR之下的异常强振幅反射同相轴推测I-BSR存在的可能性。在地震剖面上，通常把I-BSR解释为这样一条线（如图3中D、E之间的连线），该线与水合物带之下降升的强反射同相轴向上倾斜的终止端相连。这些倾斜的强反射同相轴是由气体充填地层引起的，该地层的上倾末端被水合物胶结的沉积物所封堵（Lee等，l993）[7]。

图1　S-BSR及W-BSR地震反射剖面（美国布莱克海岭）　Fig1　The Seismic section of S-BSR and W-BSR（据Lee等，1993）

图2　I-BSR地震反射剖面（美国布莱克海岭）　Fig2　The seismic section of I-BSR（据Lee等，1993）

3　振幅空白

在含水合物地层中，由于地震波速度增大，使得它与下伏地层之间的反射系数增大，在地震剖面上出现相应的强反射界面，而在其上方的含水合物层由于沉积物空隙被水合物充填胶结，使地层变得“均匀”，波阻抗差减少，地震反射剖面上通常呈现弱振幅或振幅空白带。空白程度与孔隙空间内胶结水合物数量成比例，水合物含量越高，振幅越弱，空白程度越高[8]。研究区内所有被解释为BSR之上的反射层均存在弱振幅或振幅空白带现象。因此，BSR之上出现的振幅空白现象是探测水合物沉积物，特别是没有明显S-BSR地区的水合物沉积物的地震指示。同时，地震剖面上水合物沉积物的振幅信息可以提供一种估计气体水合物数量的方法（Lee,1993）。

应该注意的是，地震剖面上振幅空白带并不总是与水合物相关。引起地震剖面上出现振幅空白带的因素还有许多，如沉积环境等。然而，多数显示有BSR存在的地震剖面也表现出不同程度的振幅空白带，并且空白带在BSR之上较集中。

图3为横穿布莱克海台顶部的一条单道地震反射剖面，显示清晰的S-BSR反射波及W-BSR反射波，BSR之上为大片反射空白或弱反射区，空白区连续性较好，被认为是水合物胶结的沉积层，空白带基本代表了水合物稳定域的厚度（Dillon,1993）。

图3　显示反射空白的地震剖面（美国布莱克海台）　Fig3　The seismic profile showing the Blanking reflections（据Dillon,1993）

4　负极性

水合物带的子波地震反射特征通常呈现负极性，即所谓的极性反转（与海底反射相反），并具有较大反射系数（Shipley等，1979;Lee等，1993）。图4为美国东南近海布莱克海岭一段具有清晰BSR显示的子波波形，具有典型的极性反转特征，即BSR反射波的波形极性与海底反射波的正好相反（海底波形向右，而BSR波形向左）。为方便分析反射特征，视向右的最大振幅值作为波峰（一个从低阻层到高阻层的反射界面），向左的最大振幅值为波谷（一个从高阻层到低阻层的反射界面）。这个异常反射界面以一对强振幅谷－峰波为特征，其波谷的视反射系数大于-01。这种成对出现的波形是典型的上下为高阻抗的低阻抗薄层的地震响应，该薄层很可能是水合物层之下的含游离气层。整个子波的反射速率为-012±004，负的反射速率表示高速层覆盖在低速层之上的反射界面。

图4　BSR波形（地震剖面范围为图1中CMP1000～1200）　Fig4　The wavelet of BSR（据Lee等，1993）

5　异常高速带

含水合物层的地层速度往往比一般的地层速度高，其速度与水合物含量有关，含量越高，速度越高[9]。从速度方面看，BSR是其上具有较高声波速度的水合物胶结沉积物与其下低速非胶结沉积物的分界面。通常，海洋中浅层沉积层的地震纵波速度为16～18km/s，如果存在水合物，地震波速度将大幅提高，可达185～25km/s，如果水合物层下面为游离气层，则地震波速度可以骤减为05～02km/s。因此，在速度剖面上，水合物层的层速度变化趋势呈典型的三段式，即上下小、中间大。

表1是根据南海北部9个声纳浮标站位资料计算出来的海底浅层层速度数据（姚伯初，1998），为195～245km/s，比正常的海洋沉积物层速度（16～18km/s）高，站位水深达420～3920m，高速层厚度大约为200～840m。

6　讨论

综上所述，海洋天然气水合物具有BSR、振幅空白、负极性和异常高速带等地震识别标志，尤其是BSR和振幅空白，被认为是天然气水合物的指示。但是，应该注意的是，BSR就像石油勘探中的“亮点”技术，并不是具有BSR就一定存在水合物；同时，许多含水合物地层也不一定具有BSR。判断一个地区是否发育天然气水合物，需要根据多方面因素综合分析，除了根据上述地震标志外，还可利用含水合物地层具有正AVO异常、高S/P速度比率及轻的碳同位素值（δ13C，通常小于-60‰）等重要d性特征和物性特征加以综合判断。此外，从电测井曲线获得的地球物理信息也是探测和评价天然气水合物层段的有用资料（Kvenvolden和Grantz,1990），包括井径、伽马射线、自然电位、电阻率、声波速度和中子孔隙度测井等（Goodman,1980）。相信在不久的将来，随着勘探和研究的不断深入，我们将会获得清晰而有效的天然气水合物识别标志，从而揭开天然气水合物的神秘面纱。

表1　南海北部9个声纳浮标站位浅层沉积物速度、水深及厚度　Table1　Velocity and thickness of the shallow sediments,calculated from the data of sonobuoy in the north margin of South China Sea

参考文献

1SloanED1998,A primer on the geological occurrence of gas hydratesin:Henriet JPedGas hydrates relevance to world margin stability and climate changeLondon the geological society,31～50

2Gornitz,V,et al,1994,Potential distribution of methane hydrate in the world oceanGlobal Biogechemical Cycles,Vol8,No3,335～346

3姚伯初，1998，南海北部陆缘天然气水合物初探，海洋地质与第四纪地质，1998,Vol18,No4,11～18。

4Yang Muzhuang,et al1999,Exploration prospect of gas hydrates in South China SeaFouth international conference on Asian Marine Geology

5张光学、文鹏飞，2000，南海甲烷水合物的地震特征研究，首届广东青年科学家论坛论文集，中国科学技术出版社。

6Miller,JJet al1991,An analysis of a seismic reflection from the base of a gas hydrate zone,offshore PeruAAPGBulletin,Vol75,No5,910～924

7Lee,MW,et al1993,Seismic character of gas hydrates on the Southeastern UScontinental marginMarine Geophysical Researches,Vol16,163-184

8ShiplyTHet al1979,Seismic evidence for widespread possible gas hydrate horizons on continental slopes and risesAAPGBulletin,Vol63,No12,2204～2213

9Hyndman,RDSpence,GD,1992,A seismic study of methane hydrate marine bottom simulating reflectorsJournal of Geophysical Research,Vol97,NoB5,6683～6698

DISTINGUISHABLE EVIDENCE FOR MARINE GAS HYDRATES

Yang Muzhuang

Abstract

To distinguish the seismic evidence of gas hydrates in seismic reflection sections is very significant for the primary exploration and researches on marine gas hydratesOn the basis of the results from foreign explorations and researches,combined with the first collection data for gas hydrates survey in China,this paper searches for the seismic evidence,analyzes their features and indicates the existence of gas hydrates,such as Bottom Simulating Reflectors（BSR）,amplitude blanking,polarity reversal and high velocity anomaly

Keywords:gas hydrates,distinguishable evidence,BSR

写英语作文，最重要的是头脑清晰不是吗就算是到了大学，这一点也不会改变。下面是我给大家整理的大学英语作文，供大家参阅!

大学英语作文：My View on the Same Amount of Holiday

A seven day long holiday has flown away Mourning my happy hours during the vacation, I've been looking forward to winter holidays People around me seemed to have enjoyed the vacation, too Then, is it fair that people have the same amount of holidays regardless of their occupations

Nevertheless, divergent views emerge on the matter in question Some people are of the opinion that laborers should have the same length of holiday They hold the view that people are born equal and should be treated equally The same amount of leisure time may eliminate the unfairness among people who might be upset psychologically if otherwise At first sight, it may seem to be an attractive idea, but it doesn't bear much analysis

To begin with, people do different kinds of jobs featuring different labor intensity and so the time needed to refresh their physical condition varies For example, blue collar workers may work longer hours before they get tired physically, while white collar workers such as doctors and teachers are more mentally stressed Unable to work continuously as long as those physical labor workers, white collars need more time to pull through the mental weariness than physical tiredness All in all, people in different jobs assume different responsibilities and have different degrees of pressure

Secondly, different tasks and duties are required for different professions, resulting in the variety of holiday periods for people in different walks of life Some employees cannot have their legal holidays and even have to work overtime, such as policemen, construction workers and people in the service lineThose professions just can't be interrupted for a mornent;other wise the society will be in a mess Even for some people in the same occupation, they cannot have the same holidays entitled to them due to many factors out of human control, such as special assignments, and health conditions etc, then not to say people in different fields

Suppose we, as teachers do, all have three months' holiday in a year Then factories have to employ considerably more workers because some of them are enjoying happy holidayThat's obviously against the principles of doing business at lower cost and efficiency

If we have only three days off in a year, then all the people in our country will be working day after day No one can squeeze time to visit those tourist resorts leisurely It goes without saying that the tourist trade and catering industry thus cannot develop rapidly and healthily

Last, the development of science and technology allows the ways of work to be changed Computer experts make their programs, doctors give patients diagnosis and tutors give students instructions at home through optical fabric cable connectionsThat's how the new term SOHO bore In other words, the way people work is not fixed so that they need different holiday phases

Some people need more time to recover from the fatigue of labor And some people cannot make ends meet though they work for 365 days in a year, for whom holiday is a waste of time

It is neither possible nor necessary for people with different jobs to have the same amount of holiday off Holiday is a kind of bonus, just as the Chinese idiom goes, distribution according to

work

简 评

冯佳莹同学的这篇作文思路清晰，分析丝丝入扣，是一篇非常优秀的议论文。

这篇作文的开头清新自然，通过自己对国庆假期的感受，进而联想到其他各行各业的人的想法，引出了本文的主题，显得毫不做作。写这篇作文时，作者显然进行了深入的思考，因而分析有理有据，闪烁着作者的真知灼见和思想火花。作者的语言很出色，句式长短交错，段落层次分明，句与句之间以及句群之间的衔接都非常清晰，作者倾向于使用then，that，those等连词和代词来衔接上下文，而没有使用过多的定语从句，既使句式的长度得到了控制，又避免了中国学生常见的定语长句(尤其是非限定性定语从句)过多的弊病。尽管作者使用了一些短语，但整篇的文体并没有因此而显得过于随意，显示了作者高超的语言驾驭能力。

(点评教师：孙东云)

大学英语作文：Midnight Journal

It's a true story in the summer of 1998 when I was attending a youth conference in L A and it feels so good to rewind it in the recollection

Fog swirls, swallowing the car in front of us Dense, impenetrable, the wall of white throws back the lights into my eyes There are four of us in a family van that is creeping along the free way from downtown L A to Lancaster Fog gets even stronger and spreads out like a blanket over the flat land It's Noah's turn to drive with complaints about the couldn't be lower visibility But he is whispering so that Eugene and Kikki can sleep still With eyes slightly closed, I am playing Bob Dylan on the disk, whose rhythm echoes the midnight prays

Well, we've truly had enough prayers just now Being off from the annual Vineyard Church International Yough Conference, I find myself deeply buried in the ferment that kids create from all over the world There are cries and whisper, laughter and tears, bless and pain, confidence and disbelief mixed together, building up a confession of the detours of soul Bible has recorded all this since centuries ago It's about God, but it's not all time religion

A boy from Panama asked me if I eat turkey as an Irish Catholic when pastor David was presenting his speech I smilingly nodded my head and turned back to Mr David He is an authentic English with dark blue pupils, eyes right fixing into his listener while talking He has truly handsome chins that show up his decency of smile and subtleness of wit

"There is only one door into heaven and Jesus said, 'I'm the door' (John 10: 9), he quoted, "The way you enter is by expressing faith in the Lord Jesus Christ Faith in Jesus is a commitment to the one whose life becomes your life and his life within you makes you a new person"

There was a dead silence for praying before a burst of weeping broke the ice A blonde girl threw herself into the arms of the peer beside her, hiding her face A boy of crew cut on the first row was bending his knees, forehead down to the ground,whispering his regrets A black girl standing right beside me was shivering her lips with tears hanging up in her eyes, showing a piece of uncertainty It was truly heart wrenching when more and more kids were peacefully touched by the power of God,harmonizing their words with tears, rather than screaming out their logo as "we love Jesus yes we dm we love Jesus how about you"

It hasn't been long before I am used to the way of praying here in the States The melancholies in the air soon put me on the way of figuring out the fact lying behind the tears It numbed me to see the fear and solitude that root deeply in this materially rich world There is a false security that drastically reshapes the country by drug, crimes, pre marital sex and racism And it seemed here every tiny little souls were certain victims Jesus wasn't truly there, but the void became the savior

It was then that the mother of pastor David rose to sing "Ave Maria" She came up to piano in a gentle pace, took a breath, closed her eyes, and delivered the goods Well, never have heard the song offered with more feeling, more passion and fervor She was seventy years old and magnificent The plain voice flowing in the air was neither scratchy nor dramatic that you might expect from an aging third rate opera singer This was actually the voice of a grandmother distilling her life into the music to honor what she loved and believed in when the last lovely note faded and silence held us firm, the "grandma" opened her eyes, smiled at us, and said, "my lord be with you"

That is basically what received from the journey of soul I told out my sins with once-in-life-time honesty I may never get a chance to bump into Pastor David and I truly miss this mid aged gentleman of deep sea pupil, although he didn't succeed making me believe that Jesus died on the cross for my sin and paid my sin debt in full

Noah is now too sleepy to drive It's my turn to stay awake in this misty of midnight Los Angeles Fog stands still when we are inching along the road of this spiritual trip I don't know what keeps me in such a wild and peaceful excitement Well, if the fog doesn't lift, may we at least have some lines and reflectors put down along the road to guide us

简 评

宗教的力量可以是无穷的。作者根据在美国的所见所闻，向我们描述了基督教对美国人的思想所起到的巨大支持与影响作用。

同时作者的美国经历不仅丰富了阅历，也使其语言功底令国内的英语学习者难以望其项背。生动的描述，地道的选词都使得该篇文章成为很好的例文供我们赏鉴。

(点评教师：张雪波)

大学英语作文：讲座与讨论

Lectures and Discussions

提纲

1 大学里有讲座式和讨论式两种授课方式。

2 两种授课方式各有利弊。

3 我的看法。

There are mainly two styles of teaching at universities -- lectures and discussions Each has its own characteristics

Lectures are actually the teacher's talk to a class for the purpose of teaching In this kind of class, the teacher plays a central role, while the students are mainly the listeners Compared with lectures, discussions are more flexible Students can examine and argue about a subject with each other in this kind of class Lectures are especially suitable for big classes hecause it will be easier to keep the students under control, while discussions are typically for small classes for the reason that they are more likely to go out of control

In a word, lectures and discussions are two quite different styles of teaching, so we should choose either of them or combine them together reasonably according to the circumstances

大学英语作文：科学与人类

Science and Human Beings

提纲

1 科学使人类进步，生活得更美好。

2 科学有时也会带来负面效应，如原子能毁掉一切。

3 就科学与人类的关系谈谈你的观点。

Science enables human beings to progress rapidly and lead a better life For example, Edison's invention of the light bulb brought our gloomy life to the end and created a bright world for us Another case in point is Watt's discovery of the steam engine, which triggered the industrial revolution in the western countries, and later all over the world

Meanwhile, sometimes science does have some side effects A good illustration of this is atomic energy, which can destroy human beings completdy when used in nuclear weapons

Anyhow, it is men who develop science And science, in turn, will do us good in general As for its side effects, I believe human beings will be rational enough to avoid them or keep them under control

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