摘要
低速带具有"自由表面、低速度、高吸收"的特征,能引起反射波能量减弱,产生很强的频散,造成近地表波场畸变,严重影响地震勘探的精度。而地震采集过程又不可避免遇到以下低速带问题:1低速带存在与否会对地震波传播造成什么影响;2近地表变化大,怎么才能得到精确的低速带参数;3存在低速带时,激发井深、最佳的药量(与子波频率有关)、最佳的耦合(与低速带速度相关)等采集参数会对采集资料的品质产生什么影响。本文通过数值模拟,集中对低速带与地震波激发的井深、速度、子波主频等核心采集参数的关系进行分析研究,同时在低速带背景下对数值模拟的网格、边界等问题也进行了讨论。模拟得到的波场特征及低速带参数影响的机理分析,对做好野外地震波激发,获取高品质的地震资料,具有理论意义和指导作用。
The complexity of near surface and geological underground structure can intensify the diversity of seismic wave field, affect the quality of seismic records strongly, especially the existence of low velocity zone(LVZ) has great effect on seismic wave field. Low velocity zone characteristic of the "free surface, low velocity and high absorption" can cause the reduction of reflection wave energy, strong dispersion and distortion of surface wave field, which affects the accuracy of seismic exploration. However the seismic acquisition will face the following problems of low velocity zone inevitably:(1) what effect will be caused by the low velocity zone to seismic wave propagation;(2) Near-surface change is big, so how to get the accurate parameters of low velocity zone;(3) when there exists the low velocity zone, what effect will seismic acquisition parameters of the shooting depth, the best dosage(related to wavelet frequency) and the best coupling(related to LVZ velocity) bring to the seismic data quality. This paper analyzes the relation of low velocity zone with shooting depth, speed, wavelet frequency(dose effect), etc. based on the seismic numerical simulation method. At the same time, this paper discusses the grid and the absorbing boundary of the seismic numerical simulation under the background of low velocity zone. The wave field characteristics and low-velocity layer parameters influencing mechanism analysis will have theoretical significance and guiding effect to the field seismic wave shooting and to get high quality seismic data.
引文
[1]陆基孟,王永刚.地震勘探原理[M].东营:中国石油大学出版社,2009.Lu JM,Wang YG.Principle of seismic exploration[M].Dongying:China University of Petroleum Press,2009.(in Chinese).
[2]Lamb H.On the propagation of tremors over the surface of an elastic solid[J].Philosophical Transactions of the Royal Society(London),1904,A203:1-42.
[3]Cagniard L.Reflexion et refraction des ondes seismiques progressives.Cauthiers-Villars,Paris,1939:Translated into English and revised by Flinn EA and Dix CH.Reflection and Refraction of Progressive Seismic Waves[M].New York:Mc Graw-Hill,1962.
[4]Robbert van Vossen,Andrew C,Jeannot T.Subsonic near-surface P-velocity and low-S-velocity observations using propagation[J].Geophysics,2005,70(4):R15-R23.
[5]夏竹,张少华,王学军.中国西部复杂地区近地表特征与表层结构探讨[J].石油地球物理勘探,2003,38(4):414-424.Xia Z,Zhang SH,Wang XJ.Discussion on near-surface characters and structure in complex area of western China[J].Oil Geophysical Prospecting,2003,38(4):414-424.(in Chinese).
[6]钱荣钧.炸药震源激发效果分析[J].石油地球物理勘探,2003,38(4):583-588.Qian RJ.Analysis of explosive effect of dynamite source[J].Oil Geophysical Prospecting,2003,38(4):583-588.(in Chinese).
[7]Hestholm S,Ruud B.2D finite-difference elastic wave modeling including surface topography[J].Geophysical Prospecting,1994,42(5):371-390.
[8]杨旭明,裴正林.复杂近地表弹性波波场特征研究[J].石油地球物理勘探,2007,42(6):658-664.Yang XM,Pei ZL.Study on feature of elastic wave field in complex near surface[J].Oil Geophysical Prospecting,2007,42(6):658-664.(in Chinese).
[9]张兵,方伍宝,孔祥宁,等.近地表速度变化对地震波传播的影响[J].勘探地球物理进展,2008,31(5):357-362.Zhang B,Fang WB,Kong XN,et al.Effect of velocity variation on seismic wave propagation in near surface[J].Progress in Exploration Geophysics,2008,31(5):357-362.(in Chinese).
[10]肖云飞,殷厚成,孙成禹,等.基于随机介质模型的近地表波场模拟及分析[J].石油物探,2011,50(4):336-344.Xiao YF,Yin HC,Sun CY,et al.Near-surface wave field simulation and analysis based on random media models[J].Geophysical Prospectingfor Petroleum,2011,50(4):336-344.(in Chinese).
[11]刘凯,童思友,刘怀山,等.复杂近地表区高分辨率地震勘探激发参数优选[J].地球物理学进展,2013,28(6):3040-3048.Liu K,Tong SY,Liu HS,et al.The optimization of excitation parameters in the high-resolution seismic exploration on a complex near surface area[J].Progress in Exploration Geophysics,2013,28(6):3040-3048.(in Chinese).
[12]汪恩华,贺振华,李庆忠.炸药激发子波信号的记录与研究[J].石油地球物理勘探,2001,36(3):352-363.Wang EH,He ZH,Li QZ.Record and study of exploded wavelet signal by explosive[J].Oil Geophysical Prospecting,2001,36(3):352-363.(in Chinese).