围压作用下岩石样品中微裂纹的闭合
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摘要
岩石中普遍存在的微裂纹对其物理性质有十分显著的影响 .为了得到不同围压条件下岩样内部微裂纹形态的变化 ,选取几种具有明显层理的岩石圈岩石样品 ,在不同围压下沿相互正交的三个方向对纵横波速度和线应变等进行了测量 ,根据O’Connell模型通过计算得出有效泊松比、裂纹密度、裂纹孔隙度和裂纹的c/a比值 (a为硬币形裂纹半长轴的长度 ,c为其半宽度 ) .结果表明 ,随着围压的增加 ,在垂直于层理面的方向上裂纹闭合最快但c/a比值不减反增 .结合显微组构分析可知 ,岩石中的裂纹表面实际上是凸凹不平的 ,在围压增加引起裂纹闭合过程中一条裂纹可以变成几个 ,长度成倍减小但宽度变化不大 ,所以在垂直于层理面的方向上c/a比值随围压增加而增大 .但另两个方向上的变化明显不同 ,应该与模型本身的适用范围有关 .通过同时测量纵、横波速度 ,计算c/a比值 ,可以对样品中微裂纹的几何形态及空间分布方式有定性或半定量的了解
Velocity and Q value of P and S waves and linear strain had been measured in three orthogonal directions,under different confining pressures,for several rocks with marked foliation and strong lattice preferred orientation. The effective Poisson's ratio,crack density,crack porosity and c/a value ( a is the length and c is the width of the coin shaped microcrack) of microcracks are calculated from the measured data by O'Connell's model. It is resulted that the closure of microcracks normal to the foliation plane is the fastest but the corresponding c/a value does not decrease,but increase,with the increase of confining pressure. It may be caused by the roughness and rugosity of the crack faces,which can make one crack into several smaller cracks with significantly shorter lengths but widths not markedly smaller,during the progressive closure of microcracks with increasing confining pressure. So, c/a value increases with confining pressure. By measuring P and S wave velocity simultaneously,we can get a general idea on the shape and spatial distribution of microcracks in the sample,basing on the calculated crack sapect ratio c/a .
Velocity and Q value of P and S waves and linear strain had been measured in three orthogonal directions,under different confining pressures,for several rocks with marked foliation and strong lattice preferred orientation. The effective Poisson's ratio,crack density,crack porosity and c/a value ( a is the length and c is the width of the coin shaped microcrack) of microcracks are calculated from the measured data by O'Connell's model. It is resulted that the closure of microcracks normal to the foliation plane is the fastest but the corresponding c/a value does not decrease,but increase,with the increase of confining pressure. It may be caused by the roughness and rugosity of the crack faces,which can make one crack into several smaller cracks with significantly shorter lengths but widths not markedly smaller,during the progressive closure of microcracks with increasing confining pressure. So, c/a value increases with confining pressure. By measuring P and S wave velocity simultaneously,we can get a general idea on the shape and spatial distribution of microcracks in the sample,basing on the calculated crack sapect ratio c/a .
引文
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[1 0 ] 刘 斌 ,王宝善 ,席道瑛等 .水饱和裂纹对地壳岩样中地震波速及各向异性的影响 .地球物理学报 ,1 999,4 2( 5) :70 2— 71 0 .LIUBin ,WANGBao Shan ,XIDao Yingetal.Theeffectsofwatersaturatedcracksonseimicvelocityandanisotropyincrustalrocks.ChineseJ.Geophys.(inChinese) ,1 999,4 2 ( 5) :70 2— 71 0 .
[1 1 ] SayersCM .Commenton“Crackmodelsforatransverselyisotropicmedium” .J.Geophys.Res.,1 993,98:1 4 2 1 1— 1 4 2 1 3.
[1 2 ] BergPA ,FruyerGJ,WilkensRH .Velocity porosityrelationshipsintheupperoceaniccrust:Theoreticalconsidera tions.J.Geophys.Res.,1 992 ,97:1 52 39— 1 52 54.
[1 3] HudsonJA .Wavespeedsandattenuationofelasticwavesinmaterialscontainingcracks.Geophys.J.R .Astron .Soc,1 981 ,64:1 33— 1 50 .
[1 4 ] O’ConnellRJ,BudianskyB .Seismicvelocitiesindryandsaturatedcrackedsolids.J.Geophys.Res,1 974 ,86(B6) :541 2— 542 5.
[2 ] KernH ,WenkHP .Fabric relatedvelocityanisotropyandshearwavesplittinginrocksfromSantaRosamylonitezone,California.J.Geophys.Res.,1 990 ,95:1 1 2 1 3— 1 1 2 2 3.
[3] BarruolG ,KernH .Seismicanisotropyandshear wavesplittinginlower crustalandupper crustalrocksfromtheIvreaZone’sexperimentalandcalculateddata .Phys.EarthPlanet.Inter .,1 996,95:1 75— 1 94 .
[4 ] 母润昌 ,高 平 ,刘若新等 ,华北地区韧性剪切带几种岩石的波速各向异性高温高压实验研究 .地球物理学报 ,1 995,38( 2 ) :2 1 3— 2 2 0 .MURun chang ,GAOPing ,LIURou Xinetal.TheexperimentalstudyofP wavevelocityanisotropyofmyloniteandit’ssurroundingrocksfromductileshearzonesofNorth Chinaatelevatedpressuresandtemperatures.ChineseJ.Geophys.(inChinese) ,1 995,38( 2 ) :2 1 3— 2 2 0 .
[5] SiegesmundS ,TakeshitaT ,KernH .AnisotropyofVpandVsinanamphiboliteofthedeepercrustanditsrelation shiptothemineralogicalandtextualcharicteristicsoftherock .Tectonophysics,1 989,1 57( 1 ) :2 5— 38.
[6] 金振民 ,JiShao cheng,金淑燕 .橄榄石晶格优选方位和上地幔地震波速各向异性 .地球物理学报 ,1 994 ,37( 4) :4 69— 4 82 .JINZhen Min,JIShao Cheng,JINShu Yan.Latticepreferredorientationofolivinesandseimicanisotropyintheup permantle.ChineseJ.Geophys.(inChinese) ,1 994 ,37( 4) :4 69— 4 82 .
[7] 嵇少丞 ,MainpriceD .晶格优选定向和下地壳地震波速度各向异性 .地震地质 ,1 989,1 1 ( 4) :1 5— 2 3.JIShao cheng ,MainpriceD .Seismicanisotropyinthelowercrustinducedbythelatticepreferredorientationsofmin erals.SeismologyandGeology (inChinese) ,1 989,1 1 ( 4) :1 5— 2 3.
[8] 刘 斌 ,葛宁洁 ,KernH等 .不同温压下蛇纹岩和角闪岩中波速与衰减的各向异性 .地球物理学报 ,1 998,4 1( 3) :371— 381 .LIUBin ,GENing Jie,KernHetal.VelocitiesandattenuationofP andS wavesandtheiranisotropiesinserpenti niteandamphiboliteunderdifferentP Tconditions.ChineseJ.Geophys.(inChinese) ,1 998,4 1 ( 3) :371— 381 .
[9] KernH ,LiuB ,PoppT .RelationshipbetweenanisotropyofP andS wavevelocitiesandanisotropyofattenuationinserpentiniteandamphibolite.J.Geophys.Res.,1 997,1 0 2 (B2 ) :30 51— 30 65.
[1 0 ] 刘 斌 ,王宝善 ,席道瑛等 .水饱和裂纹对地壳岩样中地震波速及各向异性的影响 .地球物理学报 ,1 999,4 2( 5) :70 2— 71 0 .LIUBin ,WANGBao Shan ,XIDao Yingetal.Theeffectsofwatersaturatedcracksonseimicvelocityandanisotropyincrustalrocks.ChineseJ.Geophys.(inChinese) ,1 999,4 2 ( 5) :70 2— 71 0 .
[1 1 ] SayersCM .Commenton“Crackmodelsforatransverselyisotropicmedium” .J.Geophys.Res.,1 993,98:1 4 2 1 1— 1 4 2 1 3.
[1 2 ] BergPA ,FruyerGJ,WilkensRH .Velocity porosityrelationshipsintheupperoceaniccrust:Theoreticalconsidera tions.J.Geophys.Res.,1 992 ,97:1 52 39— 1 52 54.
[1 3] HudsonJA .Wavespeedsandattenuationofelasticwavesinmaterialscontainingcracks.Geophys.J.R .Astron .Soc,1 981 ,64:1 33— 1 50 .
[1 4 ] O’ConnellRJ,BudianskyB .Seismicvelocitiesindryandsaturatedcrackedsolids.J.Geophys.Res,1 974 ,86(B6) :541 2— 542 5.
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