Seismic Anisotropy in the West Antarctic Ice Sheet
Seismic Anisotropy in the West Antarctic Ice Sheet
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南极西部冰盖的地震各向异性
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发表时间：
2013
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通讯作者：
C. Bentley
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作者：
C. Bentley
在南极洲西部的几次穿越中进行的地震反射和折射测量清楚地表明了各向异性波在冰盖中的传播。这些迹象包括显着增加的折射压缩和剪切波速度在数百米的深度在冰，从冰的底部反射的波的传播速度的入射角和地理方位角的依赖关系，以及剪切波传播速度的偏振方向的变化。所有这些特性都是在通过单个冰晶体的传播中发现的。因此，对速度变化的分析是基于这样的假设进行的，即深度处的冰的行为近似为单晶，即，c轴是完美对齐的当然，这不是一个物理上现实的假设，但将给出几乎相同的弹性波速度结构，作为具有例如30 °或更小的顶角的圆锥内的c轴的随机或轴对称分布。准备了一组将P波和S波的传播速度与冰底部的入射角相关联的表格和曲线，其中"单晶"轴的倾斜度和方位角作为参数。然后，所用的程序是寻找对应于最适合每个台站所有地震观测的一组曲线的晶体取向。所用资料包括反射P波、转换P波到S波反射波以及P波和S波折射波到达。反射的S波虽然经常出现，但由于可能的波型有四重多样性，因此并不十分确定。因此，在模型选择中未使用这些参数。在选定最佳拟合模型后，将每个观测站的相应晶体空间姿态与预期冰流方向进行比较。在可以做出最有把握的选择的那些站点，c轴方向的方位角是下坡。深度探测：回波结果129当一个以上的模型同样适合地震数据时，首选与下坡方向相对应的模型。在少数情况下，特别是在轴向倾角很小的情况下，没有这样令人满意的模型。在一些台站，有一个明确的迹象表明，P波速度最小值的入射角约为45 °，测得的速度下降到接近或甚至低于3800米/秒。一般来说，速度最小值出现在具有近垂直或近水平c轴的模型中。区分两者有四个标准：垂直c轴的最小值在50 °附近，水平c轴的最小值在40 °附近，导致P波转换为S波的平均速度不同，只有水平轴模型才能出现高速折射P波和S波到，而且只有对于具有水平c轴的模型，人们才能期望记录两种类型的剪切波（分别具有包含c轴和垂直于c轴的偏振平面）具有明显不同的速度。在两个站的首选模型暗示接近垂直的c轴：这是罗斯海流域盆地中仅有的两个站。在其中一个，伯德站，从S波到达计算的各向异性冰的深度与深钻孔中发现的1200米深度吻合得很好。最好的地震模型意味着c轴的平均倾角约为15 °;这与Gow（1969）在样品岩心中发现的近垂直方向相当吻合。在两个站处暗示的倾斜方向近似平行于表面轮廓，即，与预期流线垂直。在别林斯高晋海和威德尔海流域，三个最东面的测站的首选模型的c轴几乎水平，沿着流线指向。一个剖面的证据特别强，包括高速折射P波和S波，PS反射中的大振幅横向分量，以及与反射P波速度拟合的极好模型。横波折射波到达表明，各向异性冰在这里发现的深度不超过200米。其余9个观测站位于阿蒙森海流域盆地，观测结果表明它们的轴向倾角处于中等水平。这一点在一个车站得到了明确的证明，那里完成了两个长的垂直剖面，所有类型的到达都很好。折射的S波到达表明各向异性冰的深度约为850米。反射唯一地确定了模型c轴的下坡方向，它们的平均倾角在垂直方向以下约35 °。在该测站还清楚地表明，平均轴向方向随深度而变化。满意的模型与下坡轴向方向可以找到在所有其他站，但两个在这一地区。这两个最接近冰分水岭的区域，最好用与等高线平行方向倾斜30 °和45 °的模型c轴来拟合。作为一个整体的地震证据表明，大部分的西南极冰盖是强烈的各向异性，在大多数最好的模型
Seismic reflection and refraction measurements made during several traverses in west Antarctica have provided clear indication of anisotropic wave propagation in the ice sheet. These indications include marked increases in the refracted compressional and shear wave velocities at depths of hundreds of metres in the ice, dependence of the propagation velocities of waves reflected from the bottom of the ice on the angle of incidence and also on geographic azimuth, and a variation of shear wave propagation velocity with direction of polarization. All of these characteristics are found for propagation through a single crystal of ice. An analysis of the velocity variations has therefore been made on the assumption that the ice at depth behaves approximately as a single crystal, i.e., that there is perfect alignment of the c-axes. This is not, of course, a physically realistic assumption, but will give nearly the same elastic wave velocity structure as a random, or axially symmetric, distribution of c-axes within a cone having an apex angle of, say, 30° or less. To find the best model fitting the observations at each seismic profile, a set of tables and curves relating propagation velocity for P and S waves to angle of incidence at the base of the ice, with the inclination and azimuth of the "single crystal" axis as parameters, was prepared. The procedure used was then to seek the crystal orientation corresponding to the set of curves which best fit all the seismic observations at each station. Data employed included reflected P waves, converted P to S reflections, and P and S refracted arrivals. Reflected S-waves, although often present, were found to be not very definitive owing to a four-fold multiplicity of possible wave types. They, therefore, were not used in model selection. A best-fitting model having been selected, the corresponding crystal attitude in space was then compared at each station with the expected direction of ice flow. At those stations where the most confident selections could be made, the azimuth of the c-axis orientation was downslope. DEEP SOUNDING : ECHO RESULTS 129 Where more than one model fitted the seismic data equally well, that model was preferred which corresponded to a downslope orientation. In a few cases, particularly where the axial inclination was small, there was no such satisfactory model. At a number of stations there was a clear indication of a P-wave velocity minimum at angles of incidence of around 45°, the measured velocities dropping near or even below 3800 m/sec. Generally speaking, a velocity minimum occurs in models with either near-vertical or near-horizontal c-axes. There are four criteria for differentiating between the two: the minimum falls near 50° for vertical c-axes and near 40° for horizontal c-axes, different mean velocities for converted P to S reflections result, only for models with horizontal axes can high velocity refracted P and S wave arrivals occur, and only for models with horizontal c-axes can one expect to record both types of shear waves (with planes of polarization containing the c-axes, and normal to the c-axes, respectively) with distinctly different velocities. At two stations the preferred models implied near vertical c-axes: these were the only two stations in the Ross Sea drainage basin. At one of these, Byrd Station, the depth to the anisotropic ice calculated from S-wave arrivals agrees well with the depth of 1200 m found in the deep drill hole. The best seismic model implies a mean inclination of the c-axes of about 15°; this is in reasonably good agreement with the near-vertical orientation found in sample cores by Gow (1969). The direction of the tilt implied at both stations is approximately parallel to the surface contours, i.e., normal to the expected flow line. Preferred models for the three most easterly stations, in the Bellingshausen Sea and Weddell Sea drainage areas, have c-axes nearly horizontal, pointing along the flow lines. The evidence from one profile is particularly strong, comprising high-velocity refracted P and S waves, large amplitude transverse components in the PS reflections, and an excellent model fit to the reflected P-wave velocities. The S-wave refraction arrivals indicate that anisotropic ice is found here at a depth of no more than 200 m. Observations at most of the remaining nine stations, lying in the Amundsen Sea drainage basin, indicate intermediate axial inclinations. This is unequivocally demonstrated at one station where two long, perpendicular profiles were completed with excellent arrivals of all types. Refracted S-wave arrivals indicate a depth to anisotropic ice of about 850 m. Reflections uniquely determine a down-slope orientation of the model c-axes, their mean inclination being around 35° below the vertical. It is also clearly demonstrated at this station that the mean axial orientation changes with depth. Satisfactory models with down-slope axial orientations could be found at all other stations but two in this region. Those two, the closest to the ice divide, could best be fit by model c-axes tilted 30° and 45° in a direction parallel to the contours. The seismic evidence taken as a whole demonstrates that most of the west Antarctic ice sheet is strongly anisotropic, the best models in a majority