Influence of fluids on VP/VS ratio: increase or decrease?
Influence of fluids on VP/VS ratio: increase or decrease?
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DOI：
10.1093/gji/ggy518
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发表时间：
2019-03-01
影响因子：
2.8
通讯作者：
David, Emmanuel C.
中科院分区：
文献类型：
--
作者：
Brantut, Nicolas;David, Emmanuel C.
计算了含球形孔隙的各向同性岩石纵横波速度之比（VP/V-S）随流体饱和孔隙度增加的演化。比值V-P/V-S随孔隙度的增加而减小或增大，这取决于孔隙的纵横比α、流体与固体体积模量比zeta和岩石固体成分的泊松比nu（0）。计算临界初始泊松比nu（0，crit），分离V-P/V-S随孔隙度增加而增加（如果nu（0）<nu（0），（crit））或减小（如果nu（0）> nu（0，crit））的情况。对于薄裂纹和高度可压缩流体，nu（0），（crit）近似为0.157 zeta/alpha，而对于球形孔，nu（0，crit）为0.2 + 0.8 zeta。当nu（0）接近nu（0，临界）时，随着流体饱和孔隙度的增加，V-P/V-S的演化接近中性，并取决于孔隙形状和流体性质的细微变化。这一制度被认为是有关的部分脱水蛇纹岩在俯冲带（孔隙度的纵横比接近0.1和。在0.01 - 0.1的范围内），并且使得仅从V-P/V-S难以检测这些岩石和可能升高的流体压力。
The evolution of the ratio between P- and S-wave velocities (V-P/V-S) with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. The ratio V-P/V-S is shown to either decrease or increase with increasing porosity, depending on the aspect ratio alpha of the pores, fluid to solid bulk modulus ratio zeta and Poisson's ratio nu(0) of the solid constituents of the rock. A critical initial Poisson's ratio nu(0, crit) is computed, separating cases where V-P/V-S increases (if nu(0) < nu(0), (crit)) or decreases (if nu(0) > nu(0, crit)) with increasing porosity. For thin cracks and highly compressible fluids, nu(0), (crit) is approximated by 0.157 zeta/alpha, whereas for spherical pores nu(0, crit) is given by 0.2 + 0.8 zeta. When nu(0) is close to nu(0, crit), the evolution of V-P/V-S with increasing fluid-saturated porosity is near neutral and depends on subtle changes in pore shape and fluid properties. This regime is found to be relevant to partially dehydrated serpentinites in subduction zones (porosity of aspect ratio near 0.1 and. in the range 0.01-0.1), and makes detection of these rocks and possibly elevated fluid pressures difficult from V-P/V-S only.