4.0 GPa压力下纯橄岩弹性纵波速度和衰减的实验研究

 应用超声波透射法和超声波频谱振幅比法、在4.0 GPa压力条件下、测量了弹性纵波通过纯橄岩的波速（v_P）和品质因子值（Q_P，用于表征衰减）随压力的变化，并分析了纯橄岩内部结构的变化对波速和衰减的影响。在实验压力范围内（≤4.0 GPa），随压力升高，纯橄岩的纵波速度逐渐增大：从7.6 km/s（0.4 GPa）逐渐增大至8.5 km/s（4.0 GPa），升高了11.8%，低压时的增大幅度大于高压时的增大幅度。纯橄岩的品质因子值呈两段式线性变化：从低压区间到高压区间品质因子值的增大幅度明显变小（0.4~2.4 GPa压力范围内Q_P增大了358.5%，2.4~4.0 GPa压力范围内Q_P仅升高了7.6%）。纯橄岩的品质因子从54（0.4 GPa）升高至266.4（4.0 GPa），增大幅度达393.3%。在相对低压（0.4~2.4 GPa）条件下，纵波通过纯橄岩的速度增大、衰减降低主要是由于样品内的孔隙和微裂纹大量闭合，岩石密度增大，纵波在通过样品内孔隙和裂缝时损失的机械能降低，因此纵波通过纯橄岩的能量增大（即衰减降低），波速升高；当围压较高（2.4~4.0 GPa）时，纯橄岩内大部分裂纹已经闭合，而且矿物颗粒边界接触紧密。岩石内部的裂纹和颗粒之间由于摩擦滑动而损失的能量也变少，所以在高压时纯橄岩的波速和衰减的变化幅度变缓，但波速仍呈线性增大，而品质因子值（衰减）随压力升高几乎趋于不变。

Elastic P Wave Velocity and Attenuation in Dunite up to 4.0 GPa

Elastic P wave velocity(v_P) and quality factor value(Q_P,on behalf of attenuation) of dunite are determined using ultrasonic pulse transmission and spectral ratio methods in a multi-anvil apparatus up to 4.0 GPa at room temperature.During the experimental pressure range(0.4~4.0 GPa),v_P and Q_P increase about 11.8%(from 7.6 km/s to 8.5 km/s) and 393.3%(from 54 to 266.4).The experimental results show that v_P and Q_P are pressure dependent and a greater rising rate at the lower pressures than that at higher pressures: the increase rate of Q_p at 0.4~2.4 GPa is 358.5%,while only 7.6% at 2.4~4.0 GPa.At the lower pressures,the pores with low aspect ratio in rocks are easy to close,and almost all of them would close with the pressure increasing.As a result,the samples show the internal inherent elasticity of the whole constituent minerals themselves and the increasing rate of v_P turns lower at the higher pressures.As the pressure rising(0.4~2.4 GPa),Q_P of dunite increases because the energy loss in the pores and between the surfaces of cracks and the mineral boundaries is reduced due to the porosity decreasing and the mineral boundaries contact more tightly.While at(higher) pressures(2.4~4.0 GPa),most microcracks in dunite have been closed and mineral grains are compacted very tightly,as a result the internal frictional energy loss due to the relative motion at the grain boundaries and across crack surfaces became less,which results in small change of elastic wave attenuation in dunite.