蠕变及超声波频率下砂岩的衰减

The sandstone's attenuation under creep and ultrasonic frequency

为了在较宽频域内研究砂岩衰减变化，实验在13．2MPa的初应力水平下，测得了蠕变（1小时～3天）和超声波（470KHz、700KHz）频率下干燥的大庆、紫蓬山砂岩衰减．发现蠕变衰减随蠕变时间增大而增大，且均大于超声波频率下的衰减，最小相差亦有40％．将衰减与应力作用频率关系转化为衰减与蠕变量关系后，二者线性相关，相关系数大于95％．且据此将蠕变量向超声波频率对应的应变量外推所得的衰减值与实验测得的衰减值符合较好．实验得到的应变量与衰减的线性关系，解释了已有的0．1～1MHz下衰减近似常量的事实，亦可预测岩石从蠕变破裂所对应的最低频率到MHz频率范围内衰减变化．

Many investigations on the rock＇s attenuation have been done, which mainly concentrate on the frequencies of seismic waves, compared with this, the researches of attenuations under a lower frequency are less. In order to obtain the attenuations of sandstone under a more wider frequency domain, the attenuations of dry Taching, Zipeng mountain sandstones under the initial stress level of 13. 2MPa in different creep periods which respectively kept for 1 hour, 2 hours, 4 hours, 12 hours, 1 day, 2 days and 3 days were jointly considered with the attenuations under the ultrasonic frequencies 470 KHz, 700 KHz. As both the creep and the ultrasonic tests had a zero driving stress amplitude under the same stress, the influence factors on the attenuation caused by the stress amplitude and the initial stress were removed. Both results were only related to the frequency and could be comparable. The attenuations of creep were computed by the method of stress-strain curves, and the attenuations under ultrasonic frequencies came from the method of ratios of spectrum-amplitudes. After comparing the values of attenuations, the results showed that the attenuations of creep which increased according to the creep period were higher than ultrasonic attenuations, and the smallest difference between them was about 400/00. Usually, the difference was roughly explained by different measuring methods, there was a relative lack of the researches involving quantitative analysis of the differenc~ For the sake of a quantitative explanation to the difference, changing the relationship between attenuation and creep time into the relationship between attenuation and strain, the linear correlation coefficient was above 95~/00. The result of creep attenuation extrapolated in the smaller strain direction was very close to the ultrasonic attenuation. Consequently, it seemed that the difference was not caused by different measuring methods but by the different strains At the same time, the fact that the attenuations in 0. 1 - 1 MHz are almost constant can be explained, because the attenuation is proportional to the strain, and the strain in such frequencies approaches zero, the same strain means the same attenuation （the intercept at the origin in the line of strain- attenuation）. If the creep attenuation was extrapolated in the bigger strain direction, the attenuation of creep rupture could be found and the creep time could been estimated according to the amount of rupture creep, once the correctness of extrapolating in the direction of creep rupture was tested by experiments, then the attenuation for one sample within the lowest frequency to MHz could be obtained.