用穿透法测量声速时初始点的选择问题

用穿透方法测量岩心或固体的声速时，从测量波形中确定纵波（或横波）的到达时刻（或称初始点）比较困难，当声波的频率比较低时，其误差对测量结果影响比较大。本文用相位分析的方法研究了初始点的选择问题。首先，用各向同性较强、均匀的汉白玉制作标准件，测量其不同传播距离的波形，再用相位补偿的方法得到其传播速度；其次，利用传播速度，可从不同距离的测量波形中确定初始点及其对应的测量系统的时间延迟常数；最后，确定波形的其他位置为初始点时对应的时间延迟常数，因此，我们可以根据实际测量波形的特点选择最佳的初始点，利用其对应的时间延迟常数得到比较精确的声速。模型测量结果表明，初始点位置不同，其对应的测量系统的时间延迟常数也不同，也即测量系统的时间延迟常数是与初始点的选择相对应的。

Selection of first-break timing in using transmitting method for measuring sonic velocity

It is difficult to determine the first-break timing (or initial point) of P-wave or S-wave when using transmitting method for measuring sonic velocity of core or solid.The impact of errors on measured results is bigger when sonic frequency is lower.The paper uses phase analysis to study the problem of selecting the initial point.First,using good isotropic and homogeneous white marble for fabricating standard parts and measuring the sonic waveform from different propagation distance,then the propagation velocity can be got by using phase compensation method;second,using the propagation velocity,the first-break timing and the corresponding time-delay constant of measurement system can be determined from measured waveform at different distance;finally,the other position of waveform can be determined as time-delay constant corresponding to first-break timing;therefore,we select optimum first-break timing according to characters of practically measured waveform and get more precise sonic velocity by using corresponding time-delay constant. The model test showed that different position of first-break timing may get different time-delay constant of measurement system,in other words,the time-delay constant of measurement system is corresponding to selection of first-break timing.