压裂微震数据初至拾取的时频谱熵法

在油气田开发过程中，微震监测是获得水力压裂引起裂缝分布的一种较为有效的方法。微震的定位成像与裂缝解释需要利用有效微震信号位置，而微震信号具有低信噪比的特点，传统信号拾取方法无法有效实现较低信噪比条件下初至时刻的准确拾取。本文提出一种基于时频谱熵的初至拾取新方法，该方法首先通过S变换获取含噪信号的时频谱；然后对谱内各个采样点沿频率方向进行分帧操作，并计算每帧频段内的近似负熵值，以最小近似负熵值作为该谱点的负熵值；最后沿时间方向比较各谱点的负熵值，最小值对应的时刻即为初至时刻。本文利用不同信噪比的合成地震数据对该方法进行效果验证，并与长短时窗能量比（STA/LTA）法进行拾取结果对比，结果表明：信噪比在-5 dB时，两种方法拾取效果都很好；信噪比在-10 dB时，时频谱熵法拾取效果更好。时频谱熵法更适合低信噪比情况下的信号初至拾取。

Time-Spectral Entropy Method for Picking Up Fracturing Microseismic Data

In the process of oil and gas field development, microseismic monitoring is an effective method to obtain the fracture distribution of hydraulic fracturing. Microseismic location imaging and crack interpretation need to use the location of effective microseismic signals; however,microseismic signals have the characteristics of low signal-to-noise ratio, and the traditional signal picking methods cannot effectively achieve the accurate picking at the first arrival time under the condition of low signal-to-noise ratio. In this paper, a new method based on time-spectral entropy for picking up the initial arrival point is proposed. This method first obtains the time-frequency spectrum of the signal containing noise through S transformation;then divides each sampling point in the spectrum into frames along the frequency direction, and calculates the approximate negative entropy value in each frame frequency band, with the smallest approximate negative entropy value as the negative entropy value of the point;finally, the approximate negative entropy values of all sampling points are compared in the time direction, and the time corresponding to the minimum value is the initial arrival position. In this study, a group of synthetic seismic data are used to verify the effect of the new method, and the results are compared with those obtained by the STA/LTA method. It is concluded that the two methods are both effective when the signal-to-noise ratio is -5 dB, but the time-spectral entropy method is better when the signal-to-noise ratio is -10 dB. Thus, the time-spectral entropy method is more suitable for the first arrival signal picking under low signal-to-noise ratio.