Abstract: 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.

Key words: microseismic signals, time-frequency spectrum, frequency domain frame, negative entropy, arrival-time picking