Microseismic monitoring has become an effective means for geophysical surveys and energy industry extraction. Since the 1990s, with the advancement of optical fiber technology, interferometric optical fiber detectors have made significant progress. These detectors have advantages such as high sensitivity, wide bandwidth, resistance to electromagnetic interference, and ease of reusability, enabling high-fidelity acquisition of seismic signals.
To optimize resource utilization, it is usually necessary to adopt multiplexing techniques to construct multi-detector sensing networks. Based on the different physical properties of light waves, researchers have developed schemes such as space division multiplexing, wavelength division multiplexing, and time division multiplexing. Among them, the time division multiplexing scheme achieves a good balance in system cost performance, detector performance, and array gain. This scheme reconstructs the interference signal by modulating continuous light into pulsed light and using the time difference of the returned pulses from each detector in the array to reconstruct the interference signal. Additionally, time division multiplexing can be combined with other multiplexing techniques to construct larger-scale detector arrays. Typical time division multiplexing structures include: the traditional stepped structure, where each detector requires 3 couplers; the In-line Michelson structure, where each detector only requires 1 coupler; and the F-P cavity structure composed of optical fiber gratings (FBG). Among them, the In-line Michelson structure is widely used due to its simple structure, but the identification of the return pulses from each detector in the array still requires further research. FREITAS D et al. studied the crosstalk problem of time division multiplexing array, but their assumption that the delay parameters of each detector are the same is difficult to guarantee in practical applications. Li Shupeng et al. proposed a measurement method that is precise, but the equipment is complex and expensive.
For the time division multiplexing optical fiber detector array of the In-line Michelson structure, this paper proposes a method for measuring the return pulse delay parameters. This method extracts the difference features of interference pulses and background pulses, uses the variance vector as the positioning identifier for each detector signal, and introduces a pulse template function to smooth the variance vector to suppress noise interference. Finally, the maximum point of the correlation coefficient vector is solved to determine the delay parameters. Experimental verification based on original data with different signal-to-noise ratios shows that: when the signal-to-noise ratio is >12 dB, the method has a correct rate of 100%; when the signal-to-noise ratio drops to 7 dB, the success rate still remains above 98%; even at -3 dB of extremely low signal-to-noise ratio, it can still maintain a correct rate of over 65%.