Time delay estimation accuracy enhancement with envelope filtering in LFM-DAS

Distribute acoustic sensors (DAS) based on linear frequency modulation (LFM) has advantages flexible sensitivity adjusting and less affection from interference fading. The time delay estimation (TDE) is commonly used in interferogram movement caused by frequency shift compensation to acquire quantitative strain. While, jump demodulation error may occur when large dynamic strain is applied to detection fiber, limits the dynamic range of sensing system. In this paper, we established a frequency band characteristic model of LFM-DAS optical signal envelope, the relationship between phase change and interferogram envelope shift in different bands is investigated. The simulation and experiment result shown that envelope of high frequency band has lower energy and SNR. Low-pass Filters (LPF) related can used to stabilize the envelope shape and increased correlation coefficient. In addition, part of the jump error is eliminated, which enhances the strain demodulation accuracy. 41.2 km, 100Hz, 53.81nε sinusoidal disturbance DAS has been realized based on LFM pulse. The root-mean-square error (RMS) is 3.191, Power spectral density (PSD) is 60.14 rad²/Hz.