The sensitivity of the Sagnac interferometer for various acoustic influence coordinates was studied. The principles of the formation of a dead zone in an acoustic distributed fiber-optic sensor based on the Sagnac interferometer have been obtained and experimentally confirmed. The response of the interferometer was studied for different types of acoustic impact on the circuit: in the form of a rectangular pulse, sinusoidal, and in the form of a periodic triangular function. The nature of the change in the phase difference at the output of the Sagnac interferometer for each of them was studied. With the found value of the typical frequency = 10.8 kHz and a loop length of 20 km, numerical simulation and experimental study of the amplitude of the phase difference under acoustic impact through each 1 km of the loop in the range from 0 to 10 km were carried out. A dead zone elimination method is proposed for integrating the Sagnac interferometer into a complex monitoring system using a phase-sensitive optical time domain reflectometer.