Investigation of the dynamic range restrictions influence of the fiber-optic towed seismic streamer on the seismogram quality

In this paper, we presented results of influence of the dynamic range in fiber-optic streamer signal processing circuit on the quality of recorded seismograms. By contrast with the existing hydroacoustic systems, which based on piezoceramic transducers, dynamic range limitations in a fiber-optic towed streamer do not lead to clipping of acoustic signals, but to complex nonlinear distortions that affect both the amplitude and phase frequency characteristics of the recorded signals. Therefore, main task of this work is to assess the distortion of seismograms obtained from acoustic signals recorded under limited dynamic range conditions of fiber-optic towed seismic streamer. To solve this problem, seismic signals obtained during field tests for various types of seismic streamers in the water area of Kola Bay are used. The recorded acoustic signals in the form of digital readings, converted into radians, taking into account the known sensitivity coefficient of hydrophones of the towed seismic streamer, are converted into digital reports of optical interference signals. Interference signals in digital form are equivalent to real optical signals recorded by the receiving path in the signal processing unit of the fiber optic streamer. The digital form of the recorded acoustic signals makes it possible to amplify them by multiplying them by a given gain factor, simulating various energy levels of an acoustic source. Further, these signals served as input data in the mathematical model of the signal processing circuit of the fiber-optic towed streamer, which takes into account the fundamental limitations of the dynamic range due to the finite sampling frequency of the interference signal, the fixed frequency of the auxiliary phase modulation, and the finite bandwidth of the low-pass filters used. Thus, it is possible to simulate the process of recording acoustic signals of a fiber-optic towed seismic streamer both without distortion and under conditions of limited dynamic range. As a result, signals from the output of the processing circuit model are used to construct seismograms of the same shelf area with different levels of acoustic amplification using the reflected wave method. The results demonstrate that the limitations of the dynamic range of signal processing circuit of fiber-optic towed seismic streamer have a significant impact on the quality of seismograms, on reducing the signals detailing, and also on decreasing the amplitudes of the recorded waves (in the presented data, the amplitude decreases by 5 times). The quality of seismograms drops significantly in areas of sharp transition between layers with different densities, which generate the most distinct and strong reflected seismic vibrations. The results obtained are of great practical importance, since they allow evaluating the effect of complex nonlinear distortions of acoustic signals under the conditions of limited dynamic range of the signal processing circuit of a fiber-optic towed seismic streamer on the received seismograms. These results are presented for the first time due to the lack of world analogues of the developed fiber-optic towed seismic streamer. In addition, taking into account the known sensitivity of the fiber-optic hydrophones of the streamer, the constructed model of the signal processing path allows choosing the optimal energy of the acoustic source for seismic exploration with the most efficient use of the dynamic range of the fiber-optic streamer