Geohydroacoustic Noise Monitoring of Under-Ice Water Areas of Northern Seas

Abstract

The paper presents the results of theoretical and experimental research into the structure of geohydroacoustic wave fields generated in continuous ice-covered northern seas. A simplified mathematical model is constructed that takes into account experimental data demonstrating that the generation of different types of geohydroacoustic waves in the lithosphere–hydrosphere–ice cover system are primarily influenced by the water layer with the ice cover. The seafloor structure mainly affects the characteristics of propagating waves rather than the generation of new modes. Mathematical modeling results have laid the basis for new technologies to localize inhomogeneities in ice-covered water areas. The main distinguishing feature of this novel technology for monitoring a medium under ice-covered marine conditions is the possibility to measure noise signal parameters without active geohydroacoustic emission sources. Methods that measure the characteristics of surface-type waves are the most promising for use in northern sea conditions, in particular, microseismic sounding and noise tomography. Integration of these methods combines the recent achievements of passive geophysics and takes into account the particularities of underwater acoustics. To obtain information on the wave propagation medium, both the wave field amplitude and phase characteristics are used. To detect particular types of waves in records, spatiotemporal signal processing methods are used with the appropriate choice of frequency range. The authors describe their new-generation seismohydroacoustic information-measuring modules (embedded buoys), which are equipped with vector and molecular-electronic primary transducers. The information-measuring modules are designed for combined use with distributed ice-class arrays capable of monitoring continuous ice-covered northern seas year round. Studies of how ice-embedded information-measuring systems function, as well as verification of the obtained theoretical results, were carried out during field tests in February 2017. At each measurement point, the receiver system consisted of three reference devices that took measurements on the seafloor, in the water column, and on the ice surface. Mockups of the tested geohydroacoustic buoys were embedded at points offset by 1 km. Dropped 32 kg weights were used as the sources. Controlled perturbations in the ice experiments made it possible to obtain qualitative spectrograms of geohydroacoustic perturbations in layered structures and analyze the dispersion curves. When the fundamental bottom modes were studied, the signal source consisted of an underwater charge at a depth of 10 m. The embedded seismohydroaoustic information-measuring modules successfully passed the ice-based tests in field conditions at low temperatures, demonstrating the reliability of the obtained seismohydroacoustic information. The experimental data agree very well with theoretical estimates obtained with the created model of a layered geological medium. These studies demonstrated that natural sea noise contains useful information reflecting the internal structure of the seafloor and the water layer and led to development of the instrumental and methodological foundations of a noise technology for localizing inhomogeneities in the aquatic environment and layered bottom structures of northern seas by means of passive microseismic noise monitoring.