Reconstructing parameters of sediment layers of a shallow sea bottom using broadband seismoacoustic sources

We study the possibilities of reconstructings the parameters of the sediment layers of a shallow sea bottom from interference patterns appearing during sounding of the bottom by broadband seismoacoustic sources with a bistatic sea-bottom sounding scheme, adaptive signal reception with a horizontal array, and matched filtration of low-frequency seismoacoustic pulses excited with an air gun and interacting with the sea bottom, as well as seismoacoustic noise from a vessel in a shallow sea. The parameters of sediment layers of the sea bottom are reconstructed by statistical checking of hypotheses on the parameters of dynamic spectra obtained by parametric models. Comparative possibilities of reconstructing sea bottom parameters with bistatic and monostatic observation schemes are discussed.     

Electromagnetic and seismoacoustic signals revealed in karst caves (Central Italy)

Summary Since 1988–89 equipment for detecting electric, magnetic and seismoacoustic signals has been running inside the Amare cave. The Amare cave is placed on the southern slope of the Gran Sasso chain, that is one of the largest karst areas of the Italian Apennines. In 1992, a similar equipment was installed inside the Cervo cave. This cave is located in another karst area of the Central Apennines, at about 50 km southwestwards of the Amare cave. In both these measurements sites, the signals are recorded every ten minutes in a digital form; the equipment is able to record signals, the frequency of which ranges from some hundred Hz to some hundred kHz. The data collected up to now seem to identify two different states that we call ?quiet? and ?perturbed? state. In the quiet state only electric and magnetic signals with the highest frequencies appear. These signals are connected with radio broadcastings and with the general lightnings activity of the Earth. A perturbed state is characterized by the sudden appearance of seismoacoustic signals coupled with electric and magnetic ones. This phenomenology is connected with local processes. Rainfall, atmospheric-pressure variations and some thermal effects are responsible for these local processes. A possible model is proposed to justify the observed phenomenology: micromovements of the limestone blocks that constitute the roof of the caves are invoked for the production of seismoacoustic signals. The electrification generated by these movements is invoked for the production of electric and magnetic signals.     

The Use of Low-Frequency Noise in Passive Seismoacoustic Tomography of the Ocean Floor

Acoustical Physics - The authors have investigated the possibility of using the background seismoacoustic noise generated Scholte-type surface waves on the ocean floor in order to study the...     

Tomographic Estimation of Waterbody Parameters in the Presence of Ice Cover Using Seismoacoustic Sources

Acoustical Physics - The article presents the results of field tests of marine seismic sources and seismoacoustic areal measuring systems based on autonomous embedded buoys in Lake Ladoga under ice...     

Coherent marine seismoacoustics: New approaches to reconstructing bottom layer structures in shelf water areas

Results from the theoretical development and experimental study of the coherent seismoacoustic sounding of the sea floor under natural conditions are discussed. Ways of solving the inverse problem of estimating the geoacoustic parameters of the bottom layer by layer are proposed, and their effectiveness and robustness are analyzed numerically. Practical means of seismoacoustic profiling and reconstructing the parameters of individual layers at the sea floors of the shallow water areas using coherent hydroacoustic radiating elements of original design are demonstrated.     

Application of a mobile seismoacoustic system for studying geological structure and prospecting for inhomogeneities to a depth of 100 m

The results of a practical evaluation of a mobile seismoacoustic system intended for the search for underground engineering structures and their imaging are presented. The system was designed at the Institute of Applied Physics of the Russian Academy of Sciences and consists of an electrodynamic radiator of seismoacoustic waves that operates in the frequency range 200–800 Hz, an array of receivers, and a computer-based control unit. The signal is controlled digitally, which allows the system to store up to 10³ realizations. The signal is received by high-sensitivity seismic sensors, amplified by low-noise amplifiers, supplied to a 16-channel A/D converter, and displayed on the computer monitor. The system was used in a field experiment to determine the location of an antilandslide drainage adit.     

A high-power intrawell radiator of shear waves for coherent seismoacoustics

A brief review of investigations in the field of coherent seismoacoustics is presented, and the general requirements for seismoacoustic wave radiators intended for solving problems of remote sounding are formulated. The principle of operation of a novel high-power radiator created at the Institute of Applied Physics, Russian Academy of Sciences, for generating low-frequency seismic waves is described, and the results of the analytical and numerical modeling of this radiator are presented. The main element of the radiator is a piezoelectric cylinder executing bending vibrations in a well filled with water. The concept of sectioning the radiating cylinder for increasing the efficiency of excitation of various radiator modes and improving the matching the radiator with the medium is formulated. The results of the field measurements of the space-time structure of the seismic field generated by the sectioned radiator are presented. On the basis of these measurements, estimates of the power, efficiency, and quality factor of the radiator are obtained.     

Long-base fiber-optic deformometer for the measurement of linear deformations

Principles of a fiber-optic measurement system for the monitoring of seismoacoustic and deformation processes in large-scale physical objects and technical constructions are developed. The system is based on a low-mode fiber-optic interferometer that is sensitive to linear deformations.     

Surface-to-volume wave conversion in shallow water with a gently sloping bottom

Marine sediments support seismoacoustic surface waves, which can propagate along the seafloor, in deep and shallow water, and even onshore. Because of the strong attenuation of compressional and especially shear waves in the sediments, the surface waves can significantly contribute to the acoustic field far from the shore only through their coupling with volume waves in the water. We theoretically study the excitation of acoustic normal modes by seismoacoustic surface waves in a shallow-water waveguide with a sloping bottom consisting of unconsolidated marine sediments. It is found that the coupling primarily occurs in the vicinity of a modal cutoff. The effects of geoacoustic parameters and stratification of soft marine sediments on the efficiency of surface-to-volume conversion are investigated. Published in Russian in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 6, pp. 809–816. The article was translated by the author.     

Numerical Simulation of Hydro- and Seismoacoustic Waves on the Shelf

Acoustical Physics - A finite element method is used for three-dimensional numerical simulation of hydro- and seismoacoustic waves in shallow water, generated by harmonically oscillating sources....     

Coherent Seismoacoustics

In this paper, we briefly overview the last-decade studies in the field of coherent seismoacoustics, carried out at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS). Various schemes of the use of powerful hydroacoustic transmitters as effective seismic sources, as well as theoretical models for their analysis and numerical modeling are comparatively described. Methods for the high-resolution seismoacoustic imaging of underground inhomogeneities are discussed. These methods are based on the use of coherent signals, in particular, coded by large pseudorandom sequencies. The special attention is paid to the results of recent field experiments demonstrating the advantages and new capabilities of coherent methods of seismoacoustic diagnostics at different depths from tens of meters up to tens of kilometers.     

Surface-to-volume wave conversion in shallow water with a corrugated bottom

Acoustic transmission between points onshore or in very shallow water and points in deep water is strongly influenced by the shear rigidity of marine sediments, which control the parameters and the very existence of seismoacoustic surface waves. Previously, it was found that coupling between acoustic modes and the seismoacoustic surface waves is normally weak, although not negligible in the case of a gently sloping seafloor and soft sediments. In this paper, the previous work is extended by accounting for the small-scale roughness of the seafloor. The significant role of roughness in coupling between volume and surface waves is demonstrated. The combined effect of bottom topography, roughness, and wave attenuation in soft marine sediments on the sound propagation between points in shallow and deep water is discussed. Published in Russian in Akusticheskiĭ Zhurnal, 2008, Vol. 54, No. 3, pp. 400–407. The article was translated by the author.     

Analysis of possibilities of the acoustic forecast of natural disasters

Potential of acoustic methods for forecasting disastrous phenomena of infrafrequency dynamics in natural media is assessed. A possibility of using seismoacoustic information in tsunami propagation monitoring systems is shown.     

Use of the piezoelectric hydrophone for detecting infra-low-frequency seismoacoustic fields

A possibility of using piezoelectric sound detectors to record infra-low-frequency wave fields is demonstrated. A device for measuring seismoacoustic fields with frequencies 0.01 Hz and lower is developed. The bench tests of the hydrophone showed correctness of the technical approaches used.     

Recording seismoacoustic signals of a surface vessel with a two-coordinate strainmeter

Experimental data are analyzed, obtained during the recording of seismoacoustic signals with a two-coordinate laser strainmeter; the signals were generated by a surface vessel moving over the shelf and deep water. Based on analysis of the obtained data, we substantiate the prospect of applying shore-based laser strainmeters for the direction finding of vessels and the creation of monitoring systems for large water areas.     

Outlook for passive marine seismic exploration using infralow-frequency hydrophones

Possibilities of solving geological and geophysical problems in seas using passive seismic exploration are analyzed. A mockup of the infralow-frequency seismoacoustic field meter is developed and tested.     

Excitation of seismoacoustic waves by a harmonic force source acting at the boundary of a liquid layer and an elastic halfspace

A problem on the excitation of seismoacoustic waves in a system of a homogeneous isotropic elastic halfspace covered with a liquid layer is solved in the case of action of a source of point harmonic force on the surface of an elastic medium. Integral expressions are obtained for the radiation powers averaged over a wave period for longitudinal and transverse waves in a solid. Mode excitation is analyzed in detail. Expressions describing parts of the mode powers radiated into a liquid layer and an elastic medium are obtained. Numerical analysis of radiation powers is conducted for spherical longitudinal and transverse waves as well as for the radiation powers of seismoacoustic modes in a solid halfspace and a liquid layer. It is determined that in the conditions characteristic of bottom rocks in the case, where the basin depth is several times and more larger than the sound’s wavelength, about 2/3 of the total power is radiated into a liquid.     

A new method for reconstructing medium inhomogeneities using Rayleigh waves: Case studies

The problem of reconstructing local low-contrast inhomogeneities in the Earth’s surface layers by means of coherent Rayleigh surface waves is considered. It is shown that analysis of the frequency characteristics of shear projections in this wave on the surface allows construction of the function of inhomogeneity distribution in a specified depth range. The results from seismoacoustic reconstruction of inhomogeneities correlate with data obtained using standard geophysical methods and are confirmed by direct observations.     

Seismoacoustic emission of an oil-producing bed

Results are presented from a study of seismoacoustic emission appearing in an oil-saturated porous geological medium under the acoustic force action in a borehole. It is shown that dynamic nonlinear processes in the producing bed are activated under the internal elastic action on the stratum, changing the energy state of the medium, and this change can be seen as a change in the acoustic emission pattern. The correlation between the high-frequency part of the acoustic emission spectrum and the low-frequency one is found, indicating the development of this process in space at different scale levels.