Seismoacoustic survey of artificial inhomogeneities in the ground

The results of processing the field test data obtained for the seismoacoustic system designed at the Institute of Applied Physics of the Russian Academy of Sciences for the visualization of underground engineering structures are presented. The described experiment is the first demonstration of the use of a high-power, high-stability transmitting-receiving system for producing a coherent insonification with a frequency of 195 Hz. The receiving element of the system is a synthetic aperture array. With the use of focusing as a method of the final signal processing, an image of a tunnel lying at a depth of 30 m is obtained in three spatial cross-sections, which demonstrates the possibility of a three-dimensional, coherent, high-frequency seismic survey of engineering structures.     

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.     

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.     

Experimental investigation of potentialities of seismoacoustic sea-bottom sounding using coherent pulse signals

We describe the results of experimental investigations of the seismoacoustical sounding of the bottom structure of the Caspian Sea. They were obtained using a ship towed hydroacoustic emitter of LFM pulse signals in several frequency ranges of frequency band from 100 to 1000 Hz. Based on the high coherence and relatively high frequencies of emitted signals, the results point to feasibility of substantial improvement in noise immunity and resolution of sounding the bottom rocks?? structure at depths of up to 1000 m thanks to combined application of a series of procedures of coherent processing of incoming signals. The processing involves matched filtering of individual pulses, coherent accumulation of pulse trains within the horizontally uniform bottom area, and adaptive path accumulation of pulses accounted for inclination of individual reflecting layers. The resulting gain in noise immunity came to about 30 dB, which points to possibility of efficient use of relatively low-power (up to 100 W) coherent sources for seismoacoustic sounding of sea bottom at minimal damage to local ecology.     

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.     

Monitoring of anthropogenic noise on the shelf of Sakhalin Island during seismic survey research

The paper describes the technique and gives the results of acoustic-noise and seismoacoustic-signal-parameter measurements on a northeast shelf of Sakhalin Island, generated during seismoacoustic research at the licensed Chaivinskii site. The aim of measurements was acoustic control of the water area round an emitting vessel. Results of field measurements and 3-D simulations of seismoacoustic signal propagations on sea and land are presented.     

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.     

Frequency-shifted megawatt soliton output of a hollow photonic-crystal fiber for time-resolved coherent anti-Stokes Raman scattering microspectroscopy

Hollow-core photonic-crystal fibers (PCFs) provide soliton delivery and frequency shifting of 2.8 MW femtosecond pulses with an input central wavelength of 618 nm. The frequency-shifted megawatt soliton output of the hollow PCF is used as a high-peak-power Stokes field for coherent anti-Stokes Raman scattering (CARS) microspectroscopy, providing a dynamic range of nearly four decades for anti-Stokes signal detection, thus enabling time-resolved CARS studies of ultrafast relaxation processes on time scales from tens of femtoseconds up to tens of picoseconds.     

Some aspects of the registration of low-frequency impulse signals in air using a complex receiving system

The specific features of the formation of impulse acoustic and seismic signals, as well as their propagation to relatively short distances under various conditions of their excitation, based on the registration of consistent data on a pressure field and an oscillation speed field are discussed. It is shown that the “delayed” character of the signals, especially at low exit angles of projectiles, is due to the intense interaction of a wave excited by a shot with the ground and to multiple reflections of the signals. The use of signals registered by the channels of a vector receiver makes it possible to obtain information on the spatial distribution of the signals. When a projectile bursts in air, the sound wave that is incident on the land surface induces a seismoacoustic wave that is first registered by channels of a combined receiving unit (CRU).     

X‐ray sources for studies of ultrafast processes

Radio signals from Jupiter were first detected in 1955 in the radio range at a frequency of 22.2 MHz. The emissions were sporadic in character, and were confined to frequencies below 40 MHz. These decametric (DAM) emissions have been interpreted as coherent cyclotron radiation from electrons in the tens of keV range. The innermost jovian moon Io, which orbits Jupiter in only 1.8 days, appears to modulate the emission: both the intensity and the probability of the occurrence of bursts increase when Io is at certain locations in its orbit with respect to Jupiter and the observer. The emissions originate in Jupiter's aurora, being produced by electrons that travel along magnetic field lines. Particles that enter the atmosphere may locally excite atoms and molecules, which upon de-excitation are visible as aurora at UV and infrared wavelengths (sometimes also at X-ray wavelengths). A fraction of the electrons is reflected back along the field lines, and produces DAM emissions.     

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.     

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.     

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.     

半导体超晶格声子激光器的研究进展

太赫兹频率的相干声子在纳米尺度器件的探测和操控领域具有重要的应用价值。半导体超晶格声子激光器是实现太赫兹频率相干声子源稳定输出的重要途径。本文首先回顾了GHz到THz频率范围声学放大的多种方法,然后详细阐述了超晶格声子放大、超晶格声学布拉格镜的工作原理与设计方法以及声子激光器的阈值条件,同时总结了电抽运和光抽运结构器件的研究现状,最后简要讨论了亚太赫兹声子激光器在声-电子领域的应用。分析表明,这种能够产生强相干太赫兹声子的半导体超晶格声子激光器在纳米尺度器件的探测与成像等方面具有广阔的发展前景。     

Suppression of image artifacts in the spectral-domain optical coherence tomography

In this paper, we review methods for obtaining in vivo optical images of bio-objects with the resolution ranged from several units to tens of micrometers, namely, the methods of spectral-domain optical coherence tomography. We focus on problems that are specific of the spectral-domain optical coherence tomography, namely, the presence of coherent noise and mirror-symmetric phantoms in the image. On the basis of the most important relevant publications, we analyze possibilities of eliminating these drawbacks in modern devices.     

Enhancing the emergence rate of coherent wavefronts from ocean ambient noise correlations using spatio-temporal filters

Extracting coherent wavefronts between passive receivers using cross-correlations of ambient noise (CAN) provides a means for monitoring the seismoacoustic environment without using active sources. However, using cross-correlations between single receivers can require a long recording time in order to extract stable coherent arrivals from CAN. This becomes an issue if the propagation medium fluctuates significantly during the recording period. To address this issue, this article presents a general spatio-temporal filtering procedure to enhance the emergence rate for coherent wavefronts extracted from time-averaged ambient noise correlations between two spatially separated arrays. The robustness of this array-based CAN technique is investigated using ambient shipping noise recorded over 24?h in the frequency band [250-850 Hz] on two vertical line arrays deployed 143?m apart in shallow water (depth 20?m). Experimental results confirm that the array-based CAN technique can significantly reduce the recording duration (e.g., from 22?h to 30?min) required for extracting coherent wavefronts of sufficient amplitude (e.g., 20?dB over residual temporal fluctations) when compared to conventional CAN implementations between single pairs of hydrophones. These improvements of the CAN technique could benefit the development of noise-based ocean monitoring applications such as passive acoustic tomography.     

Coherent spin-current oscillations in transverse magnetic fields

We address the coherence of the dynamics of spin-currents with components transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We study current autocorrelations at finite temperatures and the real-time dynamics of currents at zero temperature. Besides a coherent Larmor oscillation, we find an additional collective oscillation at higher frequencies, emerging as a coherent many-magnon effect at low temperatures. Using numerical and analytical methods, we analyze the oscillation frequency and decay time of this coherent current-mode versus temperature and magnetic field.     

Search for hydrocarbons by the method of induced seismoacoustic emission in wells

Regular specific features have been observed for the first time in the response of a fluid-saturated porous medium to a high-intensity seismoacoustic field. Experimental investigations show that the observed behavior can serve as a basis for a new effective complex technology capable of detecting hydrocarbons and water in the near-well zone and selectively restoring the fluid conductivity in oil pools.     

Advance in earthquake prediction by physical simulation on the baikal ice cover

The main challenge in the prediction of tectonic earthquakes and their control is still insufficient awareness of seismotectonic processes in the lithosphere and upper mantle during the preparation of strong earthquakes. This is associated in many respects with not quite appropriate equipment for researchers. Among relevant problems is also a lack of adequate models of preparation of earthquake sources at different stages, and this retards the development of earthquake prediction methods. The paper discusses long-term research on deformation and destruction of the Baikal ice cover in the context of physical mesomechanics. With certain combinations of meteorological factors (wind, temperature, precipitation, undercurrents, etc.) responsible for deformation, major cracks of many kilometers arise in the Baikal ice cover. Their spontaneous growth often involves seismic phenomena as ice quakes whose energy reaches E _max = 10⁴–10⁷ J. The nucleation of major cracks is similar to that of rock bursts of moderate strength or weak earthquakes. It is found that ice quakes and earthquakes are both preceded by foreshocks, seismic calm for tens of minutes, aftershocks and other events against the background of accelerated creep in fractures and increased seismoacoustic activity. Research data make it possible to put forward two genetically interrelated criteria among basic factors for ice quake prediction: variations in deformation modes at convergent boundaries of ice sheets and a specific intensification mode-generation of strong foreshocks in a segment in which ice sheets are prepared for dynamic motion. We substantiate the conclusion that simpler and clearer scenarios of preparation of strong seismic events in the Baikal ice cover allow successful physical simulation of preparation of tectonic earthquakes and rock bursts and advances in their prediction. We also consider and substantiate the feasibility of techniques for more efficient seismic risk reduction.     

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.