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.

The paper presents the results of laboratory measurements of the acoustic nonlinearity parameter for a granite sample from the site of a conducted field experiment. This made it possible to completely confirm the results of the field experiment and explain the occurrence of a large scatter of values for the nonlinearity parameter in the field measurements. The size of the quadratic linearity parameter in granite rocks was determined, normalized to the volumetric concentration of fractures, which can be used for remote estimation of the fracture concentration.

Experimental results of the seismic profiling with bottom penetration up to 1000 m based on broadband signals and conducted in the Caspian Sea sites are presented. Use has been made of synchronized sequences of probing pulses with linear frequency modulation at a frequency deviation of 50 to100 Hz. The pulses were emitted by a towed sound source of an original design (acoustic power up to 300 W, frequency ranged from 100 to 1000 Hz) and received by a standard digital seismic streamer. The processing of the signals involved the matched filtering of the individual pulses and the trajectory accumulation of a long sequence of pulses lengthwise the horizontal-homogeneous reflecting layers of the bottom structure. The adaptive stacking procedure taking into account the linear inclinations of the individual layers allowed us to enlarge the stacking interval by up to 100 pulses and to increase the effective depth and the spatial resolution of the seismic profiling, which gave us a total increase of more than 30 dB in the S/N ratio. In our view, the seismic profiling using low-power (about 100 W) and broadband (up to several hundred Hz) coherent sound sources represents a promising technology for decreasing the hazardous impact on aquatic ecosystems. The approach developed is an alternative to the conventional technology of marine seismic prospecting based on powerful pulse sources of the shock type (air guns, sparkers) in the low frequency range (less than ∼200 Hz).

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.

The results of an experiment on crosswell profiling using the coherent downhole source of SH waves are presented. Seismic records were obtained during stationary disposal of the source and varying depths of the vector receiver position. The source coherence made it possible to accumulate signals, thus sub-stantially improving the quality of seismograms and measuring phases. These procedures allowed low-contrasting boundaries between layers to be distinguished. The performed digital modeling confirmed the measurement results.

The results of an experiment on crosswell profiling using the coherent downhole source of SH waves are presented. Seismic records were obtained during stationary disposal of the source and varying depths of the vector receiver position. The source coherence made it possible to accumulate signals, thus sub� stantially improving the quality of seismograms and measuring phases. These procedures allowed lowcon� trasting boundaries between layers to be distinguished. The performed digital modeling confirmed the mea� surement results.

Александр Григорьевич Литвак (к 70-летию со дня рождения), Велихов Е.П., Гапонов-Грехов А.В., Гапонов С.В., Железняков В.В., Зелёный Л.М., Кругляков Э.П., Матвеев В.А., Месяц Г.А., Сергеев А.М., Смирнов В.П., Таланов В.И., Фортов В.Е.

Reactions of an unconsolidated medium to a strong impact action are considered. A character of medium relaxation was controlled based on changes in the phase velocity of a trial Rayleigh wave. The analysis of the R-wave dispersion and its dependence on the distance to the source allowed for estimating the dimensions of a space region where the impact action was revealed. Seismoacoustic measurements showed that the time dependence of the R-wave is logarithmic.

Results of a 2004 field experiment aimed at determining the quadratic nonlinearity parameter in granite that forms the shore of the Ladoga lake are presented. The measurements were based on the observation of the nonlinear interaction between monochromatic waves excited by two hydroacoustic radiators of 1 kW each positioned near the shore. The initial level of nonlinear distortions was much lower than the level of the received difference-frequency signal. The quadratic nonlinearity parameter proved to be higher than that reported in the majority of publications. An assumption was put forward that the high nonlinearity of granite is caused by the high concentration of cracks in it. Pieces of rock were taken from the measurement site, and rect- angular samples were prepared from them. The samples were studied by the acoustic spectroscopy method in laboratory conditions. As a result, estimates of crack concentration were obtained, which proved to be consis- tent with the field measurements of the quadratic nonlinearity parameter. Thus, the possibility of estimating the crack concentration in situ from the measurements of the quadratic nonlinearity parameter was demonstrated.

Results of a 2004 field experiment aimed at determining the quadratic nonlinearity parameter in granite that forms the shore of the Ladoga lake are presented. The measurements were based on the observation of the nonlinear interaction between monochromatic waves excited by two hydroacoustic radiators of 1 kW each positioned near the shore. The initial level of nonlinear distortions was much lower than the level of the received difference-frequency signal. The quadratic nonlinearity parameter proved to be higher than that reported in the majority of publications. An assumption was put forward that the high nonlinearity of granite is caused by the high concentration of cracks in it. Pieces of rock were taken from the measurement site, and rectangular samples were prepared from them. The samples were studied by the acoustic spectroscopy method in laboratory conditions. As a result, estimates of crack concentration were obtained, which proved to be consistent with the field measurements of the quadratic nonlinearity parameter. Thus, the possibility of estimating the crack concentration in situ from the measurements of the quadratic nonlinearity parameter was demonstrated.