Coherent seismoacoustics

The main goal of seismoacoustics is the remote sensing of earth rocks and monitoring of the earth rock state by means of acoustic signals and by retrieving the properties (parameters) of a medium or its inhomogeneities. From this point of view, the development of seismoacoustics has a general radiophysical context that is the use of universal approaches to solving the inverse problems of wave diagnostics of inhomogeneous media. One of the well-known approaches is to employ coherent signals for increasing both the spatial resolution of the inner structure imaging and the sensitivity to small variations of its properties. The seismoacoustic studies based on this approach, initiated in the mid-1990s by V. I. Talanov, have led to a new promising trend at the intersection of physical acoustics and geophysics, i.e., coherent seismoacoustics.
The principal advantages provided by the use of coherent radiation sources of probing signals in seismoacoustics, which have high-stability, well-controlled radiation modes and allow one to radiate complex signals with a large base, are determined by several factors. The basic one is the potential possibility of coherent accumulation of long sequences of probing signals to increase the probing depth and/or improve the resolution at moderate (and even relatively weak) absolute levels of radiation that does not significantly distort the state of the Earth's rocks (otherwise, such measurements are not quite correct).

Study of the effects of the nonlinear interaction of seismic signals in natural granites of the Ladoga Lake (left), parametric generation of a signal at the difference frequency of two interacting waves as functions of their amplitude (right). Two powerful underwater sound transducers with frequency tuning were used as pump wave sources; signal reception
was carried out directly in the coastal rocks

Depending on specific objectives and experimental conditions, one can apply various techniques well known in other fields (e.g., radar) for the implementation of these advantages, namely, (i) formation of a signal field from several sources, or a single moved source (coherent synthesis of the source array aperture), (ii) formation of an extended receiving array based on one or several moved receivers (coherent synthesis of the receiving aperture), and (iii) radiation of a signal in the form of a long sequence of modulated trains of a special type by a single source, matched filtering of which in the receiving system provides a proportional increase in equivalent power of a single pulse and in spatial resolution of the system as a whole, as well as a combination of these methods. All of these approaches are almost not used in routine geophysical exploration because of the low coherence of conventional sources (surface and downhole vibrators, impact-effect sources, and explosions).
In the mid-1990s and early 2000s, the IAP RAS research team headed by V. I. Talanov performed a set of demonstration field experiments in the coastal zone of the Gorky Reservoir and in the Vladimir and Kaluga Regions. The results obtained showed for the first time the principal possibility of a significant increase in resolution and noise immunity of seismoacoustic sensing by the use of highly coherent probing signals and appropriate methods for their recording and processing (V. S. Averbakh, B. N. Bogolyubov, Yu. M. Zaslavsky, A. V. Lebedev, A. P. Maryshev, Yu. K. Postoenko, et al.).

Schematic of crosshole profiling: the left well — test
signal source; the right one — synthetic array
of vector receivers

Location of the Vorotilovskaya deep well
(shown by red color) on map
of the Nizhny Novgorod Region

Coherent processing of long signal sequences of different complex coherent signals (both frequency- and phase-modulated signals were exploited) and coherent synthesis of the receiving (or transmitting) aperture were effectively used in these studies.
The recent field experiments on coherent seismoacoustics have been performed in the IAP RAS suburban laboratory "Bezvodnoye" and at a specially-equipped site of the Vorotilovskaya deep well (Koverninsky District of the Nizhny Novgorod Region). In these works, the focus is on studying the coherent methods for the crosshole ground profiling. Unlike the conventional vertical seismic profiling schemes that are widely used in engineering and commercial seismoacoustics, the crosshole profiling scheme is much less employed, although it has a number of potential advantages. Among them are the almost completely eliminated (or significantly reduced) effect of the near-surface low-velocity area characterized by strong signal attenuation and a specific advantage provided by the use of SH-wave sources, i.e., the almost absent exchange effects of multiple wave transformations at the internal interfaces.
The crosshole profiling of SH-waves was carried out for the first time using precise phase measurements under field conditions in the late 2010s. As a result, low-contrast (a few per cent) layers in a ground structure, which cannot be detected by conventional methods because of the substantial overlap of probing pulse arrivals on a rather short path, were confidently resolved (V. S. Averbakh, A. V. Lebedev, S. A. Manakov, and V. I. Talanov).
Advanced methods for coherent seismoacoustics at small (tens of meters) depths were further developed. It turned out that a joint analysis of the phase velocity variance and the ratio of shift projections in the Rayleigh wave permits one to significantly improve the accuracy of reconstruction of the shear modulus and, which was not done before, to reconstruct the dependence of the Poisson ratio on depth. The revealed contrast variations in profiles of the Poisson ratio and shear stiffness, depending on the degree of water saturation of the ground, open up new possibilities for monitoring of the Earth's rocks under natural conditions and for solving the ecological problems (A. I. Konkov and S. A. Manakov).

Crosshole reconstruction of the very low-contrast (~ 3%) layers of the ground strusture, which corresponds to the presence of layers of loam, sandy loam, clay, and water-saturated sand (top to bottom), by the use of phase technique

Installing a seismoacoustic antenna at the site
of the suburban laboratory "Bezvodnoye"

Since the mid-2000s, the IAP RAS scientists together with colleagues from the O. Yu. Shmidt Institute of Physics of the Earth, RAS (Moscow) and the Institute of Volcanology and Seismology of the FEB RAS (Petropavlovsk-Kamchatsky) have initiated a seismoacoustic investigation of the Vorotilovskaya deep well (VDW) that is a unique object of geophysical research in the European part of Russia (the main well depth is 5374 m; the satellite well depth is 1498 m). The drilling activity in the VDW as a research well was promoted by interest in an unusual geological formation, namely, the impact ring structure, at the center of which two those wells were drilled (at the turn of the 1980s and the 1990s).
Major work in the VDW is related to the monitoring of downhole signals of seismoacoustic emission and the active diagnostics of deep rocks using coherent sources of test signals. In particular, the possibility of terrestrial rock diagnostics at depths of down to 3 km using relatively high-frequency coherent signals in a range of about 100—400 Hz was shown for the first time. The sources of probing signals were sonar transducers previously developed at the IAP RAS and installed either directly in the mouth of the well (at an immersion depth of down to 100 m) or in a local pond located at the VDW site. Reception was made by vector downhole receivers at different horizons, including those with an equidistant movement to depth by a standard profile shooting technique. The obtained results indicate the basic possibility of implementing the technique for active seismoacoustic diagnostics of consolidated Earth's rocks at the horizons of their natural occurrence down to a few kilome ters (A. I. Malekhanov, I. N. Didenkulov, A. P. Maryshev, A. A. Stromkov, A. N. Fokin, and V. V. Chernov).

Preparing the sonar transducer to lowering into the VDW
in the experiment on coherent crosshole sensing

Result of coherent integration and averaging (over 17 pulses) of the convolution of a vertical polarization signal with a replica of the radiated signal received in the satellite well of the VDW (1 — receiving horizon of 553 m) and in the main well (2 — receiving horizon of 3200 m). The radiation mode: a sequence of chirp pulses in a band of 76—96 Hz. Individual maxima correspond to arrivals of refracted wave pulses

An important advancement in the field of geophysical acoustics and seismoacoustics is also associated with the development of high-accuracy laboratory techniques for the acoustic diagnostics of samples of rocks and other materials. The main focus here is on developing the method for resonant acoustic spectroscopy (RAS), which has been substantially upgraded to be effective as applied for diagnostics of low-Q samples, whose modal resonances are much broader compared with high-Q materials (A. V. Lebedev and V. V. Bredikhin). The RAS method is currently used at the IAP RAS to solve the problems of determining the fracture parameters and diagnosing the initial stages of destruction, localizing the defects in a sample and measuring their parameters, diagnosing the nanocomposite materials (in collaboration with the G. A. Razuvaev Institute of Organometallic Chemistry of RAS), and studying the correlations of elastic anisotropy and anisotropy of the magnetic sensitivity of sedimentary and metamorphic rocks (in collaboration with the Institute of Tectonics and Geophysics of FEB RAS).
In particular, one of the latest results is high-precision measurements of the dependence of the elasticity tensors of porous materials for a varying degree of its liquid saturation. High-precision measurements enabled one to identify all the three key stages of saturation, i.e., condensation of a liquid in the pores, the formation of meniscus, and filling of pores by a liquid, which correspond to qualitative variations in acoustic characteristics of the material. These results were confirmed by direct data of granulometric, mineralogical, and chemical analyses performed by standard methods in the geological laboratory. The present-day research indicates interesting features of the relaxation processes in sedimentary rocks; namely, the existence of two different time scales of the relaxation of the shear modulus, the second scale corresponding to the time-logarithmic relaxation time with a time scale of a few days. Such a "coherent" analysis of the relaxation of the moduli of volume and shear stiffness yields new results which are important for determining the basic properties of slow dynamics of the relaxation processes in real rocks.

Instrument for precision measurements by the RAS method (left, 1 — receiver, 2 — examined sample, 3 — emitter) and surface fragment of a low-Q sample of rocks with a characteristic pore size of 0.001—1 mm (center). Inset — typical resonance curves: experimental dependence (black) and the dependence calculated using the model (grey). Comparison of the curves allows one to retrieve the Green's function and geo-acoustic parameters of the sample