A. S. Belyakov

Magnetoelastic Sensors and Geophones for Vector Measurements in Geoacoustics

The process of earthquake origination is associated with the action of intrinsic forces of both natural and artificial origin, which substantially change the stress fields in the Earth’s crust. These slow movements are accompanied by acoustic noise (acoustic emission). Broadband measurements of acoustic emission from naturally deposited rocks is an effective tool for an instrumental monitoring of the Earth’s crust that is aimed at earthquake forecasting. Considerable advances in this direction have become possible with the development of a new type of acoustic sensors with a velaccelerometric characteristic, for which the sensitivity increases by three orders of magnitude when the frequency increases tenfold. In geoacoustic observation systems, this makes it possible to considerably expand the amplitude-frequency range of investigation and creates new opportunities for a detailed analysis of the earthquake origination process. The results of observations of high-frequency underground acoustic noise, which were carried out in various regions of the Earth, in wells and edits, with the use of hardware/software systems containing new broadband magnetoelastic acoustic geophones, have confirmed experimentally its relation to slow deformations in the Earth’s crust. It turns out that underground hum in the frequency range from 16 to 2000 Hz contains new independent information on changes in the stressed state and is a sensitive indicator of tectonic and tidal movements. Practical investigations performed by researchers from the Institute of Volcanology, Far East Division of the Russian Academy of Sciences, in Petropavlovsk-Kamchatski have convincingly proved that acoustic noise variation may serve as a reliable seismic alert. The present publication is devoted to the history of the development and application of magnetoelastic geophones.

Geoacoustic monitoring as a means for investigating the state of the lithosphere and for earthquake forecasting

The existing approach to mitigating earthquakes and other catastrophic geological processes stipulates the determination of the site, time, and strength of the anticipated event. However, the disastrous earthquakes of recent years clearly demonstrate that even the most advanced systems of monitoring and warning for seismic and other geological hazards are insufficiently effective. However, by adding the monitoring of the Earth’s acoustic field to the existing systems of observation of so-called precursors and by restricting the observations to a certain region (thus excluding the task of determining the site), it is possible to provide a more adequate forecast of earthquakes and other catastrophic geological processes in this particular region.

Instrumental study of high-frequency seismoacoustic signal modulation in a deep drillhole

For many years we have failed to record instrumentally the background variations in seismoacoustic emission in solid consolidated rocks of the Earth’s crust at great depths (3200 m) and to confirm their relation to lunar-solar gravitation. We managed to fulfill the task only with significantly improved parameters of the recording system measurement channel. As follows from analysis of new data, the amplitudes in the diurnal cycle vary from 1930 to 2100 fm (10−15) for the band of 160 Hz and from 129.5 to 132 fm for the band of 500 Hz, while the energy model correlation coefficients are within 0.737–0.852. Hence, this fact proves once again that background variations related to lunar-solar tides are universal in occurrence, and their frequency and amplitude depend significantly on the host rocks, position, and time of observations, while the energy model fits in these variations.

What happens in the Earth’s crust between sunset and sunrise?

673 The overwhelming majority of processes on the Earth have a diurnal cycle, which is related in one way or another to the Sun. In any case, if not the processes, then their time variability is related to the rhythm of the interaction between the Earth and the Sun. There fore, regardless of the widely spread opinion about the dominating influence of the Moon on the hydroa coustic processes in the Earth’s crust, we studied, first of all, the correlation of the seismoacoustic processes in the upper part of the Earth’s crust with the influence of the Sun. It was found that the deformation pro cesses in the Earth’s crust related to the Sun’s gravita What Happens in the Earth’s Crust between Sunset and Sunrise?

Synchronization of seismoacoustic emission with deformation of the earth's upper crust

Long-term geoacoustic observations revealed an explicit synchronization in the Earth’s upper crust between the amplitude of acoustic noise at a frequency of 28‐35 Hz, which is related to seismoacoustic emission (SAE), and the calculated value (a squared rate of variation in the volume strain of the earth’s crust due to solar gravitation [1]). A joint study of these two processes, the observed SAE, and the calculated tidal deformation established that the average background seismic noise has a relatively high correlation coefficient (0.6‐0.8) with a solar component of tidal deformation (SD), but lacks stable correlation with the lunar component of tidal deformation (LD) or total deformation due to lunar‐solar tide (LSD). We found out previously that variation in SAE intensity within the mentioned frequency band at a depth of about 2000 m has good agreement (in terms of amplitude and phase) with the rate of variation in lunar‐solar disturbance of gravity [2]. As a result of a posteriori detailed analysis of data obtained in 1988 in a borehole at a depth of 390 m [3], the SAE intensity was found to have the best correlation with the sum of squared rates of variations in volume strain due to solar and lunar gravitation components (LD + SD), i.e., with the total power of the Earth’s tide. It is evident from the cited data that correlation coefficients of SAE intensity and calculated power of individual components of the tidal process in the Earth’s crust are rather significant for LD and SD components, but the correlation coefficient with the squared rate of deformation due to total lunar‐solar gravitation (LSD) yields negative values. This is evident from the following correlation coefficients between SAE and components, as follows:

New evidence of acoustic effects of radiation impacts in deep wells

Passive seismic observations were performed in the Vorotolovskaya deep well in Nizhny Novgorod oblast at a depth of 1400 m and in a well in the vicinity of Ufa at a depth of 500 m. Observations show significantly different acoustic impulse fluxes in these wells: hundreds per second in the Vorotilovskaya deep well and only few impulses per day in the Ufa well. It is supposed that it is the geological conditions that determine the character of seismoacoustic signals. A seismic observation technique is proposed for further studies of acoustic manifestations of radiation fields.

Study of acoustic noise on Valaam Island

A long-term endogenous nanomicroseismic event was registered instrumentally for the first time on Valaam Island (Lake Ladoga) in the winter of 2012. In the past, similar events in the form of acoustic emissions of separate phases have occurred from time to time and were heard by many residents and visitors to the island. This emission has long been known as barantida. The identification of the signal was based on a comprehensive analysis of the obtained seismograms and the subjective descriptions of this rare phenomenon by witnesses. It is proposed to continue instrumental observations in the mode of continuous high resolution seismoacoustic monitoring.

Joint analysis of the seismic data and velocity gravity model

We performed joint analysis of the seismic noises recorded at the Japanese Ogasawara station located on Titijima Island in the Philippine Sea using the STS–2 seismograph at the OSW station in the winter period of January 1–15, 2015, over the background of a velocity gravity model. The graphs prove the existence of a cause-and-effect relation between the seismic noise and gravity and allow us to consider it as a desired signal.

Investigation of the fine structure of weak seismoacoustic signals in the Earth’s crust

974 Our geoacoustic investigations were carried out in the central part of the East European Platform using an instrument with unique sensor characteristics [1] mounted in an uncased borehole at a depth of 508 m, which revealed the bedrock. The frequency character istic of the sensor allowed us to record reliably the acoustic oscillations in the frequency band from 1 to 1000 Hz. The internal noises of the recording channel within this band reduced to the amplitudes of the mea sured displacements were significantly (by 1–3 orders of magnitude) smaller than the measured amplitudes. In turn, this allowed us to measure the background sig nals of telluric origin, which previously could not be recorded.