rus
Issue #6/2021
G. I. Dolgikh, S. G. Dolgikh, V. V. Ovcharenko, V. A. Chupin, V. A. Shvets, S. V. Yakovenko
Features of The Use of Laser Deformographs of Classical and Pendulum Types
Features of The Use of Laser Deformographs of Classical and Pendulum Types
DOI: 10.22184/1993-7296.FRos.2021.15.6.474.483
The features of the application of laser strainmeters of the pendulum and classical types are considered on the example of recording hydroacoustic vibrations created on the shelf of the Sea of Japan by a low-frequency hydroacoustic emitter with a central frequency of 22 Hz. When analyzing the obtained experimental data, not only the ratios of the received amplitudes of seismic-acoustic vibrations by these laser strainmeters are established, but also the approximate propagation velocities of these disturbances in the upper layer of the Earth’s crust are determined.
The features of the application of laser strainmeters of the pendulum and classical types are considered on the example of recording hydroacoustic vibrations created on the shelf of the Sea of Japan by a low-frequency hydroacoustic emitter with a central frequency of 22 Hz. When analyzing the obtained experimental data, not only the ratios of the received amplitudes of seismic-acoustic vibrations by these laser strainmeters are established, but also the approximate propagation velocities of these disturbances in the upper layer of the Earth’s crust are determined.
Теги: classical-type laser strainmeter hydroacoustic signal low-frequency hydroacoustic emitter pendulum-type laser strainmeter seismic-acoustic signal гидроакустический сигнал лазерный деформограф классического типа лазерный деформограф маятникового типа низкочастотный гидроакустический излучатель сейсмоакустический сигнал.
Features of The Use of Laser Deformographs of Classical and Pendulum Types
G. I. Dolgikh, S. G. Dolgikh, V. V. Ovcharenko, V. A. Chupin, V. A. Shvets, S. V. Yakovenko
V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
The features of the application of laser strainmeters of the pendulum and classical types are considered on the example of recording hydroacoustic vibrations created on the shelf of the Sea of Japan by a low-frequency hydroacoustic emitter with a central frequency of 22 Hz. When analyzing the obtained experimental data, not only the ratios of the received amplitudes of seismic-acoustic vibrations by these laser strainmeters are established, but also the approximate propagation velocities of these disturbances in the upper layer of the Earth’s crust are determined.
Keywords: classical-type laser strainmeter, pendulum-type laser strainmeter, low-frequency hydroacoustic emitter, hydroacoustic signal, seismic-acoustic signal.
Received on: 24.09.2021
Accepted on: 08.10.2021
INTRODUCTION
In studying the nature of the emergence and development of the Earth’s deformation processes in the infrasonic and sound ranges, experimental data obtained on good equipment with high sensitivity with wide frequency and dynamic ranges are of great importance. In the infrasonic region of the spectrum, fiber, rod and wire strainmeters [1–3] were created, with the help of which the natural vibrations of the Earth were experimentally detected for the first time. In the sound range, the most common receiving systems are various seismographs created on all sorts of physical principles that have a relatively narrow frequency range, but are capable of detecting even long-period oscillations of the Earth [4]. In the last fifty years, based on Michelson and Fabry-Perot interferometers, various laser strainmeters have been developed and created, which have significantly better technical characteristics in comparison with rod, wire and quartz strainmeters [5–7]. Laser strainmeters have been created in various versions: one-dimensional and two-dimensional, equal and unequal, mobile and stationary. All of these laser strainmeters can be classified as so-called classical-type laser strainmeters. With the help of laser strainmeters of the classical type, deformation anomalies are distinguished, according to the magnitude of which, taking into account the distance to the occurred earthquakes, it is possible to determine the magnitude of the displacements of the seabed, leading to the formation of a tsunami [8]. Based on the results obtained, the deformation method for determining the tsunamigenicity of earthquakes has been substantiated [9]. The use of laser strainmeters in hydroacoustics [10] and oceanology [11] has made it possible to obtain outstanding results of a pioneering nature. All laser strainmeters of the classical type have a linear amplitude-frequency characteristic in the infrasonic and low-frequency sound ranges, but starting from some frequencies, which depends on the recorded wavelength and the length of the working arm of the laser strainmeter, in a higher frequency range their amplitude-frequency characteristic experiences beats [12]. In order to increase the sensitivity of laser strainmeters and obtain a linear amplitude-frequency response in the high-frequency range, pendulum-type laser strainmeters have been developed, the use of which in hydroacoustic studies will allow obtaining better results.
This article analyzes the results obtained during synchronous measurements by laser strainmeters of classical and pendulum types of seismoacoustic vibrations created in an elastic medium as a result of the transformation of hydroacoustic vibrations generated by a low-frequency hydroacoustic emitter at a frequency of 22 Hz.
EXPERIMENT
Experimental studies involved: a low-frequency hydroacoustic emitter with a central radiation frequency of 22 Hz, a classical-type laser strainmeter with a measuring arm length of 52.5 m, a pendulum-type laser strainmeter with a measuring arm length of 52.2 m.
A low-frequency hydroacoustic emitter is part of a 19–26 Hz radiating hydroacoustic system [13]. The emitting hydroacoustic system is designed to generate harmonic and phase-shift keyed hydroacoustic signals in a frequency band of about 1 Hz with a central frequency of the band in the range of 19–26 Hz. The amplitude of the volumetric vibrational displacements of the emitter reaches a value of 0.0123 m3. At a frequency of 20 Hz in a limitless body of water, this corresponds to a radiated acoustic power of 1000 W. The composition of the emitting hydroacoustic system includes: a radiator with an electromagnetic transducer, a frame for hanging the radiator, a cable-hose with a control pressure gauge, a power supply, an electric pump, a control hydrophone, two calibration accelerometers.
The emitter has a mass of 260 kg in air and 40 kg in water. Contains a cylindrical body and a pair of radiating pistons oscillating in mutually opposite directions and creating in-phase flows of the volumetric oscillatory velocity. Oscillations are excited by an electromagnetic converter with U-shaped typesetting halves of the core and four coils. A set of 312 cylindrical springs is clamped between the edges of the pistons, the preliminary compression of which is achieved due to a reduced pressure of 0.5 atm. air pressure in the cavity of the emitter relative to the hydrostatic pressure at the depth of its immersion. To compensate for hydrostatic pressure when submerging or lifting, a 60 m hose with a test pressure gauge and two nipples is used. The gaps between the body flanges and the piston edges are sealed with rubber-fabric collars. A battery of series-connected (in the amount of 3 to 22 pieces, depending on the required power) acid batteries with a voltage of 12 V and a capacity of 90 A*h is used as the primary sources of direct current. The power supply is a bridge key amplifier based on two half-bridge IGBT modules, equipped with a 420 µF compensating capacitor bank, a circuit breaker and a DC ammeter. In the course of the experiment, a harmonic signal was emitted at a radiation frequency of 22 Hz.
A classical-type laser strainmeter with a measuring arm length of 52.5 m is located at Cape Schultz in a hydrothermally insulated underground room at a depth of 3–5 m from the Earth’s surface. It uses a frequency stabilized helium-neon laser with short-term stability in the tenth decimal place as a light source. The measuring arm of a classical-type laser strainmeter is oriented at an angle of 18° relative to the north-south line. The main interference unit of the laser strainmeter is mounted on a concrete pedestal about 3 m long, the base of which is fixed on a rock of high density loam. The corner reflector is mounted on a concrete pedestal 1 m long, the base of which is fixed on a granite rock. All the information obtained in real time goes to the laboratory room, where, after preliminary processing, filtration and decimation, it is entered into the previously created experimental database.
A pendulum-type laser strainmeter with a measuring arm length of 52.5 m is located in the same hydrothermally insulated underground room at a depth of 3–5 m from the Earth’s surface. It uses a frequency stabilized helium-neon laser with short-term stability in the tenth decimal place as a light source. The measuring arm of the pendulum-type laser strainmeter is oriented at an angle of 18 0 relative to the north-south line. The main interference unit of the laser strainmeter is mounted on the same concrete pedestal as the classical-type laser strainmeter.
The corner reflector is mounted on a massive cube, which is part of the pendulum system. The length of the pendulum suspension is about 3 m. If we take this pendulum system for a mathematical pendulum, then the natural frequency of the pendulum system will be about 0.3 Hz. All the information received in real time goes to the laboratory room, where, after preliminary processing, filtration and decimation, is entered into the previously created experimental database.
Fig. 1 shows a map showing the scheme of the experiment, where No. 1 denotes Cape Schulz, on which the laser strainmeters are located, point No. 2 denotes the operation of the emitter at a frequency of 22 Hz at a depth of 18 m (N42°32.448, E131°02.998), point No. 3 denotes the operation of the emitter at a frequency of 22 Hz at a depth of 18 m (N42°29.657, E131°07.528).
PROCESSING AND ANALYSIS
OF THE OBTAINED EXPERIMENTAL DATA
The experimental data were placed into the experimental database with a sampling rate of 1 000 Hz. During processing, the data were filtered in order to eliminate the possible influence of powerful high-frequency components on the results of spectral processing, followed by decimation up to a sampling rate of 200 Hz. Synchronous areas of recordings of a classical-type laser strainmeter and a pendulum-type laser strainmeter were processed. Fig. 2 shows the spectra obtained during the processing of synchronous experimental data of the indicated laser strainmeters during the operation of a low-frequency hydroacoustic emitter at station 3. As can be seen from this figure, the magnitude of the received seismoacoustic signal at the radiation frequency of the hydroacoustic signal of a pendulum-type laser strainmeter is much greater (by almost an order of magnitude) than the signal value, accepted by a classical-type laser strainmeter.
In accordance with work [12], we present an equation describing the registered displacement of a classical-type laser strainmeter:
, (1)
where: u is the displacement at a point , is the projection of the wave amplitude onto the axis of the strainmeter, is the wave number, is the wavelength, is the cyclic frequency, is the frequency of the wave, is the current time, is the length of the working arm of the strainmeter, and is the coordinate of the first abutment of the strainmeter. In article [12], when editing, the second degree was lost with a sine. That is, the amplitude recorded by a classical-type laser strainmeter will be equal to:
. (2)
In accordance with Eq. (2), the recorded amplitude strongly depends on the ratio of the working arm length of a classical-type laser strainmeter and the seismoacoustic wavelength. At low frequencies, the frequency response is linear. At high frequencies, the amplitude varies from 0 to depending on the ratio of the arm length of the laser strainmeter to the recorded wavelength.
For a pendulum-type laser strainmeter, the recorded displacement can be written as:
, (3)
where , i. e. they recorded the amplitude of the wave depends on the , , and , is the quality factor of the pendulum system. The amplitude in this case can be written as:
. (4)
In the low-frequency (infrasonic) region of the spectrum, the characteristic of a pendulum-type laser strainmeter is identical to a classical-type laser strainmeter, and in the high-frequency region of the spectrum, a pendulum-type laser strainmeter is capable of measuring at all frequencies and registering the absolute wave amplitude with increasing frequency. For this pendulum-type laser strainmeter, which has a natural frequency of 0.3 Hz, the Q-factor has not been determined, but nevertheless, we can effectively process the obtained experimental data with an assessment of the sensitivity of the pendulum and classical-type laser strainmeters presented in this article.
When a low-frequency hydroacoustic emitter is operating at station 2, the ratio of the amplitudes of seismoacoustic signals at a frequency of 22 Hz hydroacoustic signals received by a classical-type laser strainmeter to the amplitudes of seismoacoustic signals received by a pendulum laser strainmeter is on average 0.15. When a low-frequency hydroacoustic emitter is operating at station 3, the ratio of the amplitudes of seismoacoustic signals at a frequency of 22 Hz hydroacoustic signals received by a classical-type laser strainmeter to the amplitudes of seismoacoustic signals received by a pendulum-type laser strainmeter is on average equal to 0.12.
Based on the experimental data obtained on a pendulum-type laser strainmeter, it is possible to determine the approximate amplitude of a seismoacoustic wave propagating in the earth’s crust as a result of the transformation of the emitted hydroacoustic signal at a frequency of 22 Hz into a seismoacoustic signal at the “water-bottom” boundary. Thus, for example, according to the graphs shown in Fig. 2, the signal amplitude recorded by a pendulum-type laser strainmeter is 2.4 · 10–3 µm. The line “station 3 – pendulum-type laser strainmeter” is oriented at an angle of 2.5° relative to the axis of the pendulum and classical strain gauges. Taking this into account, it can be argued that the amplitude of the seismoacoustic signal at the registration point is not less than 2.4 · 10–3 µm.
When the hydroacoustic emitter is operating at point 2, the magnitude of the seismoacoustic signal received by the laser strainmeter of the pendulum type at the frequency of the emitted hydroacoustic signal of 22 Hz is 1.96 · 10–3 µm. The line “station 2 – pendulum-type laser strainmeter” is oriented at an angle of 45° relative to the axis of the pendulum and classical strain gauges. Taking this into account, it can be argued that the amplitude of the seismoacoustic signal at the registration point is not less than 2.3 · 10–3 µm.
CONCLUSION
In accordance with the above, we can assert that the sensitivity of a pendulum-type laser strainmeter at a given frequency (22 Hz) is almost an order of magnitude better than the sensitivity of a classical-type laser strainmeter. In this case, the amplitude of the received seismoacoustic signal by the classical-type laser strainmeter at two stations is on average 0.15 and 0.12 of the amplitude of the pendulum-type seismoacoustic signal received by the laser strainmeter at the frequency of the emitted hydroacoustic signal (22 Hz). This relationship can be obtained when the propagation speed of a seismoacoustic wave with a frequency of 22 Hz, which is in the range of 1 250–1 300 m / s, calculated using equation (2). At the same time, we believe that the recorded seismoacoustic wave refers to a surface wave of the Rayleigh type.
Funding
The research was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (the topic of the state task is “Studying the fundamental foundations of the emergence, development, transformation and interaction of hydroacoustic, hydrophysical and geophysical fields in the World Ocean”).
REFERENCES
Starovoit O. E., Feofilaktov V. D., Shulgin L. L., Yaroshevich M. I. Quartz strainmeter of the central seismological observatory «Obninsk». Izvestiya AN SSSR. Physics of the Earth. 1971;11:85–94.
Bilham R. G. The location of Earth strain instrumentation. Phil. Trans. Roy. Soc. Lond. A. 1973; 274: 429–433.
Latynina L. A., Karmaleeva R. M. Deformographic measurements. – M.: Science. 1978. 154 p.
Petrova L. N., Linkov E. M. Spectra of long-period oscillations preceding earthquakes. Uch. Deputy LSU. L. 1978;27(392):60–66.
Bilham R. G. The location of Earth strain instrumentation. Phil. Trans. Roy. Soc. Lond. A. 1973;274:429–433.
Aleshin V. A., Dubrov M. N., Yakovlev A. P. Lazernyy interferometr dlya izmereniya deformatsiy zemnoy kory. Doklady Akademii nauk SSSR. 1980;256(6):1343–1346.
Dolgikh G. I., Kopvillem U. Kh., Pavlov A. N. Measurement of the Earth free oscillation periods with a laser strainmeter. Izvestiya Akademii Nauk SSSR. Fizika Zemli. 1983;2: 15–20.
Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Koren I. A., Ovcharenko V. V., Chupin V. A., Shvets V. A., Yakovenko S. V. Recording of deformation anomaly of a tsunamigenous earthquake using a laser strainmeter. Doklady Earth Sciences. 2007;412(1):74–76.
Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Ovcharenko V. V., Chupin V. A., Shvets V. A., Yakovenko S. V. A deformation method of tsunamigenic earthquakes definition. Doklady Earth Sciences. 2007;417(1):1261–1264.
Davydov A. V., Dolgikh G. I., Kabanov N. F. Hydroacoustic applications of laser deformographs. Acoustical physics. 1995; 41(2):201–204.
Alekseev A. V., Valentin D. I., Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Koren I. A., Ovcharenko V. V., Kholodkevich E. D., Shvets V. A., Yakovenko S. V. Registration of infragravity waves at the hydrosphere-lithosphere boundary using coastal laser strainmeter. Doklady Earth Sciences. 2003;389(2):291–293.
Dolgikh G. I. Principles of designing single-coordinate laser strainmeters. Technical Physics Letters. 2011;37(3):204–206.
Dolgikh G. I., Dolgikh S. G., Pivovarov A. A., Samchenko A. N., Shvyrev A. N., Chupin V. A., Yakovenko S. V. and Yaroshchuk I. O. A Hydroacoustic System that Radiates at Frequencies of 19–26 Hz. Instruments and Experimental Techniques. 2017;4:596–600.
Information about the authors
Grigory Ivanovich Dolgikh, Doctor of Physical and Mathematical Sciences, Academician of the Russian Academy of Sciences, dolgikh@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0002-2806-3834
Stanislav Grigorievich Dolgikh, Cand. of Physical and Mathematical Sciences, sdolgikh@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-9828-5929
Vladimir Vladimirovich Ovcharenko, Cand. of Physical and Mathematical Sciences, ovcharenko@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-7784-2140
Vladimir Alexandrovich Chupin, Cand. of Physical and Mathematical Sciences, chupin@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-5103-8138
Vyacheslav Aleksandrovich Shvets, Cand. of Technical Sciences, vshv@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0002-4752-6865
Sergey Vladimirovich Yakovenko, Cand. of Technical Sciences, ser_mail@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-8324-3849
G. I. Dolgikh, S. G. Dolgikh, V. V. Ovcharenko, V. A. Chupin, V. A. Shvets, S. V. Yakovenko
V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
The features of the application of laser strainmeters of the pendulum and classical types are considered on the example of recording hydroacoustic vibrations created on the shelf of the Sea of Japan by a low-frequency hydroacoustic emitter with a central frequency of 22 Hz. When analyzing the obtained experimental data, not only the ratios of the received amplitudes of seismic-acoustic vibrations by these laser strainmeters are established, but also the approximate propagation velocities of these disturbances in the upper layer of the Earth’s crust are determined.
Keywords: classical-type laser strainmeter, pendulum-type laser strainmeter, low-frequency hydroacoustic emitter, hydroacoustic signal, seismic-acoustic signal.
Received on: 24.09.2021
Accepted on: 08.10.2021
INTRODUCTION
In studying the nature of the emergence and development of the Earth’s deformation processes in the infrasonic and sound ranges, experimental data obtained on good equipment with high sensitivity with wide frequency and dynamic ranges are of great importance. In the infrasonic region of the spectrum, fiber, rod and wire strainmeters [1–3] were created, with the help of which the natural vibrations of the Earth were experimentally detected for the first time. In the sound range, the most common receiving systems are various seismographs created on all sorts of physical principles that have a relatively narrow frequency range, but are capable of detecting even long-period oscillations of the Earth [4]. In the last fifty years, based on Michelson and Fabry-Perot interferometers, various laser strainmeters have been developed and created, which have significantly better technical characteristics in comparison with rod, wire and quartz strainmeters [5–7]. Laser strainmeters have been created in various versions: one-dimensional and two-dimensional, equal and unequal, mobile and stationary. All of these laser strainmeters can be classified as so-called classical-type laser strainmeters. With the help of laser strainmeters of the classical type, deformation anomalies are distinguished, according to the magnitude of which, taking into account the distance to the occurred earthquakes, it is possible to determine the magnitude of the displacements of the seabed, leading to the formation of a tsunami [8]. Based on the results obtained, the deformation method for determining the tsunamigenicity of earthquakes has been substantiated [9]. The use of laser strainmeters in hydroacoustics [10] and oceanology [11] has made it possible to obtain outstanding results of a pioneering nature. All laser strainmeters of the classical type have a linear amplitude-frequency characteristic in the infrasonic and low-frequency sound ranges, but starting from some frequencies, which depends on the recorded wavelength and the length of the working arm of the laser strainmeter, in a higher frequency range their amplitude-frequency characteristic experiences beats [12]. In order to increase the sensitivity of laser strainmeters and obtain a linear amplitude-frequency response in the high-frequency range, pendulum-type laser strainmeters have been developed, the use of which in hydroacoustic studies will allow obtaining better results.
This article analyzes the results obtained during synchronous measurements by laser strainmeters of classical and pendulum types of seismoacoustic vibrations created in an elastic medium as a result of the transformation of hydroacoustic vibrations generated by a low-frequency hydroacoustic emitter at a frequency of 22 Hz.
EXPERIMENT
Experimental studies involved: a low-frequency hydroacoustic emitter with a central radiation frequency of 22 Hz, a classical-type laser strainmeter with a measuring arm length of 52.5 m, a pendulum-type laser strainmeter with a measuring arm length of 52.2 m.
A low-frequency hydroacoustic emitter is part of a 19–26 Hz radiating hydroacoustic system [13]. The emitting hydroacoustic system is designed to generate harmonic and phase-shift keyed hydroacoustic signals in a frequency band of about 1 Hz with a central frequency of the band in the range of 19–26 Hz. The amplitude of the volumetric vibrational displacements of the emitter reaches a value of 0.0123 m3. At a frequency of 20 Hz in a limitless body of water, this corresponds to a radiated acoustic power of 1000 W. The composition of the emitting hydroacoustic system includes: a radiator with an electromagnetic transducer, a frame for hanging the radiator, a cable-hose with a control pressure gauge, a power supply, an electric pump, a control hydrophone, two calibration accelerometers.
The emitter has a mass of 260 kg in air and 40 kg in water. Contains a cylindrical body and a pair of radiating pistons oscillating in mutually opposite directions and creating in-phase flows of the volumetric oscillatory velocity. Oscillations are excited by an electromagnetic converter with U-shaped typesetting halves of the core and four coils. A set of 312 cylindrical springs is clamped between the edges of the pistons, the preliminary compression of which is achieved due to a reduced pressure of 0.5 atm. air pressure in the cavity of the emitter relative to the hydrostatic pressure at the depth of its immersion. To compensate for hydrostatic pressure when submerging or lifting, a 60 m hose with a test pressure gauge and two nipples is used. The gaps between the body flanges and the piston edges are sealed with rubber-fabric collars. A battery of series-connected (in the amount of 3 to 22 pieces, depending on the required power) acid batteries with a voltage of 12 V and a capacity of 90 A*h is used as the primary sources of direct current. The power supply is a bridge key amplifier based on two half-bridge IGBT modules, equipped with a 420 µF compensating capacitor bank, a circuit breaker and a DC ammeter. In the course of the experiment, a harmonic signal was emitted at a radiation frequency of 22 Hz.
A classical-type laser strainmeter with a measuring arm length of 52.5 m is located at Cape Schultz in a hydrothermally insulated underground room at a depth of 3–5 m from the Earth’s surface. It uses a frequency stabilized helium-neon laser with short-term stability in the tenth decimal place as a light source. The measuring arm of a classical-type laser strainmeter is oriented at an angle of 18° relative to the north-south line. The main interference unit of the laser strainmeter is mounted on a concrete pedestal about 3 m long, the base of which is fixed on a rock of high density loam. The corner reflector is mounted on a concrete pedestal 1 m long, the base of which is fixed on a granite rock. All the information obtained in real time goes to the laboratory room, where, after preliminary processing, filtration and decimation, it is entered into the previously created experimental database.
A pendulum-type laser strainmeter with a measuring arm length of 52.5 m is located in the same hydrothermally insulated underground room at a depth of 3–5 m from the Earth’s surface. It uses a frequency stabilized helium-neon laser with short-term stability in the tenth decimal place as a light source. The measuring arm of the pendulum-type laser strainmeter is oriented at an angle of 18 0 relative to the north-south line. The main interference unit of the laser strainmeter is mounted on the same concrete pedestal as the classical-type laser strainmeter.
The corner reflector is mounted on a massive cube, which is part of the pendulum system. The length of the pendulum suspension is about 3 m. If we take this pendulum system for a mathematical pendulum, then the natural frequency of the pendulum system will be about 0.3 Hz. All the information received in real time goes to the laboratory room, where, after preliminary processing, filtration and decimation, is entered into the previously created experimental database.
Fig. 1 shows a map showing the scheme of the experiment, where No. 1 denotes Cape Schulz, on which the laser strainmeters are located, point No. 2 denotes the operation of the emitter at a frequency of 22 Hz at a depth of 18 m (N42°32.448, E131°02.998), point No. 3 denotes the operation of the emitter at a frequency of 22 Hz at a depth of 18 m (N42°29.657, E131°07.528).
PROCESSING AND ANALYSIS
OF THE OBTAINED EXPERIMENTAL DATA
The experimental data were placed into the experimental database with a sampling rate of 1 000 Hz. During processing, the data were filtered in order to eliminate the possible influence of powerful high-frequency components on the results of spectral processing, followed by decimation up to a sampling rate of 200 Hz. Synchronous areas of recordings of a classical-type laser strainmeter and a pendulum-type laser strainmeter were processed. Fig. 2 shows the spectra obtained during the processing of synchronous experimental data of the indicated laser strainmeters during the operation of a low-frequency hydroacoustic emitter at station 3. As can be seen from this figure, the magnitude of the received seismoacoustic signal at the radiation frequency of the hydroacoustic signal of a pendulum-type laser strainmeter is much greater (by almost an order of magnitude) than the signal value, accepted by a classical-type laser strainmeter.
In accordance with work [12], we present an equation describing the registered displacement of a classical-type laser strainmeter:
, (1)
where: u is the displacement at a point , is the projection of the wave amplitude onto the axis of the strainmeter, is the wave number, is the wavelength, is the cyclic frequency, is the frequency of the wave, is the current time, is the length of the working arm of the strainmeter, and is the coordinate of the first abutment of the strainmeter. In article [12], when editing, the second degree was lost with a sine. That is, the amplitude recorded by a classical-type laser strainmeter will be equal to:
. (2)
In accordance with Eq. (2), the recorded amplitude strongly depends on the ratio of the working arm length of a classical-type laser strainmeter and the seismoacoustic wavelength. At low frequencies, the frequency response is linear. At high frequencies, the amplitude varies from 0 to depending on the ratio of the arm length of the laser strainmeter to the recorded wavelength.
For a pendulum-type laser strainmeter, the recorded displacement can be written as:
, (3)
where , i. e. they recorded the amplitude of the wave depends on the , , and , is the quality factor of the pendulum system. The amplitude in this case can be written as:
. (4)
In the low-frequency (infrasonic) region of the spectrum, the characteristic of a pendulum-type laser strainmeter is identical to a classical-type laser strainmeter, and in the high-frequency region of the spectrum, a pendulum-type laser strainmeter is capable of measuring at all frequencies and registering the absolute wave amplitude with increasing frequency. For this pendulum-type laser strainmeter, which has a natural frequency of 0.3 Hz, the Q-factor has not been determined, but nevertheless, we can effectively process the obtained experimental data with an assessment of the sensitivity of the pendulum and classical-type laser strainmeters presented in this article.
When a low-frequency hydroacoustic emitter is operating at station 2, the ratio of the amplitudes of seismoacoustic signals at a frequency of 22 Hz hydroacoustic signals received by a classical-type laser strainmeter to the amplitudes of seismoacoustic signals received by a pendulum laser strainmeter is on average 0.15. When a low-frequency hydroacoustic emitter is operating at station 3, the ratio of the amplitudes of seismoacoustic signals at a frequency of 22 Hz hydroacoustic signals received by a classical-type laser strainmeter to the amplitudes of seismoacoustic signals received by a pendulum-type laser strainmeter is on average equal to 0.12.
Based on the experimental data obtained on a pendulum-type laser strainmeter, it is possible to determine the approximate amplitude of a seismoacoustic wave propagating in the earth’s crust as a result of the transformation of the emitted hydroacoustic signal at a frequency of 22 Hz into a seismoacoustic signal at the “water-bottom” boundary. Thus, for example, according to the graphs shown in Fig. 2, the signal amplitude recorded by a pendulum-type laser strainmeter is 2.4 · 10–3 µm. The line “station 3 – pendulum-type laser strainmeter” is oriented at an angle of 2.5° relative to the axis of the pendulum and classical strain gauges. Taking this into account, it can be argued that the amplitude of the seismoacoustic signal at the registration point is not less than 2.4 · 10–3 µm.
When the hydroacoustic emitter is operating at point 2, the magnitude of the seismoacoustic signal received by the laser strainmeter of the pendulum type at the frequency of the emitted hydroacoustic signal of 22 Hz is 1.96 · 10–3 µm. The line “station 2 – pendulum-type laser strainmeter” is oriented at an angle of 45° relative to the axis of the pendulum and classical strain gauges. Taking this into account, it can be argued that the amplitude of the seismoacoustic signal at the registration point is not less than 2.3 · 10–3 µm.
CONCLUSION
In accordance with the above, we can assert that the sensitivity of a pendulum-type laser strainmeter at a given frequency (22 Hz) is almost an order of magnitude better than the sensitivity of a classical-type laser strainmeter. In this case, the amplitude of the received seismoacoustic signal by the classical-type laser strainmeter at two stations is on average 0.15 and 0.12 of the amplitude of the pendulum-type seismoacoustic signal received by the laser strainmeter at the frequency of the emitted hydroacoustic signal (22 Hz). This relationship can be obtained when the propagation speed of a seismoacoustic wave with a frequency of 22 Hz, which is in the range of 1 250–1 300 m / s, calculated using equation (2). At the same time, we believe that the recorded seismoacoustic wave refers to a surface wave of the Rayleigh type.
Funding
The research was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (the topic of the state task is “Studying the fundamental foundations of the emergence, development, transformation and interaction of hydroacoustic, hydrophysical and geophysical fields in the World Ocean”).
REFERENCES
Starovoit O. E., Feofilaktov V. D., Shulgin L. L., Yaroshevich M. I. Quartz strainmeter of the central seismological observatory «Obninsk». Izvestiya AN SSSR. Physics of the Earth. 1971;11:85–94.
Bilham R. G. The location of Earth strain instrumentation. Phil. Trans. Roy. Soc. Lond. A. 1973; 274: 429–433.
Latynina L. A., Karmaleeva R. M. Deformographic measurements. – M.: Science. 1978. 154 p.
Petrova L. N., Linkov E. M. Spectra of long-period oscillations preceding earthquakes. Uch. Deputy LSU. L. 1978;27(392):60–66.
Bilham R. G. The location of Earth strain instrumentation. Phil. Trans. Roy. Soc. Lond. A. 1973;274:429–433.
Aleshin V. A., Dubrov M. N., Yakovlev A. P. Lazernyy interferometr dlya izmereniya deformatsiy zemnoy kory. Doklady Akademii nauk SSSR. 1980;256(6):1343–1346.
Dolgikh G. I., Kopvillem U. Kh., Pavlov A. N. Measurement of the Earth free oscillation periods with a laser strainmeter. Izvestiya Akademii Nauk SSSR. Fizika Zemli. 1983;2: 15–20.
Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Koren I. A., Ovcharenko V. V., Chupin V. A., Shvets V. A., Yakovenko S. V. Recording of deformation anomaly of a tsunamigenous earthquake using a laser strainmeter. Doklady Earth Sciences. 2007;412(1):74–76.
Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Ovcharenko V. V., Chupin V. A., Shvets V. A., Yakovenko S. V. A deformation method of tsunamigenic earthquakes definition. Doklady Earth Sciences. 2007;417(1):1261–1264.
Davydov A. V., Dolgikh G. I., Kabanov N. F. Hydroacoustic applications of laser deformographs. Acoustical physics. 1995; 41(2):201–204.
Alekseev A. V., Valentin D. I., Dolgikh G. I., Dolgikh S. G., Kovalev S. N., Koren I. A., Ovcharenko V. V., Kholodkevich E. D., Shvets V. A., Yakovenko S. V. Registration of infragravity waves at the hydrosphere-lithosphere boundary using coastal laser strainmeter. Doklady Earth Sciences. 2003;389(2):291–293.
Dolgikh G. I. Principles of designing single-coordinate laser strainmeters. Technical Physics Letters. 2011;37(3):204–206.
Dolgikh G. I., Dolgikh S. G., Pivovarov A. A., Samchenko A. N., Shvyrev A. N., Chupin V. A., Yakovenko S. V. and Yaroshchuk I. O. A Hydroacoustic System that Radiates at Frequencies of 19–26 Hz. Instruments and Experimental Techniques. 2017;4:596–600.
Information about the authors
Grigory Ivanovich Dolgikh, Doctor of Physical and Mathematical Sciences, Academician of the Russian Academy of Sciences, dolgikh@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0002-2806-3834
Stanislav Grigorievich Dolgikh, Cand. of Physical and Mathematical Sciences, sdolgikh@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-9828-5929
Vladimir Vladimirovich Ovcharenko, Cand. of Physical and Mathematical Sciences, ovcharenko@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-7784-2140
Vladimir Alexandrovich Chupin, Cand. of Physical and Mathematical Sciences, chupin@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-5103-8138
Vyacheslav Aleksandrovich Shvets, Cand. of Technical Sciences, vshv@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0002-4752-6865
Sergey Vladimirovich Yakovenko, Cand. of Technical Sciences, ser_mail@poi.dvo.ru; V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia.
ORCID: 0000-0001-8324-3849
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