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The results of seismic studies, which allow speaking of high productivity of mobile laser deformograph are given below.
INTRODUCTION
Currently, different interferometers are used at V.I.Il’ichev Pacific Oceanological Institute of Far Eastern Branch of the Russian Academy of Sciences (POI FEB RAS) for studies in order to measure low-frequency and super low-frequency variations of microdisplacements of background level [1, 2]. However, most of them are bulky, complex in installation and they require constant control during operation [3]. In order to eliminate these problems, the mobile laser deformograph based on unequal-arm Michelson interferometer was designed using up-to-date laser-interferometer methods [4]. The device includes electronic recording system, which records the variation of difference of interferometer arm lengths. The task of deformograph consists in the operation mobility with retaining of high measurement precision of low-frequency and super low-frequency variations of microdisplacements of background level. The device is distinguished by its compactness, easy adjustment, reliability, capability of autonomous operation during long periods of time with low energy consumption and low cost.
ARRANGEMENT OF MOBILE LASER DEFORMOGRAPH
Optical part of mobile laser deformograph is developed on the basis of unequal-arm Michelson interferometer, and it is similar to the relevant unit of deformograph designed earlier [5, 6]. Frequency-stabilized laser produced by Melles Griot is used in the capacity of light source; its long-term stability is equal to 10–9–10–10. The whole travel path of laser beam between the reflector and interference unit is located in light guide of polypropylene pipes. The reflector is mounted on elastic foundation. Polarized laser beam, which is later divided into two beams, is used for the measurements. One of the divided beams propagates along the measured arm, and the second beam propagates along the reference arm. The measuring part of beam propagates in light guide, gets on reflector and returns back to semitransparent plate. The reference part of beam propagates through the adjustment unit and returns back to semitransparent plate. Adjustment unit consists of the mirrors fixed on piezoceramic bedding. Then, these two beams are combined and the interference pattern is obtained; its variations are estimated using the photoelectronic equipment. The spot with interference pattern is analyzed using photodiode of recording system. The task of this device consists in recording of variations of microdeformations of earth crust within the frequency range from 0 (provisionally) to 10 000 Hz with the precision of 0.3 nm within practically unlimited range.
USE OF MOBILE LASER DEFORMOGRAPH
In order to measure the background level of geophysical noises and determine the main parameters of resonance objects within audible and infra-audible ranges the mobile laser deformograph was installed near Nakhodka. In Fig. 1 the service map "Planet Earth" is given with the point of device installation. Coordinates of the installation of deformograph measuring bench – 42°52,560′N, 132°48,643′E. Coordinates of reflector – 42 °52,562′N, 132°48,639′E.
Deformograph was installed on the surface of Earth with the penetration of optical bench bedding down to hard rock. Device construction is isolated from the environmental impact. The general operational view of device is given in Fig. 2. The digital system of recording was built-in into the deformograph; this system was developed by POI FEB RAS. This module refers to the systems of optimizing control and operates the interferometers supporting the interference pattern in intensity maximum with the help of piezoceramic elements in the feedback circuit. When reaching the limit of dynamic range, which is stipulated by the cyclic change of extremums of interference pattern, recording system generates the reset pulse reducing to zero the voltage, which is supplied to piezoceramics. Thus, the extension of device dynamic range is achieved. The value of voltage supplied to piezoceramics and number of reset pulses at certain moment of time correspond to the variation of optical path difference in interferometer arms. At the same time, the path difference has linear correlation with the displacement of earth crust. This information is sent to computer in digital form through communication line and recorded to hard disk with the set sampling frequency.
The principle of measurement of path difference on the basis of laser deformographs is the same as in deformographs, which provide similar or better sensitivity. Good stability of temperature, pressure, humidity, taking into account their variations in measurements, short range of voltages supplied to piezoceramics (up to 198 V) allow eliminating the measurement errors, which occur due to such parasitics as piezoceramics creep and hysteresis during the deformograph operation within the range from 0 (provisionally) to 1000 hz.
In the course of works, the record of earth crust microdeformations with the duration of more than 3 days was made. The record fragment of laser deformograph with the duration of about 14 h is given in Fig. 3.
The microseismic vibrations with the periods of about 8, 10 and 12 sec. were allocated from the record (Fig. 4); they correspond to the wind-driven waves of the Sea of Japan in Vostok Bay. During the data accumulation there was a cyclone accompanied by strong wind and rain above the place of laser deformograph installation. During this time the vibrations with the period of 22.5 sec. were recorded by laser deformograph; these vibrations were caused by heavy wind pressure.
When carrying out the experiment with dynamic impacts of "stroke on ground" type in the area of measurements, the records of the relevant vibration processes were obtained. The experiment was carried out with the impact loads of two types: repeated strokes (5 strokes with the pause of about 1 sec. at the distances of 30–100 m) and hard single strokes on the ground in the places of hard rock exposure. In order to eliminate the vibration reaction of components of measuring device to the impacts, the single strokes were performed at the distance of more than 100 m from equipment. The example of record area where the results of experiment with five-time repetition are reflected is given in Fig. 5. Each impact represents the process which damps in time.
Calculations of quality factor for repeated impacts are given in the table. Analysis of the central frequency of damped process vibrations showed that it coincides with the vibrations frequency stimulated during natural processes, on other words at 29.3 Hz, and it indicates the excitation of the same resonator.
CONCLUSION
Mobile laser deformograph is designed on the basis of modern laser-interference methods; it operated well during seismic studies. During the studies, which were performed near Nakhodka, the natural and stimulated processes of audible and infra-audible ranges at background level were recorded.
The works were performed with partial support of the Russian Science Foundation (agreement 14-17-00041).
Currently, different interferometers are used at V.I.Il’ichev Pacific Oceanological Institute of Far Eastern Branch of the Russian Academy of Sciences (POI FEB RAS) for studies in order to measure low-frequency and super low-frequency variations of microdisplacements of background level [1, 2]. However, most of them are bulky, complex in installation and they require constant control during operation [3]. In order to eliminate these problems, the mobile laser deformograph based on unequal-arm Michelson interferometer was designed using up-to-date laser-interferometer methods [4]. The device includes electronic recording system, which records the variation of difference of interferometer arm lengths. The task of deformograph consists in the operation mobility with retaining of high measurement precision of low-frequency and super low-frequency variations of microdisplacements of background level. The device is distinguished by its compactness, easy adjustment, reliability, capability of autonomous operation during long periods of time with low energy consumption and low cost.
ARRANGEMENT OF MOBILE LASER DEFORMOGRAPH
Optical part of mobile laser deformograph is developed on the basis of unequal-arm Michelson interferometer, and it is similar to the relevant unit of deformograph designed earlier [5, 6]. Frequency-stabilized laser produced by Melles Griot is used in the capacity of light source; its long-term stability is equal to 10–9–10–10. The whole travel path of laser beam between the reflector and interference unit is located in light guide of polypropylene pipes. The reflector is mounted on elastic foundation. Polarized laser beam, which is later divided into two beams, is used for the measurements. One of the divided beams propagates along the measured arm, and the second beam propagates along the reference arm. The measuring part of beam propagates in light guide, gets on reflector and returns back to semitransparent plate. The reference part of beam propagates through the adjustment unit and returns back to semitransparent plate. Adjustment unit consists of the mirrors fixed on piezoceramic bedding. Then, these two beams are combined and the interference pattern is obtained; its variations are estimated using the photoelectronic equipment. The spot with interference pattern is analyzed using photodiode of recording system. The task of this device consists in recording of variations of microdeformations of earth crust within the frequency range from 0 (provisionally) to 10 000 Hz with the precision of 0.3 nm within practically unlimited range.
USE OF MOBILE LASER DEFORMOGRAPH
In order to measure the background level of geophysical noises and determine the main parameters of resonance objects within audible and infra-audible ranges the mobile laser deformograph was installed near Nakhodka. In Fig. 1 the service map "Planet Earth" is given with the point of device installation. Coordinates of the installation of deformograph measuring bench – 42°52,560′N, 132°48,643′E. Coordinates of reflector – 42 °52,562′N, 132°48,639′E.
Deformograph was installed on the surface of Earth with the penetration of optical bench bedding down to hard rock. Device construction is isolated from the environmental impact. The general operational view of device is given in Fig. 2. The digital system of recording was built-in into the deformograph; this system was developed by POI FEB RAS. This module refers to the systems of optimizing control and operates the interferometers supporting the interference pattern in intensity maximum with the help of piezoceramic elements in the feedback circuit. When reaching the limit of dynamic range, which is stipulated by the cyclic change of extremums of interference pattern, recording system generates the reset pulse reducing to zero the voltage, which is supplied to piezoceramics. Thus, the extension of device dynamic range is achieved. The value of voltage supplied to piezoceramics and number of reset pulses at certain moment of time correspond to the variation of optical path difference in interferometer arms. At the same time, the path difference has linear correlation with the displacement of earth crust. This information is sent to computer in digital form through communication line and recorded to hard disk with the set sampling frequency.
The principle of measurement of path difference on the basis of laser deformographs is the same as in deformographs, which provide similar or better sensitivity. Good stability of temperature, pressure, humidity, taking into account their variations in measurements, short range of voltages supplied to piezoceramics (up to 198 V) allow eliminating the measurement errors, which occur due to such parasitics as piezoceramics creep and hysteresis during the deformograph operation within the range from 0 (provisionally) to 1000 hz.
In the course of works, the record of earth crust microdeformations with the duration of more than 3 days was made. The record fragment of laser deformograph with the duration of about 14 h is given in Fig. 3.
The microseismic vibrations with the periods of about 8, 10 and 12 sec. were allocated from the record (Fig. 4); they correspond to the wind-driven waves of the Sea of Japan in Vostok Bay. During the data accumulation there was a cyclone accompanied by strong wind and rain above the place of laser deformograph installation. During this time the vibrations with the period of 22.5 sec. were recorded by laser deformograph; these vibrations were caused by heavy wind pressure.
When carrying out the experiment with dynamic impacts of "stroke on ground" type in the area of measurements, the records of the relevant vibration processes were obtained. The experiment was carried out with the impact loads of two types: repeated strokes (5 strokes with the pause of about 1 sec. at the distances of 30–100 m) and hard single strokes on the ground in the places of hard rock exposure. In order to eliminate the vibration reaction of components of measuring device to the impacts, the single strokes were performed at the distance of more than 100 m from equipment. The example of record area where the results of experiment with five-time repetition are reflected is given in Fig. 5. Each impact represents the process which damps in time.
Calculations of quality factor for repeated impacts are given in the table. Analysis of the central frequency of damped process vibrations showed that it coincides with the vibrations frequency stimulated during natural processes, on other words at 29.3 Hz, and it indicates the excitation of the same resonator.
CONCLUSION
Mobile laser deformograph is designed on the basis of modern laser-interference methods; it operated well during seismic studies. During the studies, which were performed near Nakhodka, the natural and stimulated processes of audible and infra-audible ranges at background level were recorded.
The works were performed with partial support of the Russian Science Foundation (agreement 14-17-00041).
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