Features Of Application Of Mobile Laser Strainmeter In Winter Conditions
In connection with the need for the exploration of the Arctic, the scientists are faced with the tasks of various directions in the implementation of a number of fundamental studies with further output of the results obtained in the applied field with the development of technologies and techniques for the exploration of Arctic resources. One of the topical tasks is the task of studying the structure and composition of the sea crust of both shelf and deep-water ice-covered Arctic areas. Nowadays, active and passive acoustic methods are more suitable for these purposes [1–7]. Active acoustic methods are focused on the use of low-frequency hydroacoustic radiators, capable of generating signals of varying complexity in water. Unlike the use of air guns, explosive sources and streamers, the use of hydroacoustic radiators is the most environmentally friendly for the environment and biota. Nowadays, the receiving systems in one of the main technologies for studying the structure and composition of the sea crust for the purpose of searching for minerals are arranged in the form of sensors distributed under the sea. Such arrangement of receiving systems is a complex task for ice-covered water areas, especially without its pre-destruction. To overcome this difficulty, the receiving systems can be located in ice  or on the shore [2–4].
In papers [2–4], stationary laser strainmeters located on the shore of the Schulz Cape in the Sea of Japan are used as receiving systems. Laser strainmeters are created according to the Michelson interferometer scheme using frequency-stabilized helium-neon lasers as a radiation source. The actuating arms of laser strainmeters have lengths of 52.5 and 17.5 m [8, 9]. Together they make up a two-coordinate laser strainmeter , which can be used for direction finding of various sources of natural and artificial origin .
Stationary laser strainmeters cannot be used to solve the inversion problems of many water areas. To solve various problems that do not require high metrological support, a mobile laser strainmeter was created , where the length of the actuating arm varies depending on the tasks assigned. A frequency-stabilized helium-neon laser by Melles Griot is used as a source of radiation in a mobile laser strainmeter which has a long-term stability to the nine decimal places. This article deals with the features of using a mobile laser strainmeter in winter conditions for solving problems on studying the structure and composition of the sea crust.
DESCRIPTION OF THE EXPERIMENT AND THE RECEIVED RESULTS
Before the beginning of the experiment, a mobile laser strainmeter was installed on the shore of the Amur Bay of the Sea of Japan with the coordinates of 43°11.754′ north latitude and 131°55.141′ east longitude, see Fig. 1. In the heat-stabilized room 1, where the temperature was maintained with an accuracy of 0.5 deg, the bulk of the Michelson interferometer was located. It consisted of: 1) a laser; 2) a collimator; 3) a dividing plate PI‑100; 4) two plane-parallel mirrors fixed on piezoceramic propelling cylinders and a level-resetting system; 5) an extreme regulation system designed to control the operation of the interferometer. The angle reflector was placed in the thermally insulated box 2. The laser beam was propagated into the foam propylene pipes (3) between points 1 and 2. A computer with an analog-to-digital converter was placed in a heated room 4, were the operator was also located. All hardware was powered by a portable generator that provides stable variable voltage at the output of 220 V. The digital registration system of the laser strainmeter registered the change in the distance between the main node of the interferometer (point 1) and the angle reflector (point 2) with an accuracy of 0.3 nm. With a measuring arm length of 6 m, the ultimate sensitivity of the mobile laser strainmeter was .
At a distance of 3150 meters from the location of the mobile laser strainmeter at the point with coordinates 43° 12.391′ north latitude and 131° 52.984′ east longitude, a low-frequency hydroacoustic radiator of electromagnetic type was lowered into the propylene hole to a depth of 12 m, which produced harmonic and complex phase-shift keyed (M-sequences) signals in the water with a central frequency of 33 Hz. The low-frequency hydroacoustic radiator is the main element of the radiating system, which additionally includes a frame for suspending the radiator, a cable-hose with a control manometer, a power source, an electric pump, a control hydrophone, two calibration accelerometers, a digital-to-analog converter, a laptop. A battery of 12 V acid cells connected in series with a capacity of 90 A · h was used as a primary source of direct current (in the amount of 3 to 6 pieces, depending on the required power). The power supply source was a bridge key amplifier made on two half-bridge IGBT-modules, equipped with a compensating battery of 420 µF capacitors, a protective automatic device and a direct current ammeter. The maximum effective sound pressure that a radiating system is capable of producing is 3500 Pa (191 dB/1 µPa). All additional hardware of the radiating system was located in a minibus standing on the ice. The experimental scheme is shown in Fig. 2.
Before the beginning of the experiment, a signal model was constructed on the computer consisting of a tone signal 300 s long, a pause 30 s long and a single phase-shifted signal. The total duration of radiating packet was 485 s. The radiation of a single phase-modulated signal was separately performed additionally after each radiation series. Fig. 3 shows the dynamic spectrograms of the records of the control hydrophone and the mobile laser strainmeter when the hydroacoustic radiator operates.
The received records of a control hydrophone and a mobile laser strainmeter were subjected to additional processing in the laboratory, i. e., convolution of the control hydrophone record with the laser strainmeter record. One of the results of the convolution is shown in Fig. 4. The use of time-accurate systems in the radiating system and in the mobile laser strainmeter allowed us to objectively estimate the arrival times of the recorded signals with an accuracy of 1 ms. Three arrivals were registered for sure with time intervals from the beginning of the radiation of 0.924, 1.270 and 1.526 s. By the times of signal arrival and the distance between the radiation and reception points, the probable minimum propagation velocities were determined: 3400, 2480 and 2060 m/s. It can be assumed that: 1) a signal propagating with a velocity of 2480 m/s corresponds to a damped Rayleigh wave propagating along the "water-bottom" interface; 2) the signal propagating with a velocity of 3400 m/s corresponds to the Love wave propagating along the "bottom sediments – basalt" interface. The signal having a propagation velocity of 2060 m/s is of particular interest. It can be connected with the ice cover, and is caused by a bending mode. Alternatively, it can be ascribed to a surface wave (similar to a Rayleigh wave of decaying type) propagating along the ice-water interface.
During the experimental work on the ice and the coast of the Amur Bay of the Sea of Japan, the technique of operating a mobile laser strainmeter in winter conditions under negative air temperatures has been perfected. Based on the obtained experimental data, the minimum velocities of three signals propagating from the radiation site to the shore, which are equal to 3400, 2480 and 2060 m/s, were determined. The obtained results of the experiment demonstrated the great possibilities of this technology for studying the structure and composition of the sea crust of ice-covered Arctic water areas.
The research was carried out with partial financial support of the Far Easten program and the Russian Foundation for Basic Research (grant 16-29-02023 ofi_m, hardware modernization and experimentation).