Ocean-Bottom Seismographs Based on Broadband MET Sensors: Architecture and Deployment Case Study in the Arctic

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Article Ocean-Bottom Seismographs Based on Broadband MET Sensors: Architecture and Deployment Case Study in the Arctic

Artem A. Krylov 1,2,3,*, Ivan V. Egorov 2, Sergey A. Kovachev 1, Dmitry A. Ilinskiy 1, Oleg Yu. Ganzha 1, Georgy K. Timashkevich 1, Konstantin A. Roginskiy 1, Mikhail E. Kulikov 1, Mikhail A. Novikov 1,2 , Vladimir N. Ivanov 1, Elena A. Radiuk 1, Daria D. Rukavishnikova 1, Alexander V. Neeshpapa 2, Grigory O. Velichko 4, Leopold I. Lobkovsky 1,2,3, Igor P. Medvedev 1 and Igor P. Semiletov 3,5,6

1 Shirshov Institute of Oceanology, Russian Academy of Sciences, 36, Nakhimovskiy Prospekt, 117997 Moscow, Russia; [email protected] (S.A.K.); [email protected] (D.A.I.); [email protected] (O.Y.G.); [email protected] (G.K.T.); [email protected] (K.A.R.); [email protected] (M.E.K.); [email protected] (M.A.N.); [email protected] (V.N.I.); [email protected] (E.A.R.); [email protected] (D.D.R.); [email protected] (L.I.L.); [email protected] (I.P.M.) 2 Moscow Institute of Physics and Technology, 9, Institutsky lane, 141700 Dolgoprudny, Russia; [email protected] (I.V.E.); [email protected] (A.V.N.) 3 V.I. Il’ichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, 43, Baltijskaya St., 690041 Vladivostok, Russia; [email protected] 4 Kuban State University (KubSU), 149, Stavropolskaya St., 350040 Krasnodar, Russia; [email protected] 5 National Tomsk Polytechnic University, 30, Lenina Prospekt, 634050 Tomsk, Russia 6 National Tomsk State University, 36, Lenina Prospekt, 634050 Tomsk, Russia Citation: Krylov, A.A.; Egorov, I.V.; * Correspondence: [email protected] Kovachev, S.A.; Ilinskiy, D.A.; Ganzha, O.Y.; Timashkevich, G.K.; Abstract: The Arctic seas are now of particular interest due to their prospects in terms of hydrocarbon Roginskiy, K.A.; Kulikov, M.E.; extraction, development of marine transport routes, etc. Thus, various geohazards, including those Novikov, M.A.; Ivanov, V.N.; et al. related to seismicity, require detailed studies, especially by instrumental methods. This paper is Ocean-Bottom Seismographs Based devoted to the ocean-bottom seismographs (OBS) based on broadband molecular–electronic transfer on Broadband MET Sensors: (MET) sensors and a deployment case study in the Laptev Sea. The purpose of the study is to Architecture and Deployment Case introduce the architecture of several modifications of OBS and to demonstrate their applicability in Study in the Arctic. Sensors 2021, 21, solving different tasks in the framework of seismic hazard assessment for the Arctic seas. To do this, 3979. https://doi.org/10.3390/ s21123979 we used the first results of several pilot deployments of the OBS developed by Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP Ilyinskiy A.D. in the Laptev Sea Academic Editor: Chris Karayannis that took place in 2018–2020. We highlighted various seismological applications of OBS based on broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s), including the analysis of ambient Received: 29 April 2021 seismic noise, registering the signals of large remote earthquakes and weak local microearthquakes, Accepted: 2 June 2021 and the instrumental approach of the site response assessment. The main characteristics of the Published: 9 June 2021 broadband MET sensors and OBS architectures turned out to be suitable for obtaining high-quality OBS records under the Arctic conditions to solve seismological problems. In addition, the obtained Publisher’s Note: MDPI stays neutral case study results showed the prospects in a broader context, such as the possible influence of the with regard to jurisdictional claims in seismotectonic factor on the bottom-up thawing of subsea permafrost and massive methane release, published maps and institutional affil- probably from decaying hydrates and deep geological sources. The described OBS will be actively iations. used in further Arctic expeditions.

Keywords: ocean-bottom seismograph; molecular–electronic transfer seismometer; seismic hazard assessment; teleseismic signal; local microearthquake; ambient seismic noise; site response analysis; Copyright: © 2021 by the authors. Laptev Sea; Arctic region Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Seismic hazard assessment and seismic zonation are extremely important and are 4.0/). among the most complicated problems of seismology. These problems are relevant because

Sensors 2021, 21, 3979. https://doi.org/10.3390/s21123979 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 3979 2 of 22

of the intensification of construction in seismically active areas, including difficult to access and sparsely populated areas. This also applies to the Russian Arctic, which is now being actively developed: oil-and-gas terminals, extracting platforms, and military bases are being built. The vast Arctic shelf zones are not depicted on the normative maps of the general seismic zoning of Russia [1]. The lack of knowledge also applies to geohazards related to seismicity, such as soil liquefaction, underwater landslides, tsunamis, massive methane seepage from the sea bottom, etc. At present, the climate warms twice as fast in the Arctic region and top sea level stands, for about 5–6 thousands years longer than during the previous warm geological epochs [2]. This causes the progressive subsea permafrost thawing and consequent massive methane release in the Russian Arctic seas, especially in the East Siberian Arctic Shelf (the broadest and shallowest shelf in the World Ocean) which represents >80% of the subsea permafrost and mega-pool of hydrates [3–5]. Numerous geohazards could be related with progressive methane release, including impact on infrastructure of the Northern Sea Route, gas and oil under-water pipes, and general gas and oil exploration. Seismic hazard assessment is a set of theoretical and instrumental methods aimed at solving a wide range of tasks, from describing regional-scale tectonic processes in the study area to assessing the influence of local conditions on the propagation of seismic waves at the construction sites [6]. Instrumental studies using local networks of seismic stations are the most important source of information to solve both fundamental and applied seismological problems. The main tasks of local instrumental observations include registration of remote and local earthquakes, seismic noise analysis, obtaining the location and activity of seismogenic structures, obtaining deep structure, description of seismic regime, assessment of local site amplification, and others [7]. The peculiarities of work at sea, especially in the Arctic, lead to the need for special approaches to the design of the ocean-bottom seismographs (OBS) and to their deployment on the seabed. Since it is impossible to be sure that the seabed will be substantially flat at the deployment site of the OBS, the allowable inclination for seismic sensors should be wide enough. Sensors are required to be tolerant to unfavorable conditions of transportation, deployment, and operation. It is also difficult to accurately determine the orientation of the sensors on the seabed relative to the pole, even with digital compass modules. Therefore, it is necessary to install a network of OBS or use them in combination with on-land stations. Due to the fact that GPS synchronization of the internal clock is possible only before the deployment of OBS on the seabed, and re-synchronization when the stations dismantle is not always possible, depending on the experiment duration, then their accuracy should be high due to the use of chip-scale thermo-stated quartz or atomic clocks. Many seismological tasks require long-term deployments, resulting in the need for a corresponding power source. The first Arctic experiments showed that the ice cover significantly reduces the seismic noise caused by the wind waves [8]. Thus, the OBS must be functional for at least the ice-covered time period. At the same time, it is obvious that work in the Arctic imposes requirements on the stations for the stable operation of all equipment at low temperatures. The presence of ice cover at the Arctic seas for most of the year leads to the need for considering the probability of damaging the OBS by icebergs and stamukhi in shallow shelf waters. The Arctic shelf seabed is dotted with ice plough marks [9,10]. Therefore, the year-long installation of OBS at sites with a sea depth less than 40 m is largely unsafe. Long-term installations increase the risk of equipment loss. This leads to the need to develop reliable mechanisms for OBS recovery: hydroacoustic connection, release devices, and possible trawling schemes. The weight of the equipment also becomes an important parameter, as well as its cost. In addition to the above, the solution of seismological problems imposes significant requirements on the width of the frequency range, sensitivity, and dynamic range of the sensors and recording equipment. The purposes of the present paper are to introduce the architecture of several modifications of the OBS based on broadband molecular–electronic transfer (MET) sensors and to demonstrate their applicability in solving different seismic hazard assessment tasks. To Sensors 2021, 21, x FOR PEER REVIEW 3 of 22

logical problems imposes significant requirements on the width of the frequency range, sensitivity, and dynamic range of the sensors and recording equipment. The purposes of the present paper are to introduce the architecture of several modi- Sensors 2021, 21, 3979 fications of the OBS based on broadband molecular–electronic transfer (MET) sensors3 of 22 and to demonstrate their applicability in solving different seismic hazard assessment tasks. To do this, we used the first results of several pilot deployments of the OBS developed by the Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO do this, we used the first results of several pilot deployments of the OBS developed by RAS) and IP Ilyinskiy A.D. in the Laptev Sea, that took place in 2018–2020. the Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP The MET sensor has a number of significant advantages, such as increased reliability Ilyinskiy A.D. in the Laptev Sea, that took place in 2018–2020. in operation, absence of special transportation conditions, low energy consumption, wide The MET sensor has a number of significant advantages, such as increased reliability temperature range, insensitiveness to installation angles, and no need to fix or center the in operation, absence of special transportation conditions, low energy consumption, wide mass [11]. Recently, the use of this type of sensors has become more frequent for various temperature range, insensitiveness to installation angles, and no need to fix or center the monitoringmass [11 tasks,]. Recently, including the use in the of thisArctic type seas of sensors[12–16]. hasAs it become will be more shown frequent below, forthe various use of themonitoring sensors of tasks, this type including in OBS in also the Arcticdemonstrated seas [12 –their16]. Aseffectiveness it will be shownin solving below, seismic the use problems,of the sensors such as of registration this type in of OBS remote also demonstrated and local earthquakes, their effectiveness seismic noise in solving analysis, seismic andproblems, assessment such of aslocal registration site amplification, of remote under and local the earthquakes,severe conditions seismic of the noise Arctic analysis, seas. and assessment of local site amplification, under the severe conditions of the Arctic seas. 2. Instrumentation 2.1.2. Broadband Instrumentation MET Seismometers 2.1.The Broadband OBS are METequipped Seismometers with two types of broadband MET sensors depending on the modification:The OBS CME-4111 are equipped (120 s) with and two CME-4311 types of (60 broadband s), developed MET sensors by R-sensors, depending Dolgo- on the prudny,modification: Russia [17]. CME-4111 An electrochemical (120 s) and CME-4311sensing element (60 s), of developed CME-4111/4311 by R-sensors, seismome- Dolgo- tersprudny, consists Russia of two [17 pairs]. An electrochemicalof platinum mesh sensing electrodes element that of CME-4111/4311form a four-electrode seismometers anode-cathode-cathode-anodeconsists of two pairs of platinum (ACCA) mesh system. electrodes To protect that form the a four-electrodeadjacent electrodes anode-cathode- from contact,cathode-anode permeable (ACCA) dielectric system. spacers To protectare used. the The adjacent whole electrodes assembly from is immersed contact, permeable in an iodine–iodidedielectric spacers electrolyte are used.that can The move whole freel assemblyy in one isdirection immersed along in ana round iodine–iodide or square elec- channeltrolyte through that can the move electrode-spacer freely in one system direction (see along Figure a round 1). or square channel through the electrode-spacer system (see Figure1).

A C C A

S S S

Figure 1. The sensing element in electrolyte (denoted: A—anodes, C—cathodes, S—spacers). Figure 1. The sensing element in electrolyte (denoted: A—anodes, C—cathodes, S—spacers). As soon as a small potential difference, say 0.2–0.6 V, is applied to both pairs of electrodes,As soon as a reversiblea small potential chemical difference, reaction of say oxidation 0.2–0.6 of V, iodine is applied at the anodes:to both pairs of electrodes, a reversible chemical reaction− of oxidation− of− iodine at the anodes: I3 + 2e → 3I (1)

and a reduction of iodine at the cathodes:

3I− − 2e− → I− (2) 3 take place [18]. This results in a charge transfer between anodes and cathodes via the electrolyte ions in the solution. The above-mentioned potential is sufﬁcient to bring the electrochemical process to saturation, so the current becomes limited by the volumetric rate Sensors 2021, 21, x FOR PEER REVIEW 4 of 22

I + 2e → 3I (1) and a reduction of iodine at the cathodes:

3I − 2e → I (2) Sensors 2021, 21, 3979 4 of 22 take place [18]. This results in a charge transfer between anodes and cathodes via the electrolyte ions in the solution. The above-mentioned potential is sufficient to bring the electrochemical process to saturation, so the current becomes limited by the volumetric rateof supplyof supply of theof the active active component component to to the the electrodes electrodes rather rather than than the the applied applied potential. potential. In Inturn, turn, the the supply supply rate rate is is determined determined by by the the ion io diffusionn diffusion in in the the stationary stationary electrolyte. electrolyte. In In the thesteady steady state, state, a symmetric a symmetric concentration concentration distribution distribution of the of activethe active component component is observed, is ob- which results in an equality of the currents J1 and J2 ﬂowing through the pairs of electrodes served, which results in an equality of the currents J1 and J2 flowing through the pairs of (Figure2a). electrodes (Figure 2a).

V=0 V>0 V<0

J1=J2 J1>J2 J1

b c a FigureFigure 2. 2. TheThe active active component component concentration distributiondistribution in in the the electrolyte electrolyte (( a(()a steady) steady state, state, (b )( andb) and (c) moving(c) moving to the to rightthe rightand and left, left, respectively). respectively).

AsAs the the electrolyte electrolyte starts starts moving moving through through th thee sensing sensing element, element, the the convective convective flow flow is is alwaysalways directed directed from from a acathode cathode to to an an anode anode for for one one pair pair of of electrodes electrodes and and in in the the opposite opposite directiondirection for for another. another. Therefore, Therefore, the the concentr concentrationation distribution distribution of of the the active active component component showsshows asymmetry, asymmetry, which which results results in in a acurrent current difference difference related related to to the the amplitude amplitude and and the the directiondirection of of the the electrolyte electrolyte motion motion (Figure (Figure 2b,c)2b,c) [19]. [19]. BeingBeing employed employed in thethe conditioncondition of of saturation, saturation, molecular–electronic molecular–electronic sensing sensing elements ele- mentsshow show a high a conversion high conversion factor that factor can registerthat can the register smallest the motion smallest exposures, motion suchexposures, as those suchof the as naturalthose of seismic the natural Earth seismic signals overEarth the signals noise over of accompanying the noise of electronicaccompanying circuitry. electronic However,circuitry. the output response of a molecular–electronic sensing element shows neither a velocity-flatHowever, northe anoutput acceleration-flat response of response. a molecular–electronic The frequency-dependent sensing element response shows gives neitherrise to a the velocity-flat task of frequency nor an equalizationacceleration-flat of theresponse. sensing The element frequency-dependent response. The actual re- sponseparameters gives ofrise a sensorto the dependtask of frequency on its inner equalization geometry and of displaythe sensing a noticeable elementrange, response. even Thefor actual two identically parameters made of a sensors sensor ofdepend one batch. on its Therefore, inner geometry the task and of achievingdisplay a noticeable the desired range,accuracy even can for only two beidentically performed made by means sensors of of measuring one batch. the Therefore, cell transfer thefunction task of achiev- directly. ing theApart desired from accuracy this, the can electrochemical only be perfor reactionsmed by showmeans exponential of measuring dependence the cell transfer of their rate vs. temperature [20,21]. Due to this dependence and a change of the sensor mechan- function directly. ical parameters over temperature, the sensitivity of sensing elements show frequency- Apart from this, the electrochemical reactions show exponential dependence of their dependent behavior over temperature. Typical velocity vs. frequency responses of a rate vs. temperature [20,21]. Due to this dependence and a change of the sensor mechan- horizontal sensor at three different temperatures are provided in Figure3. As a result, the ical parameters over temperature, the sensitivity of sensing elements show frequen- task of equalizing the sensor frequency response should contain both frequency-dependent cy-dependent behavior over temperature. Typical velocity vs. frequency responses of a and temperature-dependent elements. horizontal sensor at three different temperatures are provided in Figure 3. As a result, the The molecular–electronic sensors’ construction differs whether a sensor is equipped task of equalizing the sensor frequency response should contain both frequen- with a force feedback mechanism or not. As a rule, the sensors without a force feedback cy-dependent and temperature-dependent elements. mechanism are of horizontal type, which is in place in both CME-4111 and CME-4311, The molecular–electronic sensors’ construction differs whether a sensor is equipped with a rigid case made of ceramics. Despite the technical possibility of such improvement, with a force feedback mechanism or not. As a rule, the sensors without a force feedback the vertical sensors of CME-4111 seismometers are not equipped with a force feedback as well. This was done intentionally to achieve the lowest power consumption. The compensation of temperature and frequency dependence for no-feedback sensors can only be performed by means of a combination of low-pass and high-pass filters with temperature-dependent elements. The force feedback mechanism in the vertical sensor of CME-4311 contains a firmly fixed permanent magnet and a coil, which in turn is fixed on a moving part of a sensor. The force feedback mechanism is provided by an interaction of the magnet with a current Sensors 2021, 21, x FOR PEER REVIEW 5 of 22

mechanism are of horizontal type, which is in place in both CME-4111 and CME-4311, with a rigid case made of ceramics. Despite the technical possibility of such improvement, the vertical sensors of CME-4111 seismometers are not equipped with a force feedback as well. This was done intentionally to achieve the lowest power consumption. The compensation of temperature and frequency dependence for no-feedback sensors can only be performed by means of a combination of low-pass and high-pass filters with Sensors 2021, 21, 3979 temperature-dependent elements. 5 of 22 The force feedback mechanism in the vertical sensor of CME-4311 contains a firmly fixed permanent magnet and a coil, which in turn is fixed on a moving part of a sensor. The force feedback mechanism is provided by an interaction of the magnet with a current ﬂowingflowing inin thethecoil. coil. So,So, thethe circuitrycircuitry ofof aasensor sensor withwith aa force force feedback feedback contains contains a a current current ampliﬁer,amplifier, which which on on the the one one hand hand allows allows to to improve improve the the stability stability and and the the evenness evenness of of the the frequencyfrequency response, response, but but on on the the other other hand, hand, requires requires more more power power consumption consumption to keep to keep the currentthe current in the in feedback the feedback coil.The coil. introduction The introduction of a force-feedback of a force-feedback mechanism mechanism into a sensor into a allowssensor reducingallows reducing the requirements the requirements that are imposed that are onimposed the precision on the of precision the frequency of the and fre- thequency temperature and the stabilization.temperature stabilization.

FigureFigure 3. 3.The The amplitude-frequencyamplitude-frequency response response of of a a typical typical sensing sensing element element at at different different temperatures. temperatures.

SinceSince thethe powerpower consumptionconsumption isis thethe most most important important when when designing designing autonomous autonomous subseasubsea systems,systems, therethere is is a a trade-off trade-off between between the the precision precision of of an an instrument instrument and and its its power power demands.demands. A A CME-4111, CME-4111, which which is ais three-component, a three-component, no-feedback no-feedback broadband broadband seismometer, seismo- consumesmeter, consumes only 7 mAonly from 7 mA a 12-Vfrom powera 12-V source,power whilesource, the while same the size same CME-4311, size CME-4311, which haswhich just has one just force-feedback one force-feedback and two and no-feedback two no-feedback components, components, needs 8.5 needs mA in8.5 a mA steady in a state.steady However, state. However, the vertical the vertical component component of CME-4311 of CME-4311 has has a ± a0.5 ±0.5 dB dB tolerance tolerance in in the the middlemiddle ofof a a passband passband and and± ±11 dBdB atat the the slopes, slopes, compared compared to to± ±11 andand± ±22 dBdB ofof CME-4111,CME-4111, respectively.respectively. The The comparison comparison shows shows that thethat improvement the improvement of precision of precision has been achievedhas been byachieved introduction by introduction of a force feedback.of a force feedback. TheThe bandwidthsbandwidths of the sensors employed employed in in th thee study study are are 0.0083 0.0083 (120 (120 s) s)to to50 50Hz Hz for forCME-4111 CME-4111 and and 0.0167 0.0167 (60 (60 s) s)to to 50 50 Hz Hz for for CME-4311. The noisenoise performanceperformance ofof bothboth seismometersseismometers is is almost almost identical, identical, with with a a slight slight rise rise of of noise noise density density of of the the vertical vertical channel channel ofof CME-4311CME-4311 inin higherhigher frequenciesfrequencies (Figure(Figure4 ).4). This This rise rise of of noise noise density density is is of of electronic electronic ampliﬁer origin and results from a higher gain in the forward ampliﬁcation circuit needed Sensors 2021, 21, x FOR PEER REVIEWamplifier origin and results from a higher gain in the forward amplification circuit6 of 22 forneeded a system for a with system a negative with a feedback.negative feedback. The parameters The parameters of both seismometers of both seismometers are listed are in Table1. listed in Table 1.

FigureFigure 4. 4.The The self-noise self-noise of of CME-4111 CME-4111 (left (left) and) and CME-4311 CME-4311 (right (right) seismometers.) seismometers.

Table 1. Technical parameters of CME-4111 and CME-4311 seismometers.

Parameter CME-4111 CME-4311 Vertical, with feedback, Type No feedback Horizontal, no feedback Sensitivity 4000 V/(m/s) 2000 V/(m/s) Type of output signal analog, differential analog, differential Number of orthogonal components 3 3 Maximum output signal ±20 V ±10 V Maximum input signal ±5 mm/s ±5 mm/s Bandwidth 0.0083 (120 s) to 50 Hz 0.0167 (60 s) tp 50 Hz 12 V DC 12 V DC Power supply voltage (10.5–16 V acceptable) (9.5–16 V acceptable) Current consumption 7 mА 8.5 mА Output impedance 2 × 500 Ohm 2 × 500 Ohm Self-noise see Figure 4 see Figure 4 Dynamic range at 1 Hz 123.5 dB 123.5 dB Vertical, 0.15% Nonlinearity at 1 Hz 0.5% Horizontal, 0.5% Maximum inclination during installation ±15° ±15° Temperature range −12–+55 °С −12–+55 °С Housing material Stainless steel (optional) Stainless steel (optional) Housing dimensions, diameter/height 146/90 mm 146/90 mm Weight 3.1 kg 2.6 kg Connector type on the housing to connect IDCC-10MR (10 pin) IDCC-10MR (10 pin) the cable

Both seismometers optionally have the same type and size, light stainless-steel case, and are pin-to-pin compatible (Figure 5).

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Table 1. Technical parameters of CME-4111 and CME-4311 seismometers.

Parameter CME-4111 CME-4311 Vertical, with feedback, Type No feedback Horizontal, no feedback Sensitivity 4000 V/(m/s) 2000 V/(m/s) Type of output signal analog, differential analog, differential Number of orthogonal components 3 3 Maximum output signal ±20 V ±10 V Maximum input signal ±5 mm/s ±5 mm/s Bandwidth 0.0083 (120 s) to 50 Hz 0.0167 (60 s) tp 50 Hz 12 V DC 12 V DC Power supply voltage (10.5–16 V acceptable) (9.5–16 V acceptable) Current consumption 7 mA 8.5 mA Output impedance 2 × 500 Ohm 2 × 500 Ohm Self-noise see Figure4 see Figure4 Dynamic range at 1 Hz 123.5 dB 123.5 dB Vertical, 0.15% Nonlinearity at 1 Hz 0.5% Horizontal, 0.5% Maximum inclination during ±15◦ ±15◦ installation Temperature range −12–+55 ◦C −12–+55 ◦C Housing material Stainless steel (optional) Stainless steel (optional) Housing dimensions, 146/90 mm 146/90 mm diameter/height Weight 3.1 kg 2.6 kg Connector type on the housing to IDCC-10MR (10 pin) IDCC-10MR (10 pin) connect the cable

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Both seismometers optionally have the same type and size, light stainless-steel case, and are pin-to-pin compatible (Figure5).

Figure 5. The CME-4111/CME-4311 seismometer in an optional stainless-steel case (Courtesy of Figure 5. The CME-4111/CME-4311 seismometer in an optional stainless-steel case (Courtesy of R-sensors LLC). R-sensors LLC).

2.2. OBS Modifications for Shallow Waters The MPSSR ocean-bottom seismograph developed by the IO RAS is suitable for a wide range of tasks, including seismological monitoring, active and passive seismics and high-resolution seismoacoustic investigations. The design of the MPSSR and its external view are presented in Figure 6. The MPSSR is equipped with two three-component seismometers and a hydrophone. The first three-component seismometer is broadband and consists of CME-4311 MET sensors produced by R-sensors, with a frequency range of 0.0167–50 Hz. The second three-component seismometer is short-period and consists of SV-10 and SH-10 classic electromechanical geophones, with a frequency range of 10–200 Hz (analogy of GS-20DX), placed in gimbal. The Hydrophone 5007 m included in the package was also developed by the IO RAS, Moscow, Russia and has a frequency range of 0.04–2500 Hz. The basic parameters of the Hydrophone 5007 m are presented in Table 2. The recorder URS-S is based on the ARM microcontroller STM32F103. It is supposed to be a part of autonomous devices intended for seismic research. It can also be used wherever multi-channel acquisition of analog signals with a reference to absolute time is required, with low power consumption. Table 3 shows the main characteristics of the recorder URS-S. The features of the recorder include its relatively low power consumption and the presence of a built-in high-precision thermo-stated reference frequency generator MXO37/8P [22], in combination with a GPS interface, which makes it possible to time-tie the received records to absolute time. The Trimble AcutimeTM GG antenna is used for receiving a synchronizing GPS signal. The sample rate can be chosen from 20 up to 800 Hz. The use of a combined anti-aliasing filter significantly increases the dynamic range of the recorder in relation to the dynamic range of the ADC. An ordinary SD memory card with the capacity of up to 64 Gb is used for data storing. The recorder is powered from a single positive-polarity power supply with the conversion into voltages required for the recorder circuits to operate. The power supply voltage is 6.5–32 V, and the consumption in the recording mode is about 20 mA, at a supply voltage of 12 V. In addition, the recorder generates the voltages required for the operation of external analog sensors, which are the sources of the recorded signals, as well as the supply voltage of the GPS receiver. A round and flat battery package is located in the central part of the station and can be equipped with various types of alkaline or lithium batteries. The MPSSR is also equipped with a separate digital compass module based on the digital three-axis magnetometer, MAG3110. The module has its own internal built-in

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2.2. OBS Modiﬁcations for Shallow Waters memoryThe MPSSR and a separate ocean-bottom power seismograph supply, with developed the voltage by of the 9 IOV and RAS the is suitablebattery forcapacity a wide of range200 mAh. of tasks, It only including works seismologicalfor the first 12 monitoring,h after turning active on andthe OBS passive to capture seismics the and moment high- resolutionwhen the seismoacousticOBS touches the investigations. seabed and to Thesave design the orientation of the MPSSR data. and its external view are presentedThe MPSSR in Figure has 6duralumin. The MPSSR spherical is equipped housing with with two three-componenta diameter of 444 seismometers mm designed and a hydrophone. The ﬁrst three-component seismometer is broadband and consists of for depths up to 3000 m. The housing is rigidly attached to the concrete ballast to improve CME-4311 MET sensors produced by R-sensors, with a frequency range of 0.0167–50 Hz. the traction on the seabed. The current equipment is not self-pop-up and the deployment The second three-component seismometer is short-period and consists of SV-10 and SH-10 in shallow waters is conducted with the use of external acoustic release and buoys, ac- classic electromechanical geophones, with a frequency range of 10–200 Hz (analogy of cording to the deployment scheme presented in Figure 7. The deployment scheme allows GS-20DX), placed in gimbal. The Hydrophone 5007 m included in the package was also trawling a rope laid on the seabed between the OBS and the ballast-buoy system if the developed by the IO RAS, Moscow, Russia and has a frequency range of 0.04–2500 Hz. The acoustic release does not work after long-term operation. Thus, the current equipment basic parameters of the Hydrophone 5007 m are presented in Table2. and the deployment scheme imply work on the shelf at depths of no more than 100 m.

FigureFigure 6. 6.The The designdesign ofof thethe MPSSRMPSSR ocean-bottom seismograph developed developed by by the the IO IO RAS RAS (a ()a )and and its itsexternal external view view on on the the R/V R/V Akademik Akademik Mstislav Mstislav Keldysh, autumnautumn 20182018 (b(b).). (1) (1) Three-component Three-component broadbandbroadband seismometer seismometer CME-4311, CME-4311, (2) three-component(2) three-component short-period short-period seismometer seismometer (SV-10 and(SV-10 SH-10) and placedSH-10) in placed gimbal, in (3) gimbal, Hydrophone (3) Hydrophone 5007m, (4) recorder5007m, (4 URS-S,) recorder (5) digital URS-S, compass (5) digital module, compass (6) batteries module, block,(6) batteries (7) protective block, half-cover(7) protective for hydrophone, half-cover fo (8)r hydrophone, duralumin sphere, (8) duralumin (9) concrete sphere, ballast. (9) concrete ballast. The recorder URS-S is based on the ARM microcontroller STM32F103. It is supposed to be a part of autonomous devices intended for seismic research. It can also be used wherever multi-channel acquisition of analog signals with a reference to absolute time is required, with low power consumption. Table3 shows the main characteristics of the recorder URS-S. The features of the recorder include its relatively low power consumption and the presence of a built-in high-precision thermo-stated reference frequency generator MXO37/8P [22], in combination with a GPS interface, which makes it possible to time-tie the received records to absolute time. The Trimble AcutimeTM GG antenna is used for receiving a synchronizing GPS signal. The sample rate can be chosen from 20 up to 800 Hz. The use of a combined anti-aliasing ﬁlter signiﬁcantly increases the dynamic range of the recorder in relation to the dynamic range of the ADC. An ordinary SD memory card with the capacity of up to 64 Gb is used for data storing. The recorder is powered from a single positive-polarity power supply with the conversion into voltages required for the recorder circuits to operate. The power supply voltage is 6.5–32 V, and the consumption in the recording mode is about 20 mA, at a supply voltage of 12 V. In addition, the recorder generates the voltages required for the operation of external analog sensors, which are the sources of the recorded signals, as well as the supply voltage of the GPS receiver. A round Figure 7. MPSSR ocean-bottom seismograph deployment scheme on the shallow shelf with a submerged buoy.

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memory and a separate power supply, with the voltage of 9 V and the battery capacity of 200 mAh. It only works for the first 12 h after turning on the OBS to capture the moment when the OBS touches the seabed and to save the orientation data. The MPSSR has duralumin spherical housing with a diameter of 444 mm designed for depths up to 3000 m. The housing is rigidly attached to the concrete ballast to improve the traction on the seabed. The current equipment is not self-pop-up and the deployment in shallow waters is conducted with the use of external acoustic release and buoys, according to the deployment scheme presented in Figure 7. The deployment scheme allows trawling a rope laid on the seabed between the OBS and the ballast-buoy system if the acoustic release does not work after long-term operation. Thus, the current equipment and the deployment scheme imply work on the shelf at depths of no more than 100 m.

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and ﬂat battery package is located in the central part of the station and can be equipped with various types of alkaline or lithium batteries. The MPSSR is also equipped with a separate digital compass module based on the digital three-axis magnetometer, MAG3110. The module has its own internal built-in memory and a separate power supply, with the voltage of 9 V and the battery capacity of 200 mAh. It only works for the ﬁrst 12 h after turning on the OBS to capture the moment when the OBS touches the seabed and to save the orientation data. The MPSSR has duralumin spherical housing with a diameter of 444 mm designed for depths up to 3000 m. The housing is rigidly attached to the concrete ballast to improve the traction on the seabed. The current equipment is not self-pop-up and the deployment in shallowFigure 6. waters The design is conducted of the MPSSR with ocean-bottom the use of external seismograph acoustic developed release by and the buoys, IO RAS according (a) and its toexternal the deployment view on the scheme R/V Akademik presented Mstislav in Figure Keldysh,7. The deployment autumn 2018 scheme (b). (1) allows Three-component trawling abroadband rope laid onseismometer the seabed CME-4311, between (2) the three-comp OBS andonent the ballast-buoy short-period systemseismometer if the (SV-10 acoustic and releaseSH-10) doesplaced not in gimbal, work after (3) Hydrophone long-term operation.5007m, (4) recorder Thus, theURS-S, current (5) digital equipment compass and module, the (6) batteries block, (7) protective half-cover for hydrophone, (8) duralumin sphere, (9) concrete deployment scheme imply work on the shelf at depths of no more than 100 m. ballast.

Figure 7. MPSSR ocean-bottom seismograph deployment scheme on the shallow shelf with a Figure 7. MPSSR ocean-bottom seismograph deployment scheme on the shallow shelf with a submerged buoy. submerged buoy.

Table 2. Basic parameters of the Hydrophone 5007 m.

Frequency Band 0.04–2500 Hz Sensitivity at 15 Hz 7.2 ± 0.5 mV/Pa Dynamic range 100 dB Sensor diameter 50 mm Maximum depth up to 5000 m

Table 3. Main characteristics of the recorder URS-S.

A number of Analog Channels 4 (Basic), 8 (Extended) A number of digital channels 1 (for reference to absolute time) Sample rates, Hz 20, 25, 40, 50, 80, 100, 160, 200, 400, 800 Time synchronization GPS interface Temperature stability of the quartz generator ±5 × 10−9 Dynamic range 85–90 dB Memory SD card up to 64 Gb Power supply voltage 6.5–32 V Power consumption 20 mA (12 V) Sensors 2021, 21, 3979 9 of 22

There is also a short-period Typhoon ocean-bottom seismograph with a frequency band of 1–50 Hz. It is a smaller modiﬁcation of the MPSSR with the housing diameter of 350 mm and it is equipped with one three-component seismometer CME-3311 (see Table4) and the Hydrophone 5007 m. The recorder URS-S is also used in it. When installing on the seaﬂoor, the MPSSR and Typhoon stations are usually accom- panied by autonomous wave recorders, which are attached to a rope not far from the OBS. The wave recorder ARW-K14-1 is equipped with a quartz baro-sensitive element, in which the membrane is bent by the action of the column pressure of the liquid, deforming a strong sensitive piezo-element attached to it. Thus, the absolute pressure at the point of measurement can be calculated. The wave recorder is also equipped with a temperature compensation system, which allows the temperature to be measured at the same time as the pressure. The pressure is recorded at a sampling rate of 1 Hz. Another device is a RBR virtuoso3D wave logger [23], equipped with a Keller pressure sensor, which is also based on the use of a piezoelectric quartz sensor as a baro-sensitive element. It records the pressure values of the water column and translates them into the variable depth of the location. The recording frequency is 1 Hz.

Table 4. Main technical parameters of the CME-3311 seismometer.

Frequency Band 1–50 Hz Sensitivity 2000 V/(m/s) Dynamic range at 1 Hz 118 dB Power supply voltage 12V (10.5–16 V acceptable) Current consumption 25 mA Temperature range −12–+55 ◦C Maximum inclination during installation ±15◦

2.3. OBS Modification for Deep Waters Unlike the MPSSR and Typhoon models described above, the GNS-C model is used for scientific applications of the IO RAS in deep waters. It was developed by the IO RAS and IP Ilinskiy A.D. The GNS-C employs the 120 s MET sensor CME-4111, produced by R-sensors. GNS-C is a self-pop-up OBS for the water depths ranged up to 6000 m and is suitable for studying seismicity, tectonics and deep structure, down to the middle mantle. The design of the GNS-C and its external view are presented in Figure8. The general characteristics of the GNS-C are shown in Table5. The notable capabilities of the station are: • Multiple seabed deployment/recoveries without opening or recharging the node. • Automatic clock synchronization with GPS signals through the instrument case. Built- in GPS receiver, activated automatically after surfacing node from the seabed. • Wireless on/off power switch. No need to open the node case after the transport and seabed recovery, all preparation and tests before deployment could be performed on a vessel deck. • Wireless user interface. Fast data download after the node retrieval to a ship deck. • Automatic and manual tests of power supply, seismic recorder and acoustic release. In addition to the three-component seismometer CME-4111, the station is equipped with a low-frequency hydrophone produced by the Experimental Design Bureau of Oceanological Engineering, Russian Academy of Sciences (EDBOE RAS). A brief technical specification of the hydrophone is presented in Table6. The OBS has a GNS-4-channel seismic recorder with a 24-bit ADC converter for each channel. Table7 shows the main technical characteristics of the GNS recorder. There is a built-in, high-precision thermo-stated reference frequency generator MXO37/8 and GPS interface with the use of a Trimble Silvana or uBlox UC530M receiver. Hydroacoustic connection is maintained with the onboard autonomic acoustic unit, powered from 220 V or from internal batteries in a waterproof case. The acoustic link range is up to 10 km, and the frequency range of an acoustic link is 9–13 kHz. Sensors 2021, 21, x FOR PEER REVIEW 10 of 22

• Multiple seabed deployment/recoveries without opening or recharging the node. • Sensors 2021, 21, 3979 Automatic clock synchronization with GPS signals through the instrument10 case. of 22 Built-in GPS receiver, activated automatically after surfacing node from the seabed. • Wireless on/off power switch. No need to open the node case after the transport and seabed recovery, all preparation and tests before deployment could be performed on aGNS-C vessel isdeck. also equipped with additional sensors, such as two temperature sensors, •a currentWireless measurement user interface. sensor, Fast a data 3D-compass download module, after the a node digital retrieval pressure to sensora ship deck. and an •analog Automatic pressure and manometer. manual tests Additional of power OBS supply, sensors seismic are shown recorder in Table and8 acoustic. release.

Figure 8. Design of the GNS-C ocean-bottom seismograph developed by by IO RAS and IP Ilinskiy A.D. ((aa)) andand its its external external view view on on the the R/V R/V Akademic Akademic Mstislav Mstislav Keldysh, Keldysh, autumn autumn 2018 (2018b). (1) (b Three-). (1) Three-componentcomponent broadband broadband seismometer seismometer CME-4111, CME-4111, (2) recorder (2) recorder GNS, (3) GNS, acoustic (3) acoustic modem, (4)modem, batteries (4) batteriesblock, (5) block, acoustic (5) hydrophone,acoustic hydr (6)ophone, seismic (6) hydrophone, seismic hydrophone, (7) penetrator, (7) penetrator, (8) glass spherical(8) glass spherical housing housingwith diameter with diameter 430 mm, 430 (9) anchor,mm, (9) (10)anchor, lamp, (10) (11) lamp, ﬂag, (11) (12) flag, anchor (12) release, anchor (13)release, plastic (13) case. plastic case.

TableIn 5. additionGeneral characteristics to the three-component of the GNS-C seismometer ocean-bottom station.CME-4111, the station is equipped with a low-frequency hydrophone produced by the Experimental Design Bureau of OceanologicalDeep-Water Engineering, Radio Transparent Russian AcademyGlass Sphere of Sciences of 43 cm Diameter(EDBOE InlaidRAS). with A brief Durable technical specificationInstrument of the Case hydrophone is presented in TablePlastic 6. Shell The OBSSeismic has a recorder GNS-4-channel seismic recorder 4 channels, with 24-bit a 24-bit ADC ADC for each converter channel for each channel. Table 7 shows the main technicalWireless characteristics USB. OS Windows of the GNS client recorder. program forThere the is a User interface/data backup link built-in, high-precision thermo-stated reference frequencydevice generator management MXO37/8 and GPS interface withUser the terminal use of a Trimble Silvana or uBlox Tablet, UC530M Laptop receiver. or Desktop PC Hydroacoustic connection is maintained with the onboard autonomic acoustic unit, Molecular electronic sensors 3C, 0.0083 (120 s) to 50 Hz, powered from 220 V or from internal batteries in a waterproof case. The acoustic link Seismic sensors one deep-water low-frequency hydrophone (0.067 Hz range is up to 10 km, and the frequency range of an acoustic(15 s) tolink 30 is kHz) 9–13 kHz. GNS-C is also equipped with additional sensors, such as two temperature sensors, a Release command Acoustic call or automatically by preset time current measurement sensor, a 3D-compass module, a digital pressure sensor and an Electrochemical with mechanical support (salt and fresh analog pressureBallast anchor manometer. release Additional OBS sensors are shown in Table 8. waters) Self-dissolving into natural sea components after TableBallast 5. General anchor environmentalcharacteristics featureof the GNS-C ocean-bottom station. surveying (24 kg weight in air) or metallic Glass Sphere of 43 cm Diameter Inlaid with Du- Deep-WaterContinuous Radio recordingTransparent time Instrument Case Up to 13 months (lithium batteries) rable Plastic Shell Seismic recorder 3.6 V Primary4 channels, lithium-thionyl 24-bit ADC chloride for each (Li-SOCl2). channel Battery type High-energyWireless D-size USB. cell-type OS Windows LS 33,600 client and program battery for User interface/data backup link protection board the device management Detecting equipmentUser terminal on the sea Radio beacon withTablet, GPS coordinatesLaptop or Desktop transmission, PC ﬂash surface Molecularlight electronic (at night), sensors ﬂag 3C, 0.0083 (120 s) to WeightSeismic sensors 50 Hz, 38 kgone in deep-water air (without low-frequency anchor) hydro- Depth rangephone up (0.067 to 6000 Hz m(15 s) to 30 kHz) Release command Acoustic call or automatically by preset time

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Table 6. Main technical features of the hydrophone produced by the EDBOE RAS.

Sensitivity with Preampliﬁer 200 V/bar Frequency range From 0.067 Hz (15 s) to 30 kHz Self-noise to input Mean square noise in a range of 1 Hz to 1 kHz, 0.06 µBar Maximal operation depth 6000 m Physical size 4.2 cm long and 4.0 cm in diameter

Table 7. Main technical characteristics of the GNS recorder.

A Number of Input of Differential Channels 4 Supported data sampling 0.25, 0.5, 1, 2, 4 ms Dynamic Range 153 dB Built-in GPS/GLONASS navigation receiver, Time synchronization powered off during recording Temperature stability of the quartz generator ± 5 × 10−9 SD ﬂash card of 32 Gb (expandable up to Memory 128 Gb) Power supply voltage 8.5–20 V Wireless USB (non-active during data Main user interface acquisition)

Table 8. Additional GNS-C sensors.

Temperature Measurement 2 Temperature Sensors Current measurement AD8218 sensor Compass LSM 303DLHC (3D accelerometer) Pressure BMP180 (Digital Pressure sensor) Wireless USB dongle for communication Alerion Wireless USB Magnetic switch on|off of the OBS Gerkon KЭM-2A GPS|GLONASS receiver Trimble Silvana or uBlox UC530M

3. OBS Deployment Case Study in the Arctic and the First Results 3.1. Scientific Cruises to the Laptev Sea The Laptev Sea is one of the most interesting regions for complex scientific research. It is the most seismically active water area among the Russian Arctic seas [24]. In addition, in the Laptev Sea and East-Siberian Sea, a large number of methane outflows from the seabed have been found, both on the shelf and on the continental slope [3–5]. Methane, in the case when it has an endogenous, deep origin, can come from great depths along faults and reach the level of occurrence of gas hydrates, where it mixes with near-surface methane. At the same time, weak microearthquakes occurring in such areas indicate active faults. Moreover, seismotectonic events can affect the intensity of gas vents [25–30]. The study of such phenomena is part of the geohazard assessment, including the purely seismic hazard assessment. Since 2018, the program of seismological research has been included in a series of scientific cruises to the Laptev Sea on the R/V Akademik Mstislav Keldysh organized by the V.I. Ilichov Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences and the IO RAS. The seismological work program has been aimed to determine the seismic and seismotectonic characteristics of the Laptev Sea region in the context of the relationship of the tectonic processes with the discharge of bubble methane from the bottom by registering local microearthquakes, remote teleseismic events and ambient seismic noise on the shelf and the continental slope of the Laptev Sea. In addition, new data on seismicity Sensors 2021, 21, x FOR PEER REVIEW 12 of 22

vents [25–30]. The study of such phenomena is part of the geohazard assessment, including the purely seismic hazard assessment. Since 2018, the program of seismological research has been included in a series of scientific cruises to the Laptev Sea on the R/V Akademik Mstislav Keldysh organized by the V.I. Ilichov Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences and the IO RAS. The seismological work program has been aimed Sensors 2021, 21, 3979 to determine the seismic and seismotectonic characteristics of the Laptev Sea region12 of 22 in the context of the relationship of the tectonic processes with the discharge of bubble methane from the bottom by registering local microearthquakes, remote teleseismic events and ambient seismic noise on the shelf and the continental slope of the Laptev Sea. andIn addition, present tectonics new data of theon Laptevseismicity Sea and region pres areent extremely tectonics necessary of the Laptev for detailed Sea region seismic are hazardextremely assessment necessary of thefor detailed region. seismic hazard assessment of the region. DuringDuring thethe expeditions,expeditions, the OBS OBS were were depl deployedoyed for for a year a year on onthe the shelf shelf and and the thecon- continentaltinental slope slope of ofthe the Laptev Laptev Sea Sea (the (the techni technicalcal goal goal of of the seismological workwork program).program). By now, the records of two one-year campaigns have been obtained. Figure9 shows the By now, the records of two one-year campaigns have been obtained. Figure 9 shows the long-term deployment sites for the AMK-73 cruises (2018) and the AMK-78 cruise (2019). long-term deployment sites for the AMK-73 cruises (2018) and the AMK-78 cruise (2019). The locations, types and operation periods of the OBS are shown in Table9. The locations, types and operation periods of the OBS are shown in Table 9.

FigureFigure 9. 9.The The OBS OBS long-term long-term deployment deployment sites sites in in the the Laptev Laptev sea. sea. ( 1()1) Permanent Permanent TIXI TIXI broadband broadband seismicseismic stationstation (included(included inin Global Seismograph Network Network [31]), [31]), (2 ()2 )the the OBS OBS on on the the inner inner shelf shelf deployed in the AMK-73 cruise (2018), (3) the OBS on the slope deployed in the AMK-73 cruise (2018), deployed in the AMK-73 cruise (2018), (3) the OBS on the slope deployed in the AMK-73 cruise (4) the OBS on the outer shelf deployed in the AMK-78 cruise (2019). (2018), (4) the OBS on the outer shelf deployed in the AMK-78 cruise (2019). Table 9. The coordinates, types and dates of operation for the OBS on the shelf and continental Tableslope 9. ofThe the coordinates,Laptev Sea in types 2018–2020 and dates (for of the operation station locations, for the OBS see on Figure the shelf 9). and continental slope of the Laptev Sea in 2018–2020 (for the station locations, see Figure9). Site Type Latitude Longitude Depth Operation Period SiteSt4 Type MPSSR Latitude 75.422° N Longitude 127.391° E Depth 42 m 6 Oct Operation 2018 to Period8 Feb 2019 St4St5 MPSSR MPSSR 75.422 75.431°◦ N N 127.391 129.132°◦ E E 42 40 m m 6 6 OctOct 20182018 toto 88 FebMar 2019 2019 St5Slope MPSSR GNS-C 75.431 77.308°◦ N N 129.132 120.610°◦ E E 40 350 m m 15 6 Oct Oct 2018 2018 to to 8 31 Mar May 2019 2019 SlopeSt3 GNS-C MPSSR 77.308 76.392°◦ N N 120.610 125.660°◦ E E 350 51 m m 15 9Oct Oct 2018 2019 to to 31 5 MayJan 2020 2019 St3 MPSSR 76.392◦ N 125.660◦ E 51 m 9 Oct 2019 to 5 Jan 2020 Typ2 Typhoon 76.834◦ N 127.688◦ E 61 m 10 Oct 2019 to 9 Feb 2020

3.2. Recording of Ambient Seismic Noise in the Laptev Sea In the study of seismic noise on the records of the OBS, it has been found that the ambient noise on the seabed signiﬁcantly depends on the wind waves and the ice-cover conditions both on the shelf and the continental slope. We compared spectrograms of the signals obtained by different sensors of the OBS, signals obtained by the wave recorders and ice concentration curves derived from the reanalysis data. Sensors 2021, 21, x FOR PEER REVIEW 13 of 22

Typ2 Typhoon 76.834° N 127.688° E 61 m 10 Oct 2019 to 9 Feb 2020

3.2. Recording of Ambient Seismic Noise in the Laptev Sea In the study of seismic noise on the records of the OBS, it has been found that the ambient noise on the seabed significantly depends on the wind waves and the ice-cover Sensors 2021, 21, 3979 13 of 22 conditions both on the shelf and the continental slope. We compared spectrograms of the signals obtained by different sensors of the OBS, signals obtained by the wave recorders and ice concentration curves derived from the reanalysis data. Continuously,Continuously, every every second second sea sea level level records record haves have been been formed formed based based on the on data the fromdata thefrom ARW-K14-1 the ARW-K14-1 and the and RBR the virtuoso3D RBR virtuoso3D tide gauges tide gauges for 365 for and 365 363and days, 363 days, respectively. respec- Then,tively. we Then, obtained we obtained the data the on data the intra-annualon the intra-annual variability variability of the levelof the fluctuations level fluctuations in the centralin the partcentral of thepart Laptev of the Laptev Sea shelf Sea in shelf 2018–2019 in 2018–2019 (ARW-K14-1) (ARW-K14-1) and in theandnorthern in the northern part of thepart Laptev of the Sea Laptev shelf Sea in 2019–2020 shelf in 2019–2020 (RBR virtuoso3D). (RBR virtuoso3D). To analyze To the analyze dependence the dependence of the short- of periodthe short-period level fluctuations level fluctuations (wind waves, (wind swell) waves, on ice cover,swell) spectrograms on ice cover, of spectrograms annual sea level of recordsannual insea the level range records of periods in the range from 2of toperiod 20 s haves from been 2 to constructed.20 s have been The constructed. spectrograms The ofspectrograms annual sea levelof annual records sea le havevel records been calculated have been with calculat Welch’sed with method Welch’s [32 method], using [32], the Kaiser–Besselusing the Kaiser–Bessel spectral window spectral in twowindow days, in with two 50% days, overlap. with 50% Daily overlap. ice concentration Daily ice con- data havecentration been provided data have by been the provided European by Organization the European for Organization the Exploitation for the of MeteorologicalExploitation of SatellitesMeteorological (EUMETSAT), Satellites with (EUMETSAT), a grid resolution with a ofgrid 25 kmresolution [33]. of 25 km [33]. FigureFigure 10 10 shows shows thethe spectrogramsspectrograms ofof thethe wavewave recorders’recorders’ data and ambient seismic noisenoise obtained obtained fromfrom somesome channelschannels ofof thethe OBSOBS operatingoperating inin twotwo campaigns,campaigns, 2018–2019 andand 2019–2020. 2019–2020. It can be be seen seen from from the the tide tide gauges’ gauges’ records records that that ice-cover ice-cover smooths smooths out the out thewind wind waves waves (spectral (spectral periods periods 3–8 3–8s) above s) above the thedeployment deployment sites. sites. Spectrograms Spectrograms of the of thehorizontal horizontal and and vertical vertical CME-4311 CME-4311 sensors sensors show show that that the thewind wind waves waves in ice-free in ice-free time time pe- periodsriods cause cause pronounced pronounced seismi seismicc noise, noise, with with two two clear clear spectral spectral period period bands: bands: 5–10 5–10 s s (primary(primary microseisms microseisms causedcaused by direct direct transmis transmissionsion of of the the surface surface pressure pressure variations variations to tothe the seafloor) seafloor) and and 2–3 2–3 s s (secondary (secondary double-frequency double-frequency microseisms microseisms caused caused by by standing wavewave field) field) [34 [34].]. Ice-cover Ice-cover smooths smooths out out the the wind wind waves waves and and therefore therefore significantly significantly reduces re- theduces microseisms the microseisms level. However, level. However, more long-period more long-period oscillations oscillations (spectral (spectral periods periods larger thanlarger 30 than s) are 30 present s) are inpresent both ice-free in both and ice-free ice-covered and ice-covered time periods time and periods can be and caused can be by infragravitycaused by infragravity waves [35]. waves [35].

the vicinity of the St3 and Typ2 sites, (d) vertical MET sensor CME-4311 (2019–2020) for the site St3, (e) the hydrophone EDBOE RAS for the site Slope (2018–2019). Black solid line—ice concentration (%) at the deployment sites according to [33]. 3.3. Registration of the Teleseismic Signals Registration of large remote seismic events is one of the most important purposes of the instrumental seismological studies. Teleseismic signals, both body waves and surface Sensors 2021, 21, 3979 14 of 22

waves, are used for a broad range of tasks, including structural studies by different methods and seismic micro-zonation. We have obtained a signiﬁcant number of teleseismic signals in the Laptev Sea recorded by the broadband CME-4111 and CME4311 seismic sensors and also by the short-period CME-3311 sensors. Figure 11 shows the example of waveforms and FFT spectra of the earthquake with M = 6.3 that occurred in Alaska (2019-11-24 00:54:01 UTC) [36] recorded by the hydrophone and three-component CME-4311 channels of the MPSSR station at the site St3. Figure 12 shows the example of waveforms and FFT spectra of the earthquake with M = 7.1 that occurred in Alaska (2018-11-30 17:29:29 UTC) [36] recorded by the hydrophone and three-component CME-4111 channels of the GNS-C station at the site Slope. There are clear onsets of body waves and surface waves on the waveforms. Spectra shows a broad frequency band of recorded earthquake signals and seismic noise. Table 10 contains the list of remote earthquakes that are recorded at St4 and St5 sites, and their P- and S-wave onsets are clear enough to check it by the AK135 Travel Time Tables (campaign 2018–2019). The main information, such as origin time, magnitude, coordinates and depths, has been obtained from the USGS catalog [36]. Table 11 contains the same data for the remote earthquakes that were recorded at St3 and Typ2 sites (campaign 2019–2020).

Table 10. Remote earthquakes registered at St4 and St5 sites and checked by the AK135 Travel Time Tables (campaign 2018–2019).

◦ Depth, Distance, Time, UTC M Latitude Longitude Region km St4 St5 2018-11-18 6.8 −17.87 −178.93 540 99.0 98.6 Fiji 20:25:46.590 2018-11-25 6.3 34.36 45.74 18 54.8 55.3 Iran 16:37:32.830 2018-11-30 7.1 61.35 −149.96 46.7 30.2 29.7 Alaska 17:29:29.330 2018-12-01 6.4 −7.38 128.71 136 82.9 82.9 Indonesia 13:27:21.080 2018-12-05 New 7.5 −21.95 169.43 10 100.9 100.6 04:18:08.420 Caledonia 2018-12-20 7.3 55.10 164.70 16.56 24.7 24.4 Russia 17:01:55.150 2019-01-05 5.9 51.33 −178.12 30 32.1 31.7 Alaska 18:47:11.740 2019-01-06 6.6 2.26 126.76 43.21 73.2 73.2 Indonesia 17:27:18.980 2019-01-08 6.3 30.59 131.04 35 44.9 44.9 Japan 12:39:30.950 2019-01-15 6.6 −13.34 166.88 35 92.0 91.7 Vanuatu 18:06:34.300 Prince 2019-01-22 6.3 −10.41 119.02 24 86.0 86.1 Edward 05:10:03.480 Islands region 2019-02-01 6.7 14.68 −92.45 66 86.7 86.4 Mexico 16:14:12.329 2019-02-02 6 −2.85 100.07 20 79.9 80.2 Indonesia 09:27:36.030 Sensors 2021, 21, 3979 15 of 22

Table 11. The remote earthquakes registered at St3 and Typ2 sites and checked by the AK135 Travel Time Tables (campaign 2019–2020).

◦ Depth, Distance, Time, UTC M Latitude Longitude Region km St3 Typ2 2019-11-20 6.3 53.13 153.68 496 25.6 25.6 Russia 08:26:08.017 2019-11-20 6.2 19.45 101.36 10 58.3 58.9 Thailand 23:50:43.955 2019-11-24 6.3 51.38 −175.51 20 33.3 33.0 Alaska 00:54:01.053 2019-11-23 6.2 1.64 132.81 5 74.9 75.2 Indonesia 12:11:15.564 2019-11-26 6.4 41.51 19.53 22 53.5 53.7 Mamurras 02:54:12.872 2019-12-02 6 51.19 −178.10 28 32.9 32.6 Amatignak 05:01:54.821 2019-12-15 6.8 6.70 125.17 18 69.7 70.1 Philippines 06:11:51.155 2019-12-20 6.1 36.54 70.46 212 46.6 47.3 Afghanistan 11:39:52.874 2019-12-23 6 50.52 −129.76 10 44.6 43.9 Canada 20:56:23.555 2019-12-23 6 50.61 −129.94 10 44.4 43.8 Canada 19:49:43.086 2019-12-23 5.7 50.54 −129.83 10 44.5 43.9 Canada 19:13:25.075 2019-12-25 6.3 50.61 −129.96 6.58 44.4 43.8 Canada 03:36:01.626

3.4. Registration of the Signals from Local Earthquakes Local earthquakes’ distribution is an important source of information about present tectonic features of the region, location of seismogentic structures, active faults and their characteristics. Since the local instrumental studies are usually short-term, mainly weak microearthquakes with a magnitude M < 3 and relatively high frequency are recorded. Thus, seismic sensors must have appropriate sensitivity, dynamic range and frequency bands for high-quality recording of both long-period teleseismic signals and short-period signals from local microearthquakes. Figure 13 shows the example of waveforms and the FFT spectra of the local microearthquake with P-wave arrival at 2019-11-11 01:58:31 UTC, recorded by the hydrophone and three-component CME-4311 channels of the MPSSR station at the site St3. Since the earthquake signal is buried in long-period seismic noise, the bandpass filter of 2–45 Hz was applied for demonstrating the waveforms. Figure 14 shows the distribution of the earthquake epicenters in the Laptev Sea region obtained from the joint regional catalog (ISC [37], USGS [36], “Earthquakes of Russia” database [38]) and the catalog of the events that were most clearly recorded by the OBS (2018–2019). The OBS data confirm a general shift of the epicenter’s cloud towards the eastern part of the Laptev Sea, which was observed according to regional catalogs. This shift also implies the existence of a transcurrent fault zone. The approximate location of the supposed transcurrent fault on the outer shelf is characterized by a significant number of methane seeps [3]. Thus, this issue is complex and demands as much data as possible and careful seismotectonic interpretation. Sensors 2021, 21, x FOR PEER REVIEW 14 of 22

Figure 10. The spectrograms of the ambient seismic noise obtained from the records of: (a) wave recorder ARW-K14-1 (2018–2019) for the vicinity of the St4 and St5 sites (see Figure 9), (b) horizontal MET sensor CME-4311 (2018–2019) for the site St5, (c) wave recorder RBR virtuoso3D (2019– 2020) for the vicinity of the St3 and Typ2 sites, (d) vertical MET sensor CME-4311 (2019–2020) for the site St3, (e) the hydrophone EDBOE RAS for the site Slope (2018–2019). Black solid line—ice concentration (%) at the deployment sites according to [33].

3.3. Registration of the Teleseismic Signals Registration of large remote seismic events is one of the most important purposes of the instrumental seismological studies. Teleseismic signals, both body waves and surface waves, are used for a broad range of tasks, including structural studies by different methods and seismic micro-zonation. We have obtained a significant number of teleseismic signals in the Laptev Sea recorded by the broadband CME-4111 and CME4311 seismic sensors and also by the short-period CME-3311 sensors. Figure 11 shows the example of waveforms and FFT spectra of the earthquake with M = 6.3 that occurred in Alaska (2019-11-24 00:54:01 UTC) [36] recorded by the hydrophone and three-component CME-4311 channels of the MPSSR station at the site St3. Figure 12 shows the example of waveforms and FFT spectra of the earthquake with M = 7.1 that occurred in Alaska (2018-11-30 17:29:29 UTC) [36] recorded by the hydrophone and three-component CME-4111 channels of the GNS-C station at the site Slope. There are clear onsets of body waves and surface waves on the waveforms. Spectra shows a broad frequency band of recorded earthquake signals and seismic noise. Table 10 contains the list of remote earthquakes that are recorded at St4 and St5 sites, and their P- and S-wave onsets are clear enough to check it by the AK135 Travel Time Tables (campaign 2018–2019). The main information, such as origin time, magnitude, Sensors 2021, 21, 3979 coordinates and depths, has been obtained from the USGS catalog [36]. Table 11 contains16 of 22 the same data for the remote earthquakes that were recorded at St3 and Typ2 sites (campaign 2019–2020).

FigureFigure 11. 11.Waveforms Waveforms and and FFT FFT spectra spectra of theof earthquakethe earthquake with with M = 6.3M that= 6.3 occurred that occurred in Alaska in Alaska (2019- 11-24(2019-11-24 00:54:01 00:54:01 UTC) [36 UTC)] obtained [36] byobtained the hydrophone by the hydrophone and three-component and three-component CME-4311 channels CME-4311 of channels of the MPSSR station at the site St3: blue line—the spectra of the earthquake, green Sensors 2021, 21, x FOR PEER REVIEWthe MPSSR station at the site St3: blue line—the spectra of the earthquake, green line—the15 spectra of 22 of line—the spectra of the seismic noise preceding P-wave arrival. the seismic noise preceding P-wave arrival.

FigureFigure 12. 12.Waveforms Waveforms and and FFT FFT spectra spectra of of the the earthquake earthquake with with M M = 7.1= 7.1 that that occurred occurred in Alaskain Alaska (2018- 11-30(2018-11-30 17:29:29 17:29:29 UTC)[ 36UTC)] obtained [36] obtained by the hydrophone by the hydrophone and three-component and three-component CME-4111 CME-4111 channels of channels of the GNS-C station at the site Slope: blue line—the spectra of the earthquake, green the GNS-C station at the site Slope: blue line—the spectra of the earthquake, green line—the spectra line—the spectra of the seismic noise preceding P-wave arrival. of the seismic noise preceding P-wave arrival. Table 10. Remote earthquakes registered at St4 and St5 sites and checked by the AK135 Travel TimeFigure Tables 15(campaign shows 2018–2019). the cumulative recurrence curves for the eastern part of the Laptev Sea according to the joint regional catalog (ISC, USGS, “Earthquakes of Russia” database) Depth, Distance, ° Time, UTC M Latitude Longitude Region km St4 St5 2018-11-18 6.8 −17.87 −178.93 540 99.0 98.6 Fiji 20:25:46.590 2018-11-25 6.3 34.36 45.74 18 54.8 55.3 Iran 16:37:32.830 2018-11-30 7.1 61.35 −149.96 46.7 30.2 29.7 Alaska 17:29:29.330 2018-12-01 6.4 −7.38 128.71 136 82.9 82.9 Indonesia 13:27:21.080 2018-12-05 7.5 −21.95 169.43 10 100.9 100.6 New Caledonia 04:18:08.420 2018-12-20 7.3 55.10 164.70 16.56 24.7 24.4 Russia 17:01:55.150 2019-01-05 5.9 51.33 −178.12 30 32.1 31.7 Alaska 18:47:11.740 2019-01-06 6.6 2.26 126.76 43.21 73.2 73.2 Indonesia 17:27:18.980 2019-01-08 6.3 30.59 131.04 35 44.9 44.9 Japan 12:39:30.950 2019-01-15 6.6 −13.34 166.88 35 92.0 91.7 Vanuatu 18:06:34.300 2019-01-22 Prince Edward Is- 6.3 −10.41 119.02 24 86.0 86.1 05:10:03.480 lands region 2019-02-01 6.7 14.68 −92.45 66 86.7 86.4 Mexico 16:14:12.329

SensorsSensors 2021 2021, ,21 21, ,x x FOR FOR PEERPEER REVIEWREVIEW 1717 of of 22 22

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Figure 15 shows thethe cumulativecumulative recurrencerecurrence curves curves for for the the eastern eastern part part of of the the Laptev Laptev Sea according to the joint regional catalog (ISC, USGS, “Earthquakes of Russia” database) andSea theaccording joint regional to the catalogjoint regional combined catalog with (I theSC, catalog USGS, of“Earthquakes the events that of wereRussia” most database) clearly and the joint regional catalogcatalog combinedcombined withwith thethe catalog catalog of of the the events events that that were were most most recordedclearly recorded by the OBS by the (2018–2019). OBS (2018–2019). It is clear It thatis clear the OBSthat the data OBS signiﬁcantly data significantly contribute con- to clearly recorded by the OBS (2018–2019). It is clear that the OBS data significantly con- completenesstribute to completeness of the events of the catalog events in thecatalog magnitude in the magnitude range of 1–3. range of 1–3. tribute to completeness of the events catalog in the magnitude range of 1–3.

FigureFigure 13.13. Waveforms (bandpass (bandpass filter ﬁlter 2–45 2–45 Hz Hz applied) applied) and and the the FFT FFT spectra spectra of the of thelocal local micro- mi- croearthquakeFigureearthquake 13. Waveforms with with P-arrival P-arrival (bandpass at 2019-11-112019-11-11 filter 2–45 01:58:3101:58:31 Hz applied) UTC, UTC,obtained and obtained the byFFT theby spectra hydrophonethe hydrophoneof the local and three-micro-and earthquakethree-component with CME-4311P-arrival channelsat 2019-11-11 of the MPSSR01:58:31 stat UTC,ion at theobtained site St3: by blue the line—the hydrophone spectra andof component CME-4311 channels of the MPSSR station at the site St3: blue line—the spectra of the three-componentthe earthquake, green CME-4311 line—the channels spectra of of the the MPSSR seismic statnoiseion preceding at the site P-wave St3: blue arrival. line—the spectra of earthquake, green line—the spectra of the seismic noise preceding P-wave arrival. the earthquake, green line—the spectra of the seismic noise preceding P-wave arrival.

Figure 14. The distribution of the earthquake epicenters in the Laptev Sea region: red circles—the Figurejoint regional 14. The catalog distribution by ISC of [37], the USGS earthquake [36] and epicenters “Earthquakes in the of Laptev Russia” Sea database region: [38], red blue circles— cir- Figure 14. The distribution of the earthquake epicenters in the Laptev Sea region: red circles—the thecles—the joint regional catalog catalogof the events by ISC that [37], were USGS mo [36st] clearly and “Earthquakes recorded by of the Russia” OBS (2018–2019), database [38 yellow], blue joint regional catalog by ISC [37], USGS [36] and “Earthquakes of Russia” database [38], blue cir- stars—methane seeps location on the outer shelf [3], dotted line—supposed boundary between the circles—thecles—the catalog catalog of of the the events events that that were were mo mostst clearly recordedrecorded byby thethe OBSOBS (2018–2019), (2018–2019), yellow yellow Eurasian and North American tectonic plates [39], solid line—supposed transcurrent fault. stars—methanestars—methane seeps seeps location location on on the the outer outer shelf shelf [ 3[3],], dotted dotted line—supposed line—supposed boundary boundary between between the the EurasianEurasian and and North North American American tectonic tectonic plates plates [ 39[39],], solid solid line—supposed line—supposed transcurrent transcurrent fault. fault.

Sensors 2021, 21, x FOR PEER REVIEW 18 of 22 Sensors 2021, 21, 3979 18 of 22

FigureFigure 15. 15. CumulativeCumulative recurrence recurrence curves curves for for eastern eastern part part of of the the Laptev Laptev Sea: Sea: blue blue circles—according circles—according toto the the joint joint regional regional catalog catalog by by ISC ISC [37], [37], US USGSGS [36] [36 ]and and “Earthquakes “Earthquakes of of Russia” Russia” database database [38], [38], red red stars—thestars—the joint joint regional regional catalog catalog combined combined with with th thee catalog catalog of of the the events events that that were were most most clearly clearly recordedrecorded by by the the OBS OBS (2018–2019). (2018–2019).

3.5.3.5. Site Site Response Response Analysis Analysis SiteSite response response analysis analysis is is a apart part of of seismic seismic microzonation. microzonation. It Itis is conducted conducted both both by by empiricalempirical methods, methods, nume numericalrical modeling modeling and and instrumental instrumental studies studies [40]. [40]. The The site site response response analysisanalysis for for the the water water areas hashas itsits peculiarities.peculiarities. Some Some investigations investigations based based on on the the seafloor sea- floorseismic seismic records’ records’ analysis analysis show show that that the the vertical vertical components components of theof the seafloor seafloor motions motions are aresignificantly significantly lower lower than than those those of the of onshorethe onshore motions motions near thenear P-wave the P-wave resonant resonant frequencies, frequencies,caused by caused the water by the layer water above layer the above station the [41 station–43]. [41–43]. TheThe OBS OBS records records of of local local earthquakes earthquakes in in the the Laptev Laptev Sea Sea allow allow us us to to demonstrate demonstrate this this featurefeature of of marine marine site site response response analysis. analysis. Figure Figure 16 16 shows shows V/H V/H curves curves (Fourier (Fourier spectra spectra and and responseresponse spectra) spectra) for for 10 10 local local earthquakes earthquakes obta obtainedined at at St3 St3 and and Typ2 Typ2 sites. sites. A A similar similar clear clear dropdrop of of V/H V/H curves curves at atfrequencies frequencies of of 3–10 3–10 Hz Hz is isobserved observed for for both both sites, sites, which which are are located located atat a asignificant significant distance distance but but under under a asimilar similar water water column column above above the the sites sites (51 (51 and and 61 61 m, m, respectively).respectively). The The theoretical theoretical estimate estimate of of the the P-wave P-wave resonant resonant frequencies frequencies is is 5–7 5–7 Hz Hz for for thesethese depths depths (obtained (obtained by by the the formula formula f f= = c/4H, c/4H, where where c cis is the the sound sound speed speed in in water water and and HH is is the the depth depth of of the the sea sea [42]). [42]). Thus, Thus, the the es estimatedtimated and and the the observed observed values values are are in in good good agreement.agreement.

SensorsSensors2021 2021,,21 21,, 3979 x FOR PEER REVIEW 1919 of 2222

FigureFigure 16.16. V/H spectral spectral ratio ratio curves of 10 local earthquakes obtained atat thethe St3St3 sitesite ((aa,,bb)) andand forfor thethe Typ2Typ2 sitesite ((cc,d,d).). GrayGray lines—thelines—the earthquakeearthquake spectra,spectra, redred line—theline—the averageaverage curvecurve forfor thethe FFTFFT spectralspectral ratios,ratios, blueblue line—theline—the averageaverage curvecurve forfor thethe responseresponse spectralspectral ratios.ratios. 4. Discussion and Conclusions 4. Discussion and Conclusions In this article, we have highlighted various seismological applications of the OBS In this article, we have highlighted various seismological applications of the OBS based on the broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s). There are based on the broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s). There are several modifications of the OBS developed by the IO RAS and IP Ilinskiy A.D., depending several modifications of the OBS developed by the IO RAS and IP Ilinskiy A.D., de- on the permissible depths and frequency ranges. The OBS records obtained during the pending on the permissible depths and frequency ranges. The OBS records obtained pilot installations in the Laptev Sea were used to demonstrate the applicability of the OBS during the pilot installations in the Laptev Sea were used to demonstrate the applicability for seismic hazard assessment tasks in the Arctic seas. of the OBS for seismic hazard assessment tasks in the Arctic seas. Two field campaigns in 2018–2019 and in 2019–2020 resulted in high-quality seismic Two field campaigns in 2018–2019 and in 2019–2020 resulted in high-quality seismic records. When using the standard alkaline battery packs, we have obtained 3–5 months of records. When using the standard alkaline battery packs, we have obtained 3–5 months recordings. Using lithium batteries allows for making 7–8 months of recordings. It turned outof recordings. that the OBS Using recording lithium capabilities batteries onallows the Arctic for making shelf and 7–8 the months upper of slope recordings. are highly It dependentturned out onthat the the level OBS of recording ambient seismiccapabilities noise, on which, the Arctic in turn, shelf is and influenced the upper by theslope wind are waves.highly Adependent strong noise on the level, level at least of ambient in the autumn seismic ice-free noise, which, time period, in turn, practically is influenced leads by to thethe impossibilitywind waves. ofA obtainingstrong noise high-quality level, at leas recordst in the of autumn earthquakes. ice-free Ice-cover time period, smooths practi- out windcally leads waves to and the significantlyimpossibility reduces of obtaining noise hi level.gh-quality Thus, itrecords is recommended of earthquakes. to conduct Ice-cover the OBSsmooths recording out wind on thewaves Arctic and shelf significantly in ice-covered reduces time noise periods. level. For Thus, this, it itis isrecommended necessary to deployto conduct the the OBS OBS in September–October recording on the Arctic and shelf to dismantle in ice-covered in a year, time or periods. at least For inJune. this, it At is thenecessary same time, to deploy the requirements the OBS in September–October for the reliability of and the to power dismantle supply in anda year, dismantling or at least mechanismsin June. At the are same significantly time, the increased. requirements Theadditional for the reliability possibility of the of trawling power supply stations and in suchdismantling conditions mechanisms is quite useful. are significantly Our experience increased. has shown The additional high efficiency possibility of this of method trawl- ofing the stations OBS dismantling in such conditions on the shelfis quite after useful. an annual Our experience setting. However, has shown the high risks efficiency of losing OBSof this are method quite high, of the especially OBS dismantling in shallow on waters the shelf due after to the an presence annual setting. of stamukhi. However, the risksThe of losing main characteristicsOBS are quite of high, the broadbandespecially METin shallow sensors, waters such du ase permissible to the presence installa- of tionstamukhi. angles, temperature range, sensitivity, dynamic range and frequency band, appeared

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to be suitable for obtaining the OBS records under the Arctic conditions to solve the seismological problems. The OBS deployments in the Laptev Sea resulted in a significant number of clear teleseismic arrivals, both body waves and surface waves within a broad frequency range. It allows conducting structural studies, for example with such approaches as the receiver function technique or surface wave group velocity inversion. For these methods, useful teleseismic waves have frequencies in the range of 0.1 to 1 Hz. In addition to teleseismic signals, we have received a significant number of short- period signals from local microearthquakes. In tectonically active regions, such as the Laptev Sea region, the local microearthquakes’ distribution is crucial for describing present seismotectonic processes and for seismic hazard assessment. Another application of the OBS recordings for seismic hazard assessment is the site response analysis. Since this is a spectral approach, a wide frequency range is required. The obtained OBS records demonstrated peculiarities of the offshore site response analysis, such as the decrease in response at the frequencies close to the P-wave resonant frequency, caused by the water layer above the OBS. Seismological applications highlighted in the paper have promising first results. Each topic can be further developed as a separate in-depth study. Especially, it concerns the possible link between the tectonic processes and bottom-up progressive thawing of subsea permafrost, which can accelerate massive methane release in the Siberian Arctic Seas [5,44]. Recent studies show the “mixing” origin of this methane in the innermost shelf [45] and deep termogenic origin of the escaping gas in the outer shelf of the Laptev Sea [46]. New information on the microearthquakes’ distribution and the structure of the lithosphere in this area may shed light on the mechanisms of this phenomenon. Another promising direction is a deeper study of the relationship of microseisms with meteorological parameters, which will allow a more detailed description of the mechanisms of their generation. In addition to the fundamental research, a purely engineering direction is no less important. The study of the peculiarities of the influence of the sea soils and water column on the propagating seismic waves will be useful in the future construction of the extracting infrastructure in the perspective Arctic regions. The described OBS have demonstrated efficiency in solving a wide range of seismological problems and will be actively used in further Arctic expeditions.

Author Contributions: Conceptualization, A.A.K.; methodology, A.A.K.; software, A.A.K., M.E.K. and M.A.N.; validation, A.A.K.; formal analysis, A.A.K.; investigation, A.A.K., I.V.E., S.A.K., D.A.I., M.E.K., M.A.N., G.O.V., D.D.R., E.A.R. and A.V.N.; resources, A.A.K., D.A.I., O.Y.G., M.E.K., V.N.I. and G.K.T.; data curation, A.A.K., I.P.S. and D.A.I.; writing—original draft preparation, A.A.K.; writing—review and editing, all authors; visualization, A.A.K., I.V.E., D.A.I. and A.V.N.; supervision, A.A.K., L.I.L., I.P.S., K.A.R. and I.P.M.; project administration, A.A.K., K.A.R., I.P.S. and I.P.M.; funding acquisition, A.A.K., I.V.E., K.A.R. and I.P.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the Russian Foundation for Basic Research, project no. 20-05-00533 (collecting and processing the ocean bottom seismograph records, description of the basics of OBS modification for shallow water); the Russian Foundation for Basic Research, project no. 19-37-90106 (description of the basics of MET sensors and their technical features); the Russian Foundation for Basic Research, project no. 20-35-90096 (collecting and processing the records of the wave recorders); the Russian Science Foundation, project no. 20-77-00034 (earthquake site response analysis using OBS records); the Russian State Assignment, project no. 0128-2021-0011 (description of the basics of OBS modification for deep water); the Russian Science Foundation, project no. 21-77- 30001 (tectonic application of OBS records, description of the connection of tectonics with methane seepage, climatic consequences of permafrost degradation regarding seismotectonic activities). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Sensors 2021, 21, 3979 21 of 22

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