The use of a portable seismic station on the territory of Karelia

. This paper provides examples of usage, as well as the technical specifications of the portable seismic station Cossack Ranger II. Examples of in-field use are presented, demonstrating the convenience, flexibility and mobility of this type of equipment.


Introduction
The seismic hazard in Karelia is determined by seismic activity within the Kandalaksha-Dvina paleorift and the Ladoga-Botnian shear zone.These zones exhibit increased layering, the modification of the Moho boundary, and an increase in the thickness (up to 10-15 km) of the transition layer in the lower crust, reaching depths to the Moho boundary (50-60 km) [1].Anthropogenic activities accelerate dynamic processes by increasing weak and very weak seismic events.Currently, there is evidence of technogenic seismicity in the Kostomuksha ore region, where over 1 billion tons of ore have already been extracted.
The seismicity in Karelia is considered insignificant, and seismic maps of the Fennoscandian region depict isolated events, both weak in terms of instrumental observations and historical records [2].Starting from the Late Precambrian, the main features of the recent tectonic stage are determined by the tectonic regime.Its characteristics include the manifestation of a positively directed type of development and glacioisostatic compensatory movements during active deglaciation of the ice sheet.The uplift of the shield and the high level of horizontal compressive stress in the Earth's crust are determined by the presence of an asthenospheric lens in the lower horizons of the Earth's crust and horizontal compression influenced by the Mid-Atlantic Ridge.These factors also explain the increased seismicity of the Fennoscandia relative to the territory of the Russian Plate [3].
One of the main parameters that determine the quality of any seismic station is the level of inherent seismic receiver noise and the dynamic range of the recorder.The adjustment of the period of inherent noise in sensors such as the SM3-KV is done manually after their final installation, which is quite inconvenient.To eliminate differences between sensors in a station network, the Cossack Ranger I (CR-I) structure introduced a software equalizer, which is also present in the new Cossack Ranger II (CR-II) version.Geophones can be used in a significant number of cases as short-period seismic receivers [4].The Cossack Ranger II three-component high-frequency seismic station was tested for not only studying the region's seismicity but also for practical applications.

Geophone as a seismic receiver
In order to study the seismic regime and create a modern geodynamic model of Karelia, seismic observations were organized in the Geophysics Laboratory of the Institute of Geology, Karelian Research Center of the Russian Academy of Sciences, starting in 1999.From October 26, 1999, to October 19, 2001, experimental observations were conducted here using the NARS CSD20 digital broadband seismic station developed by Utrecht University (Netherlands).Due to the low seismic activity in Karelia and the small magnitudes of earthquakes, as well as equipment and data collection imperfections, only limited information reflecting the real picture of seismicity in Karelia was obtained.Specifically, it was not possible to accurately determine the epicenter location and time of seismic events.The energy of a significant portion of weak earthquakes is concentrated in the high-frequency range of the spectrum, around 5-20 Hz, and the amplitude of seismic waves leaving the source is relatively small.The signals from such earthquakes become faint against the background of natural and anthropogenic noise even at distances of several hundred kilometers.In such circumstances, short-period seismic stations with good frequency resolution and precise synchronization with world time are required to create a seismological network.In 2000, the automatic station Cossack Ranger I (CR-I) [5], which met these requirements, was installed at the Petrozavodsk seismic station.
Rapid progress in electronics led to the emergence of high-performance compact microcontrollers and more accurate analog-to-digital converters than those used in CR-I.Therefore, in 2003, the development of a new version of the Cossack Ranger data acquisition system, Cossack Ranger II (CR-II), was initiated.This development was completed by the beginning of 2005 [3].The versatility of the structure, precise (to microseconds) synchronization with world time, and high data quality provided by CR-II determined its wide application in the PGI KRC RAS observatories for collecting various types of geophysical data.Cossack Ranger II is a three-component seismic station based on GS-11D geophones, consisting of four modules: seismometer, ADC, synchronizer and GPS receiver.The data recorder can be any type of computer from a personal computer to a laptop supporting USB or RS232 port. Figure 1 shows a simplified structural diagram of the seismic station based on CR-II and figure 2 shows the acceleration response.The acceleration of particle conversion is performed by a three-component seismometer with an electromagnetic transducer.When developing the Cossack Ranger station, the emphasis was placed on transferring the maximum number of functions from the hardware to the software part.The external device management and data collection programs operate under the Linux operating system.

Seismic monitoring
The most expensive component of modern seismological observations is the seismometer itself, and the seismic receiver based on GS-11D geophones (Figure 3) was able to address this issue.In the year 2000, the Cossack Ranger seismic station was installed at the "Petrozavodsk" station, and a total of 4 such stations were subsequently installed throughout Karelia.Typically, a series of the most suitable locations (based on external and geological conditions) are chosen for the installation of a seismic station, where an analysis of the microseismic background is conducted, resulting in the selection of the most appropriate location [6].Since 2006, the Cossack Ranger II data acquisition systems have equipped the regional seismic observation network of the Institute of Geology of the Karelian Research Centre of the Russian Academy of Sciences.In addition to the shortperiod station "Petrozavodsk" (PTRZE), three digital stations have been installed: BELME (Belomorsk), KOSTE (Kostomuksha), PITKE (Pitkäranta) (Figure 3).Having proven itself as a stationary seismic station, the Cossack Ranger, based on GS-11D geophones, was tested as a mobile portable station on a military range.At the military range near the village of Novaya Vilga, ammunition that has reached the end of its service life is removed from service and disposed of.There was a need to determine the permissible levels of seismic impact to ensure the seismic stability of the structures under construction.For existing buildings and structures, the goal was to determine the number of simultaneously detonated munitions to ensure seismic stability up to the corresponding design seismicity level, as factors such as physical deterioration of structures, accumulation of damage, reinforcement corrosion, and others contribute to a decrease in the load-bearing capacity of buildings, especially when subjected to seismic loads [7].In 2010, registration of ammunition explosions was carried out in the village of Novaya Vilga using the portable seismic station Cossack Ranger II of the Institute of Geology of the Karelian Research Centre of the Russian Academy of Sciences.The analysis of the obtained results allowed for the assessment of the intensity of seismic impacts transmitted from the ground to the structures, the selection of the optimal quantity of munitions, and the calculation of safe blast locations.Thanks to the work carried out from June to September 2010, expired ammunition was safely destroyed without causing damage to the residents of Novaya Vilga.
Acoustic impact in quarries is considered weak (up to 500 m) since the main energy of the explosion is expended on rock fragmentation and, to some extent, determines the quality of the explosion [8].Seismic and sonic wave effects of the explosion were recorded at the mill of the Kondopoga Bakery Combine at two levels: on the ground (seismic sensor installed on the asphalt surface oriented azimuthally towards the explosion point) and on the 6th floor of the mill (seismic sensor oriented in the same azimuth).Two sets of seismic recorders based on GS-11D geophones were used for measurements.In seismology, the seismic effect of an earthquake is primarily determined by three parameters: the level of amplitudes, the spectral composition, and the duration of vibrations.The latter factor (in the case of resonance) can be crucial for the destruction of structures, whereas short-term loading, even with a very high peak acceleration value, may not be dangerous.The task of assessing seismic hazard, seismic risk and seismic protection is complicated by poorly predictable effects of resonant amplification of seismic vibrations by near-surface soils.Studies have shown that when the maximum frequency of vibrations in the seismic impact coincides with the natural frequencies of buildings and structures, resonant phenomena can occur, which can be one of the causes of damage and destruction to residential buildings and industrial structures.Recommendations were provided to the company conducting drilling and blasting operations to reduce the amount of explosive material.
The portable seismic station was used to calculate the time in the source and analyze the peculiarities of the recordings.A field trip was made to the quarries of the Kostomuksha GOK.The purpose of the work was to obtain accurate information that could be used in the processing of seismic events.Observations were made on preparation, charging, and blasting.The obtained results were compared with the data from the national seismic networks of Finland and Norway.The portable seismic station installed in the Kostomuksha GOK quarries provided new data on the source time and other characteristics [9,10].These values will allow us to calculate a more accurate value of the travel time to other stations in the region, which can be used in the processing and identification of Kostomuksha explosions.

Conclusion
The software determined the flexibility of configuration and ease of operation of the system as a whole.The versatility of the structure, precise synchronization with global time to fractions of microseconds, and the high quality of the obtained data have determined its wide application not only at the Geophysical Observatory of the Karelian Research Centre of the Russian Academy of Sciences but also at the seismic stations of the Karelian network.Subsequently, the Cossack Ranger II seismic station was involved in various field observations in the territory of Karelia, where it demonstrated its convenience, flexibility, and operational mobility, enabling launch and installation by a single person.A network covering a certain area consisting of seismic recorders of this type allows for a prompt assessment of the variability of seismic field behavior, identification of the most active areas, and formulation of predictive conclusions.Work is being carried out to assess the possibility of localizing seismic events in space using only energy characteristics of the recorded seismic field.

Fig. 1 .
Fig. 1.Structural diagram of the CR-II seismic station and scheme for the restoration of the CR-II station with world time.Developer Yu.V. Fedorenko, 2005.

Fig. 2 .
Fig. 2. A response to the accelerationof the portable seismic station Cossack Ranger II.