A self-contained seismic station that has a modular structure adjustable to different operational conditions—like onshore, offshore to 500-m depth, and at transition zones—has been developed and field tested. The station operation frequency band is 1–300 Hz, which is wider than that of the majority of seismic stations based on using standard (10 Hz) geophones. Such improvement was achieved through the use of molecular-electronic transfer seismic sensors that allow for covering a low-frequency part of the spectrum that is needed for broadband processing and receiving information on subsurface formation. Basically, the system includes a module of sensing elements, a module of digital electronics, and a battery module. Optionally, a self-surfacing module could be used. The field test of the station was performed in August 2016 in the Sea of Azov.
There are a number of autonomous seismic stations designed for use onshore, in a transition zone, and on the continental shelf up to 700–750-m depth (FairfieldNodal 2016; Geospace Technologies 2012; Hovland 2016). Usually systems that were developed to work in marine environments and transition zones are unsuitable for deployment on land due to their weight and size. At the same time, onshore stations are not suitable for use in marine environments due to the lack of water tightness. This means that depending on the environmental conditions, a different specific type of station is required. For example, if it is necessary to survey an area that changes from onshore to the offshore and has deep channels (like the situation on the northeastern shore of Mozambique), then a set of three types of stations must be used. Another limitation of typical systems is the limited frequency response of standard electromechanical sensors, which exhibit poor sensitivity to frequencies below 10–12 Hz.
To overcome the existing problems, we developed a self-contained seismic station (SCSS) that has a modular structure adjustable for different operational conditions onshore, offshore to depths down to 500 m, and transition zones. The frequency band of our station is 1–300 Hz. If compared to standard 10-Hz geophones, it covers a lower-frequency part of the spectrum that is needed for broadband processing and receiving information from a deep subsurface formation. The system has a modular structure that allows us to deploy only those modules necessary to acquire data in a given environment. Basically, the system includes a module of sensing elements (MSE), a module of digital electronics (MDE), and a battery module (BM). Optionally, a self-surfacing module (SSM) could be used.
The MSE consists of the three-component molecular-electronic transfer (MET) seismic sensor MTSS-2003 (R-sensors 2014; Agafonov et al. 2014), a 2-Hz commercial hydrophone (Benthowave Instrument Inc. 2016), auxiliary sensors, and a signal conditioning electronic board. The MET seismic sensor is critical for achieving low operation frequencies. The MET operating principles are described by Huang et al. (2013) and Agafonov et al. (2015). The operation of the MET sensors is characterized by the use of a liquid inertial mass of a water-based solution of potassium iodide or lithium iodide with a small amount of molecular iodide. The sensing element is a microscopic four-electrode electrochemical cell. There is a voltage applied between electrodes, and the electrical current passing between these electrodes is sensitive to the liquid motion produced by inertial forces. The sensors are very rugged due to the lack of delicate mechanical parts and provide a very high conversion coefficient in a low-frequency portion of the spectrum. The MTSS-2003 model is equipped with a force-balance feedback mechanism intended to optimize measurement stability under varying environmental parameters, including temperature stability in the −30° to +65°C operation range (Zaitsev et al. 2016). The sensor scheme and actual view of the three-component unit are presented in Fig. 1. The experimental Bode plots of the MTSS-2003 sensor response compared to the Bode plots for a standard 10-Hz geophone are shown in Fig. 2, confirming a significantly wider frequency operating range. In the system we also include auxiliary sensors, like the STMicroelectronics LSM303 three-component accelerometer and magnetic sensor used to measure the system orientation relative to the gravity and magnetic field direction (STMicroelectronics 2013).
The basic functions of the MDE are analog-to-digital conversion of sensor signals, time synchronization, and data recording at the internal memory. These functions are provided by the system consisting of a microcontroller, an analog-to-digital converter, a GPS/Global Navigation Satellite System (GLONASS) module, a precision quartz generator, and a flash memory drive. In addition, the system includes a Wi-Fi module. The information received by the accelerometer and magnetic sensor is used for further processing and interpretation of seismic signals to get true vertical, north, and east motions regardless of the accidental orientation of the seismic system on the seafloor. The total consumption of the MDE in an autonomous mode does not exceed 400 mW.
The quality of the data recording is primarily determined by the characteristics of the analog-to-digital converter (ADC). The ADS131E04 chip by Texas Instruments combined with a source of Analog Devices ADR444 voltage reference is used in the system (Texas Instruments 2012; Analog Devices 2016). The ADC contains four 24-bit sigma-delta converters operating synchronously and driven by a common digital logic. With the output sampling rate of 1000 Hz and no gain, this combination gives a dynamic range of 122 dB that is equivalent to 20.4 noise-free bits.
For time synchronization based on the GPS time, the following system is used: uBlox MAX-7 for the GPS module, the GK152-UN quartz generator by the Russian research-and-production company BMG PLUS, and the microcontroller based on the ARM Cortex-M architecture (Ublox 2014; BMG PLUS 2005). The microcontroller measures the quartz generator frequency on the base of the precise time stamps from the GPS module. This frequency is adjusted by varying the voltage that is formed by a microcontroller pulse-width modulation (PWM) output. The system operates on the principle of feedback, that is, the controlling voltage depends on the measured error of the quartz generator. The quartz generator frequency is 2048 kHz, which corresponds to a 488-ns period. The GPS module provides a pulse-per-second (PPS) signal with the precision of tens of nanoseconds in a steady reception of satellite signals. Thus, the microcontroller counter achieves maximum precision of ±0.5 counts of quartz pulses in a second. For a 2-MHz clock this corresponds to a tuning precision better than 1 ppm. In the water where the GPS signals are not available, the precision of the timing is determined by the temperature instability of the quartz generator, which is 0.2 ppm °C−1 (BMG PLUS 2005). Assuming that the stability of the water temperature near the seafloor is a hundredth of a degree (Lachenbruch and Marshall 1968) and that the linear component of drift can be eliminated by comparing the exact start and end times of the record, the rough estimate of the timing error associated with a 30-day deployment is ±5 ms.
A micro Secure Digital (SD) memory card is used as internal data storage. The maximal storage volume is 32 GB; the File Allocation Table 32 (FAT32) file system is used. This volume is sufficient for autonomous data recording for 30 days at a sampling frequency of 1000 Hz. The data are recorded in an internal format, which is a series of interlaced raw ADC samples and every-minute headers that contain service information and outputs from the additional sensors. The recorded files contain all the necessary information to be converted into common seismic formats, such as the mini Standard for the Exchange of Earthquake Data (miniSEED) and the Society of Exploration Geophysicists (SEG-Y).
The station is linked to a central station either via wired (USB) or wireless connection (Wi-Fi). Both types of connections allow for monitoring the state of the station, transmitting commands and parameters, and observing real-time sensor signals. The USB connection also allows for a quick download of the recorded data at the USB high speed rate of up to 480 Mb s−1. An advantage of a wireless connection to stations deployed outdoors is that the station operator is not obliged to remove the protective cover in order to connect the cable for checking the station. Thus, the station always remains protected from moisture, sand, and dust.
There are two modes of system operation—the command mode and the calendar mode. In the command mode, data recording can be started and stopped by the user; the station must be connected to a PC via wired or wireless connection. This mode is suitable for field operations onshore as well as for testing the station. The calendar mode ensures autonomous operation of the station.
The housing of the SCSS (Fig. 3) is a set of two hermetic cylinders. The sensors and MDE are placed in one cylinder, and the battery module in the second one. The connection between the cylinders is made by a bent pipe connected tightly with the housing covers and serving as a handle to carry the station. The units are electrically connected using a cable passing through the handle. The cylinders are made of stainless steel to ensure corrosion resistance. An impact-resistant plastic housing was developed to provide additional protection when using the system in harsh field environments.
The SCSS field tests were conducted together with the Institute of Physics of the Earth of the Russian Academy of Sciences expedition to study an underwater mud volcano located near the town of Golubitskaya, in the Krasnodar region of Russia. An array of stations included 17 locations on the seafloor and one reference point onshore (see Fig. 4). The seismic stations were placed at the reference point and the two ends of the profile: −1800 and +1800 m.
The seismic stations provided a synchronous 68-h recording of seismic and hydroacoustic signals at the scheduled points. The coordinates of the underwater locations as well as the precise time signals and ambient temperature were recorded. In particular, the earthquake that occurred in Ukraine on 7 August 2016 was recorded (Russian Academy of Science Geophysical Survey 2016). The distance to the earthquake epicenter was about 200 km. The seismogram is shown in Fig. 4, bottom part. For the seismic geophone channels, the spectra are presented in Fig. 5, thus showing the significant contribution of the low-frequency components into the recorded signal.
Thus, the seismic stations have proven their suitability for seismic and seismological studies on the sea bottom and in the transition zone, and have proved useful for recording low-frequency seismic events like regional earthquakes.
The study was supported by the Russian Ministry of Science and Education, Project ID RFMEFI57814X0013.