A Compact Ocean Bottom Electromagnetic Receiver and Seismometer

Joint marine electromagnetic (EM) and seismic interpretation are widely used for offshore gas hydrate and petroleum exploration, produce better estimates of lithology and fluids, and decrease 10 the risk of low gas saturation. However, joint data acquisition is not commonly employed. Current marine EM data acquisition depends on an ocean bottom electromagnetic receiver (OBEM) and current seismic exploration methods use seismometers. Joint simultaneous data acquisition can decrease costs and improve efficiency; yet conventional independent data receivers have several drawbacks, including large size, high costs, position errors, and low operational efficiency. To address these limitations, we 15 developed a compact ocean bottom electromagnetic receiver and seismometer (OBEMS). Based on existing ocean bottom E-field receiver (OBE) specifications, including low noise levels, low power consumption, and low clock-drift error, we integrated two induction coils for the magnetic sensor and a three-axis omnidirectional geophone for the seismic sensor and assembled an ultra-short base line (USBL) transponder as the position sensor, which improved position accuracy and operational 20 efficiency while reducing field data acquisition costs. The resulting OBEMS has a noise level of 0.1 nV/m/rt (Hz) at 1 Hz in E-field and 0.1 pT/rt (Hz) at 1 Hz in B-field and a 30 day battery lifetime. It also supports a WiFi interface for configuring data acquisition parameters and data download. Offshore acquisition was performed to evaluate the system’s field performance during offshore gas hydrate exploration. The OBEMS functioned effectively throughout operation and field testing. The OBEMS 25 therefore functions as a low cost, compact, and highly efficient joint data acquisition method.

, which are dependent on data post-processing. Position errors may lead to reduced inversion accuracy.
KMS and GeoSYN developed a GEOSYN/KMS 870-VectorSeisEM Broadband-Ocean bottom station, which is a broadband 4C seismic/6C electromagnetic node for shallow and deep water geophysical 80 applications. Using a single survey vessel, Petroleum Geo-Services Inc. can acquire high-density EM data simultaneously with 2D GeoStreamer® seismic data, or high-density 3D EM data over existing, or planned 3D seismic data. Offshore high-density joint EM and seismic acquisition and integrated data analysis represents a stepwise change in the application of EM technology. Both technologies seek to mitigate risk when searching for and extracting oil and gas. During 2010, we acquired coincident 85 marine CSEM and OBS data when PGS conducted one of the first field trials of their towed streamer EM system at the Troll field in the Norwegian North Sea (Zhdanov et al. 2012). The towed-streamer EM system allows CSEM data to be acquired simultaneously with seismic data over very large areas, resulting in higher production rates and lower costs than for conventional CSEM acquisition.
China University of Geosciences (Beijing) (CUGB) developed an OBEM in 1998(Deng et al., 2003. 90 During the past 20 years, CUGB has successfully used its OBEM equipment in deep EM surveys for gas hydrate mapping and hydrocarbon exploration (Wei et al., 2009;Jing et al., 2016). The OBEM has also been widely used for marine magnetotelluric and CSEM measurements. The current OBEM system has an acoustic telemetry modem and folding-arm mechanism (Chen et al., 2015), with low noise levels and low time drift errors. In 2014, CUGB developed a micro OBE for low cost and highly 95 efficient data acquisition (Chen et al., 2017). To achieve joint EM and seismic data acquisition, the instrument was upgraded from an existing micro OBE by: 1) integrating a three-axis omni-directional geophone for seismic parameter measurements; 2) installing two induction coils for horizontal magnetic field component measurements; and 3) installing an ultra-short baseline (USBL) transponder for tracking seafloor position as the system ascends after release. 100 The OBEMS has been mechanically optimized to satisfy all technical requirements for simultaneous joint seismic and electromagnetic data acquisition. This technical advancement permits enhanced modeling and simultaneous interpretation of both datasets, which minimizes acquisition costs. The advantages of this OBEMS include 1) lower cost and higher efficiency of both the instrument and offshore data acquisition, as the same cost includes more nodes needed to improve horizontal 105 resolution and 2) a smaller seafloor instrument position error, which decreases the inversion error. The OBEMS system can also replace an OBEM as the receiver in marine CSEM surveys. In addition, the OBEMS can be used for OBS observations. In the future, a hydrophone will be added to the OBEMS system to allow measurement of the acoustic pressure field.

Instrument specifications 110
To achieve joint EM and seismic data acquisition with the goals of reducing the data acquisition cost and improving operational efficiency, we developed a new OBEMS. The OBEMS was then used to record seafloor EM field and motion signals. Figure 1 shows a schematic of the system. The OBEMS consists of a nylon frame, two glass spheres, a red flag, a transducer, a USBL transponder, a data logger, a battery, a geophone, an electrode, and an anchor. The equipment is fixed on a nylon frame 115 https://doi.org/10.5194/gi-2019-25 Preprint. Discussion started: 23 September 2019 c Author(s) 2019. CC BY 4.0 License. measuring 105cm  55cm  65cm. All electronics are installed inside a 17-inch glass sphere except the transducer and USBL, while the other glass sphere provides buoyancy. We used Ag/AgCl electrodes to measure the electric voltage in the Ex dipole and Ey dipole. The E-field noise level was 0.1 nV/m/rt (Hz) at 1 Hz (at a working water depth of 1000m) with a 12m dipole. 8 Hz omni-directional geophones were used to record earthquakes and natural seismicity in active tectonic areas. The geophone 120 sensitivity is 78.5 V/m/s at a 15 Hz natural frequency, and the internal resistance is 3100 Ω. Four sets of six 18650 Li-ion batteries for 25.2 V batteries supply power to the data logger circuit. One independent 16.8 V battery supplies the acoustic telemetry modem (ATM) module. The power consumption is approximately 1W. The power supply module supports data acquisition for ~30 days at a maximum sampling rate of 2400 Hz. 125 The OBEMS data logger has a 24 bit analog-to-digital converter for each of the two electrical field components and the three-axis geophone components. The attitude and heading reference system (AHRS) module records the pitch, roll, and heading while the instrument is on the seafloor. The OBEMS has two parallel release mechanisms. The transducer connects with the ATM for acoustic telemetry. When the ATM receives the release command, the burn-wire mechanism release is driven, 130 and the anchor releases after 10 minutes. Additionally, a USBL transponder responder and motordriven release were also installed on the OBEMS. The transponder is designed for positioning remotely operated vehicles (ROVs), towed fish, and other mobile targets in water depths up to 4000 m and is equipped with an omni-directional transducer for a wide range of general USBL tracking applications.
The transponder is available with acoustically controlled output lines suitable for an external motor 135 drive. This transponder integrated the USBL transponder, release, and the internal depth sensor to improve USBL position performance.
To keep the design of the OBEMS simple and compact, the seismometers were indirectly coupled to the seafloor via the sphere, release hardware, anchor, and the spring used to connect the OBS to the anchor. While seismometers work best when they are in direct contact with the Earth (Mà nuel et al., 140 2012), this design has been proven effective in collecting data at long shot-receiver offsets. Coupling the instrument to the seafloor is extremely important, as the geophone, which measures movement of the seafloor, is located inside the sphere rather than deployed on the seafloor. To further optimize coupling, a weight in the form of a cross with a U-profile was used to ensure good penetration of the anchor weight into the seafloor. 145 To provide the highest possible accuracy, time was recorded to the nearest millisecond. Each OBEMS uses a microprocessor-compensated oscillator (MCXO) as a stable clock reference, for which the drift can be as little as 2 ms/day. Following each deployment, the offset is measured to compensate for the total time drift.
The size of the anchor is 110cm x 60cm x 6cm and it weighs 136kg in air and 78kg in water. During 150 deployment, the weight of the OBEMS in water is 42 kg, and -36 kg when it ascends. The descent and ascent velocities of the OBEMS are approximately 1 m/s and 0.8 m/s, respectively. Table 1 shows the specifications of the OBEMS system.  Figure 3 shows a schematic of the OBEMS data logger. The data logger is based on a 24-bit analog-to-155 digital converter (ADC) for each channel. Different analog pre-amplifiers were used for electric field, magnetic field, and geophone measurements. The data logger contains eight channels. Each channel integrates a pre-amplifier and 24-bit ADC. The pre-amplifier for the E-field channel is an ultra-low noise chopper amplifier; the self-noise level is approximately 0.6 nV/rt(Hz) at 1 Hz, the gain is 1200, and the -3 dB bandwidth is 100 s to 100 Hz. The pre-amplifier for the geophone is a different amplifier 160 with a gain of 3.3. The input range of the ADC is ±5 Vpp to match the pre-amplifier output range. The ADC module is based on an eight-channel, 24-bit ADC: the ADS1282. The ADS1282 is a one-channel, high dynamic range, fourth order Δ-Σ modulator, with a digital filter for data decimation and interfacing with the microcontroller module which provides a dynamic range of 130 dB at a 250 Hz sample rate, and a total harmonic distortion (THD) of -122 dB. 165 Micro-control unit (MCU) A is the master MCU, which is used to set sample rate, configure the ADC register, write data to the SD card, communicate with a computer via a WiFi module, and to communicate with the slave MCU B, the AHRS module, and the GPS module via a serial port. The complex programmable logic device (CPLD) parallel reads converted data from eight ADCs and series data awaiting MCU A data transfer. The MCU A employs an internal direct memory acess (DMA) 170 controller and writes data to the SD card. The MCXO is from Vectron, with a low power consumption of 3.3 V & 12 mA and high frequency stability of approximately ±20 ppb from 0°C to 50°C . The CPLD generates a 2.4576 MHz clock as the ADC master clock. The sampling rate can be set to 2400 Hz, 600 Hz, or 150 Hz. MCU B is used as the slave MCU for communication with ATM, driving the burn wire current source, 175 measuring battery capacity, and as a pressure sensor inside the glass sphere. When the ATM receives the release command, MCU B drives the current source and provides 500 mA to the burn-wire release.
A charger module converts the external DC 28V to charge each Li-ion battery set independently.
The collected data are stored on an SD card. To download the data there is no need to open the glass sphere or use an Ethernet wire. A computer can connect to the data logger to download data and 180 configure acquisition parameters using the onboard WiFi module. The capacity of the SD card is 32 GB, and can be expanded to 128 GB. The sampling rate was set to 150 Hz, generating 400 MB of data per day. An effective download speed of 3 MB/s was achieved, which allows 30 days of data (approximately 12 GB) to be downloaded in less than 67 min. After the OBEMS has been released from its anchor and is floating at the surface, it can be recovered using radio signals that can be 185 detected by a 165 MHz Very High Frequency (VHF) direction finder at distances of up to 5 km, even in poor visibility. The flashing light inside the sphere is especially useful for recovery at night. the Guangzhou Marine Geological Survey and CUGB. The scientific goal of the cruise was to map gas hydrates using a marine EM method, while also using CSEM to determine the electrical structure 500 m below the seafloor. To achieve this, 20 previously developed OBEMs and a towed CSEM transmitter (Wang et al., 2017) were utilized during the cruise. To evaluate the overall performance of the 195 developed receiver, two OBEMS were also used.
The Qiongdongnan region of the South China Sea is located 170 km southeast of Sanya. The seafloor is an ocean basin with a depth of 1700-1800m. Figure 7 shows a map of the experiment, which included 22 receivers, where QH-R19 and QH-R21 indicate the two newly developed OBEMS, and other labels represent the existing OBEMs. All receivers were equipped with a USBL transponder. 200 When the receiver started its descent to the seafloor, the transponder tracked its position. If the depth of the receiver did not change substantially, the location result was assumed to indicate the true position.
After all receivers were deployed, two transmitter lines were towed for different waveforms: a single square waveform at 8 Hz and multiple frequencies synthesized with a 0.5 Hz fundamental frequency.
After 10 days on the seafloor, all 20 OBEMs and two OBEMS were successfully recovered. 205 We estimated the MT responses (apparent resistivity and phase difference) using the robust estimate method. To calculate the MT responses at site QH-R19, Figure 6 presents the respective computed MT responses for the site over a period from 10s to 10000s. The data quality is excellent down to periods of approximately 10000s. At high frequency ranges, the seafloor responses for both asymptote modes were close to 1Ωm. 210 The CSEM data acquisition employed a towed CSEM transmitter that generated horizontal electrical dipoles (HED) with a length of 300m, while the altimeter of the towed transmitter body was approximately 20~50m. The transmitters were used at a single frequency of 8Hz and multiple frequencies synthesized with a fundamental frequency of 0.5Hz. The transmitter was equipped with a depth sensor, an altimeter, and an acoustic transponder. The transmitter transmitted at 450 A. The 215 OBEMS received CSEM data, following which the horizontal E-field and B-field component fast Fourier transfer (FFT), current data FFT, instrument calibration, and field component rotation were performed. The Ex component of the site QH-R19 signal spectrum is presented in Fig. 8. The result of the short time Fourier transform (STFT) shows the two towed CSEM lines (8 Hz towed line and 0.5Hz towed line) clearly. Figure 9 shows the MVO of the horizontal EM component at site QH-R19. The 8 220 Hz data are above the instrumental noise floor at a 3.5 km range.
After CSEM data acquisition using a towed transmitter source, seismic data acquisitions were carried out by testing the functionality of the OBEMS using an air gun as the source, the partial results of which are shown in Fig. 10. The vertical peaks in each channel correspond to the acquisition of the reflected and refracted acoustic signals generated by the artificial source. The recordings show clear 225 vibration signal arrivals, which demonstrate the proper functioning of all three geophone channels.

Conclusions
To achieve joint marine EM and seismic data acquisition, we installed an OBEMS based on an existing micro-OBE receiver, which consisted of two induction coils for horizontal magnetic field component measurement and a three-axis omni-directional geophone for recording movement of the seafloor in all 230 https://doi.org/10.5194/gi-2019-25 Preprint. Discussion started: 23 September 2019 c Author(s) 2019. CC BY 4.0 License. directions, and assembled a USBL transponder for seafloor position tracking. The final system included four electrodes, three geophones, two induction coils, two glass spheres, a USBL transponder, a motor drive release, an integrated ATM, a burn-wire release mechanism, a recovery beacon (LED, radio modem, VHF radio), and an expanded Wi-Fi module for data transfer. The data logger and battery are contained inside a 17-inch glass sphere. The proposed OBEMS architecture exhibited low noise, low 235 clock drift, and low-power specifications. The OBEMS has been mechanically optimized to satisfy all technical requirements for simultaneous acquisition of seismic and electromagnetic data. However, the following minor technical improvements will be made in future research: 1) The autonomy of the instrument will be extended to 60 days of data acquisition and 90 days spent on the seafloor. 240 2) A hydrophone will be installed to achieve a fully integrated all-in-one receiver.
3) The performance of the WiFi module will be enhanced.
As these preliminary tests have shown, OBEMS technology is capable of high quality MT, CSEM, and artificial seismic data acquisition. The future instrument will add a hydrophone and lengthen the working time on the seafloor (2-3 months) to the existing advantages of the OBEMS (low cost, easy 245 deployment, small size, high efficiency). This development will again be accomplished by cooperation between GMGS and CUGB.

Data availability
The raw data of experiment are available upon request (ck@cugb.edu.cn).

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Diagram shows the structural design inside the glass sphere, with omni-directional geophones in the lowest layer, then the Li-ion battery sets, Acoustic Telemetry Modem (ATM), and the data logger. All print circuit boards are covered with a magnetic shielding box. Ferrite sheets, with 0.01mm thick film on one side and 0.02mm thick adhesive tape on the other, were glued inside the shielding box. These ferrite sheets function primarily as suppressors, blocking EM noise at lower frequencies and absorbing it at higher frequencies.