Evaluations of an ocean bottom electro-magnetometer and preliminary results offshore NE Taiwan

The first stage of field experiments involving the design and construction of a low-power consumption ocean bottom electro-magnetometer (OBEM) has been completed, which can be deployed for more than 180 d on the seafloor with a time drift of less than 0.95 ppm. To improve the performance of the OBEM, we rigorously evaluated each of its units, e.g., the data loggers, acoustic parts, internal wirings, and magnetic and electric sensors, to eliminate unwanted events such as unrecovered or incomplete data. The first offshore deployment of the OBEM together with ocean bottom seismographs (OBSs) was performed in NE Taiwan, where the water depth is approximately 1400 m. The total intensity of the magnetic field (TMF) measured by the OBEM varied in the range of 44 100–44 150 nT, which corresponded to the proton magnetometer measurements. The daily variations in the magnetic field were recorded using the two horizontal components of the OBEM magnetic sensor. We found that the inclinations and magnetic data of the OBEM varied with two observed earthquakes when compared to the OBS data. The potential fields of the OBEM were slightly, but not obviously, affected by the earthquakes.


Introduction 21
Marine electromagnetic exploration is a geophysical prospecting technique used to 22 reveal the electrical resistivity features of the oceanic upper mantle down to depths of 23 several hundreds of kilometers in different geologic and tectonic environments, such as 24 in areas around mid-oceanic ridges, areas around hot-spot volcanoes, subduction zones, 25 and normal ocean areas between mid-oceanic ridges and subduction zones zones (

Units of the OBEM and their specifications 101
The OBEM is recovered by releasing its anchor from the seafloor via an on-board 102 acoustic command. The OBEM is returned to the sea surface via buoyancy when the 103 anchor is released. There are two typical release mechanisms available for OBEMs to 104 unlock their anchors: spin motor and burn-wire systems (Kasaya and Goto, 2009). The 105 OBEM uses the burn-wire system because it weighs less than the spin motor system. 106 The acoustic controller and transducer use ORE #B980175 ASSY PCB and #D980709, 107 respectively, manufactured by EdgeTech, USA, for the corresponding functions of 108 OBEM recovery and underwater ranging. The ASSY PCB acoustic controller uses a 109 binary FSK encoder, including the commands "RELEASE1," "RELEASE2," 110 "DISABLE," "ENABLE," and "OPTIONAL1." The frequency of the acoustic range 111 ranges from 7.5 kHz to 15 kHz in increments of 0.5 kHz with a sensitivity of 80 dB re 112 1uPa. The #D980709 transducer can work at a depth of 6,000 m and in environments 113 from −10°C to +40°C. 114 115 The EdgeTech 8011M model acoustic commander (8011M) is used on board to send 116 the "ENABLE" command to open the ranging function, the "RANGE" command to 117 measure the distance between the OBEM and the research vessel, the "DISABLE" 118 command to close the ranging function, and the "RELEASE1" command to activate 119 the burn-wire system to release the anchor. The "RELEASE1" command persists for 120 15 min unless terminated by the "OPTIONAL1" command. 121

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We selected the RF-700A and ST-400A NOVATECH models for the radio and flash 123 beacons, respectively, for use in the OBEM. The maximum deployment depth for these 124 models is 7,300 m. The radio beacon is turned ON by sending a VHF signal, and the 125 flush beacon is turned ON at atmospheric pressure of less than 1 atm (equal to a depth 126 of 10 m below the sea surface) in a dark environment. The beacons are also turned OFF 127 at a depth of 10 m or at atmospheric pressure of less than 1 atm, respectively. These two 128 beacons have four independently installed C-type alkaline batteries that allow for six 129 days of continuous operation at maximum; this power supply differs from that of the 130 data logger. The two independent power supply layouts allow the beacons to properly 131 operate even if the power supply for the data logger fails. An on-board radio scanner 132 Page 6 of 35 pages detects the signal transmitted from the radio beacon at a distance of 6.4-12.9 km when 133 the OBEM is floating on the surface. These two beacons can assist in locating the 134 OBEM on the sea surface in both daytime and nighttime. 135 136 TL-5930 model lithium batteries manufactured by TADIRAN are used for the OBEM, 137 with specifications of 3.6 V, 19 Ah, and D-type with characteristics of high energy 138 density and a low self-discharge rate suitable for long periods of operation. Figure 2  139 shows a block diagram of the OBEM data logger. The ADC1278EVM model is a 24- Hz clock that supports a timing error smaller than 3 s y −1 . Even though the time base 150 module supports a very small timing error of 3 s y −1 , the data logger clock is still 151 synchronized with the GPS on deck for timing corrections after recovering the OBEM. 152 The maximal capacity of the SD card is 64 GB and can support data storage for more 153 than one year with a sampling rate of 10 SPS. 154 155 Two 17-in glass VITROVEX spheres manufactured by Nautilus Marine Service GmbH, 156 Germany, are used for the OBEM. These glass spheres contain the fluxgate and tiltmeter 157 (sensor ball) and the seven channels of the Amp & LPF, data logger, #B980175 ASSY 158 PCB acoustic controller, and batteries (instrument ball) and can be deployed at a depth 159 of 6,000 m and support a total buoyancy of 52 kg. The instrument and sensor balls, the 160 silver chloride electrodes, and the burn-wire system are connected via waterproof 161 cables. There is a pressure-vacuum valve outside the glass spheres that allows a pumped 162 vacuum to be preserved at 0.7 atm; self-fusing butyl rubber tape is used to fill the suture 163 zone between the half glass spheres. In addition, two crossed stainless-steel bands are 164 used to improve the waterproofing of the glass spheres and cover the orange PE cases. 165 Page 7 of 35 pages Four PVC pipes with lengths of 2 m are combined to form the OBEM platform for the 166 electric receivers, and the silver chloride electrodes are installed at the ends of the pipes. 167 A 60-kg nonmagnetic anchor is attached to the bottom of the OBEM platform and 168 catches via a releasing mechanism. The anchor can be released using the burning-wire 169 system to recover the OBEM. Figure 3 shows a photograph of the OBEM platform.

Calibrations of the background noise of the data logger and the Amp & LPF 179
The background noise of the data logger is defined as 180 where n is a data point and A1 to An indicate the amplitudes of the data points, 1 to n, the sensitivity, linearity, and error. The average sensitivity is 655,968.5 counts/V with 242 a maximum error smaller than 1.35%. Figure 5 shows a calibration of the electric 243 channels (EX and EY) checking the sensitivity, linearity, and error. The average 244 sensitivity is 135,856,047.8 counts/V with a maximum error smaller than 0.8%. Figure  245 6 shows a calibration of the tiltmeter channels (TX and TY) checking the sensitivity, 246 linearity, and error. The average sensitivity is 1,677,710.6 counts/V with a maximum 247 error smaller than 0.25%. The noise level of the data logger is 57.8 dB, whereas its 248 dynamic range is 80.2 dB at 10 Hz. 249 250

Evaluation of the current consumption 251
The power supplies of the OBEM consist of two 7.2-V battery packs in a series 252 connection with two 3.6-V lithium batteries. One battery pack is for the data logger and 253 converts to ±5 VDC and +3.3 VDC . The other pack is for the sensors and converts to 254 ±5 VDC and +12.0 VDC. Two +7.4-VDC output current batteries were measured for 255 their current consumption measurement using two ammeters connecting the two +7.4-256 V battery packs. Table 2 shows the current consumption of the OBEM system. The 257 maximum current consumptions of the data logger and sensors are 32 mA and 105 mA, 258 respectively. The total power consumption is less than 1 W, which corresponds to 259 expectations. 260

Evaluation of the electrodes 262
Two pairs of silver chloride electrodes are used for the OBEM. We first put a pair of 263 electrodes separated by a fixed distance within a tank filled with seawater to check the 264 status of the electrodes. Second, we measured the electrical potential and impedance of 265 the electrodes using a digital volt-ohm-milliammeter (VOM) (Fig. 7). Third, we sent a 266 swept sine signal to check the frequency responses of the electrodes, as shown in Fig.  267 8. Fourth, we input a DC voltage to check the electrode-induced voltages, as shown in 268

Evaluation of the acoustic transceiver and its transducer 284
We selected the large-scale Breeze Canal in New Taipei City for testing because it has 285 few obstacles and is suitable for evaluating the functions of the 8011M. The Breeze 286 Canal has a length of approximately 800 m and is located in a straight river with a depth 287 of 2-5 m. The distance between the transducer and the acoustic transceiver was 288 approximately 630 m, and the layout for the field test is shown in Fig. 10. The testing 289 procedure for the transducers is described below. The results are listed in Table 4. We then checked the acoustic transceivers after all of the transducers were successfully 302 checked; the testing procedure for the acoustic controller is described below. The results 303 are listed in Table 5. 304 1. Change the acoustic controller, and record its serial number in a notebook. 305 2. Send the "ENABLE" command via the 8011M, and then count the response 306 beeps. 307 3. Send the "RANGE" command via the 8011M five times, and record the distance 308 of each ranging. We deployed six broadband BBYBs and one OBEM near a small submarine volcano 331 area in the OT offshore NE Taiwan (Fig. 11) on 03/26/2018 for a submarine observation 332 to evaluate all the OBEM units. All the equipment was successfully recovered after one 333 month of deployment. Figure 12