Abstract. 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.

Abstract. The first stage of field experiments involving the design and construction of a low-power consumption ocean bottom electro-magnetometer (OBEM) has been completed. 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 evaluations of the procedure included the following. Data logger: digitizer sensitivity, linearity, and errors Acoustic transceiver: “ENABLE,” “DISABLE,” “RANGE,” “RELEASE1,” “RELEASE2,” and “OPTION1” functions Magnetic sensor: sensitivity of the fluxgate and orthogonality Electrical receiver: potential voltage, impedance, and frequency responses Power consumption: the maximum operating current of two sets of batteries Deployment and recovery procedures on deck We confirmed the optimal performance of the OBEM after repeatedly testing the procedures. The first offshore deployment of the OBEM together with ocean bottom seismographs (OBSs) was performed in NE Taiwan, where the water depth is approximately 1,400 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 of 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.