Development of 9000 m level hadal OBEM

Taking the sea area near the southern part of the Mariana Trench as a typical area is crucial for deep structural research in marine geology and geophysics. The magnetotelluric (MT) method has advantages such as large detection depth, sensitive to low resistance reactions, low cost, and high efficiency. The application of MT in deep water requires instruments with high reliability and stability, low noise, wideband, low power consumption, and miniaturization. The ocean bottom electromagnetic receiver (OBEM), as one of the important instruments for MT in deep water observation, its performance directly affects the quality of detected data.

In response to the shortcomings of the existing 6000 m level OBEM, there is an urgent need to develop a 9000 m level hadal OBEM. According to the requirements, we have focused on overcoming the challenges of weak E-field measurement technology, low-power and low-noise M-field measurement technology, low-power underwater acoustic release technology, and water surface large-scale recycling technology. We have achieved lower noise, longer underwater operation time, and efficient operations, providing reliable and stable instruments for hadal MT observation.

We have developed a chopper amplifier that matches deep water E-field sensors, analyzed the causes of injected charges, and adopted a scheme that combines peak filtering technology and dead zone technology to suppress residual misalignment generated during the chopper modulation, effectively reducing 1/f noise in the circuit, expanding the input range, and improving input impedance.

An orthogonal fundamental mode fluxgate based on digital demodulation is developed. Digital closed-loop real-time processes such as high-precision ADC, digital synchronous demodulation, digital integration, and high-precision large dynamic range DAC are used to reduce the switching charge noise introduced by analog circuits. Developing adaptive closed-loop feedback control algorithms to achieve fast feedback compensation with low noise and large dynamic range can help improve key parameters such as noise bandwidth, and input range of sensors.

We adopt a deep-water acoustic release system, pressure-resistant acoustic transducer, and control module prototype. Hydroacoustic communication controls the opening of the constant current source and the electrocorrosion decoupler. This solution reduces the size of the instrument and only relies on a single glass ball to achieve the floating of the instrument. The system integrates commands such as status query, electrocorrosion on and off. The status information includes distance, electrocorrosion status, battery voltage, etc. The propagation distance of acoustic signals is greatly increased, improving the success rate of underwater acoustic communication.

The glass ball is equipped with a beacon module, which is controlled by acoustic signals to activate the AIS, achieving real-time transmission of the OBEM position. Besides, high-power LED flashing is controlled to facilitate nighttime recycling and further reduce the cost of offshore operations.

In August 2023, 5000 m level test was conducted in the southern South China Sea. It is preliminarily verified the MT measurement, which has been improved in terms of low power consumption, low noise, and adaptability to deep sea. In the future, we will conduct test verification in deeper sea.