ABSTRACT

Ocean-bottom electromagnetic (EM) receivers are used to record EM signals for marine magnetotelluric and controlled-source EM offshore data acquisition. These marine EM data are used for offshore gas hydrate and petroleum exploration. Although many conventional receivers are used for offshore data acquisition, they have deficiencies, such as a large size, high cost, and low operational efficiency. To address these limitations, we have developed a micro ocean-bottom E-field (micro-OBE) receiver. It reduces costs and deck-space use, while providing improved horizontal resolution and operational efficiency. Based on conventional receiver specifications, including low noise levels, low power consumption, and low clock-drift error, we reduced the receiver size, provided a low-cost release mechanism, integrated an acoustic telemetry module, improved the operational efficiency, and reduced the field acquisition cost. The resulting micro-OBE is comprised of a compact nylon frame, 17 inch glass sphere, logging system, batteries, recovery beacon, burn-wire release mechanism, transducer, electrode, red flag, and anchor (which eliminates the heavy and expensive acoustic release). It has no alloy aluminum press case, and it contains a minimal number of glass spheres, watertight cables, and connectors. Its size is 70×50×50  cm (not including the electrode arm, red flag, and anchor). Its weight in air is 70 kg. It has a noise level of 0.1  nV/m/rt (Hz) at 8 Hz, and it provides 33 days of battery lifetime. Offshore data acquisition was performed to evaluate the micro-OBE field performance during an offshore gas-hydrate experiment. Our results indicate that the receiver effectively functioned throughout the operation and offshore data acquisition. Micro-OBE was thus verified as providing a low cost, compact size, and high operational efficiency.

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