[G02-2] Gravimetry

[G02-2-06] Development of a high-accuracy gravity measurement system onboard a moving autonomous underwater vehicle

^○Takemi Ishihara¹, Masanao Shinohara¹, Akito Araya¹, Tomoaki Yamada¹, Toshihiko Kanazawa², Hiromi Fujimoto³, Satoshi Tsukioka⁴, Shinobu Omika⁴, Kenji Uehira², Masashi Mochizuki², Tsuyoshi Yoshiume⁴, Kokichi Iizasa⁵ (1.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan, 2.National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan, 3.Tohoku University, Sendai, Japan, 4.Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan, 5.Institute of Industrial Science, The University of Tokyo, Tokyo, Japan)

We have developed an underwater gravity measurement system onboard the autonomous underwater vehicle (AUV) Urashima (JAMSTEC). The gravimeter is mounted on a gimbal unit and housed in a pressure-proof sphere vessel made of titanium alloy. The gravity sensor is kept at a constant temperature of 60.4 degree C and covered with a sheet of permalloy for magnetic shielding. In order to obtain precise gravity data, a Gaussian low-pass filter with the width of 100-300 s is applied to 100 Hz output of the gravity sensor, and corrections are made using various measurements: correction of vertical acceleration with high-accuracy water pressure gauge data, Eotvos and free water corrections with the AUV's navigation data, correction of location difference between the gravimeter and water pressure gauge with the AUV's pitch and roll angle data, and Bouguer correction with the detailed topographic data collected by the Urashima's multi-beam bathymetric system. A detailed and high-accuracy gravity survey of seafloor mineral deposit area can be carried out by moving the AUV along planned track lines 50-100 m above the seafloor. After the first successful sea experiment in September 2012, good quality data were collected by three experiments in mineral deposit areas in the Okinawa Trough and the Izu-Ogasawara Arc. Almost noiseless gravity anomalies were obtained by the above data processing of the collected data including those from constant altitude track lines above rough seabottom topography, where large amplitude vertical acceleration was observed. We obtained a good result with an rms crossover error of about 0.1 mGal in a favorable survey condition: measurements at a constant depth of 1550 m in relatively flat Izena Caldera of the Okinawa Trough. High anomalies with amplitudes < a few mGal, which suggest occurrences of high-density material, possibly mineral deposits, are observed in the Bouguer anomaly map of the southern part of Izena Caldera.