High-resolution gravity measurement aboard an autonomous underwater vehicle

Abstract

We have developed an underwater gravity measurement system that uses an autonomous underwater vehicle (AUV) for exploration of seafloor mineral deposits. An improved air/sea gravimeter mounted on a gimbal mechanism in a pressure capsule 50 cm in diameter was installed in the AUV Urashima. We carried out 11 AUV dives in Sagami Bay as well as in deep-sea mineral deposit areas offshore Japan. The AUV was navigated at a constant speed of 2 knots and at either constant depth or constant altitude above the sea bottom. We obtained high-resolution Bouguer anomaly data through processing of gravity data, water pressure data, and the AUV’s navigational data, including pitch and roll motion. In addition to a vertical acceleration correction using a precise pressure meter with an in situ conversion factor from pressure to depth, three additional corrections were made to the gravimeter data: corrections for the effects of a spatial separation and a time delay between the depth sensor and the gravimeter, and an adjustment to the conversion from pressure increments to depth increments. These new corrections allowed us to obtain high-resolution data in a constant-depth survey of the southern Izena Hole at a depth of approximately 1550 m. The data had a 0.1 mGal rms crossover difference after leveling. This survey revealed two high Bouguer anomaly areas with amplitudes of 1–2 mGal. A model calculation suggested that the anomalies result from the presence of two buried cylindrical high-density mineral deposits. Our data processing method allows the detection of mineral deposits located at or just beneath the seafloor using gravity data collected aboard AUVs. However, further improvements in resolution are desirable, particularly for surveys in areas with rugged topography.