University of Alaska Fairbanks, United States

We developed an autonomous electromagnetic flowmeter as part of a cross-hole hydrogeologic experiment using subseafloor borehole observatories (CORKs) that penetrate into the volcanic ocean crust. The cylindrical flowmeter is adapted from a conventional industrial tool and hardened for use at water depths up to 6000 m. In addition, the electronics were modified with a new power controller, and a data logger and communication board was added to enable data storage and long-term, autonomous use for up to eight years. The flowmeter generates a magnetic field and measures a voltage gradient that is created across the orifice as water moves through it. This kind of tool is ideally suited for use in the deep sea, particularly for measuring hydrothermal fluids emanating from the ocean crust, because it requires no moving parts, places no obstructions along the flow path, gives total flow volume as well as instantaneous flow rate, and is highly accurate across a large dynamic range, including bi-directional flow. This flowmeter was deployed on a CORK wellhead using an adapter and ring clamp system located above a 4-inch ball valve. The ball valve can be opened to permit flow (from an overpressured formation) out of the CORK and into the overlying ocean. A polyvinyl chloride "chimney" positioned vertically above the flowmeter is instrumented with autonomous temperature loggers to permit an additional estimate of fluid flow rates with time, based on heat loss during fluid ascent, and to facilitate fluid sampling. Calibration of the new flowmeter was completed in two stages: tank testing using a pump at flow rates of 0.5 to 1.2 L/s, and by lowering the flowmeter on a wireline at sea at rates equivalent to 0.5 to 5.2 L/s. A cross plot of apparent and reference flow rates obtained during calibration indicates a highly linear instrument response. Comparison of instantaneous (once per minute) and integrated (total flow) data collected during calibration indicates good agreement, although the instantaneous data tended to be noisy because of irregularity of flow (turbulence). The flowmeter was deployed in Summer 2011 on a CORK installed in IODP Hole 1362B, on the eastern flank of the Juan de Fuca ridge. Once the flowmeter was attached to the wellhead, the underlying ball valve was opened, which allowed overpressured fluids from the permeable ocean crust to flow upward and out of the seafloor at 5 to 10 L/s (estimated rate). Changes in formation fluid pressure resulting from this flow are being monitored in four additional CORKs located 310 to 2320 m away from Hole 1362B, which will allow large-scale, directional assessment of formation properties. The flowmeter is recording data for instantaneous flow rate and total flow once per hour, and will be recovered to permit collection and analysis of experimental data during a servicing visit in Summer 2012.