Monitoring periodic and episodic flow events at Monterey Bay seeps using a new optical flow meter

Abstract

To enable testing of the hypothesis that fluid flow from cold seeps responds to episodic tectonic events, we have developed a flow meter with a temporal resolution on the order of minutes. The Optical Tracer Injection System (OTIS) measures flow rates through the sediment-water interface (SWI) of 0.1 to >500 m/yr ±10%, samples fluid for postrecovery chemical analysis, and is adaptable for use as a long-term real-time monitoring station. Laboratory and in situ testing demonstrate that the instrument meets temporal accuracy and resolution requirements necessary to detect transient flow events. In a deployment at an active seep site in Monterey Bay, the OTIS measured flow rates with tidal period variability. Time series analysis indicates flow rate and tide height records are in phase. This flow rate response to tides, together with seep fluid temperatures measured at the SWI of greater than 9°C, suggests the seep is connected via a highly permeable fracture or other conduit to an overpressured aquifer at a depth of at least 110 m. We infer the hydrogeology of this well-like system is further complicated by additional fluid sources since 3He/4He ratios are elevated, and there is a lack of correlation between flow rate and temperature records at frequencies lower than tidal frequencies. This short-duration deployment demonstrates the OTIS's potential to capture flow and chemistry transients associated with earthquakes and creep in the offshore environment. Copyright 2007 by the American Geophysical Union.