A novel optical sensor designed to measure methane bubble sizes in situ

Abstract

This work presents a novel design for an optical bubble size sensor that is rugged, economical to build, and capable of accurately measuring methane bubble sizes in aquatic environments over long deployment periods. The sensor intercepts rising gas bubbles, elongates them in a thin glass tube, and routes elongated bubbles past an optical detector. The optical detector records information on bubble rise velocity and travel time, which can be combined with the flow path geometry to calculate bubble volume at flow rates up to 3 bubbles/second. The sensor circuitry is powered by 6‐V C alkaline battery packs and is cased in a waterproof housing built from commercially available PVC pipe fittings. Laboratory testing indicates the sensor can accurately measure bubble volumes up to 1 mL in volume. We deployed the sensor in a lake with a history of methane ebullition and gathered data on bubble size distributions (most bubbles were between 0.025 mL and 0.2 mL) as well as the precise timing of bubbling events. The sensor also includes an optional gas collection system to allow for bulk gas sampling.

Ephemerality of discrete methane vents in lake sediments

“…The dependence on flow rate and bubble size matched predictions from an analytical model of the expected sonar response to a constant bubble stream rising through a horizontally oriented multibeam sonar. The expected response from a bubble stream with a wide but constant bubble size distribution was calculated by adapting a method that was developed for single and split beam sonar [Muyakshin and Sauter, 2010;Veloso et al, 2015], assuming a constant bubble size distribution measured in UML with an optical bubble sizer [Delwiche et al, 2015] and assuming that plumes are too sparse to be influenced significantly by multiple scatter reflections (see supporting information). The relative uncertainty in flux estimates is estimated to be ∼70% (see supporting information).…”

Persistence of bubble outlets in soft, methane‐generating sediments

“…Although these bubbles constitute 79% of the bubbles by number, they contribute only 3.3% of the total volume released. The frequency of such small bubbles is large compared with other in‐field estimates of the BSD in UML, which found <1% smaller than 2 mm in diameter [ Delwiche et al , ]. Part of that discrepancy may be attributed to a lower detection limit for bubble sizes for the instrument used in that study, as bubbles smaller than 2 mm in diameter were not tested in the calibration of the device, and it is also likely that many of the small bubbles were spuriously detected by our method.…”

“…In a record of sonar‐detected ebullition in Upper Mystic Lake (UML), MA, a characteristic spatial structure of apparently discrete seeps was observed over periods up to 1 day in duration, but over longer periods the occurrence of spatially independent venting made the pattern indistinguishable from a completely spatially random process [ Scandella et al , ]. Other studies on the UML, a dimictic, eutrophic kettle lake north of Boston, MA, include a 4 month record of ebullition from five surface‐moored bubble traps that showed surprising synchronicity in hydrostatically triggered ebullition episodes, despite the sensors' being located over water depths ranging from 9 to 25 m [ Scandella et al , ; Varadharajan and Hemond , ; Delwiche et al , ]. In contrast with this wealth of observations at the field scale, fine‐scale observations of the spatial distribution of ebullition from natural sediments are lacking.…”

An enhanced bubble size sensor for long‐term ebullition studies

“…We note that in Delwiche et al (2015) we attributed the sensor's inability to accurately measure large bubble sizes to the tendency for large bubbles to break apart within the glass funnel stem. However, further analysis of the data showed that typically at least 97% of the original gas volume is contained in a single large bubble, followed by 1-3 small bubbles, Fig.…”

“…Upgraded extension funnel In addition to the data cable modifications, we also updated the first generation detachable extension funnel described in Delwiche et al (2015) to improve performance in the field (Fig. 3).…”