Chad A. Greene

Laboratory investigation of a passive acoustic method for measurement of underwater gas seep ebullition

Passive acoustic techniques are of interest as a low-power means of quantifying underwater point-source gas ebullition. Toward the development of systems for logging natural seep activity, laboratory experiments were performed that exploited the bubbles Minnaert natural frequency for the measurement of gas flow from a model seep. Results show agreement among acoustic, optical, and gas trap ebullition measurements over the range of emission rates from 0 to 10 bubbles per second. A mathematical model is proposed to account for the real gas behavior of bubbles which cannot be approximated as ideal, such as methane at marine depths exceeding 30 m.

Antarctic Mapping Tools for Matlab

We present the Antarctic Mapping Tools package, an open-source Matlab toolbox for analysis and plotting of Antarctic geospatial datasets. This toolbox is designed to streamline scientific workflow and maximize repeatability through functions which allow fully scripted data analysis and mapping. Data access is facilitated by several dataset-specific plugins which are freely available online. An open architecture has been chosen to encourage users to develop and share plugins for future Antarctic geospatial datasets. This toolbox includes functions for coordinate transformations, flight line or ship track analysis, and data mapping in georeferenced or projected coordinates. Each function is thoroughly documented with clear descriptions of function syntax alongside examples of data analysis or display using Antarctic geospatial data. The Antarctic Mapping Tools package is designed for ease of use and allows users to perform each step of data processing including raw data import, data analysis, and creation of publication-quality maps, wholly within the numerical environment of Matlab. Graphical abstractDisplay Omitted HighlightsAMT is an open-source Matlab toolbox for Antarctic data analysis and display.AMT lets users create fully scriptable, publication-quality maps in Matlab.AMT incorporates publicly available datasets through user-generated plugins.

Wind causes Totten Ice Shelf melt and acceleration

Wind upwells warm water from the deep ocean off the East Antarctic coast, leading to ice-shelf melt and glacier acceleration. Totten Glacier in East Antarctica has the potential to raise global sea level by at least 3.5 m, but its sensitivity to climate change has not been well understood. The glacier is coupled to the ocean by the Totten Ice Shelf, which has exhibited variable speed, thickness, and grounding line position in recent years. To understand the drivers of this interannual variability, we compare ice velocity to oceanic wind stress and find a consistent pattern of ice-shelf acceleration 19 months after upwelling anomalies occur at the continental shelf break nearby. The sensitivity to climate forcing we observe is a response to wind-driven redistribution of oceanic heat and is independent of large-scale warming of the atmosphere or ocean. Our results establish a link between the stability of Totten Glacier and upwelling near the East Antarctic coast, where surface winds are projected to intensify over the next century as a result of increasing atmospheric greenhouse gas concentrations.

Basal channels drive active surface hydrology and transverse ice shelf fracture

Ice shelf basal channels cause transverse fractures that can be exacerbated by surface rivers, culminating in calving. Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel–driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilization.

The homemade didgeridoo: An ancient acoustic resonator.

The didgeridoo is an ancient Aboriginal instrument traditionally crafted from Eucalyptus trees which have been hollowed out by termites. The instrument is an excellent example of a simple one‐dimensional acoustic resonator with a fundamental resonance corresponding to one‐half wavelength. Today, the ubiquity and low cost of PVC pipes make didgeridoos inexpensive, easy to find, and trivial to construct from materials found at any hardware store. Students will have fun learning to make sounds with the instrument—perhaps they will even learn the technique of circular breathing—all while exploring fundamental principles of acoustics. As further investigation students can hum as they play, resulting in “beats, the speed of which can be adjusted by changing the pitch of the hum.

Toward passive acoustic remote sensing of ocean‐bottom gas seeps.

Cold seeps are gas vents that are found in the ocean, often along continental shelves near sediment‐borne methane hydrates. Methane hydrates and methane gas seeps are of particular interest both for their potential use as an energy source and for their possible contribution to global climate change. This work is an initial step toward passively locating cold seeps and quantifying their gas flow rates using acoustic remote sensing techniques. Results are presented from laboratory experiments in which gas fluxes were determined from the radiated acoustic signature of a model seep. The physical principle that supports the technique and its accuracy will be discussed. [Work supported by ONR.]

A simple experiment for understanding resonant air columns.

One goal of Project Listen Up is to enhance children’s understanding of acoustics through engagement in basic acoustics experiments. This work proposes an experiment that examines the resonant frequencies of an air column. Equipment for this experiment consists of a small tunable oscillator circuit connected to a loudspeaker, which can also be used for other experiments in the kit. The experimenter will provide a cardboard tube, rubber band, plastic wrap, and sand. Resonance is demonstrated by placing the loudspeaker at the base of the tube, covering the top of the tube with a plastic wrap membrane, and placing sand grains on top of the membrane. As frequency is adjusted, the sand grains are excited at the resonance frequencies of the tube. Frequencies at which sand motion is observed can be compared to a simple algebraic model, which relates the length of the tube to the resonance frequencies of the system. An optional advanced experimental track investigates the effect of varying the length of the tube....

Measurements of sound speed in bubbly liquids under high‐pressure conditions.

Methane hydrates occur naturally on the ocean bottom and in the upper layers of sediment on continental shelves. Seismic surveying could be used to locate methane hydrates; however, their low‐frequency acoustic properties are not well‐known. In addition, these properties can vary dramatically depending on whether the methane is in a gas or solid phase. As a step toward better understanding the three‐phase case of gassy sediments in water, the two‐phase case of methane gas bubbles in water was investigated. Wood’s equation is often used to model sound propagation in bubbly liquids and has been widely verified by experiments at atmospheric pressure. However, there is little information in the literature verifying the validity of Wood’s equation at high pressures. Low‐frequency (0.5–10‐kHz), resonator‐based sound speed measurements were obtained for air bubbles in water and methane bubbles in water under pressures ranging from 1 to 10 atm at room temperature. The results are presented and compared to the pre...

A demonstration of acoustic damping using bubbly liquid for Project Listen Up.

Acoustic resonances are exploited in many natural and human‐made systems, and demonstrations of the phenomena have been presented previously [Greene et al., J. Acoust. Soc. Am. 124, 2568 (2008); Argo IV et al., J. Acoust. Soc. Am. 125, 2625 (2009)]. In practice, resonance is always accompanied by energy loss, which limits the maximum excursion of a system. When sufficiently large, this loss can alter the resonance frequency of the system. An air bubble in water is an example of a lossy acoustic resonator and is well‐suited for demonstrating acoustic damping in the Listen Up paradigm. For the proposed demonstration, students will observe the resonance of a water‐filled drinking glass by striking it with a metal spoon to produce a strong ringing sound. They will then introduce damping into the system by using a stream of water to entrain bubbles which will deaden the ringing response of the system. The primary mechanism for the acoustic energy dissipation is conversion into heat during bubble oscillation. A...

A Helmholtz resonator experiment for the Listen Up project.

The Listen Up project seeks to develop an educational booklet and apparatus that will be used to foster interest in acoustics and teach basic acoustical concepts to middle school students. The packaged kit must be low cost; hence this experiment uses a 16 oz plastic water bottle as a Helmholtz resonator, a disposable syringe, and an inexpensive pitch pipe. Understanding the Helmholtz resonator is fundamental to many applications in acoustics. A simple algebraic model of the device, derived from a mechanical mass‐spring analogy, relates the volume of air in the bottle to the resonance frequency of the system, and hence the pitch that is produced when one blows over the bottle opening. The volume of air inside the bottle can be easily and accurately controlled by adding water with a graduated syringe. The dependence of pitch on the volume of air in the bottle can be systematically demonstrated, and the validity of the model can be checked by comparing the sound produced to musical notes from the pitch pipe....