EAGER: Development of a Prototype 2D Acoustic Tomography System for Rapid Temperature Measurements in Diffuse Hydrothermal Effluent
Mid-ocean ridges, the boundaries between separating tectonic plates, are some of the most volcanically active features on Earth. Along many ridges, storage of molten magma in shallow chambers results in heating of seawater stored in the porous crust. Thermally buoyant, this seawater rises through the crust, is chemically altered by water-rock interactions, and finally exits the seafloor at hydrothermal vents. It is now recognized that this type of deep-sea hydrothermal circulation plays a key role in controlling long-term ocean chemistry, the thermal and chemical structure of the oceanic crust, and the evolution of unique and diverse chemosynthetic ecosystems found nowhere else on the planet. However, quantifying the biological and chemical impact of hydrothermal circulation requires knowledge of the volume, heat, and chemical fluxes exiting the seafloor, which are notoriously difficult to quantify. One large source of uncertainty in flux estimates lies in the widespread distribution of lower-temperature (300°C), "black smoker" style vents. Existing methods to quantify the flux of diffuse hydrothermal venting are limited by small measurement areas, measurements of a single quantity (e.g., only temperature or only velocity), or are invasive and alter the flow as they measure it. This project will develop new technologies for improved measurement of diffuse venting. Researchers will collaborate with University of Idaho, School of Engineering undergraduate students as part of their senior year Capstone Design Course to develop a prototype two-dimensional (2D) acoustic tomography system that can rapidly measure the temperature of anomalously warm, upwelling diffuse fluids across a ~1 m2 area. Development of these technologies will lead to future construction of a deep-sea measurement system targeting diffuse hydrothermal venting.
The primary goal of this project is to develop a prototype two-dimensional (2D) acoustic tomography system capable of a time series of rapid (1 Hz) measurements of the temperature of anomalously warm, upwelling fluids at a spatial resolution of centimeters across a ~1 square meter area. The proposed system will consist of ~20-25 acoustic immersion transducers fixed at known spacing on a square, rigid frame. A subset of the transducers (~4-6) will emit staggered acoustic chirp pulses and the system will measure the pulse travel time between the emitting and receiving transducers. Using travel-time acoustic tomography, these travel times will be converted to sound speed and finally temperature throughout the 2D domain. Development will begin with a two-transducer, 1D system to test appropriate signal frequencies (e.g., kHz or MHz), chirp type (increasing or decreasing frequency), transducer shapes (e.g., spherical, planar, cylindrical), and effective measurement rates. Building upon these results, the final prototype will utilize the optimum transducer shape and pulse frequencies. After construction, the system will undergo submergence tests at the University of Idaho with controlled sources of warm, buoyantly rising fluids. To calculate fluid temperatures, we will evaluate several travel-time inversion methods including the algebraic reconstruction technique (ART), multiplicative algebraic reconstruction technique (MART), and simultaneous iterative reconstruction technique (SIRT). Final results will be compared to thermocouple measurements distributed throughout the 2D measurement domain.