The influence of tidal level on periodicity of an undersea geyser: An application of acoustic Doppler current meter

Abstract

The periodicity of an under water geyser in the Taketomi submarine hot spring in Yaeyama archipelago, Japan, has been investigated. We concluded the time cycle of the geyser responds to the tide, after conducting the time series analysis of the upward velocity taken by an acoustic Doppler current meter. The time cycles in high and low tide were 66 s and 41 s, respectively. In accordance with the Bunsen's vertical tube theory, and assuming underground depth of heated domain was just 10 m below the sea bottom, that was backed up by the averaged integration of upward flux per each eruption, we also denoted the condition that temperature of heat source (Θ) of over 200°C and the recharge water temperature (θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) of 130.57 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C could sustain the observed time cycles in high and low tide. These estimations of Θ were consistent with the previous estimations of the “deep underground water temperature” at the Taketomi Submarine Hot Spring, which ranged from 160 to 200 °C as reported by geochemists. Additionally, time variability of the relative bubble density and the relative bubble flux from the undersea geyser was investigated, utilizing the ultrasonic echo intensity and the upward velocity. A relational expression between the relative bubble density and the ultrasonic echo intensity were derived, after assuming that the (a)bubbles could be replaced by simple rigid spheres with a constant radius, (b)the bubble's radius was enough larger than the wavelength of the incident ultrasonic wave, and (c)the sound absorption by the seawater was constant. We demonstrated that at high tide, at erupting state the relative bubble density had large value, while at halting state it had quite small value. At low tide, however, the relative bubble density had almost always positive value, thus the bubbles were always present in the seawater. And the similarity between the relative bubble density and the upward velocity confirmed that the bubbles in the sea water were transported by the fluid discharged from the underwater geyser.