火山監視カメラの映像にもとづく噴出物の温度推定

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For better estimation of the temperature of red-hot volcanic ejecta from their images taken by online digital cameras at night, we studied the applicability of a new pyrometer to the images. Our pyrometer applies the theory of black-body radiation to the color of red-hot object. We examined the color of red-hot volcanic ejecta, a heated basaltic rock with known temperature, and infra-red LEDs. Because of the sensitivity to infra-red ray, a digital camera Nikon D40 can visualize hot basalt specimen at temperatures much lower (ca. 370℃) than the naked eyes (500〜550℃). As a side-effect of this capability, color data of the hot basalt discord from the isothermal color lines calculated from the black-body radiation and the CIE colorimetric system. Night photographs taken by the online digital camera aimed at the explosive ash eruptions of Asama volcano (2 a.m. 2 Sep., 2009) indicated that the color of red-hot volcanic ejecta was disturbed by the influence of infra-red ray in the same manner as observed on the heated basalt specimen in the laboratory. As a result, the temperature of the volcanic ejecta would be much lower than their appearance in digital images (e.g., 1000℃) but higher than the detection limit (370℃). Comparison of the color of red-hot volcanic ejecta and isothermal color lines revealed that some of the volcanic ejecta fell along the 600℃ isotherm line of the black-body radiation, which suggests that volcanic cloud reduces the influence of infra-red ray. The estimated temperature (600℃) is consistent with those deduced from petrological observation and thermodynamic computation (600〜700℃). We conclude that color analysis of the images of red-hot ejecta taken by online digital cameras are useful in temperature evaluation when images suffer less influence of infra-red ray.
2012-03-30

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