IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_2E-P1

Date/Time: July 24 Poster

A low-cost SO2 imager with the use of digital cameras of consumer use
Takeshi Hashimoto1 , Akihiko Terada2 , Mitsumu Ejiri3 , Takuji Nakamura3 , Makoto Abo4
1

Institute of Seismology and Volcanology, Hokkaido University, Japan, 2 Volcanic Fluid ResearchCenter, Tokyo Institute of Technology, Japan, 3 National Institute of Polar Research, Japan, 4 Graduate
School of System Design, Tokyo Metropolitan University, Japan
E-mail: hasimoto@mail.sci.hokudai.ac.jp

Sulfur dioxide in volcanic plumes is generally hard to recognize with human eyes. In recent years, ground-based
remote sensing of SO2 is mostly performed with DOAS instruments which measure the UV spectra along the lineof sight (e.g. McGonigle et al., 2002). Besides, so-called SO2 cameras which composed of a CCD device andband-pass filters have been developed for imaging the SO2 concentration in a plume (e.g. Mori and Burton, 2006).
The prime feature of the latter system is the visualization of SO2 gas as a snapshot, in which a plume shape isinstantly recognized. Such an easy visualization of SO2 may contribute to the quick awareness of harmful gas fromthe vent as well as to the monitoring of volcanic activity. However, high-spec CCD devices are generally expensive
and this is one of the reasons that prevent such SO2 cameras to be widely used by non-specialists. In this study,
we produced an SO2 imager of a low-cost version by means of digital cameras of consumer use.

In our system, we introduced the model D70 (Nikon), as a light-receiving CCD device. The dimension, number ofpixels, and brightness resolution of the CCD are 23.7 by 15.6 mm, 6.27 M pixels, and 12 bit, respectively. Somemodifications are necessary in order to enable the camera to sense the UV lights. First of all, a built-in optical
low-pass filter must be removed manually. Appropriate lens and filters which permit UV bands should be chosen.
We used a compound lens (Nikon UV-105 mm, F4.5) and band-pass filters (FWHM: 9 nm, max transmission rate:65%, blocking: 10-5 ) to extract the 310 and 330 nm bands separately, with additional two band-pass filters foreach to cut unwanted wavelengths. The former band has a higher absorbance by sulfur dioxide than the latter.
We adopted simultaneous shooting system with the use of two cameras to overcome a somewhat long exposure
time (5 to 10 sec) required. Additional ND filters were added to adjust the exposure times of the two bands to beapproximately the same. Since images can be taken in the same way as ordinary photographing and stored in
the build-in memory card, this device can operate without external power or control PCs, and thus, is suitable formobile use.

Calibration of our device with SO2 cells of known column concentration confirmed a comparable absorption
coefficient to the device which was previously invented by Mori and Burton (2006). We also performed a field testof the system in Sakurajima volcano, southwestern Japan and confirmed its validity as an SO2 imager. However,a special care with the UV scattering in front of a plume (e.g. Mori et al., 2006) is necessary for fully quantitative
applications in a field operation. A simultaneous operation with DOAS system might be another option to ensure
the quantitative capability. Meanwhile, further cost-down is also possible by replacing an optical system into asingle silica lens by compromising on the optical distortion and aberration.

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