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 +  .+ῌ/+ ῐ ῏ \\NOPQ,FRSTFUVWXFYZ [,**3 H 2 I -+ JKL ,*+* H + I +/ JKMAtmospheric Correction in EDM by Using the JMA Numerical Weather Model :Application to Measurement at Asamayama Volcano
ῌῌAkimichi TAKAGIῌ,ῌῌ, Keiichi FUKUIῌ, Toshiki SHIMBORIῌ and Sei IIJIMAῌ
Electro-optical distance measurement (EDM) and Global positioning System (GPS) observation are appliedto monitor precise time variation of the ground deformation at active volcanoes. But observations using
electromagnetic waves such as these are accompanied by errors associated with inhomogeneity of refractive index
along the propagation path in atmosphere. In particular, the inhomogeneity in troposphere degrades theaccuracy of positioning. An improved atmospheric correction method in EDM was developed, based on theJapan Meteorological Agency (JMA) operational mesoscale analysis (MANAL) for numerical weather prediction. In this method, the precise velocity and ray path of propagated lights are estimated from the adequate
vertical proﬁle of refractive index by MANAL. Consequently distance along the bowing ray path measured by
EDM is corrected to be precise slope distance. Applying this procedure to EDM data at Asamayama volcano, theseasonal ﬂuctuation caused by inhomogeneity of refractive index in atmosphere was removed entirely.
At Asamayama volcano, very small eruptions occurred in August ,**2 since the latest ,**. eruption, andthen a small eruption occurred in February ,**3. Based on the EDM observation by Meteorological ResearchInstitute and Karuizawa Weather Station, we detected that the slope distance had been shortened since August,**2. Slope distances from the observation site to reﬂectors were corrected by using MANAL in this correction
method. Though slope distances have increased in length at a rate of +ῌ1 mm per year since the ,**. eruptions,
ground deformation turned over to inﬂation in August ,**2 and slope distances shortened to /ῌ,2 mm per ﬁvemonths by January ,**3.
In order to account for those observation data, we assumed a pressure source beneath the summit crater,
whose depth and volume increase were estimated to be at a height of ,-2* m above sea level (,** m under thesummit) and +/,-** m-, respectively.
By developing this atmospheric correction method in EDM with the use of the JMA’s numerical weathermodel, it became possible to precisely detect ground displacements and thus to reliably estimate their sources.
Therefore, this method is very e#ective to monitor activity of volcanoes.
Key words : Atmospheric Correction, EDM, JMA numerical weather model, Asamayama volcano, MANAL -*/ῌ**/,  +ῌ+!"#$%&'("#)
Seismology and Volcanology Research Department,
Meteorological Research Institute, +ῌ+ Nagamine,Tsukuba, Ibaraki, -*/ῌ**/, Japan.
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Present address : Earthquake and Disaster-Reduction
Research Division, Research and Development Bureau,
Ministry of Education, Culture, Sports, Science andTechnology - Japan.
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Asamayama Resident O$ce for Volcanic Disaster Mitigation, JMA, +1*0ῌ2, Kitakura, Nagakura, Karuizawa,Nagano, -23ῌ*+++, Japan.
Corresponding author : Akimichi Takagi
e-mail : atakagi@mext.go.jp