a forward and inverse modeling approach using thermal imagery

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MODELING APPROACH USING. THERMAL IMAGERY. Matteo Cerminara, in collaboration with Tomaso Esposti Ongaro, Sébastien Valade & Andy Harris ...
A FORWARD AND INVERSE MODELING APPROACH USING THERMAL IMAGERY Matteo Cerminara, in collaboration with Tomaso Esposti Ongaro, Sébastien Valade & Andy Harris

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Image 1

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Image Processing

Electromagnetic model

time-averaged 1D Plume model

Time-averaging and resampling

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U(z)

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turbulent entrainment

1D balance: Mass, Momentum Energy

- Image dilatation - Image rotation

Experimental TIR Image

b(z) (z),T(z)

control volume

- Radiative heat transfer (Schwarzshild's equation) - Electromagnetic absorption (Mie's theory) - Material emissivity

Synthetic TIR Image

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Inversion model 1D fit (axis) 2D fit (image)

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Plume properties retrieval

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(e.g., mass eruption rate, gas content, mean particle diameter)

GRAPHICAL ABSTRACT

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MOTIVATION Measure eruption source parameters: MER, Temp., Density, GSD… Invert data globally, using information from the whole TIR video Measure indirectly what cannot be seen directly: opaque plumes This is possible by coupling: infrared emission/absorption ⟷ fluid dynamics of volcanic plumes 3

EMISSION/ABSORPTION OF AN HETEROGENEOUS MEDIUM TIR wavelengths ≈ 7-14 μm Optical regime for d > 8 μm Absorption coefficient of particles proportional to their bulk density Schwarzschild’s equation

Optical thickness

specific absorption coefficient for particles:
 A1mm ≈ 1 m2/kg
 A0.1mm ≈ 10 m2/kg 4

VOLCANIC GASES ABSORPTION COEFFICIENTS 1

spectral response water vapor SO2 CO2

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ACO = 5.908 m2/kg

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AH

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2 O = 0.1145 m /kg

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0.2 ASO = 5.139 m2/kg

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spectral response

specific absorption coefficent [m2/kg]

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Absorption coefficient of gases depends also on the camera spectral response visibility of Air with 
 1 g/kg of H2O is approximatively 10 km 5

Thus, having the temperature T and the phases density ρ along an optical ray allows to find the TIR intensity IL impacting the camera In volcanic ash plumes, T and ρ can be found in the 3D spatial domain through 1D integral models A fast analytical plume model is used to find T and ρ in a fast way, once 7 parameters are given at the “vent”: FORWARD APPROACH entrainment velocity radius temperature ash mass fraction water mass fraction specific absorption coefficient of the mixture (depending on GSD, and mass fractions of SO2 and CO2) 6

DATA FROM SANTIAGUITO ERUPTION OCCURRED IN 2005

Averaged image Fluctuations Time window: 45-255 s Eddy turnover time ≈ 5 s 7

INVERSION PROCEDURE A cost function is identified, measuring the “distance” between the observed and measured averaged image
 
 
 Its minimum is searched using the genetic algorithm,
 to find the best vent parameters fitting the observation, globally 8

SYNTHETIC TIR IMAGE
 AND DIFFERENCE WITH OBSERVATION 100

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SYNTHETIC VS OBSERVED
 ALONG THE PLUME AXIS 110 100 90 80

T [ C]

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Optical depth:
 The plume
 is opaque

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SHAPE OF THE MINIMUM: THE INVERSION IS ROBUST 3

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INVERSION RESULTS 
 ERUPTION SOURCE PARAMETERS MER: 7.5±0.8 tons/s

Plume radius: Mass flux: Velocity: Temperature: Air mass fraction: H2O mass fraction: Ash mass fraction: GSD Sauter diameter:

23±1 m 13±2 tons/s 7.5±0.9 m/s 103±3° C 40±6 % 20±3 % 41±6 % 2.3±0.8 mm

Mass:1.9±0.4 Ktons 1/3 H2O 2/3 Ash d = 450 μm, σ=500 μm
 (log-normal) D = 2.34±0.09
 (power law) 12

INPUT DATA NEEDED

Averaged TIR image of the plume: min. time window ≈ 10D/U TIR image of the background atmosphere (good visibility needed) Atmospheric condition profiles SO2/H2O and CO2/H2O mass fractions (for GSD retrieval)

FUTURE DIRECTIONS Compare with steam-ash experimental plumes in air Application of the technique to other volcanic eruptions Fast and robust method

application to real-time monitoring

Use TIR data with thinner wavelength window Couple the TIR model with 3D plume models (ASHEE)
 synthetic TIR videos of the plume (umbrella?)

THANK YOU!

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