Using volcanic gases to understand open-vent volcanoes
Open-vent volcanoes include the most active on Earth and are characterized by their persistent outgassing and the occurrence of moderate explosive activity between major eruptions. Forecasting eruptive activity at open-vent volcanoes is particularly challenging owing to the small changes in geoph...
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Format: | Thesis-Doctor of Philosophy |
Language: | English |
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Nanyang Technological University
2023
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Online Access: | https://hdl.handle.net/10356/168337 |
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Institution: | Nanyang Technological University |
Language: | English |
Summary: | Open-vent volcanoes include the most active on Earth and are characterized by
their persistent outgassing and the occurrence of moderate explosive activity between
major eruptions. Forecasting eruptive activity at open-vent volcanoes is particularly
challenging owing to the small changes in geophysical parameters which
typically proceed eruptions. Changes in the emission rate and composition of
plume gases however, presents a critical opportunity to detect changes before the
arrival of magma at the surface. Subsequent identification of periodic trends implies
that periodicities are an underlying component of volcanic plume degassing
which could provide fundamental insights into the processes controlling volcanic
degassing, and in turn, aid our ability to forecast eruptions. One of the only ways
to obtain a continuous record of the plume gas is by ground-based remote sensing
using UV-spectrometers where the favoured approach for quantifying emissions of
SO2 is by Differential Optical Absorption Spectroscopy, a technique based on the
Beer-Lambert-Bouguer law which relates the attenuation of light to properties of
the material through which it has travelled. The challenge for volcanologists arises
due to the absorption signatures of several trace gases in the spectra, which are
present in various unknown quantities, many of which absorb light at shared wavelengths.
To isolate the absorption due to SO2 from that of other trace gases, as
well as atmospheric and instrumentals effects, a complex fitting routine is required
where expertise is not only essential for the selection of suitable fit parameters but
also for visual inspection of the fit results.
Here, I present a novel approach which instead exploits the well-defined spatial frequencies
in UV spectra. Although the wavelength ranges of trace gas absorption
cross sections overlap, the occurrence of their narrow-band absorption features is
not uniform and when viewed in the wavenumber domain their absorption signatures
are distinct. I present a simple linear model to quantify SO2 absorption by
detection of its wavelet transform and demonstrate its ability to return expected
slant column densities of SO2 which are comparable to those achieved using a traditional
DOAS algorithm. I further capitalise on the spatial frequencies contained
in the recorded UV spectra to track volcanic SO2 without the need for external reference
spectra. By correlating spectra in the frequency domain, I provide a simple means to identify spectra which contain the absorption signature of the volcanic
plume. Use of wavelet coherence provides an alternative means to verify fit results
and to establish a suitable wavelength range for spectral analysis. I test the possibility
to analyse spectra for trace gases which are typically present at much lower
concentrations, and explore the use of the linear model to quantify bromine monoxide
(BrO) and chlorine dioxide (OClO), which offer the potential to detect magma
emplaced at shallow depth. Finally, I test the existence of periodicities in the SO2
emission rate from a range of volcanoes with vastly different characteristics. I use
time series data acquired by the Network of Volcanic and Atmospheric Change
(NOVAC) and the Lomb-Scargle periodogram, to identify significant periodicities
in the SO2 emission rates of 17 of the 28 volcanoes analysed. However, I find that
most of these periodicities are also present in the plume speeds used to determine
SO2 emission rates and are related to intra- and inter-seasonality in global trade
winds and not volcanic in origin. |
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