13,105 research outputs found
Magma and fluid migration at Yellowstone Caldera in the last three decades inferred from InSAR, leveling and gravity measurements
We studied the Yellowstone caldera geological unrest between 1977 and 2010 by investigating
temporal changes in differential Interferometric Synthetic Aperture Radar (InSAR), precise spirit leveling and
gravity measurements. The analysis of the 1992–2010 displacement time series, retrieved by applying the SBAS
InSAR technique, allowed the identification of three areas of deformation: (i) the Mallard Lake (ML) and Sour
Creek (SC) resurgent domes, (ii) a region close to the Northern Caldera Rim (NCR), and (iii) the eastern Snake
River Plain (SRP). While the eastern SRP shows a signal related to tectonic deformation, the other two regions
are influenced by the caldera unrest. We removed the tectonic signal from the InSAR displacements, and we
modeled the InSAR, leveling, and gravity measurements to retrieve the best fitting source parameters. Our
findings confirmed the existence of different distinct sources, beneath the brittle-ductile transition zone, which
have been intermittently active during the last three decades. Moreover, we interpreted our results in the light
of existing seismic tomography studies. Concerning the SC dome, we highlighted the role of hydrothermal
fluids as the driving force behind the 1977–1983 uplift; since 1983–1993 the deformation source transformed
into a deeper one with a higher magmatic component. Furthermore, our results support the magmatic nature
of the deformation source beneath ML dome for the overall investigated period. Finally, the uplift at NCR is
interpreted as magma accumulation, while its subsidence could either be the result of fluids migration outside
the caldera or the gravitational adjustment of the source from a spherical to a sill-like geometr
Full waveform inversion with extrapolated low frequency data
The availability of low frequency data is an important factor in the success
of full waveform inversion (FWI) in the acoustic regime. The low frequencies
help determine the kinematically relevant, low-wavenumber components of the
velocity model, which are in turn needed to avoid convergence of FWI to
spurious local minima. However, acquiring data below 2 or 3 Hz from the field
is a challenging and expensive task. In this paper we explore the possibility
of synthesizing the low frequencies computationally from high-frequency data,
and use the resulting prediction of the missing data to seed the frequency
sweep of FWI. As a signal processing problem, bandwidth extension is a very
nonlinear and delicate operation. It requires a high-level interpretation of
bandlimited seismic records into individual events, each of which is
extrapolable to a lower (or higher) frequency band from the non-dispersive
nature of the wave propagation model. We propose to use the phase tracking
method for the event separation task. The fidelity of the resulting
extrapolation method is typically higher in phase than in amplitude. To
demonstrate the reliability of bandwidth extension in the context of FWI, we
first use the low frequencies in the extrapolated band as data substitute, in
order to create the low-wavenumber background velocity model, and then switch
to recorded data in the available band for the rest of the iterations. The
resulting method, EFWI for short, demonstrates surprising robustness to the
inaccuracies in the extrapolated low frequency data. With two synthetic
examples calibrated so that regular FWI needs to be initialized at 1 Hz to
avoid local minima, we demonstrate that FWI based on an extrapolated [1, 5] Hz
band, itself generated from data available in the [5, 15] Hz band, can produce
reasonable estimations of the low wavenumber velocity models
Anelastic sensitivity kernels with parsimonious storage for adjoint tomography and full waveform inversion
We introduce a technique to compute exact anelastic sensitivity kernels in
the time domain using parsimonious disk storage. The method is based on a
reordering of the time loop of time-domain forward/adjoint wave propagation
solvers combined with the use of a memory buffer. It avoids instabilities that
occur when time-reversing dissipative wave propagation simulations. The total
number of required time steps is unchanged compared to usual acoustic or
elastic approaches. The cost is reduced by a factor of 4/3 compared to the case
in which anelasticity is partially accounted for by accommodating the effects
of physical dispersion. We validate our technique by performing a test in which
we compare the sensitivity kernel to the exact kernel obtained by
saving the entire forward calculation. This benchmark confirms that our
approach is also exact. We illustrate the importance of including full
attenuation in the calculation of sensitivity kernels by showing significant
differences with physical-dispersion-only kernels
Space-weighted seismic attenuation mapping of the aseismic source of Campi Flegrei 1983-84 unrest
Peer reviewedPublisher PD
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