31 research outputs found
Inversion of crosshole seismic data in heterogenous environments : Comparison of waveform and ray-based approaches
High-resolution tomographic imaging of the shallow subsurface is becoming
increasingly important for a wide range of environmental, hydrological
and engineering applications. Because of their superior resolution
power, their sensitivity to pertinent petrophysical parameters, and
their far reaching complementarities, both seismic and georadar crosshole
imaging are of particular importance. To date, corresponding approaches
have largely relied on asymptotic, ray-based approaches, which only
account for a very small part of the observed wavefields, inherently
suffer from a limited resolution, and in complex environments may
prove to be inadequate. These problems can potentially be alleviated
through waveform inversion. We have developed an acoustic waveform
inversion approach for crosshole seismic data whose kernel is based
on a finite-difference time-domain (FDTD) solution of the 2-D acoustic
wave equations. This algorithm is tested on and applied to synthetic
data from seismic velocity models of increasing complexity and realism
and the results are compared to those obtained using state-of-the-art
ray-based traveltime tomography. Regardless of the heterogeneity
of the underlying models, the waveform inversion approach has the
potential of reliably resolving both the geometry and the acoustic
properties of features of the size of less than half a dominant wavelength.
Our results do, however, also indicate that, within their inherent
resolution limits, ray-based approaches provide an effective and
efficient means to obtain satisfactory tomographic reconstructions
of the seismic velocity structure in the presence of mild to moderate
heterogeneity and in absence of strong scattering. Conversely, the
excess effort of waveform inversion provides the greatest benefits
for the most heterogeneous, and arguably most realistic, environments
where multiple scattering effects tend to be prevalent and ray-based
methods lose most of their effectiveness
Evaluation of the viability and robustness of an iterative deconvolution approach for estimating the source wavelet during waveform inversion of crosshole ground-penetrating radar data
A major issue in the application of waveform inversion methods to
crosshole ground‐penetrating radar (GPR) data is the accurate estimation
of the source wavelet. Here, we explore the viability and robustness
of incorporating this step into a recently published time‐domain
inversion procedure through an iterative deconvolution approach.
Our results indicate that, at least in non‐dispersive electrical
environments, such an approach provides remarkably accurate and robust
estimates of the source wavelet even in the presence of strong heterogeneity
of both the dielectric permittivity and electrical conductivity.
Our results also indicate that the proposed source wavelet estimation
approach is relatively insensitive to ambient noise and to the phase
characteristics of the starting wavelet. Finally, there appears to
be little to no trade‐off between the wavelet estimation and the
tomographic imaging procedures
Analysis of an iterative deconvolution approach for estimating the source wavelet during waveform inversion of crosshole ground-penetrating radar data
A major issue in the application of waveform inversion methods to
crosshole georadar data is the accurate estimation of the source
wavelet. Here, we explore the viability and robustness of incorporating
this step into a time-domain waveform inversion procedure through
an iterative deconvolution approach. Our results indicate that, at
least in non-dispersive electrical environments, such an approach
provides remarkably accurate and robust estimates of the source wavelet
even in the presence of strong heterogeneity in both the dielectric
permittivity and electrical conductivity. Our results also indicate
that the proposed source wavelet estimation approach is relatively
insensitive to ambient noise and to the phase characteristics of
the starting wavelet. Finally, there appears to be little-to-no trade-off
between the wavelet estimation and the tomographic imaging procedures
Waveform inversion of crosshole georadar data: influence of source wavelet variability and the suitability of a single wavelet assumption
Waveform-based tomographic imaging of crosshole georadar data is a
powerful method to investigate the shallow subsurface because of
its ability to provide images of electrical properties in near-surface
environments with unprecedented spatial resolution. A critical issue
with waveform inversion is the a priori unknown source signal. Indeed,
the estimation of the source pulse is notoriously difficult but essential
for the effective application of this method. Here, we explore the
viability and robustness of a recently proposed deconvolution-based
procedure to estimate the source pulse during waveform inversion
of crosshole georadar data, where changes in wavelet shape with location
as a result of varying near-field conditions and differences in antenna
coupling may be significant. Specifically, we examine whether a single,
average estimated source current function can adequately represent
the pulses radiated at all transmitter locations during a crosshole
georadar survey, or whether a separate source wavelet estimation
should be performed for each transmitter gather. Tests with synthetic
and field data indicate that remarkably good tomographic reconstructions
can be obtained using a single estimated source pulse when moderate
to strong variability exists in the true source signal with antenna
location. Only in the case of very strong variability in the true
source pulse are tomographic reconstructions clearly improved by
estimating a different source wavelet for each transmitter location
Source-wavelet estimation during full-waveform inversion of ground-penetrating radar data
A major issue in the application of waveform inversion methods to
crosshole ground-penetrating radar (GPR) data is the accurate estimation
of the source wavelet. Here, we explore the viability and robustness
of incorporating this step into a recently published time-domain
inversion procedure through an iterative deconvolution approach.
Our results indicate that, at least in non-dispersive electrical
environments, such an approach provides remarkably accurate and robust
estimates of the source wavelet even in the presence of strong heterogeneity
of both the dielectric permittivity and electrical conductivity.
Our results also indicate that the proposed source wavelet estimation
approach is relatively insensitive to ambient noise and to the phase
characteristics of the starting wavelet. Finally, there appears to
be little to no trade-off between the wavelet estimation and the
tomographic imaging procedures
Detection of the fire blight biocontrol agent Bacillus subtilis BD170 (Biopro®) in a Swiss apple orchard
Fire blight, caused by Erwinia amylovora, is a major disease threat to apple, pear and other pome fruit worldwide. The disease is widespread in Europe and has recently become established in Switzerland. Antibiotics are the most effective controls used in North America but these are not permitted for agricultural use in most European countries. A newly registered biological control product Biopro®, based on the antagonist Bacillus subtilis strain BD170, is being used as an alternative strategy for fire blight management. A specific molecular marker was developed for monitoring the spread of this agent on blossoms after Biopro® spray application in a Swiss apple orchard throughout the bloom period for 2years. Direct spraying resulted in efficient primary colonisation of pistils in flowers that were open at the time of treatment. Subsequent bacterial dissemination (secondary colonisation) of flowers that were closed or at bud stage at the time of treatment was observed but was found to be dependent on the timing of treatments relative to bloom stage in the orchard. Foraging honeybees were shown to be disseminators of Biopro®. We also report detection of the biocontrol agent in honey collected from hives where bees were exposed by placing Biopro® at the entrance or in the hatching nest and from hives that were simply placed in sprayed orchards.ISSN:0929-1873ISSN:1573-846
Full-waveform inversion of cross-hole ground-penetrating radar data to characterize a gravel aquifer close to the Thur River, Switzerland
Cross-hole radar tomography is a useful tool for mapping shallow
subsurface electrical properties viz. dielectric permittivity and
electrical conductivity. Common practice is to invert cross-hole radar
data with ray-based tomographic algorithms using first arrival
traveltimes and first cycle amplitudes. However, the resolution of
conventional standard ray-based inversion schemes for cross-hole
ground-penetrating radar (GPR) is limited because only a fraction of the
information contained in the radar data is used. The resolution can be
improved significantly by using a full-waveform inversion that considers
the entire waveform, or significant parts thereof. A recently developed
2D time-domain vectorial full-waveform crosshole radar inversion code
has been modified in the present study by allowing optimized acquisition
setups that reduce the acquisition time and computational costs
significantly. This is achieved by minimizing the number of transmitter
points and maximizing the number of receiver positions. The improved
algorithm was employed to invert cross-hole GPR data acquired within a
gravel aquifer (4-10 m depth) in the Thur valley, Switzerland. The
simulated traces of the final model obtained by the full-waveform
inversion fit the observed traces very well in the lower part of the
section and reasonably well in the upper part of the section. Compared
to the ray-based inversion, the results from the full-waveform inversion
show significantly higher resolution images. At either side, 2.5 m
distance away from the cross-hole plane, borehole logs were acquired.
There is a good correspondence between the conductivity tomograms and
the natural gamma logs at the boundary of the gravel layer and the
underlying lacustrine clay deposits. Using existing petrophysical
models, the inversion results and neutron-neutron logs are converted to
porosity. Without any additional calibration, the values obtained for
the converted neutron-neutron logs and permittivity results are very
close and similar vertical variations can be observed. The full-waveform
inversion provides in both cases additional information about the
subsurface. Due to the presence of the water table and associated
refracted/reflected waves, the upper traces are not well fitted and the
upper 2 m in the permittivity and conductivity tomograms are not
reliably reconstructed because the unsaturated zone is not incorporated
into the inversion domain