Full-waveform inversion of surface ground penetrating radar data and coupled hydrogeophysical inversion for soil hydraulic property estimation
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Beschreibung
Non-invasive electromagnetic methods are increasingly applied for a wide range of
applications in geophysical engineering, infrastructure characterization and environmental
and hydrological studies. A variety of geophysical techniques are routinely used to estimate
medium properties, monitor shallow soil conditions and provide valuable estimates of soil
water content and the soil hydraulic parameters needed for the understanding of the highly
dynamic hydrological processes in the subsurface.
Traditionally, estimates of the soil water content are obtained using the subsurface
permittivity and conductivity in combination with petrophysical relationships such as the
Complex Refractive Index Model (CRIM) or empirical relationships such as Topp's
equation and Archie's law. Here, especially surface ground penetrating radar (GPR) is a
technique that enables a quick and effective mapping of the subsurface dielectric
permittivity. Although GPR has the potential to return permittivities and conductivities for
the same sensing volume at the field scale, estimates of the conductivity based on
conventional ray-based techniques that only use part of the measured data and simplified
approximations of the reality contain relatively large errors. Full-waveform inversion (FWI)
overcomes these limitations by using an accurate forward modeling and inverts significant
parts of the measured data to return reliable quantitative estimates of both permittivity and
conductivity.
In this work, we introduce a novel full-waveform inversion scheme that is able to reliably
estimate permittivity and conductivity values from surface GPR data. It is based on a
frequency-domain solution of Maxwell’s equations including far-, intermediate- and
near-fields assuming a three-dimensional, horizontally layered model of the subsurface, and
requires a starting model of the subsurface properties as well as the estimation of a source
wavelet. Although the full-waveform inversion is relatively independent of the permittivity
starting model, inaccuracies in the conductivity starting model result in erroneous effective
wavelet amplitudes and therefore in erroneous inversion results, since the conductivity and
wavelet amplitudes are coupled. Therefore, the permittivity and conductivity are updated
simultaneously with the phase and amplitude of the source wavelet. Here, optimizing the
medium properties and reducing the misfit is carried out using a gradient free approach.
This novel FWI is applied the analysis of ground waves and reflected waves. In the case of
synthetic single layered and waveguide data, where the starting model differs significantly
from the true model parameter, we were able to reconstruct the obtained model properties
and the effective source wavelet. For measured waveguide data, different starting values
returned the same quantitative medium properties and a data-driven effective source
wavelet.
Autor*in
Sebastian Busch