Time-lapse ground penetrating radar full-waveform inversion to detect
tracer plumes: Numerical study
Abstract
The movement and spreading of contaminated groundwater plumes and their
mixing with non-contaminated water is strongly influenced by the
heterogeneity of the aquifer properties, which may vary strongly over
small spatial scales. Thus, imaging these small-scale features and
monitoring transport of tracer plumes at a fine resolution is of
interest to characterize transport processes in aquifers. Full-waveform
inversion (FWI) of crosshole ground penetrating radar (GPR) measurements
can provide an aquifer characterization at decimeter-scale resolution.
The method produces images of both relative dielectric permittivity
(εr) and bulk electrical conductivity
(σb), which related to hydraulic aquifer
properties and tracer distributions. To test the potential of time-lapse
GPR FWI for imaging tracer plumes, we conducted a numerical experiment
of tracer transport in a heterogeneous aquifer. Concentration was
converted to saline and desalinated tracers, which changed
σb, and to ethanol, which changed both
εr and σb. The simulated
εr and σb distributions in
a crosshole plane were considered to simulate GPR data. These data were
subsequently used to reconstruct εr and
σb distributions using the crosshole 2D GPR FWI.
Tracer concentrations were retrieved from the inverted
εr and σb models using
information about petrophysical parameters. GPR FWI
εr images could recover preferential paths of
~0.2 m width, while the σb images
resolved structures up to ~ 0.2-0.3 m. The results
highlight that changes in εr, e.g., ethanol and
hot water, can be used to image transport processes with high resolution
by time-lapse GPR FWI, while the accuracy of the recovery of
σb is limited.