High resolution aquifer characterization using crosshole GPR full‐waveform tomography: Comparison with direct‐push and tracer test data

Abstract

Limited knowledge about the spatial distribution of aquifer properties typically constrains our ability to predict subsurface flow and transport. Here we investigate the value of using high resolution full‐waveform inversion of cross‐borehole ground penetrating radar (GPR) data for aquifer characterization. By stitching together GPR tomograms from multiple adjacent crosshole planes, we are able to image, with a decimeter scale resolution, the dielectric permittivity and electrical conductivity of an alluvial aquifer along cross sections of 50 m length and 10 m depth. A logistic regression model is employed to predict the spatial distribution of lithological facies on the basis of the GPR results. Vertical profiles of porosity and hydraulic conductivity from direct‐push, flowmeter and grain size data suggest that the GPR predicted facies classification is meaningful with regard to porosity and hydraulic conductivity, even though the distributions of individual facies show some overlap and the absolute hydraulic conductivities from the different methods (direct‐push, flowmeter, grain size) differ up to approximately one order of magnitude. Comparison of the GPR predicted facies architecture with tracer test data suggests that the plume splitting observed in a tracer experiment was caused by a hydraulically low‐conductive sand layer with a thickness of only a few decimeters. Because this sand layer is identified by GPR full‐waveform inversion but not by conventional GPR ray‐based inversion we conclude that the improvement in spatial resolution due to full‐waveform inversion is crucial to detect small‐scale aquifer structures that are highly relevant for solute transport.

Time-lapse ground-penetrating radar full-waveform inversion to detect tracer plumes: A numerical study

“…The Krauthausen test site in Germany has been investigated by well logging, time-lapse tracer measurements (Vereecken et al, 2000;Kemna et al, 2002), cone penetration tests (Tillmann et al, 2008), direct-push hydraulics measurements (Gueting et al, 2017), and highresolution GPR-FWI (Gueting et al, 2015). These investigations allowed to derive the spatial variability of the basic aquifer parameters, transport properties, hydraulic conductivity and porosity.…”

“…These investigations allowed to derive the spatial variability of the basic aquifer parameters, transport properties, hydraulic conductivity and porosity. As a first stage of the 3D model created by Gueting et al (2018), co-located data of GPR and grain size along 3 boreholes were used to derive an empirical relationship between GPR properties and grain size (Gueting et al, 2017). This relationship was used to predict the distribution of three grain size facies at locations where no grain size data exist.…”

“…Hydraulic and transport parameters related to the Krauthausen aquifer were adapted from Gueting et al (2017) and Vanderborght et al (2005) as shown in Table 1. A natural hydraulic gradient of −0.002 (from Müller et al, 2010) is applied between the xz planes of the model to stimulate flow along the y-axis.…”

“…Crosshole planes during the tracer x y z infiltration are inverted by FWI, and models are compared to the background FWI model to calculate the differences in permittivity and conductivity. The background true model (Figures 2a, b) which is used in a forward model to calculate the observed data is taken from a GPR-FWI plane as estimated by Gueting et al, 2017. The permittivity true model (Figure 2a) is the same for all scenarios for the reason given in the former subsection.…”

Reconstruction of Three-Dimensional Aquifer Heterogeneity from Two-Dimensional Geophysical Data

“…For example, Gueting et al (2017) The present study builds upon the work by Gueting et al (2017) and addresses the question if the GPR cross-sections provide enough information to reconstruct the three-dimensional aquifer structure using MPS. For this purpose, the facies distribution along the GPR cross-sections is assumed to reflect the true aquifer structure; the goal is to reconstruct the three-dimensional volume between these cross-sections.…”