3-D characterization of high-permeability zones in a gravel aquifer using 2-D crosshole GPR full-waveform inversion and waveguide detection

Abstract

Reliable high-resolution 3-D characterization of aquifers helps to improve our understanding of flow and transport processes when small-scale structures have a strong influence. Crosshole ground penetrating radar (GPR) is a powerful tool for characterizing aquifers due to the method's high-resolution and sensitivity to porosity and soil water content. Recently, a novel GPR full-waveform inversion algorithm was introduced, which is here applied and used for 3-D characterization by inverting six crosshole GPR cross-sections collected between four wells arranged in a square configuration close to the Thur River in Switzerland. The inversion results in the saturated part of this gravel aquifer reveals a significant improvement in resolution for the dielectric permittivity and electrical conductivity images compared to ray-based methods. Consistent structures where acquisition planes intersect indicate the robustness of the inversion process. A decimetre-scale layer with high dielectric permittivity was revealed at a depth of 5-6 m in all six cross-sections analysed here, and a less prominent zone with high dielectric permittivity was found at a depth of 7.5-9 m. These high-permittivity layers act as low-velocity waveguides and they are interpreted as high-porosity layers and possible zones of preferential flow. Porosity estimates from the permittivity models agree well with estimates from Neutron-Neutron logging data at the intersecting diagonal planes. Moreover, estimates of hydraulic permeability based on flowmeter logs confirm the presence of zones of preferential flow in these depth intervals. A detailed analysis of the measured data for transmitters located within the waveguides, revealed increased trace energy due to late-arrival elongated wave trains, which were observed for receiver positions straddling this zone. For the same receiver positions within the waveguide, a distinct minimum in the trace energy was visible when the transmitter was located outside the waveguide. A novel amplitude analysis was proposed to explore these maxima and minima of the trace energy. Laterally continuous low-velocity waveguides and their boundaries were identified in the measured data alone. In contrast to the full-waveform inversion, this method follows a simple workflow and needs no detailed and time consuming processing or inversion of the data. Comparison with the full-waveform inversion results confirmed the presence of the waveguides illustrating that full-waveform inversion return reliable results at the highest resolution currently possible at these scales. We envision that full-waveform inversion of GPR data will play an important role in a wide range of geological, hydrological, glacial and periglacial studies in the critical zone.

Morphological, hydrological, biogeochemical and ecological changes and challenges in river restoration – the Thur River case study

“…These interpretations were only made possible by modeling borehole effects (Doetsch et al, 2010a, b); incorporating known lithological interfaces (Coscia et al, 2011); and by employing dedicated filtering strategies to remove unwanted contributions to the geophysical monitoring data (Coscia et al, 2012a). We found that imaging techniques based on full-waveform inversion hold considerable promise, as they offer unprecedented resolution capabilities, but their reliability, especially in terms of the resolved electrical conductivity, is still a subject of ongoing research (Klotzsche et al, 2013). An investigation of the utility of self-potential monitoring to follow groundwater dynamics in the hyporheic zone was inconclusive, mainly because of thick clay deposits under the aquifer that led to very low electrical field strengths and, hence, low signal levels .…”

Two-Dimensional Linear Inversion of GPR Data with a Shifting Zoom along the Observation Line

“…The problem at hand inserts into the framework of microwave tomography from GPR data, which has been largely investigated in the literature, both from the point of view of the imaging algorithms [30][31][32][33] and from the point of view of the effects of the soil and antennas on the data [34][35][36]. Applications to cases in the field have been also presented, e.g., in [37,38].…”

Quantitative multi-layer electromagnetic induction inversion and full-waveform inversion of crosshole ground penetrating radar data

“…For a transect covering 25 m length and 10 m depth consisting of 5 crosshole planes, GPR data were inverted using the ray-based and fullwaveform inversion as described by Klotzsche et al (2013Klotzsche et al ( , 2010 and Oberröhrmann et al (2013). Densely spaced cone penetration tests (CPT), located close to the GPR transect, were used to validate and interpret the obtained images Quantitative Multi-Layer Electromagnetic Induction Inversion and Full-Waveform Inversion of Crosshole Ground 847 obtained from GPR.…”