Detection of spatially limited high‐porosity layers using crosshole GPR signal analysis and full‐waveform inversion

Abstract

High-permittivity layers, related to high-porosity layers or impermeable clay lenses, can act as low-velocity electromagnetic waveguides. Electromagnetic wave phenomena associated with these features are complicated, not well known and not easy to interpret in borehole GPR data. Recently, a novel amplitude analysis approach was developed that is able to detect continuous low-velocity waveguides and their boundaries between boreholes by using maximum and minimum positions of the trace energy profiles in measured GPR data. By analyzing waveguide models of different thickness, dip, extent, permittivity, and conductivity parameters, we extend the amplitude analysis to detect spatially limited or terminated waveguides. Waveguides that show high-amplitude elongated wave trains are most probably caused by a change in porosity rather than a change in clay content. In a crosshole GPR data set from the Boise Hydrogeophysical Research Site, two terminated wave-guiding structures were detected using the extended amplitude analysis. Information gained from the amplitude analysis improved the starting model for fullwaveform inversion which imaged the lateral extent and thickness of terminated waveguides with high resolution. Synthetic data calculated using the inverted permittivity and conductivity models show similar amplitudes and phases, as observed in the measured data, which indicates the reliability of the obtained models. Neutron-Neutron logging data from three boreholes confirm the changes in porosity and indicate that these layers were high-porosity sand units within low-porosity, poorly sorted sand, and gravel units.

Mapping sand layers in clayey till using crosshole ground-penetrating radar

“…The trace energy, defined as the sum of the squared amplitudes (e.g., Klotzsche et al, 2014), could instead have been used. But for the purpose of the analysis presented here, the amplitude of the first positive cycle gives an adequate representation of the trace energy, and is more robust to non-stationary low-frequency noise.…”

“…This approach is similar to the work of Klotzsche et al (2014). Here, traces having amplitude values below and above 15000 mV are assumed to have travelled through clay and sand, respectively.…”

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

“…Recent advances in geophysical measurement and inversion techniques allow imaging the subsurface with unprecedented resolution and coverage [ Binley et al ., ]. A particularly promising method with regard to the detailed characterization of porous aquifers is full‐waveform inversion of cross‐borehole GPR data [ Ernst et al ., ; Klotzsche et al ., ; Gueting et al ., ]. This method uses a transmitting and a receiving antenna which are placed in two adjacent boreholes, several meters apart from each other.…”

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

“…The starting relative permittivity and conductivity tomograms are usually obtained using a standard ray-based inversion that is employing the first-arrival times and first-cycle amplitudes. Applying the full-waveform inversion on several crosshole GPR data sets acquired in gravel aquifers in Switzerland and the U.S.A. shows that subwavelength thickness low-velocity wave guiding layers can be correctly imaged (Klotzsche et al, 2014(Klotzsche et al, , 2012, whereas ray-based inversion techniques are not able to image these thin waveguide layers because they only exploit the first-arrival times and first-cycle amplitudes. Converting the permittivity results into porosity and comparing them with NeutronNeutron logging data showed a good correspondence (Klotzsche et al, 2014.…”