Crosshole GPR Research Articles

Source-Independent Waveform Inversion Method for Ground Penetrating Radar Based on Envelope Objective Function

For the full waveform inversion, it is necessary to provide an accurate source wavelet for forwarding modeling in the iteration. The source wavelet estimation method based on deconvolution technology can solve this problem to some extent, but we find that the estimated source wavelet is not accurate and needs to be manually corrected repeatedly in the iteration. This process is highly operator-intensive, and the update process is time-consuming and increases the potential for errors. We propose a source-independent waveform inversion (SIEWI) scheme for cross-hole GPR data, and use the envelope objective function combined with this method to effectively reduce the nonlinearity of inversion. The residual field used by SIEWI to construct the gradient inherits the characteristics of the envelope wavefield. Compared with full waveform inversion (FWI), SIEWI is more robust and less sensitive to frequency components and inaccurate source wavelet. To avoid cycle jumping, the multi-scale strategy effectively utilizes the properties of convolutional wavefields. In one iteration, the wavefield is decomposed into multiple frequency bands through multiple convolutions in the time domain to construct a multi-scale inversion strategy that preferentially inverts low-frequency information.

GPR full-waveform inversion of a variably saturated soil-aquifer system

Characterizing the critical zone is highly important to improve the mapping of small-scale spatial heterogeneity of soil properties and the understanding of flow and transport processes. Thereby, crosshole GPR full-waveform inversion of experimental data can significantly improve imaging results of permittivity and electrical conductivity compared to standard ray-based inversion approaches. Recently, the full-waveform inversion was used to characterize the saturated domain of several different aquifers based on highly resolved permittivity maps at the decimeter-scale. Until now it was not possible to invert the entire depth range of the variably saturated soil-aquifer system including the unsaturated and saturated part of the subsurface because of the high medium contrast between the unsaturated and saturated zone and differences in borehole fillings. Here, we propose a crosshole GPR full-waveform inversion that simultaneously inverts for both the saturated and unsaturated part of the subsurface. In the proposed method, we estimate four effective source wavelets rather than one for transmitter and receiver antennae combinations being present in both saturated, both unsaturated, or saturated-unsaturated and unsaturated-saturated, respectively. The GPR full-waveform inversion that investigated the soil-aquifer system is adapted accordingly, where different source wavelets are used during the inversion depending on the transmitter and receiver being present in the saturated and/or unsaturated zone. The new full-waveform inversion was applied to image a gravel aquifer and higher resolution images were obtained compared to standard ray-based inversion. Moreover, the measured and modeled data showed a good correspondence in both shape and amplitude for antennae in the saturated domain. For transmitters and receivers in the unsaturated zone, the shape of the modeled data is also in good correspondence with the measured data, whereas in the amplitude still a misfit can be noticed. Therefore, five different starting model tests were performed to improve the results in the unsaturated domain and to define the optimal solution for the full-waveform inversion. These tests indicated that either more a priori information is needed or more data is necessary to reliable invert the unsaturated zone. Nevertheless, we have demonstrated that the new GPR full-waveform inversion for variably saturated soil-aquifer systems for different borehole fillings can be used to image the entire depth range from the critical zone and can be applied to a wide field of applications.

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

AbstractHigh‐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 full‐waveform 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.

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

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

Crosshole GPR full-waveform inversion of waveguides acting as preferential flow paths within aquifer systems

High-contrast layers caused by porosity or clay content changes can have a dominant effect on hydraulic processes within an aquifer. These layers can act as low-velocity waveguides for GPR waves. We used a field example from a hydrological test site in Switzerland to show that full-waveform inversion of crosshole GPR signals could image a subwavelength thickness low-velocity waveguiding layer. We exploited the full information content of the data, whereas ray-based inversion techniques are not able to image such thin waveguide layers because they only exploit the first-arrival times and first-cycle amplitudes. This low-velocity waveguide layer is caused by an increase in porosity and indicates a preferential flow path within the aquifer. The waveguide trapping causes anomalously high amplitudes and elongated wavetrains to be observed for a transmitter within the waveguide and receivers straddling the waveguide depth range. The excellent fit of amplitudes and phase between the measured and modeled data confirms its presence. This new method enables detailed aquifer characterization to accurately predict transport and flow and can be applied to a wide range of geologic, hydrological, and engineering investigations.

Fracture imaging within a granitic rock aquifer using multiple-offset single-hole and cross-hole GPR reflection data

The sparsely spaced highly permeable fractures of the granitic rock aquifer at Stang-er-Brune (Brittany, France) form a well-connected fracture network of high permeability but unknown geometry. Previous work based on optical and acoustic logging together with single-hole and cross-hole flowmeter data acquired in 3 neighboring boreholes (70-100 m deep) have identified the most important permeable fractures crossing the boreholes and their hydraulic connections. To constrain possible flow paths by estimating the geometries of known and previously unknown fractures, we have acquired, processed and interpreted multifold, single- and cross-hole GPR data using 100 and 250 MHz antennas. The GPR data processing scheme consisting of time-zero corrections, scaling, bandpass filtering and F-X deconvolution, eigenvector filtering, muting, pre-stack Kirchhoff depth migration and stacking was used to differentiate fluid-filled fracture reflections from source-generated noise. The final stacked and pre-stack depth-migrated GPR sections provide high-resolution images of individual fractures (dipping 30-90{\deg}) in the surroundings (2-20 m for the 100 MHz antennas; 2-12 m for the 250 MHz antennas) of each borehole in a 2D plane projection that are of superior quality to those obtained from single-offset sections. Most fractures previously identified from hydraulic testing can be correlated to reflections in the single-hole data. Several previously unknown major near vertical fractures have also been identified away from the boreholes.