Inverting surface GPR data using FDTD simulation and automatic detection of reflections to estimate subsurface water content and geometry

Abstract

A new inversion scheme for common-offset ground-penetrating radar measurements at multiple antenna separations was proposed, which is intermediate between inverting of picked reflectors using ray-tracing and full-waveform inversion. The measurements are modeled similarly to the real data using 2D finite-difference time-domain simulations. These simulations are obtained with a parameterized model of the subsurface that consists of several layers with constant dielectric permittivity and an explicit representation of the layers’ interfaces. Then, reflections in the modeled and in the real data are detected automatically, and the reflections of interest of the real data are selected manually. The sum of squared residuals of the reflections’ traveltime and amplitude is iteratively minimized to estimate subsurface water content and geometry, i.e., the position and shape of the layer interfaces. The method was first tested with a synthetic data set and then applied to a real data set. The comparison of the method’s result with ground-truth data showed an agreement with the subsurface geometry within [Formula: see text] and with the water content, a difference less than [Formula: see text] volume.

Soil hydraulic material properties and subsurface architecture from time-lapse GPR

“…Similar to Buchner et al (2012), we extract the signal travel time t and the according amplitude A at M samples of the GPR signal (events)…”

“…Therefore, Buchner et al (2012) tested all possible combinations of events, using the one with the minimal summed absolute travel time difference. However, this is only feasible for a small number of events.…”

“…2.3), the χ 2 function is not convex and may even be discontinuous at some points, as the number of associated events M is not necessarily constant during the minimization process. To compensate for adding and removing simulated or measured events, Buchner et al (2012) introduced tagging. If the number of measured events is smaller than the number of the simulated events, the simulated events that are not associated are excluded.…”

“…Using the full antenna signal may lead to many local minima prohibiting a reliable identification of the global minimum (e.g., ford et al, 2014). In contrast, filtering the radargram with convolution approaches to determine travel time and amplitude of a limited number of events may even allow the application of efficient locally-convergent algorithms (e.g., Buchner et al, 2012).…”

“…The time-lapse measurement data was acquired with a single channel on-ground bistatic antenna operating at a center frequency of 400 MHz. Similar to Buchner et al (2012), we solve Maxwell's 20 equations in 2D and employ a new semi-automatic heuristic approach to extract travel time and amplitude of relevant reflections. This allows the optimization procedure to focus on the relevant information in the radargram and decreases the number of local minima.…”