Imaging small-scale, fast subsurface flow processes: field examples and practical guidelines for GPR monitoring

Among the different near-surface geophysical methods, ground-penetrating radar (GPR) is particularly promising for investigating small-scale (centimeters to meters) and fast (seconds to minutes) subsurface flow processes. Technical developments in GPR systems and data acquisition, such as real-time signal digitization, real-time positioning techniques, and multichannel GPR systems, enable repeated 2D or 3D GPR measurements (2D/3D GPR monitoring) in various environments with a temporal resolution on orders of minutes and a spatial resolution of centimeters to decimeters. Another advantage of GPR is the ability to link temporal changes in the GPR signal to variations in soil water content, because the propagation velocity of the GPR signal depends on the dielectric permittivity and, thus, water content.

However, GPR monitoring experiments must be carefully designed to collect high-quality datasets. The experimental setup must provide accurate positioning, consistent high spatial and temporal sampling, and minimal variations in GPR antenna coupling. Furthermore, following a special data processing and data quality analyses schedule is important to obtain reliable interpretation results. Such a data analysis must focus on identifying, evaluating, and suppressing amplitude fluctuations and time shifts in the recorded GPR data that are unrelated to changes in the subsurface (time-lapse noise).

To provide a GPR monitoring strategy that incorporates all the aforementioned points, we present two field examples of GPR monitoring in combination with irrigation experiments to image subsurface flow processes: a 2D hill-slope scale experiment and 3D plot-scale experiment. These examples demonstrate our general measurement setup and schedule for repeatable GPR data collection and data analysis, as well as a first-order, attribute-based data interpretation. We also highlight important practical points to consider for performing and analyzing such GPR monitoring experiments.

Our field examples demonstrate the great potential of GPR monitoring to image and investigate subsurface flow processes. We also provide a practical guide for successfully performing GPR monitoring experiments to promote the application of GPR monitoring to study hydrological subsurface processes.