Near Surface Geophysics - 3 - GPR in Civil and Environmental Engineering: Recent Methodological Advances, 2019

This study focusses on examining the behaviour of an innovative asphalt pavement which is created as a solar energy collector, using non‐destructive testing involving ground‐penetrating radar. The concept of heat exchanger is based on the use of drainage asphalt in the bonding layer through which a heated fluid flows via gravity to de‐ice the roadway. In order to develop hydrothermal models for a test site representing such a pavement, the saturated porous layer assumption was required when the water level in each tank (up‐ and downstream of the structure) was same as the top of the porous layer. Two ground‐penetrating radar techniques were tested at this test site: a ground‐coupled impulse radar and an air‐coupled stepped‐frequency radar. The impulse ground‐penetrating radar, a high‐efficiency non‐destructive testing technique which is widely used in civil engineering, provides accurate geometric information, especially for pavement investigation. In the second innovative approach, air‐coupled stepped‐frequency radar was combined with full‐waveform inversion to obtain quantitative information, while retrieving the electromagnetic properties of the successive pavement layers.

We concentrated on the early‐stage water imbibition in the pavement structure using the impulse ground‐penetrating radar to estimate the fluid transfer velocity and both ground‐penetrating radar techniques to verify the saturated porous layer assumption in the steady state. Ground‐coupled radar enabled us to follow the water front and to capture different water‐transfer behaviours in the porous asphalt layer. Our observation could be explained by the vertical topology of the upper watertight interface. Air‐coupled stepped‐frequency ground‐penetrating radar presented similar results to ground‐coupled ground‐penetrating radar but provided a quantitative estimate of the changes within the porous layer during the test.

The Cathedral of St John in Valetta, Malta, represents a unique monument of historical importance. In this paper, we present the results of a detailed ground penetrating radar campaign performed in this Cathedral. The campaign was aimed at investigating the distribution of buried tombs under the mosaic floor of the main nave and of the lateral chapels of the Co‐Cathedral. The floor of the church shows a continuity of grave inscriptions in the main nave as well as in all lateral chapels. It was suspected, based on available historical documents, that only a part of the present‐day inscriptions corresponds to the original sepultures or tombs. The ground penetrating radar results presented here further highlight this conjecture and offer additional information regarding this important monument.

Detection of leakage in a buried water pipe is a crucial issue, as underground pipes become aged and these pipes are often used as pathways for aggressive chemical agents. Non‐destructive geophysical methods for identifying such underground leakages are of vital importance. Leakage detection in buried water pipes is sometimes carried out through specific acoustic methods. In this research, we have used ground‐penetrating radar as an alternative to such acoustic methods, because of the high sensitivity of electromagnetic waves to the presence of water in soil. This paper presents a methodology, illustrated by field‐scale ground‐penetrating radar experiments. Four water leakage points in a buried, ductile, iron, main‐water pipe were pre‐designed within a full‐scale (20 m long × 10 m wide) experimental set up. This facility was paved half by reinforced concrete and half by other pavement blocks. Four ground‐penetrating radar antennae with five nominal frequencies 200, 250, 400, 600 and 900 MHz were tested. Using a modified algorithm for common offset ground‐penetrating radar antennae, in this work, we measure the changes in electromagnetic wave velocity and wave reverberation, which sense upward and downward leakages, respectively. Leakages could be identified most clearly in the 600 MHz ground‐penetrating radar data. Our result suggests two potential applications in terms of detecting and defining the extent of multiple leakages in old water pipes, and testing and commissioning new pipes before and after pressurized tests.

A three‐dimensional ground‐penetrating radar survey was performed on the altar wall, which faced the risk of collapse, of a 16th‐century church in Michoacán, Mexico. The ground‐penetrating radar survey with high‐frequency antennas aimed to locate structural cracks and texture arrangements, the latter through the interpretation of the patterns in the electromagnetic data. The results indicate that the superficially visible cracks or fractures, except for the lateral ones at each end of the wall, disappear at a depth of between 15 and 25 cm. The lower part of the wall below a height of 3.97 m presents an area with multiple diffractions, which suggests that the masonry is made of ‘stone and mud’ and the tilt appears precisely where the adobe wall starts. Using 1500 and 900 MHz antennas, the identified texture of the wall shows at least three leaves or layers with diverse materials and a wooden beam embedded in the wall. Additionally, the survey near the church with a 200 MHz antenna supports the hypotheses that local construction materials were used here. The results inspire us to continue applying ground‐penetrating radar, a non‐destructive method, to diagnose the walls and structures of historical monuments before any intervention.

The genesis of alternate gravel bars in straightened rivers and braided rivers is closely coupled to the formation of scours. Scours can lead to riverbank failure, initiate lateral river dynamics and strongly impact the interaction between surface and subsurface flow and transport. It is, therefore, critical to account for scours in the design of flood protection measures. However, there is still little knowledge on the formation and characteristics of such scours, especially on the dynamic relationship between the riverbed morphology and the scours. Bathymetric riverbed surveys conducted at medium to low discharge may underestimate the real scour sizes and shapes if scours filled with sediments during waning discharge. Furthermore, the literature suggests that gravel bars and scours can jointly migrate on the riverbed, resulting in cut‐and‐fill sedimentary structures in the near subsurface. These cut‐and‐fill structures cannot be observed from the surface. We investigate with ground‐penetrating radar the presence of scour fills below the riverbed of two gravel‐bed rivers and study any indications of bar and scour migration. Thus, two ground‐penetrating radar surveys were conducted, one on the emerged part of an alternate bar of the channelized Alpine Rhine River (Switzerland) and the other on a flat gravel surface in a braided reach of the Tagliamento River (Italy) that is near the pristine state. At both sites, a scour fill could be clearly identified below a 2‐m‐thick sediment layer. The imaged part of the scour of the Alpine Rhine River (30 × 100 × 4.5 m) is located at the front end of the gravel bar next to the riverbank. This scour was only partially imaged by ground‐penetrating radar and is in reality significantly deeper and larger. The scour of the Tagliamento River (20 × 30 × 2.5 m) shows a clear internal structure consisting of inclined, planar cross‐beds that merge tangentially with the lower‐bounding erosional surface of the scour. At the light of the literature, we compare these two scours in terms of sedimentary processes resulting from the complex interplay between scours and gravel bars. The study findings offer a promising research avenue on the dynamic relationship between discharge, sediment transport, scour and bar formation and migration.

Precast concrete elements are commonly employed in the construction industry, however failing to comply with manufacturers’ guidelines and poor construction practice can lead to loss of efficiency compromising the usability of the building. This paper presents two case studies where ground penetrating radar surveys, performed to investigate the cause of failure of precast concrete elements, were supported by the finite difference time domain numerical approach. In the first case, the model was built after the survey for a better understanding of the complex reflection patterns unexpectedly experienced and to provide a clear interpretation; in the second case, the numerical simulation was performed prior to the survey, according to the information already available on the precast unit. The synthetic radargrams were then used as a valuable reference to assess the precast element internal conditions: on site, the comparison of the real radargrams with the synthetic ones allowed to address safely the intrusive works necessary to determine the concrete quality and during the processing step, any deviation from the ideal ground penetrating radar response gave potentially an indication of anomalies in the assembly operations that could be identified. The finite difference time domain method should then be considered as complementary to ground penetrating radar surveys aimed to investigate precast elements.

Ground‐penetrating radar is widely used to provide highly resolved images of subsurface sedimentary structures, with implications for processes active in the vadose zone. Frequently overlooked among these structures are tunnels excavated by fossorial animals (e.g., moles). We present two repeated ground‐penetrating radar surveys performed a year apart in 2016 and 2017. Careful three‐dimensional data processing reveals, in each data set, a pattern of elongated structures that are interpreted as a subsurface mole tunnel network. Our data demonstrate the ability of three‐dimensional ground‐penetrating radar imaging to non‐invasively delineate the small animal tunnels (∼5 cm diameter) at a higher spatial and geolocation resolution than has previously been achieved. In turn, this makes repeated surveys and, therefore, long‐term monitoring possible. Our results offer valuable insight into the understanding of the near‐surface and showcase a potential new application for a geophysical method as well as a non‐invasive method of ecological surveying.

Ground penetrating radar has been used extensively in near‐surface studies to detect underground objects and features typically located within a few metres beneath the surface. In urban areas, ground penetrating radar is widely used to study buried utilities such as pipes and cables. A more recent and unconventional application of ground penetrating radar is the detection of tree roots, which can interact negatively with the human infrastructure in a number of ways. However, the geophysical study of tree roots has proven quite challenging and site‐specific. Most tree roots (even coarse roots) have a small diameter and are hard to resolve through geophysical methods. In addition, the sheer amount of potential variability regarding the tree species, age, size, health and the subsurface environment (e.g., soil or a man‐made material such as concrete or asphalt) makes it very hard to implement a one‐size‐fits‐all approach. This is where robust, easily customizable forward models can be of assistance, indicating the range of detectable geophysical contrast and the limitations of the method, as well as the suitable antenna frequencies. Here, a vector network analyser with a commercial open‐ended coaxial probe was used to take direct measurements of the relative permittivity of freshly cut tree root segments at frequencies from 50 MHz to 3 GHz. The results were used as inputs to ground penetrating radar forward modelling using gprMax open source software, depicting various realistic scenarios which could be encountered in actual field surveys. The developed models help better understand the applicability, potential and limitations of ground penetrating radar surveys for detecting tree roots in different environments, aiding the development of future surveys. The notable variability in the tree roots is a significant consideration for surveys and forward models.