Nonlinear inversion of multifrequency GPR data in tomographic configurations

The accurate tomographic reconstruction of structures starting from Ground Penetrating Radar (GPR) data is useful in many real-world scenarios, ranging from the characterization of buried regions to the inspection of tree trunks. Unfortunately, the practical application of advanced inverse-scattering methods requires an accurate modeling of the GPR system, and in particular of the antenna and antenna-medium interactions [1]. In this work, the combination of an advanced antenna modelling technique with a nonlinear multifrequency inversion method is investigated from an experimental point of view. The GPR measurements, acquired with a lightweight radar system prototype in different configurations, are processed with a hybrid reconstruction approach that aims at combining the benefits of qualitative processing and quantitative inversion techniques [2]. The reconstruction of cylindrical targets buried in a sand box and in free space are considered, evaluating the influence of the antenna and its modelling on the inversion. Results are promising and allow to draw indications about the applicability of the proposed method to GPR configurations. [1] A. De Coster and S. Lambot, “Full-wave removal of internal antenna effects and antenna-medium interactions for improved ground-penetrating radar imaging,” IEEE Transactions on Geoscience and Remote Sensing, 2019. [2] F. Boero et al., “Microwave tomography for the inspection of wood materials: imaging system and experimental results,” IEEE Transactions on Microwave Theory and Techniques, vol. 66, no. 7, pp. 3497–3510, Jul. 2018

Soil hydraulic constraints on transpiration

Plants are continuously subject to changes in climate and soil conditions and their ability to promptly adapt to these changes is a key trait for the resilience against drought stress. These adaptation mechanisms take place locally at different temporal scales, ranging from seconds (e.g. for stomatal closure) to weeks (for growth and development). Stomatal regulation controls most plant carbon acquisition and regulates 60% of terrestrial water fluxes going from the soils to the atmosphere through plants. The primary function of stomata regulation is thought to prevent cavitation in the vascular system. When the soil dries out, its hydraulic conductivity decreases by several orders of magnitude and large gradients in water potential develop around the roots. The drop in water potential around the root is extremely abrupt and it depends on soil properties, transpiration rates and active root length. Practically, there is a threshold soil water potential beyond which the transpiration losses can no longer be sustained by the water flow in soils. At this critical point the water potential at the root-soil interface drops to very negative values extremely fast and so makes the xylem water potential, with consequent high risks of embolism. Our hypothesis is that plants close stomata when the gradients in soil water potential around the roots start to significantly influence the overall soil-plant hydraulic conductance. Practically, this means the relation between transpiration rate and leaf water potential, at a given soil water potential, deviates from a linear relation. This hypothesis implies that soil hydraulic conductivity is a primary constraint to transpiration. The objectives of this presentation are: 1) to provide theoretical and experimental data on the role of soil hydraulics on the relationship between transpiration, leaf water potential and soil water potential; 2) to discuss in what conditions soil hydraulics cause non-linearities in the relationship between transpiration and leaf water potential; 3) to provide experimental evidences that stomata close when the relationship between transpiration and leaf water potential becomes non linear; and 4) to propose below-ground strategies that plants developed to cope with these soil hydraulic constraints

Model-based assessment of the stochastic effect of landslides on cosmogenically-derived catchment-averaged denudation rates

In-situ cosmogenic radionuclides (CRN) have become a widely used tool in the geomorphic community to constrain geomorphic process rates. In many applications, catchment-wide denudation rates have been derived from in-situ produced 10Be concentrations in samples of river sand. These applications assume that sediment production and delivery rates within the catchment are reasonably steady and that river sediment is well mixed and all parts of the catchment are represented in proportion to their denudation rates. These assumptions are not necessarily met in active mountain ranges where deep-seated landslides or gully systems are contributing with deeply shielded soil or regolith to the river network. In this study, we analyse the variation in CRN-derived denudation rates as a function of the magnitude and frequency of geomorphic events. We first modified the CRN dynamics model of Yanites et al. (2009) to represent landslides as a proper spatial Poisson process and then adapted the code for the open-source programming language R. Then, we designed scenarios with different landscape and landslide configurations, taking into account the following four variables: the drainage area, the background erosion rate, the landslide return period and the landslide area distribution coefficient. The latter defines the shape of the magnitude-frequency landslide distribution curve, i.e. the probability of occurrence of landslides of a given magnitude. Finally, we run the scenarios for a time span of 200 kyr, and iterated 100 times each scenario to simulate the intrinsic variability of geomorphic events in landslide-prone landscapes. All scenarios show that the CRN-derived denudation rates are equal or higher than the volumetrically-derived denudation rates. The overestimation of CRN-derived denudation rates is larger when large landslides are more likely to occur and the landslide return period is shorter. An increase in background erosion reduces the landslide effect on CRN surface and river sediment concentrations. In addition, the stochastic approach shows that the variability of CRN-derived denudation rates is strongly controlled by the landslide area distribution coefficient and landslide return period. As such, the magnitude-frequency distribution of landslides influences both the variability and the accuracy of CRN-derived denudation rates. Our results show that this parameter, along with the landslide return period, should be taken into account when assessing the validity of CRN-derived denudation rates

Characterizing soil piping networks in Loess-derived soils using ground-penetrating radar

Soil piping refers to the formation of sub-surface pipes due to the erosive action of water flowing through the soil. These natural pipes can be considered as the largest category of macropores and may form subterranean networks with significant hydrological connectivity. Despite their substantial impacts on water transfer in numerous locations around the world, the origin and hydrological functioning of soil pipes is not yet fully understood, in particular in Loess-derived soils. One of the main limitations regarding the study of this singular process for this purpose is the characterization of the pipe networks (defining the number, position, dimension and connectivity of pipes). In this context, non-invasive sub-surface imaging using ground-penetrating radar (GPR) seems to be a promising technique. This research presents results from 3D, high-resolution GPR surveys performed in Loess-derived soils in order to characterize pipe networks with little prior information about their location. The adopted methodology relies on high spatial resolution scanning, 3D sub-surface imaging and automated detection of reflection hyperbolas using a 200 MHz centre-frequency antenna. Two small watersheds known to be affected by piping were investigated in Sippenaeken and Kluisbergen (Belgium). Over the two scanned zones, results revealed various continuous network patterns. Even though the most obvious patterns corresponded to recent or past anthropic activities (e.g., drainage pipes), validation tests confirmed that the chosen methodology may be used for pipe network characterization as the presence of two small pipes extending over tens of meters could be confirmed (a fairly small pipe was detected and validated over more than 100 m). Nevertheless, the presence of numerous artefacts and the high variability in size, depth and orientation of the pipes imply that GPR may only be effective at detecting pipes larger than those commonly observed at the present study sites (<10 cm in diameter). The results also point to the fact that operating at different frequencies may be advantageous to cope with such pipe variability

Together4Water: Testing a Citizen Science water monitoring project in Tunisia

Citizen Science (CS) has been emerging in the last decade as a new field of environmental monitoring involving a direct collaboration between everyday citizens and scientists. The concept is now also introduced in hydrology. In Tunisia, several recent governmental efforts aimed at reinforcing the existing official water-related information through the renovation of the Tunisian monitoring systems. However, the lack of reliable hydrological data still an issue. This major point of concern can be partially addressed through a CS approach. In this study, we present results of the test phase of the Together4Water initiative, a water resources CS project that was launched in Tunisia in 2018. We monitored river flow, rainfall and water quality in a test area of the Medjerda catchment using cost-effective and/or public available sensors. For river flow we used the ‘Discharge app’, for rain simple manual pluviometers and for water quality simplified water quality strips. We used a stepby- step approach to target, to engage and to train citizens on using the monitoring tools and transmitting the data to a centralized online platform. The collected CS data are compared with data from the governmental reference stations. Preliminary results yield a good agreement between CS river flow data collected at two sites (Slouguia and Medjez) and the reference stations (correlation coefficient R ranges between 0.8 and 0.97 for all citizens). For rainfall, measurements collected by citizens in eight locations correlate well with reference data (R ranges between 0.95 and 0.98). Finally, CS water quality data (PH, NO3, NO2, KH, GH and Cl2) are also consistent with the laboratory measurements (R ranges between 0.75 and 0.8). In addition, uncertainty of the CS data are compared with the uncertainty associated with the official governmental data. We conclude that the Together4Water CS test phase delivered a consistent hydrometrological data set. The variability between the citizens’ measurements can be explained by many factors such as the location of pluviometers for rainfall observation, the wind and light reflection for river flow measurement using Discharge app, and colors identification for the water quality strips. The CS approach is considered promising to complement existing Tunisian monitoring systems, and also to enhance innovation, adaptation, and local capacity building in the Tunisian water sector

Tree root system mise-à-la-masse (MALM) forward modelling with explicit representation of root structure

Agricultural yields critically depend on performance of the root system. Tree root system architecture and its development in time have indeed a key-impact under drought and nutrient stress conditions. Therefore, there is a need to monitor or phenotype root system, which is hidden in the soil. Electrical resistivity methods are promising for this task (Whalley et al., 2017) except that there is ambiguity in differentiating soil and root structures due to overlapping electrical conductivities of soil and root. Numerous studies also report the need for more accurate ERT methods of quantification of the root system at the plant scale, especially under field conditions (Ni et al.,2018). We previously demonstrated experimentally on a woody species (vineyard plant and orange trees) that current density distribution in the soil-root continuum could differ significantly depending on whether current is injected into the stem of the tree or in the soil surface close to stem (Mary et al., 2018). We explained this phenomenon by assuming that an injection into the stem is most likely to exit the root system only at fine root locations. Here, we present a preliminary modelling study using an explicit representation of root structure in the MALM forward modelling as a follow-up work to support the assumptions made and to understand better how this approach can be made more robust for field-scale root phenotyping. In this study, we quantified the sensitivity of the Mise-à-la-masse coupled with the ERT method to image the root system in respect to different soil water content (after Root Water Uptake). We also estimated how the contrast of resistivity between roots (for a range of resistance in inner layer of root, outer bark structure as well as fine roots), and different types of soil influence the geophysical measurements

An updated seismicity map of the Sicily Channel through improved seismic networks on the Maltese islands and Central Mediterranean

The Sicily Channel, bordered by the Sicilian and Tunisian coastlines, the Sicily-Malta escarpment to the east, and the Maghrebian thrust front to the west, is a tectonically interesting region, encompassing a NE-directed extensional process superimposed on the NW-directed thrust of Africa onto Europe. The extension is bathymetrically represented in the form of deep NW-SE oriented grabens in the sea floor, reaching a depth of over 1000 m. The nature of the rifting process is still controversial. The grabens have been investigated from the geophysical and geological aspects, but poorly studied in terms of active seismicity, which is usually referred to as sparse or insignificant in the scientific literature. [excerpt]peer-reviewe

Innovative geophysical measurements on the cultural heritage patrimony in Lecce, Italy

The bilateral project "Non-invasive investigations to enhance the knowledge and the enjoyment of cultural heritage", conducted together by the Institute for Archaeological and Monumental Heritage IBAM-CNR and the Department of Geoscience of the University of Malta, has allowed to schedule performed integrated geophysical prospections [1] both in Italy and in Malta, part of which have been already performed. In this contribution, we will describe a work regarding the Roman amphitheatre of Lecce, Italy, where ground penetrating radar and passive seismic measurements were performed. The amphitheatre of Lecce is a particular monument because it is only partially visible. In particular, its memory was lost and its traces appeared causally when digging for a building work at the beginning of last century. It was not possible to excavate it entirely because of the presence of subsequent structures, by now historical in their turn, of renaissance-baroque style. In particular, the "ambulacra", i.e. the corridors under the steps that originally surrounded the entire arena, are walled at a certain point and it is not known if they continue or not beyond the current limit up to form a closed ring, or if they collapsed. In particular, part of these ambulacra prolong under the current St Oronzo square, and we have performed geophysical investigation in the square and in particular in an area where the buried ambulacra, if still present should pass. The investigations reveals that it is probable that, at least partially, the ambulacra continue, but it is not possible to establish if they join from both sides and still form an elliptical ring under the square, also because the vertex of the ellipsis should be placed under a church where we could not perform prospections at the moment.peer-reviewe

Evaluating the performance of UAV photogrammetry with PPK positioning in topographic reconstruction and change-detection

Unmanned aerial vehicles (UAVs) are being increasingly used to provide high resolution imagery which, through photogrammetric method Structure-from-Motion (SfM), can be used for high resolution topographic reconstruction. PPK (Post-Processing Kinematic) positioning solution offers the potential of accurate image georeferencing without ground control points (GCPs). By integrating consumer-grade cameras and an onboard RTK/PPK GNSS receiver, we managed to have a low-cost and effective approach involving topographic change-detection. Here, we evaluated the positional accuracy and reproducibility of DSMs generated from PPK-SfM workflow with two camera setups by conducting multiple UAV surveys over a test field. Our results showed that the PPK solution has the same accuracy (mean: ca. 0.01 m, RMSE: ca. 0.03 m) as the more traditional georeferencing method based on ground control points. Furthermore, our results indicated that camera properties (i.e. focal length, resolution, sensor quality) have a large impact on the accuracy: a DSLR camera (Canon EOS), with 0.63 cm px-1 GSD, had 0.011 m planimetric error and 0.012 m altimetric error; while a cheap and light Action camera (GoPro) with 3.11 cm px-1 GSD had 0.019 m planimetric error and 0.024 m altimetric error at the same flight height. The repeatability of DSM construction was assessed by a DoD (DEMs of Difference) analysis. The LoD95% and LoDmin were ca. ±0.08 m and ca. ±0.04 m for the DSLR camera and ca. ±0.16 m and ca. ±0.08 m for the action camera (for a flight altitude of 45 m). The level of detection substantially improved when reducing the UAV flight altitude. We found that tie point density greatly controlled the error of the topography reconstruction for the DSLR camera. By exploiting the relation between error and tie point density, we demonstrated that a spatially explicit DoD threshold, according to the local surface condition, can greatly improve surface change detection. Volume estimation was conducted over the plowing area and obtained comparable results from DSLR and action camera datasets. Overall, the application of a PPK-UAV workflow provides a high-precision and high-efficiency solution in surveying and geomorphological applications

Attenuation relationships for the peak ground acceleration and velocity for the central Mediterranean area

The Central Mediterranean region is one of the most seismically active region within Europe. The Maltese islands lie in the centre of this tectonically active region which is characterized by numerous complex geological processes. However the islands have been negatively affected by the number of registered earthquakes in the past. Most of the larger earthquakes were occurred in Eastern Sicily, the Sicily Channel and even as far as the Hellenic arc. [excerpt]peer-reviewe