Saturated hydraulic conductivity determined by on ground mono-offset Ground-Penetrating Radar inside a single ring infiltrometer

In this study we show how to use GPR data acquired along the infiltration of water inside a single ring infiltrometer to inverse the saturated hydraulic conductivity. We used Hydrus-1D to simulate the water infiltration. We generated water content profiles at each time step of infiltration, based on a particular value of the saturated hydraulic conductivity, knowing the other van Genuchten parameters. Water content profiles were converted to dielectric permittivity profiles using the Complex Refractive Index Method relation. We then used the GprMax suite of programs to generate radargrams and to follow the wetting front using arrival time of electromagnetic waves recorded by a Ground-Penetrating Radar (GPR). Theoretically, the 1D time convolution between reflectivity and GPR signal at any infiltration time step is related to the peak of the reflected amplitude recorded in the corresponding trace in the radargram. We used this relation ship to invert the saturated hydraulic conductivity for constant and falling head infiltrations. We present our method on synthetic examples and on two experiments carried out on sand soil. We further discuss on the uncertainties on the retrieved saturated hydraulic conductivity computed by our algorithm from the van Genuchten parameters

Ground Penetrating Radar: Analysis of point diffractors for modeling and inversion

International audienceThe three electromagnetic properties appearing in Maxwell's equations are dielectric permittivity, electrical conductivity and magnetic permeability. The study of point diffractors in a homogeneous, isotropic, linear medium suggests the use of logarithms to describe the variations of electromagnetic properties in the earth. A small anomaly in electrical properties (permittivity and conductivity) responds to an incident electromagnetic field as an electric dipole, whereas a small anomaly in the magnetic property responds as a magnetic dipole. Neither property variation can be neglected without justification. Considering radiation patterns of the different diffracting points, diagnostic interpretation of electric and magnetic variations is theoretically feasible but is not an easy task using Ground Penetrating Radar. However, using an effective electromagnetic impedance and an effective electromagnetic velocity to describe a medium, the radiation patterns of a small anomaly behave completely differently with source-receiver offset. Zero-offset reflection data give a direct image of impedance variations while large-offset reflection data contain information on velocity variations

Estimating Saturated Hydraulic Conductivity from Surface Ground-Penetrating Radar Monitoring of Infiltration

In this study we used Hydrus-1D to simulate water infiltration from a ring infiltrometer. We generated water content profiles at each time step of infiltration, based on a particular value of the saturated hydraulic conductivity while knowing the other van Genuchten parameters. Water content profiles were converted to dielectric permittivity profiles using the Complex Refractive Index Method relation. We then used the GprMax suite of programs to generate radargrams and to follow the wetting front using arrival time of electromagnetic waves recorded by a Ground-Penetrating Radar (GPR). Theoretically, the depth of the inflection point of the water content profile simulated at any infiltration time step is related to the peak of the reflected amplitude recorded in the corresponding trace in the radargram. We used this relationship to invert the saturated hydraulic conductivity for constant and falling head infiltrations. We present our method on synthetic examples and on two experiments carried out on sand. We further discuss the possibility of estimating two other van Genuchten parameters, n and \alpha, in addition to the saturated hydraulic conductivity.Comment: 4th International Conference "HYDRUS Software Applications to Subsurface Flow and Contaminant Transport Problems," Praque : Czech Republic (2013

Choqek’iraw y el valle del Apurímac: hábitat y paisajes prehispánicos tardíos. Una investigación en curso

La presente nota tiene como propósito presentar una investigación arqueológica en curso sobre el valle medio del río Apurímac y la cordillera de Vilcabamba. Esta región se conoce por haber sido el escenario de relaciones diplomáticas y belicosas entre los últimos inkas y la Corona Española durante el siglo 16. Nuevos estudios muestran la complejidad de la ocupación previa a la Conquista, la cual es objeto específico de nuestro interés. En 2007, se llevó a cabo un proyecto de prospección exten..

“Sur le chemin d’Apurimaq”

Au carrefour de la géographie, de l’archéologie et de l’ethnohistoire, cet article s’intéresse à la géographicité de la culture inca des Andes préhispaniques. La réflexion est fondée sur l’étude de la section de la grande route inca qui traversait la vallée de l’Apurimac, non loin de Cuzco dans les Andes centrales. La région du pont franchissant la rivière était associée à une figure mythique du monde inca. Cette étude indique que le mythe avait pour objet de spécifier la perception du paysage expérimenté par les voyageurs qui parcouraient le chemin au sein d’un environnement qui était ainsi territorialisé dans sa double dimension matérielle et idéelle. Ainsi, l’histoire du wak’a de la rivière Apurimac met en relief les dimensions symboliques et pratiques de la territorialité inca. Dans une civilisation porteuse d’une science géographique prémoderne sans cartographie topographique, les mythes, l’architecture du paysage et les mouvements rituels étaient, en effet, des piliers des systèmes de représentation et de structuration géographique du territoire.“On the Apurimaq Road”At the juncture between geography, archeology and ethno-history, this article on the « geographicity » of Incan culture in the pre-Hispanic Andes focuses on the section of the main road, which crossed the Apurimaq valley, not far from Cuzco in the central Andes. The area around the bridge that crosses the river was associated with a mythical Incan figure. The myth specifies the perception of the landscape experienced by travelers on this road in an environment that was « territorialized » in its double dimension, material and ideal. The story about the wak’a of the Apurimaq River sheds light on the symbolic and practical dimensions of Incan territoriality. In a civilization endowed with a premodern science of geography without cartography, myths, landscape architecture and ritual movements were the pillars of systems of representation and of a geographical « structuration » of the territory

Where does a glacier end ? GPR measurements to identify the limits between the slopes and the real glacier area. Application to the Austre Lovénbreen, Spitsbergen -- 79°N

International audienceGlacier limits are usually mapped according to a spatial discrimination based on color of remote sensing images or aerial photography. What appears like ice (white or light colored areas) at the end of the ablation period (end of summer) corresponds to the glacier, while what appears as rock (dark areas) is identified as the slope. This kind of visual discretization seems to be insufficient in the case of small arctic glaciers. Indeed, the slopes have been described as very unstable parts of glacial basins. Debris are generated by the inclination of the slopes, and reach the glacier surface. Thus, the visible limit does not correspond to the ice extension: a significant amount of ice is potentially covered by rock debris, enlarging the actual glacier surface with respect to the observed area. Hence, we apply Ground Penetrating Radar (GPR) measurements for mapping, beyond the central parts of the glacier, the steep slopes of the Austre Lovénbreen (Spitsbergen, 79°N). The aim is to assess the discrepancy between the limits extracted from remote sensing methods -- aerial photography, satellite images and derived digital elevation models -- and the GPR data which exhibit significant ice thickness at locations considered outside the glacier itself. The ice is observed to extend typically from 25 to 30 meters, and up to 100~meters, under the slopes. These measurements allow for a new determination of the rock/ice interface location following criteria beyond the visual and morphological characteristics seen from the surface, as obtained by remote sensing techniques or in-situ observations

Multi-scale analysis of water alteration on the rockslope stability framework

International audienceWater is an important weathering factor on rock discontinuities and in rock mass mechanical behavior. The increase of rainfall in frequency or in intensity highlights some problems on the rock slope stability analysis. The aim of this paper is the multi scale analysis of the chemical impact of water on rock (surface roughness and matrix). In this study we show how water induces degradation and thus may decreases the stability of the discontinuous rock mass. Water is known to have significant erosion and dissolution effects on rock surface or rock matrix. The chemical features of water such as temperature, pH or salinity make it a 'good' candidate to rock degradation. This study has two main components. The first one is the study of water-solid chemical mechanisms and the other is the analysis of the mechanical response of the discontinuity modified by the water alteration. The stability of the rock mass is naturally a function of the type and the space distribution of discontinuities. The study aims also to evaluate the effect of water flow on the rock slope stability and it is performed at two space scales: laboratory (micro scale and macro scale) and in situ scales. The last one is still under investigation and will be presented

High density coverage investigation of The Austre LovénBreen (Svalbard) using Ground Penetrating Radar

COMInternational audienceA three week field survey over April 2010 allowed for the acquisition of 120 Ground Penetrating Radar (GPR) profiles, adding to a 40 km long walk across an Arctic glacier. The profiles were acquired using a Mal°a equipment with 100 MHz antennas, walking slowly enough to record a 2.224 s trace every 30 cm on the average. Some acquisitions were repeated with 50 MHz or 200 MHz antenna to improve data quality. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing, to pick the reflection arrival time from the ice-bedrock interface. Traveltimes were converted to ice thickness using a velocity of 0.17 m/ns. Dual-frequency GPS mapping and snow coverage thickness were acquired during the same survey. Using interpolation methods, we derived the underlying bedrock topography and evaluated the ice volume

Deriving ice thickness, glacier volume and bedrock morphology of the Austre Lovénbreen (Svalbard) using Ground-penetrating Radar

International audienceThe Austre Lovénbreen is a 4.6 km2 glacier on the Archipelago of Svalbard (79°N) that has been surveyed over the last 47 years in order of monitoring in particular the glacier evolution and associated hydrological phenomena in the context of nowadays global warming. A three-week field survey over April 2010 allowed for the acquisition of a dense mesh of Ground-penetrating Radar (GPR) data with an average of 14683 points per km2 (67542 points total) on the glacier surface. The profiles were acquired using a Mala equipment with 100 MHz antennas, towed slowly enough to record on average every 0.3 m, a trace long enough to sound down to 189 m of ice. One profile was repeated with 50 MHz antenna to improve electromagnetic wave propagation depth in scattering media observed in the cirques closest to the slopes. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing including migration, to pick the reflection arrival time from the ice-bedrock interface. Snow cover was evaluated through 42 snow drilling measurements regularly spaced to cover all the glacier. These data were acquired at the time of the GPR survey and subsequently spatially interpolated using ordinary kriging. Using a snow velocity of 0.22 m/ns, the snow thickness was converted to electromagnetic wave travel-times and subtracted from the picked travel-times to the ice-bedrock interface. The resulting travel-times were converted to ice thickness using a velocity of 0.17 m/ns. The velocity uncertainty is discussed from a common mid-point profile analysis. A total of 67542 georeferenced data points with GPR-derived ice thicknesses, in addition to a glacier boundary line derived from satellite images taken during summer, were interpolated over the entire glacier surface using kriging with a 10 m grid size. Some uncertainty analysis were carried on and we calculated an averaged ice thickness of 76 m and a maximum depth of 164 m with a relative error of 11.9%. The volume of the glacier is derived as 0.3487±0.041 km3. Finally a 10-m grid map of the bedrock topography was derived by subtracting the ice thicknesses from a dual-frequency GPS-derived digital elevation model of the surface. These two datasets are the first step for modelling thermal evolution of the glacier and its bedrock, as well as the main hydrological network