This article addresses the comments of Sakamato et al. on Greenhalgh et al.'s "Modeling and migration of 2-D georadar data: A stationary phase approach". Here, Greenhalgh et al. emphasizes the comprehensiveness of their approach over Sakamato et al.'s SEABED algorithm.
Crosshole resistivity imaging is increasingly used in civil engineering, groundwater and environment investigations: the cost of equipment is low and the field measurements are easy to perform. Such arrays are able to yield valuable information on the variation of resistivity between the boreholes (e.g. Daily and Owen, 1991). Resistivity imaging using buried electrodes permits a greater accuracy and resolution than what can be obtained with surface arrays. The strong imprints of near surface inhomogeneities are reduced whereas the resolution at depth is increased since the sensors are closer to the structures of interest. Nevertheless, crosshole resistivity imaging surveys are frequently prohibited on the working site. There is either one single borehole available or the distance between two boreholes is too large for crosshole resistivity investigations (the distance between the boreholes should be comparable to the length of the boreholes). In this case, surface-to-borehole (or borehole-to-surface) resistivity measurements are performed by placing a current source on the surface (or downhole) and measuring the potential field in a borehole (or on the surface). Geophysicists are probably more familiar with surface arrays since they have been using Wenner, Schlumberger or dipole-dipole configurations for many years. Resistivity surveys with non-conventional arrays (e.g. surface-to-borehole arrays) are more rarely carried out. This prompts us to find a tool for designing such surveys.
Vertical electric soundings, 2D resistivity imaging and several logging measurements were performed at Kappelen test site to identify the various geolelectric facies that allowed determining the tabular and horizontal structure of the aquifer. The surface-based geoelectric methods allowed for a reliable characterization of the overall structure and the geometry of the aquifer, while geophysical logging methods allowed for inferring detailed hydrogeophysical characteristics, such as the electrical resistivity, total porosity, global and matrix density and hydraulic conductivity. The synoptic interpretation and integration of this broad and diverse database allows for constraining the key hydrological characteristics and hence forms the basis for the detailed hydraulic modelling of flow and transport process.
To assess whether features in 2D imaging results are demanded by the data or are artefacts of the<br>inversion process, a special inversion algorithm was applied to process DOI (Depth Of Investigation)<br>index maps. This method carries out two inversions of the same data set using different values of the<br>reference resistivity. The two inversions reproduce the same resistivity values in areas where the data<br>contain information about the resistivity of the subsurface whereas the final result depends on the<br>reference resistivity in areas where the data do not constrain the model. This calculation can be also<br>performed for borehole-to-borehole or borehole-to-surface surveys. In this case, regions of investigation<br>can be outlined. Without DOI maps, interpretation of models can be sometimes difficult, nonrepresentative<br>and dangerous. As can be inferred from field examples, the DOI maps prevent overinterpretation<br>or misinterpretation of inversion results in electrical imaging studies. The DOI map helps<br>explaining the occurrence of erratic and non-geologic structures at depth. It also says how deep we can<br>see into an inverted resistivity profile. In this paper, the implementation of the algorithm is first<br>described and the methodology is then illustrated with 2D surface and borehole electrical resistivity<br>imaging applied to civil engineering and hydrogeological surveys.
Permafrost occurs on the scree slope of Creux du Van even if the mean annual air temperature is +5.5°C. A permanent frozen body is present only in the lower half and sunniest part of the slope. The main process leading to the strong thermal anomaly of the ground occurs during the cold winter periods. At that time, a direct relationship between the air temperature measured in a channel at the bottom of the talus slope and the air temperature outside indicates that air is aspirated into the ground allowing the overcooling of the interior of the scree. The talus slope acts like a chimney where natural advection occurs in winter due to air density contrast between cold and dense outside air and warm and light inside air. In summer, the chimney ef- fect is reverse and the permanent expiration of cold air prevents the vegetation from growing normally.
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