This paper aims to test, in a quantitative way, the different approaches that can be applied to improve the contact resistance problem in a debris environment for the acquisition of electrical resistivity tomography. We collected various datasets on the same investigation line in a blocky ground surface of a landslide deposit, using different coupling systems: single electrodes placed between the boulders, adding extra electrodes in parallel and drilled single electrodes inside the blocks. We performed the measurements in natural dry conditions, then we added salt water nearby the electrodes hammered among the boulders and we filled the drilled holes with a conductive carbomer‐based gel. The results clearly demonstrate that using salt water significantly reduces the contact resistances, but also that, if salt water is not available, we can collect a good quality dataset in dry conditions by connecting more electrodes in parallel. Drilling the electrodes directly inside the boulders decreases the data quality but, if necessary, we demonstrate that the use of a commercial carbon polymer gel can provide a marked improvement in contact resistances.
Abstract. Frozen layers within the subsurface of rock glaciers are generally assumed to act as aquicludes or aquitards. So far, this behavior has been mainly defined by analyzing the geochemical characteristics of spring waters. In this work, for the first time, we experimentally confirmed this assumption by executing an infiltration test in a rock glacier of the Southern Alps, Italy. Time-lapse electrical resistivity tomography (ERT) technique monitored the infiltration of 800 L of saltwater released on the surface of the rock glacier; 24 h ERT monitoring highlighted that the injected water was not able to infiltrate into the underlying frozen layer.
Lateral velocity variations in the near-surface reflect the presence of buried geological or anthropic structures, and their identification is of interest for many fields of application. Surface wave tomography (SWT) is a powerful technique for detecting both smooth and sharp lateral velocity variations at very different scales. A surface-wave inversion scheme derived from SWT is here applied to a 2-D active seismic dataset to characterize the shape of an urban waste deposit in an old landfill, located 15 km South of Vienna (Austria). First, the tomography-derived inverse problem for the 2-D case is defined: under the assumption of straight rays at the surface connecting sources and receivers, the forward problem for one frequency reduces to a linear relationship between observed phase differences at adjacent receivers and wavenumbers (from which phase velocities are straightforwardly derived). A norm damping regularization constraint is applied to ensure a smooth solution in space: the choice of the damping parameter is made through a minimization process, by which only phase variations of the order of the average wavelength are modelled. The inverse problem is solved for each frequency with a weighted least-squares approach, to take into account the data error variances. An independent multi-offset phase analysis (MOPA) is performed using the same dataset, for comparison: pseudo-sections from the tomography-derived linear inversion and MOPA are very consistent, with the former giving a more continuous result both in space and frequency and less artefacts. Local dispersion curves are finally depth inverted and a quasi-2-D shear wave velocity section is retrieved: we identify a well-defined low velocity zone and interpret it as the urban waste deposit body. Results are consistent with both electrical and electromagnetic measurements acquired on the same line.
Abstract. Continuous frozen layers within the subsoil are generally assumed to act as aquicludes or aquitards. So far, this behavior has been mainly defined analyzing the geochemical characteristics of spring waters. In this work, for the first time, we experimentally confirmed this assumption by executing an infiltration test in a rock glacier of the Southern Alps, Italy. Time-lapse electrical tomography (ERT) technique was adopted to monitor the infiltration of a huge amount of water spilled on the surface of the rock glacier. 24 hours ERT monitoring highlighted that the injected water was not able to infiltrate into the underlying frozen layer.
<p>The growing demand for renewable energy leads to an increase in the development of alternative energy applications. In this way, shallow geothermics assumes an important role in the global energy transition of building air conditioning. The design of Ground Source Heat Pumps (GSHP) requires a multidisciplinary approach including a good understanding of the underground geological setting, such as hydrogeological aspects and heat flow conditions. Classic monitoring strategies often rely on local and point-based measurements to monitor changes of underground temperature in time, with the limit of not succeeding in a whole delimitation of the Thermal Active Zone (TAZ). In this context, Electrical Resistivity Tomography (ERT) can bring relevant information on the temperature distribution for monitoring the induced thermal plume within BHEs (Borehole Heat Exchangers) systems. Geophysics helps the understanding of the thermal processes, in order to front the difficulties arising from Ground Source Heat Pumps (GSHP) implementation. Thermal conductivity and electrical resistivity depend equally in a complex way on different common subsurface and environmental attributes such as, among the main, mineralogical composition, grain size, density, porosity and saturation. Besides, thermal conductivity increases significantly with temperature in wet ground, by making it clear a relationship between both parameters.</p><p>ERT is particularly sensitive to the porous medium temperature and, when applied in time-lapse (TL), could provide spatially distributed information on the changes over time of water content, salinity or temperature. For this reason, in this work we monitored the complex TAZ temporal evolution during a heat injection experiment using a 3D time-lapse ERT survey, arranged in a reduced scale physical model. For a better understanding of measured electrical resistivity values, focused on mapping the extent of a geothermal plume around a borehole, a specific laboratory device was utilized. Grain size distribution, bulk density and saturation of the porous medium are known and established, as well as reliable temperature values acquired through sensors with which calibrate the ERT results. Thus, changes in resistivity can be interpreted to track the evolution of the plume of heated water and used to estimate the temperature change. The propagation of the heat plumes into the ground is also highly sensitive to interstitial water flow rate, thus also this condition was recreated and monitored varying the hydraulic gradient in the experimental device.</p><p>The present work aims to demonstrate the ability of ERT to provide complementary insights about the sub-surface spatio-temporal dynamic for monitoring the extension of TAZ caused by BHEs probes. In addition, the detailed scale adopted and the variable control within a laboratory setup ease the study of the interaction between thermal and electrical properties.</p>
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