Monitoring a shallow geothermal experiment in a sandy aquifer using electrical resistivity tomography: a feasibility study

Geothermal resources, especially groundwater resources, are increasingly used around the world. It is therefore necessary to improve the characterisation of thermal parameters of the aquifers. This is done mostly with thermal response tests. In this work, we monitored with electrical resistivity tomography a geothermal test on the campus of Ghent University (Belgium). We injected warm water (45°C) into a sandy aquifer where the groundwater has a temperature of 10°C at a rate of 100 liter/hour during three days. Laboratory measurements indicated that we could expect at most a change of 2%/°C of the water electrical conductivity. The time-lapse series of electrical images show clearly the thermal plume corresponding to the injected water with a maximum change of minus 20% after 72 hours of injection. A comparison with a geothermal model shows that the anomaly is well detected but also distorded due to the inversion regularization (smoothness constraint). ERT enabled to follow the evolution of the injected heated water with more spatial coverage that traditionnal techniques. ERT is thus an interesting tool to monitor the exploitation of geothermal resources. In the future, it will be necessary to improve the inversion process to use quantitavely such results in a calibration process

What does the interface on the fresh-saltwater distribution map of the Belgian coastal plain represent?

Knowing the distribution between fresh and saline groundwater is imperative for a sustainable and integrated management of water resources in coastal areas. The Belgian coastal plain forms no exception. Based on vertical electrical soundings (VES) and available geological data maps were published in 1974 and 1989 that show the depth to the interface between the freshwater and the underlying saltwater. This interface was defined as the 1.5 g/L total dissolved solids surface and interface depth was depicted using classes based on depth intervals. Recent developments in geophysical prospection make it possible to collect data with a high spatial resolution. The comparison with 1974 and 1989 maps is thereby not so straightforward as it seems. Therefore, this paper elucidates the nature of the mapped interface and its relation with salinity. The interface, as determined by VES, is the boundary between a freshwater and saltwater electrical distinct layer. The resistivity of this interface can vary between 1 and 75 Omega.m. However, this translates in a narrow range of salinities for which 1.5 g/L represents a mean. Comparing new data with the old maps thus takes more than simply comparing the depth to a certain resistivity value

The hydrogeology of the military inundation at the 1914-1918 Yser front (Belgium)

Protection against flooding by the sea, drainage of rainwater and integrated management of groundwater and surface-water resources are key issues in low-lying coastal areas. However, under exceptional circumstances, knowledge to keep coastal areas dry and habitable can be used otherwise. Inundation for military purposes is such an example. The hydrogeology of the inundation at the Yser River, Belgium, during the Great War is studied. The inundation started in October 1914 to stop the German advance and lasted until 1918. A water balance and groundwater model are combined to derive the water balance before and during the inundation and to study the impact on the groundwater system. It is concluded that a number of hydrogeological factors contributed to the effectiveness of the inundation. Most importantly, the low-permeability subsoil facilitated loss of inundation water mainly by evaporation. Further, the normal water management strategy of the area (aimed at evacuating excess water towards the sea) was reversed to keep water between the opposing armies. However, the duration of the inundation meant a reorganization of the drainage of areas not inundated; truly an exercise in integrated water management

Parameter estimation based on vertical heat transport in the surficial zone

Measured groundwater temperatures in the surficial zone are dependent on the properties of porous media and vertical flow velocity. Sensitivity analyses, collinear diagnostics and an inverse numerical solution to the one-dimensional heat transport equation are used to determine which parameters can be estimated from temperature measurements in the surficial zone. This is done for heat transport in the saturated zone considering a constant vertical flow velocity. The use of temperature profiles, temperature time-series and temperature envelopes are considered. There is an important difference between a conduction and a convection dominated system. Sensitivity analysis shows that temperature measurements are sensitive to effective thermal conductivity and heat capacity and are insensitive to effective porosity and thermal dispersivity. In a conduction dominated system, temperature is also insensitive for vertical velocity. Collinear diagnostics show that in a conduction dominated system, only the combination of heat capacity and effective thermal conductivity, the thermal diffusivity, can be derived. In a convection dominated system, both the vertical velocity and the effective thermal conductivity can, theoretically, be derived

Recharge assessment by means of vertical temperature profiles: analysis of possible influences

Temperature profiles (temperature as function of depth) can be used to derive vertical flow velocities or recharge rates. In many cases, solutions to the one-dimensional heat transport equation are used, considering steady state boundary conditions. Factors which can influence the derivation of the mean vertical flow velocity are studied here. Therefore, an explicit finite-difference approximation to the 1D heat transport equation coupled with an inverse scheme was used to interpret temperature profiles. Measurement error (larger than 0.05 °C) can result in important deviation of the derived mean flow velocity. Variation of vertical flow velocity as function of time leads to asymmetric temperature envelopes. Yearly variation in vertical flow velocities or temperature variations of the recharge water also results in asymmetric temperature envelopes. Interpretation of these asymmetric envelopes leads to important differences between derived and actual mean vertical flow velocities