Title: Time-lapse electrical resistivity tomography for the dynamic characterization of hydrogeological systems
Abstract: The saturated and unsaturated zones deeper than a few meters from the surface are difficult to study using traditional hydrogeological techniques alone (conventional soil/water sampling, in-situ test, laboratory measurements): these techniques are invasive, can modify water circulation and may provide too sparse information about subsurface processes, due to the localized and limited nature of their dataset (based on few boreholes).
This is the reason why non-conventional methods are gaining popularity; in particular, a key role is played by geophysical methods: they may provide 2D/3D information (both on the subsurface spatial structures - static information - and on the fluid presence and motion - dynamic information), are non-invasive (or minimally invasive), cost-effective, and faster than traditional hydrological techniques. The most commonly applied techniques are: electrical resistivity tomography (ERT), both from the surface and in boreholes and ground penetrating radar (GPR), again in surface and borehole applications.
A wide range of hydrological and hydrogeological objectives may be achieved by geophysical methodologies. These objectives can be categorized into three important areas: 1) hydrogeological mapping, 2) hydrological parameter estimation, and 3) hydrological process monitoring.
The first area (hydrogeological mapping) aims to define aquifer and aquitard geometry, water table level, fault and fracture zones, and fresh-salt water interfaces. Hydraulic conductivity K (i.e. the most important hydrological parameters) is largely controlled by the heterogeneity of the subsurface; therefore, the delineation of geological structures (paleo-channels, fractures, aquitards and aquicludes), that may constitute or not preferential flow paths becomes highly important in the characterization of the shallow subsurface.
The second area (hydrological parameter estimation) concerns the quantitative approaches used to estimate water content, water quality, volumetric-effective parameters. In order to translate the geophysical data into information of hydrological use, two approaches in particular can be adopted:
a. empirical relationships can be used to link measurable geophysical quantities (e.g. electrical resistivity or permittivity) directly to hydrological parameters (e.g. hydraulic conductivity);
b. geophysical quantities are translated into hydrological quantities (e.g. moisture content or solute concentration) that are used in turn to calibrate hydrological models and determine hydrological parameters.
The first approach is typical of early applications and is often of limited value due to inconsistent assumptions. The second approach (and the most common nowadays, also applied in this study) is conceptually more robust: it recognizes that hydrological parameters are defined by hydrological constitutive models and cannot be measured via techniques that are not based on the same physical–mathematical equations.
The third area (hydrological process monitoring) consists of imaging subsurface property changes, caused by natural or forced processes. Time-lapse measurements (i.e. data re-acquired in the same place at different times) image the dynamic transformations and have therefore the possibility of investigating the flow and transport processes acting in the shallow subsurface.
The data shown here concern the results obtained by this hydrogeophysic approach at three different (in terms of static and dynamic properties) experimental sites: the first two sites are both located in North-Eastern Italy (Veneto Region), in a water works well field near Valdobbiadene (Treviso) and in a natural reserve near Villaverla (Vicenza); the third pilot field is placed in a contaminated area in the northern part of the city of Trento (Trentino – Alto Adige Region), also known as “Trento-Nord” site.
The three case - studies proved to be successful examples of the application of boreholes and surface geophysical techniques to support conventional hydrogeological and environmental methods, in the present case represented by saline tracer tests, in complex environments. The ERT surveys yielded important results in this sense, ERT being very sensitive to conductivity contrasts at depth and thus ranking as a powerful tool in studying subsurface dynamics; however, some difficult aspects related to data collection, inversion and interpretation have to be considered. We performed different pilot-scale tests to gain some general insight into the quality of information that can or cannot be retrieved by such an integrated application.
Publication Year: 2012
Publication Date: 2012-01-30
Language: en
Type: article
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