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PhD Thesis Defence by Andrea Palacios

16 September 2021 @ 3:00 pm - 5:00 pm

The PhD student Andrea Palacios, from the Groundwater and Hydrogeochemistry research group, will defend her thesis next 16th September at 15:00h. If you want to attend, please click on this link.

Title: Geologically-constrained joint inversion of heads, tracer and ERT data for process visualization.

DirectorsJesús Carrera Ramírez and Juanjo Ledo.

Thesis CommitteeSebastián Olivella, Frédéric Nguyen, Majken Looms, Kamini Singha and Daniel Fernández García

Abstract

Seawater intrusion (SWI) consists in the movement of seawater (SW) into freshwater (FW) aquifers, contaminating drinking water resources. SWI, along with the parallel reduction of Submarine Groundwater Discharge may lead to ecological impacts beyond the reduction of FW resources. Water salinity is the critical physical property to identify SWI. The salinity contrast between FW () and SW (35g/L) is high enough for salinity and, therefore, water electrical conductivity (ECw), to be indirectly measured using geophysical techniques such as geophysical logs (e.g. induction) or electrical and electromagnetic methods (e.g. electrical resistivity tomography, ERT). Although the context of SWI sounds ideal for the use of geophysics, ERT displays poor resolution in depth.
We propose using cross-hole ERT (CHERT) to enhance resolution, placing the electrodes in depth along the boreholes. We tested it for the first time in a SWI context, at the Argentona experimental site, some 40 km NE of Barcelona. Results of the 2-years time-lapse CHERT monitoring showed that the use of CHERT and surface ERT increased the model resolution, and the bulk EC (ECb) values from CHERT were validated with induction logs from the site. We were able to image the seasonal fluctuations of groundwater flux that cause the SW-FW interface to move seawards during periods of high flux or landwards during periods of low flux; as well as the salinization of the aquifer due to an intense drought in the study area during the monitoring period. Two short-term events were also imaged: a decrease in ECb related to a heavy rain event, and an increase in ECb in the beach area related to storm surges.
We built a hydrogeophysical model to characterize the Argentona site using all available data types. The model couples a density-dependent flow and transport simulator with a geoelectrical solver through a petrophysical relation. This model was calibrated by minimizing the misfit between observed and simulated hydraulic heads, salt concentrations and apparent ECb. The calibration was done on four time stages: a pseudo steady-state period, a model warm-up period for the introduction of time-varying boundary conditions, a calibration period covering the first year of the Argentona site monitoring, and a validation period covering the second year. The latter was used to assess the prediction capability of the models. The procedure allowed us to update the original conceptual model and demonstrate the importance of even the finest silt-rich layers. Then, three inverse problems were performed on the updated conceptual model: a) using the traditional point measurements of heads and salinity; b) adding the time variations of heads and the spatial differences of salinity to address common issues of using heads and salinity measurements taken from boreholes in coastal aquifers; and c) adding the apparent ECb from the time-lapse CHERT. We discuss the value of using time variations of heads, instead of only head absolute values; as well as on the use of spatial differences of salt concentrations. The model calibrated using all types of data (heads, salinity and ECb) had the best prediction capability and the model was able to reproduce the main events observed during the two years of monitoring of the Argentona site.
Numerical dispersion prevents the model from simulating FW (0-1 g/L), which affects calibration. To tackle this issue, we propose to use an alternative conversion from water salinity to ECw that corrects for numerical dispersion when computing ECb. The methodology consists in applying and calibrating the error function to reduce salinity in the FW zone, and increase it in the SW zone. The error function calibration can also change the width of the FW-SW interface. This conversion improved the model prediction capability and led to a set of parameters less affected by numerical dispersion (e.g. estimated petrophysical parameters are within the expected range).

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Date:
16 September 2021
Time:
3:00 pm - 5:00 pm
Event Category:
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