DOI: 10.20350/digitalcsic/12500
title: [Dataset] Time-lapse cross-hole electrical resistivity tomography (CHERT) for monitoring seawater intrusion dynamics in a Mediterranean aquifer

description: The corresponding 2.5D electrical forward and inverse problem is solved on an unstructured mesh with tetrahedral elements using BERT (Boundless Electrical Resistivity Tomography) (Rucker et al. 2006, Gunther et al. 2006) and pyGIMLi (Generalized Inversion and Modeling Library) (Rucker et al. 2017).
description: Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but it is strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease in the zone of interest. The objective of this study is to evaluate the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After two years of monitoring, we observe variability of SWI at different time scales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-trivial salinity profiles in open boreholes (step-wise profiles really reflect the presence of fresh water at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics.
description: Name of raw data files: chert_argentona_1.txt chert_argentona_2.txt chert_argentona_3.txt chert_argentona_4.txt chert_argentona_5.txt chert_argentona_6.txt chert_argentona_7.txt chert_argentona_8.txt chert_argentona_9.txt chert_argentona_10.txt chert_argentona_11.txt chert_argentona_12.txt chert_argentona_13.txt chert_argentona_14.txt chert_argentona_15.txt chert_argentona_16.txt chert_plus_surfaceert_argentona.txt Content of raw data files: List of electrodes X and Z positions (with topography), and list of quadripoles (a, b, m, n) with measured resistance (r), error (err), geometric factor (k), and apparent resistivity (rhoa). This files are formatted to go through the BERT program for 2.5D ERT modeling and inversion. The “chert_plus_surfaceert_argentona.txt” raw file combines cross-hole and surface ERT data from Sep 8th, 2015. Name of inverse data files: chert_argentona_sigma_model_1.vector chert_argentona_ sigma_model_2. vector chert_argentona_ sigma_model_3. vector chert_argentona_ sigma_model_4. vector chert_argentona_ sigma_model_5. vector chert_argentona_ sigma_model_6. vector chert_argentona_ sigma_model_7. vector chert_argentona_ sigma_model_8. vector chert_argentona_ sigma_model_9. vector chert_argentona_ sigma_model_10. vector chert_argentona_ sigma_model_11. vector chert_argentona_ sigma_model_12. vector chert_argentona_ sigma_model_13. vector chert_argentona_ sigma_model_14. vector chert_argentona_ sigma_model_15. vector chert_argentona_ sigma_model_16. Vector chert_plus_surfaceert_argentona.vector (Reference Model for Time-lapse Inversion)