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Hydrogeophysical Monitoring of Groundwater Level Changes Induced by Tides in a Shallow Beach Aquifer
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, Near Surface 2011 - 17th EAGE European Meeting of Environmental and Engineering Geophysics, Sep 2011, cp-253-00043
- ISBN: 978-90-73834-15-6
Abstract
A joint geophysical-hydrogeological experiment was performed to monitor groundwater level changes in a shallow aquifer caused by tidal loading. Electrical resistance tomographies (ERT) were recorded every 45 minutes for two days and consisted of a profile of 41 electrodes, using a 1-m spacing dipole-dipole array protocol. Eleven piezometers were installed with a separation of 10m; pressure data was recorded at 2 min interval. Other hydrogeological data included tide levels, temperature, water electrical conductivity, and porosity estimations. Time-lapse resistivity imaging shows subtle changes in resistivity close to the sea water front and in a region of large lateral gradient of resistivity thought to be the limit between waters of contrasting salinity. Reasons for the small water-induced vertical resistivity variations were investigated using numerical modelling. It is shown that ERT may not resolve changes smaller than 25 cm over a depth of 3m using the survey acquisition parameters. Coupling hydrogeological data with groundwater ERT allowed displaying a likely contact between saline and fresh groundwater. Measured hydraulic conductivities were used to model the effect of tidal loading on the piezometric level and the computed damping and time-lag of the tide induced effects with distance inland fit well with the observed data.