3D Characterization of a Coastal Freshwater Aquifer in SE Malta (Mediterranean Sea) by Time-Domain Electromagnetics

Abstract:

Electromagnetic (EM) geophysical methods are well equipped to distinguish electrical

resistivity contrasts between freshwater-saturated and seawater-saturated formations. Beneath

the semi-arid, rapidly urbanizing island of Malta, offshore groundwater is an important potential

resource but it is not known whether the regional mean sea-level aquifer (MSLA) extends offshore.

To address this uncertainty, land-based alongshore and across-shore time-domain electromagnetic

(TDEM) responses were acquired with the G-TEM instrument (Geonics Ltd., Mississauga, ON,

Canada) and used to map the onshore structure of the aquifer. 1-D inversion results suggest

that the onshore freshwater aquifer resides at 4–24 m depth, underlain by seawater-saturated

formations. The freshwater aquifer thickens with distance from the coastline. We present 2D and

3D electromagnetic forward modeling based on finite-element (FE) analysis to further constrain

the subsurface geometry of the onshore freshwater body. We interpret the high resistivity zones

that as brackish water-saturated bodies are associated with the mean sea-level aquifer. Generally,

time-domain electromagnetic (TDEM) results provide valuable onshore hydrogeological information,

which can be augmented with marine and coastal transition-zone measurements to assess potential

hydraulic continuity of terrestrial aquifers extending offshore.