Time-Domain Electromagnetic Imaging of Coastal Aquifers at Malta and New Zealand with Implications for Their Onshore-Offshore Connectivity
Abstract
Several studies documented in the geophysics literature have applied the time-domain electromagnetic (TDEM) method for coastal and marine aquifer characterization, and for environmental and hydrological investigations. By imaging the subsurface conductivity (or its reciprocal, resistivity), the electromagnetic (EM) methods provide information about spatial variations in geomaterial properties. The main objective of this dissertation is to enhance the capability of terrestrial TDEM geophysical methods employing a controlled-loop source for onshore-offshore groundwater connectivity investigations. I developed 2D and 3D forward modeling capabilities based on finite-element (FE) analysis to show that TDEM responses can provide constraints on the geometry of the onshore freshwater body and potential flow paths offshore. Two case studies, from Malta and New Zealand respectively, are presented herein. TDEM responses have been acquired with the Geonics G-TEM instrument and used to map the geometry of the mean sea-level aquifer (MSLA) in SE Malta. A similar method is used to interpret data acquired inland at the top of the sea cliffs and along the beach at Canterbury coast, South Island of New Zealand. Forward modeling of the TDEM data based on FE algorithm is utilized to generate geoelectrical models that can be used to better understand the coastal hydrology. The subsurface geometry of the onshore freshwater body within the MSLA along the SE Malta coast is identified from 2D and 3D EM forward modeling results. In a similar manner, the utility of the TDEM geophysical method along with forward modeling demonstrates the connections between groundwater conduits and the coastal gullies in terms of directing freshwater discharge offshore along the mid Canterbury coast. This EM imaging technique has the capability of solving this similar types of problems in the other places to provide valuable information for future water management of the groundwater reserves.
Citation
Pondthai, Potpreecha (2020). Time-Domain Electromagnetic Imaging of Coastal Aquifers at Malta and New Zealand with Implications for Their Onshore-Offshore Connectivity. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192820.