# Integrated Geophysical Methods for Shallow Aquifers Characterization and Modelling
## 1 Introduction  
Aquifers stock about 31.4% of the freshwater on the Earth, provide about 50% of current potable water supply, constitute the sole water source in many areas, support groundwater-dependent ecosystems, and present more resilience than surface watercourses to the negative effects of climate change and anthropogenic activities. However, groundwater use is growing, and signs of degradation are increasingly reported.  
Groundwater research is increasingly using geophysical prospecting surveys for aquifer characterization in different hydrogeological environments. They have become a part of the holistic strategy to define aquifer geometry and some transient groundwater features, which are necessary data to implement numerical tools for the modelling of groundwater quantity (flow) and quality (salinity, pollution) dynamics. Geophysical prospecting techniques are non-invasive, usually inexpensive to apply, and useful for providing the accurate and fast subsurface information required for detailed groundwater research over multiple observation scales.  
Electrical, electromagnetic, and seismic geophysical techniques are widely used for aquifer characterization. The first two are typically used to deduce aquifer geometry and certain transient groundwater features such as piezometric level, freshwater-saltwater interface, and pore water conductivity, whereas the latter are mostly used to deduce aquifer geometry and certain steady aquifer hydraulic parameters. The integrated use of different techniques reduces the ambiguity of interpretations, especially when the conductive structures and pore-filling fluids (natural and human-induced) are subjected to the temporal dynamics of water content and dissolved ions. Integration can also be referred to using external data (e.g., geotechnical soundings logs, geochemical tracers, physical parameters) to improve and/or validate the geophysical models. Different scientific software platforms with friendly interfaces, robust algorithms for data inversion, and tools for uncertainty analysis are available.  
In this broad hydro-geophysical framework, this Special Issue aimed to attract specialized researchers in applied geophysical prospecting techniques for groundwater research, with a special focus on near-surface geophysical prospecting applications for shallow groundwater research. The accepted papers included (i) geophysical prospecting surveys as a part of the holistic strategy for aquifer conceptualization and modelling, (ii) integrated near-surface geophysical prospecting techniques and time-lapse approaches to reduce the ambiguity of hydrogeological interpretations, (iii) experimental field operational designs, and (iv) case studies surveying saturated and unsaturated media for methodological and conceptual purposes. Other papers contributed to the state of the art of the geophysicaltechniques through specific study cases covering (i) hydrogeological environments such as groundwater-dependent ecosystems (GWEs) and urbanized areas in different countries; (ii) aquifer typologies in coastal and inland areas such as weathered Paleozoic crystalline bedrocks, Mesozoic to Neogene carbonated terrains, Neogene and Quaternary volcanic formations, and Neogene and Quaternary detrital sediments; and (iii) climates including humid, sub-humid and semiarid. The used techniques were (i) electrical, such as electrical resistivity tomography (ERT), floating electrical resistivity (FER), and vertical electrical sounding (VES); (ii) electromagnetic such as ground penetrating radar (GPR), time-domain electromagnetic (TDEM), and frequency-domain electromagnetic (FDEM), including low-frequency magnetotelluric sounding (L-MTS); and (iii) seismic, such as multichannel analysis of surface waves (MASW), refraction microtremor (REMI), and vertical seismic refraction (VSR).