Approximately 40% of the world's population currently lives in a coastal zone, meaning within 100 km of a coast. An increasing trend in population and economic activity is causing the pressure on coastal ecosystems to grow, for example due to land cover change, increasing pollutant loads, and the introduction of invasive species (United Nations, 2007).

Furthermore, changes in climate can affect coastal zones, for example through changes in sea level. Sea-level rise can form a hazard for (dense) population centers near the coast: “a high population concentration in the lower elevation coastal zone (defined as less than 10 m elevation) increases a country's vulnerability to sea-level rise and other coastal hazards such as storm surges” (United Nations, 2007). In combination with rapid population growth, it can also put increasing pressure on freshwater resources and groundwater systems.

Coastal zones are also of interest from an energy perspective. Currently, a major shift can be observed to sources and storage of renewable energy. At the same time, hydrocarbons are still an important ingredient of the energy mix, thereby playing a role in the success of the energy transition. It is expected that “fossil fuels will continue to make up a significant share of the energy mix by 2050, partly because of how they combine affordability and security of supply” (Kienzler et al., 2023). Coastal areas such as deltaic systems often contain major oil and gas reservoirs. Many variables related to a delta's mode of formation determine the geometry, size, and internal architecture of deltas. For optimal hydrocarbon production, a complete understanding of the characteristics and variations of an individual delta's reservoir is required, for example for proper reservoir management and well design (Slatt, 2006).

The preceding serves to provide a brief context to highlight and emphasize the importance of studying coastal zones. Coastal areas are often characterized by dynamic space- and time-dependent conditions. As such, they are complex systems to study and understand. By obtaining an increased understanding of the key elements and dynamics of these complex coastal systems, one can identify changes to these systems and assess their potential consequences for humans and the environment. The field of geophysics provides important tools to strengthen our knowledge of coastal systems, for example through enhanced geophysical imaging and monitoring technologies.

This special section presents a selection of studies, giving an idea of the breadth of coastal geophysics.

To open the special section, Pastoressa et al. introduce SWAN — a surface-towed modular controlled-source electromagnetic (CSEM) system for mapping submarine groundwater discharge and offshore groundwater resources. It is a low-cost, modular, surface-towed hybrid time-frequency domain CSEM system capable of detecting offshore freshened groundwater and submarine groundwater discharge (SGD) down to water depths of 100 m, which can help to improve the understanding of offshore groundwater systems and their interactions with onshore systems along global coastlines.

Paepen et al. focus on fresh SGD at the land-marine interface by combining on- and offshore techniques. They assess the variability in the fresh SGD footprint near the Belgian coastline by means of electrical resistivity tomography (ERT) and continuous resistivity profiling. The difficult working conditions of the highly dynamic North Sea make the survey one of the first of its kind. The presence and size of dune formations that constitute freshwater resources along the shore seem to control the occurrence, footprint, and quantity of SGD.

Abdelrehim et al. investigate an important relationship between the subsurface hydrogeological conditions and the geomorphology of Padre Island on the Texas Gulf Coast, with a focus on the influence of human development. It is important to better understand these systems because barrier islands provide a first line of defense for coastal communities against storms, hurricanes, and sea-level rise. The authors measured apparent electrical conductivities using frequency-domain electromagnetic (FDEM) surveys and spatially correlated them with the island's morphology, obtained from a 1 m resolution digital elevation model. The authors show that anthropogenic activities have modified hydrologic patterns, introduced conductive materials, and altered vegetation cover and soil composition in various zones of the island.

Araujo et al. focus on an FDEM calibration for improved detection of sand intrusions in river embankments. Obtaining a better understanding of sand intrusions is important because these intrusions represent severe threats for river embankments due to possible flow paths during floods causing instability. The authors introduce an approach of multiarray electromagnetic induction calibrated by ERT and test it at a study site near Venice and Padua, Italy.

Finally, Parra shifts the attention to a much smaller scale, focusing on challenges related to the complex pore structure of carbonate aquifers and the interpretation of geophysical data. The author aims to identify and characterize the key flow zones of the aquifer at the Port Mayaca site, which is part of the Floridan aquifer system, a large source of ground water in South Florida. The author introduces an algorithm based on the work of Berryman (1980, 1995) to invert P-wave and S-wave velocity logs to determine secondary porosities and aspect ratios of vugs and cracks. The secondary porosity/pore aspect ratio is integrated with permeability and microresistivity logs. Formation microimager logs are used to identify vugs and fractures and with Stoneley-wave permeability to delineate water production zones. The final interpretation is validated with flow-meter data.