Combined inversion of land and marine electrical resistivity tomography for submarine groundwater discharge and saltwater intrusion characterization

Electrical resistivity tomography (ERT) is particularly suited for the characterization of saltwater intrusion and submarine freshwater discharge, given its sensitivity to the electrical conductivity of pore water. A comprehensive survey might combine land ERT collected during low tide and marine ERT acquired with lower sensitivity due to the presence of a seawater layer, most often, inverted separately. In this contribution, we propose a new methodology to jointly invert land and marine ERT acquired along a same profile, bringing them to a common situation at high tide. We first estimate the electrical resistivity distribution under the land profile. Then, we compute a corrected data set for land ERT, simulating a data set collected with the same level of water as the marine ERT. Finally, both land and marine ERT data sets are inverted jointly. We show that the approach allows to better image the freshwater/saltwater distribution across the shoreline

Determining the optimal focusing parameter in sparse promoting inversions of EMI surveys

If the magnetic field caused by a magnetic dipole is measured, the electrical conductivity of the subsurface can be determined by solving the inverse problem. For this problem a form of regularisation is required as the forward model is badly conditioned. Commonly, Tikhonov regularisation is used which adds the $\ell_2$-norm of the model parameters to the objective function. As a result, a smooth conductivity profile is preferred and these types of inversions are very stable. However, it can cause problems when the true profile has discontinuities causing oscillations in the obtained model parameters. To circumvent this problem, $\ell_0$-approximating norms can be used to allow discontinuous model parameters. Two of these norms are considered in this paper, the Minimum Gradient Support and the Cauchy norm. However, both norms contain a parameter which transforms the function from the $\ell_2$- to the $\ell_0$-norm. To find the optimal value of this parameter, a new method is suggested. It is based on the $L$-curve method and finds a good balance between a continuous and discontinuous profile. The method is tested on synthetic data and is able to produce a conductivity profile similar to the true profile. Furthermore, the strategy is applied to newly acquired real-life measurements and the obtained profiles are in agreement with the results of other surveys at the same location. Finally, despite the fact that the Cauchy norm is only occasionally used to the best of our knowledge, we find that it performs at least as good as the Minimum Gradient Support norm

Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone

Submarine groundwater discharge (SGD) is an important gateway for nutrients and pollutants from land to sea. While understanding SGD is crucial for managing nearshore ecosystems and coastal freshwater reserves, studying this discharge is complicated by its occurrence at the limit between land and sea, a dynamic environment. This practical difficulty is exacerbated by the significant spatial and temporal variability. Therefore, to capture the magnitude of SGD, a variety of techniques and measurements, applied over multiple periods, is needed. Here, we combine several geophysical methods to detect zones of fresh submarine groundwater discharge (FSGD) in the intertidal zone, upper beach, dunes, and shallow coastal area. Both terrestrial electrical-resistivity tomography (ERT; roll-along) and marine continuous resistivity profiling (CRP) are used from the shallow continental shelf up to the dunes and combined with frequency domain electromagnetic (FDEM) mapping in the intertidal zone. In particular, we apply an estimation of robust apparent electrical conductivity (rECa) from FDEM data to provide reliable lateral and vertical discrimination of FSGD zones. The study area is a very dynamic environment along the North Sea, characterized by semi-diurnal tides between 3 and 5 m. CRP is usually applied in calmer conditions, but we prove that such surveys are possible and provide additional information to primarily land-bound ERT surveying. The 2D inversion models created from ERT and CRP data clearly indicate the presence of FSGD on the lower beach or below the low-water line. This discharge originates from a potable freshwater lens below the dunes and flows underneath a thick saltwater lens, present from the dunes to the lower sandy beach, which is fully observed with ERT. Freshwater outflow intensity has increased since 1980, due to a decrease of groundwater pumping in the dunes. FDEM mapping at two different times reveals discharge at the same locations, clearly displays the lateral variation of the zone of discharge, and suggests that FSGD is stronger at the end of winter compared to the beginning of autumn. ERT, CRP, and FDEM are complementary tools in the investigation of SGD. They provide a high-resolution 3D image of the saltwater and freshwater distribution in the phreatic coastal aquifer over a relatively large area, both off- and onshore

Geochemical characterization of groundwater and saltwater intrusion processes along the Luy River, Binh Thuan, Vietnam

With an average annual rainfall of 800-1150 mm/year, the Binh Thuan province is one of the driest places in Vietnam. The quantity and quality of groundwater play a significant role in the agriculture, aquaculture development and daily life of the local communities. In 2012, the national centre for water resources (Nawapi, 2012) delineated the seawater intrusion extent in Binh Thuan based on the total dissolved solids (TDS) content of water samples taken from shallow boreholes. The threshold of 3g/L and 1.5g/L were exceeded in the estuaries of the Luy, Long Song and Ca Ty rivers. In recent years, the prolonged droughts combined with the sea level rise and the over-extraction of groundwater during the dry season increased dramatically the seawater intrusion process especially in the estuaries of the province. The geochemistry of groundwater in the Luy River catchment was studied to investigate the contamination of the aquifers and identify the processes taking place. From 1991 to 2015, 98 water samples had been taken from the wells in the area in both dry and rainy seasons. 71% of the water samples were fresh while 21% and 5% were lightly saline and moderately saline respectively. In summer 2020, 110 new water samples from both shallow and deep wells were collected in the Luy river catchment in wells from 3m to 40m. The TDS values are ranging from 105 to 23080 mg/L and can be classified into 4 groups: freshwater (48%), slightly saline (40%), moderately saline (8%) and very saline (4%). The samples show that the seawater intrusion expands not only horizontally at shallow depth along the river but also deeper down the aquifer in most of the study area, what is also confirmed by geophysical data. Freshwater samples were mostly collected at a depth lower than 10m. The chemical composition of water samples was analyzed showing evidence of seawater intrusion, but also the occurrence of freshening processes within the study area. Together with the presence of saltwater at larger depths, this points towards a situation more complex than previously thought. Saltwater intrusions are likely not only related to interaction with the river estuary, but also to the presence of fossil saltwater in the aquifer, and to groundwater pumping and irrigation practices

Combining teaching and research: a BIP on geophysical and archaeological prospection of North Frisian medieval settlement patterns

We performed a research-oriented EU Erasmus+ Blended Intensive Program (BIP) with participants from four countries focused on North Frisian terp settlements from Roman Iron Age and medieval times. We show that the complex terp structure and environment can be efficiently prospected using combined magnetic and EMI mapping, and seismic and geoelectric profiling and drilling. We found evidence of multiple terp phases and a harbor at the Roman Iron Age terp of Tofting. In contrast, the medieval terp of Stolthusen is more simply constructed, probably uni-phase. The BIP proved to be a suitable tool for high-level hands-on education adding value to the research conducted in on-going projects

Electrical resistivity tomography and continuous resistivity profiling in front of Raversijde, Wenduine, Blankenberge, and Zwin

Land ERT (electrical resistivity tomography) and marine CRP (continuous resistivity profiling) data in the areas of Raversijde, Wenduine, Blankenberge, and Zwin.The combination of two geophysical techniques is used to assess the fresh- and salt water distribution in the phreatic aquifer. It enable to locate zone of saltwater intrusion and fresh submarine groundwater discharge. The data was collected in the framework of the PhD research of Marieke Paepen, Laboratory of Applied Geology and Hydrogeology (Ugent). Logistical support was provided by VLIZ for the marine surveys

Land and marine ERT data set collected across the shoreline at the Westhoek nature reserve

Land and marine ERT data set collected respectively in low and high tide conditions. An overlapping exists in the intertidal zone. This data set is used as a demonstration example of the combined inversion of land and marine ERT. The data correspond to land and marine ERT data collected in the Western Belgian coastal plain next to the French border. This site has been largely studied in the past for its characteristic distribution of fresh- and saltwater. The large recharge in the dune area located to the South, combined with the low slope (1.1%) of the beach and the large tidal range (about 5 m) is responsible for the presence of a saltwater lens above fresh water in the intertidal zone. The total profile is about 800 m long, with 102 electrodes spaced every 5 m, for a total of 505 m on land (limit of the low water line). Land data were collected using a multiple-gradient array at low tide (-0.15 m). There is a 200 m overlap with the marine profile, collected at high tide (3.41 mTAW), which continues 300 m further offshore. The marine data were collected through continuous resistivity profiling, using 13-electrode cables with 15 m spacing and a reciprocal Wenner-Schlumberger array. Data were collected on average every 3.2 m, together with the height of the water column (bathymetry)