For about three decades, airborne electromagnetic (AEM) systems have been used for groundwater exploration purposes. Airborne systems are appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralization of the water, electromagnetic systems are suitable for providing information about the aquifer structures and water quality, respectively.More helicopter than fixed-wing systems are used in airborne groundwater surveys. Helicopterborne frequency-domain electromagnetic (HEM) systems use a towed rigid-boom. Helicopterborne time-domain (HTEM) systems, which use a large transmitter loop and a small receiver within or above the transmitter, are generally designed for mineral exploration purposes but recent developments have made some of these systems usable for groundwater purposes as well.The quantity measured, the secondary magnetic field, depends on the subsurface conductivity distribution. Due to the skin-effect, the penetration depths of the AEM fields depend on the system characteristics used: high-frequency data/early-time channels describe the shallower parts of the conducting subsurface and the low-frequency data/late-time channels the deeper parts. Typical investigation depths range from some ten metres (conductive grounds) to several hundred metres (resistive grounds), where the HEM systems are appropriate for shallow to medium deep (about 1-100 m) and the HTEM systems for medium deep to deep (about 10-400 m) investigations.Generally, the secondary field values are inverted into resistivities and depths using homogeneous or layered half-space models. As the footprint of AEM systems is rather small, one-dimensional interpretation of AEM data is sufficient in most cases and single-site inversion procedures are widely used. Laterally constrained inversion of AEM data often improves the stability of the inversion models, particularly for noisy data. Higher dimensional inversion is still not possible for standard-size surveys.Based on typical field examples the advantages as well as the limitations of AEM surveys compared to long-established ground-based geophysical methods used in groundwater surveys are discussed. In a case history from a German island an airborne frequency-domain system is used to successfully locate freshwater lenses on top of saltwater. An example from Denmark shows how a timedomain system is used to locate large-scale buried structures forming ideal groundwater aquifers. ity on a) the salinity of the groundwater, i.e., the groundwater quality, and b) the clay content of the subsurface, i.e., the aquifer conditions and protection level (e.g., Kirsch 2006).The application of geoelectrical and electromagnetic methods on ground (e.g., McNeill 1990;Binley and Kemna 2005;Everett and Meju 2005;Ernstson and Kirsch 2006) has a long tradition in groundwater exploration. AEM, however, was introduced for mineral exploration and -compared to that -airborne groundwater exploration is ...

Abstract. In deltaic areas with saline seepage, freshwater availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with saline seepage and the mechanisms that control their occurrence and size. Our findings are based on different types of field measurements and detailed numerical groundwater models applied in the south-western delta of the Netherlands. By combining the applied techniques we could extrapolate measurements at point scale (groundwater sampling, temperature and electrical soil conductivity (TEC)-probe measurements, electrical cone penetration tests (ECPT)) to field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual mixing zone between infiltrating fresh rainwater and upward flowing saline groundwater. The mixing zone is best characterized by the depth of the centre of the mixing zone D mix , where the salinity is half that of seepage water, and the bottom of the mixing zone B mix , with a salinity equal to that of the seepage water (Cl-conc. 10 to 16 g l −1 ). D mix is found at very shallow depth in the confining top layer, on average at 1.7 m below ground level (b.g.l.), while B mix lies about 2.5 m b.g.l. The model results show that the constantly alternating upward and downward flow at low velocities in the confining layer is the main mechanism of mixing between rainwater and saline seepage and determines the position and extent of the mixing zone (D mix and B mix ). Recharge, seepage flux, and drainage depth are the controlling factors.

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Seawater intrusion has often resulted in scarce fresh groundwater resources in coastal lowlands. Careful management is essential to avoid the overexploitation of these vulnerable fresh groundwater resources, requiring detailed information on their spatial occurrence. Airborne electromagnetics (EM) has proved a valuable tool for efficient mapping of ground conductivity, as a proxy for fresh groundwater resources. Stakeholders are, however, interested in groundwater salinity, necessitating a translation of ground conductivity to groundwater salinity. This paper presents a methodology to construct a high-resolution (50 × 50 × 0.5 m 3 ) 3D voxel model of groundwater chloride concentration probability, based on a large-scale (1800 km 2 , 9640 flight line kilometres) airborne EM survey in the province of Zeeland, the Netherlands. Groundwater chloride concentration was obtained by combining pedotransfer functions with detailed lithological information. The methodology includes a Monte Carlo based forward uncertainty propagation approach to quantify the inherent uncertainty in the different steps. Validation showed good correspondence both with available groundwater chloride analyses, and with ground-based hydrogeophysical measurements. Our results show the limited occurrence of fresh groundwater in Zeeland, as 75% of the area lacks fresh groundwater within 15 m below ground surface. Fresh groundwater is mainly limited to the dune area and sandy creek ridges. In addition, significant fresh groundwater resources were shown to exist below saline groundwater, where infiltration of seawater during marine transgressions was hindered by the presence of clayey aquitards. The considerable uncertainty in our results highlights the importance of applying uncertainty analysis in airborne EM surveys. Uncertainty in our results mainly originated from the inversion and the 3D interpolation, and was largest at transition zones between fresh and saline groundwater. Reporting groundwater salinity instead of ground conductivity facilitated the rapid uptake of our results by relevant stakeholders, thereby supporting the necessary management of fresh groundwater resources in the region.

Airborne electromagnetic (AEM) surveys can contribute substantially to geologic mapping and target identification if good-quality multifrequency data are produced, properly evaluated, and displayed. A set of multifrequency EM data is transformed into a set of apparent resistivity (ρ a ) and centroid depth (z * p ) values, which then are plotted as a sounding curve. These ρ a (z * p ) curves commonly provide a smoothed picture of the vertical resistivity distribution at the sounding site. We have developed and checked methods to enhance the sensitivity of sounding curves to vertical resistivity changes by using new definitions for apparent resistivity and centroid depth. One of these new sounding curves with enhanced sensitivity to vertical resistivity contrasts is plotted from ρ NB , z * s values derived from differentiation of the ρ a ( f ) curve with respect to the frequency f . This approach is similar to the Niblett-Bostick transform used in magnetotellurics. It not only enhances vertical changes in resistivity but also increases the depth of investigation.Sounding curves can be calculated directly from EM survey data and can be used to generate a resistivitydepth parasection. Based on such a section, it can be decided whether a Marquardt-type inversion of the AEM data into a 1-D layered half-space model is adequate. Each sounding curve can be transformed into an initial step model of resistivity as required for the Marquardt inversion. We have inverted data from sedimentary sequences with good results. For data from a dipping conducting layer and a dipping plate, we have found that the results depend on the right choice of the starting model, in which the number of layers should be large rather than too small. Complex resistivity structures, however, often are represented better by using the sounding-curve results than with the parameters of a layered half-space.