We present a fully developed, fast approximate method for 1D inversion of time-domain electromagnetic data. The method is applied to a helicopterborne transient electromagnetic data set from the Toolibin Lake area of Western Australia using the lateral parameter correlation method to ensure lateral smoothness of the inverted models.The method is based on fast approximate forward computation of transient electromagnetic step responses and their derivatives with respect to the model parameters of a 1D model. The inversion is carried out with multi-layer models in an iterative, constrained least-squares inversion formulation including explicit formulation of the model regularization through a model covariance matrix. The method is 50 times faster than conventional inversion for a layered earth model and produces model sections of concatenated 1D models and contoured maps of mean conductivity in depth intervals almost indistinguishable from those of conventional inversion.To ensure lateral smoothness of the model sections and to avoid spurious artefacts in the mean conductivity maps, the inversion is integrated with the lateral parameter correlation method. In this way, well determined parameters are allowed to influence the more poorly determined parameters in the survey area.Applied to the Toolibin data set, the inversion produces model sections and conductivity maps that reveal the distribution of conductivity in the area and thereby the distribution of salinity. This information is crucial for any remediation effort aimed at alleviating the salinization problems.

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We have investigated potential rockslides in Western Norway using a time- and cost-efficient airborne electromagnetic (AEM) survey approach. The study area comprises phyllite, a low-grade metamorphic rock type that tends to be reworked to clay in disturbed zones. Mapping these electrically conductive clay zones was the aim of the AEM survey. Based on indications that precipitation drives the reported rockslide movements, the local municipality and regional hydroelectricity company are evaluating the option of draining the unstable area to a nearby hydropower reservoir using a drainage tunnel of more than 10 km. We conducted the AEM mapping survey to locate the sliding planes and to investigate the tunnel corridor for areas with potential tunneling hazards. Spatially constrained inversion of the data set (250 km) reveals extended conductive zones interpreted as sliding planes and/or gneiss/phyllite interface. Detailed follow-up of these initial results is planned with targeted percussion drilling and ground resistivity surveys.

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Coastal hydrology is becoming the focus of increasing interest for several reasons. Hydrogeological models need good boundary conditions at the coast line, and with the expected sea level rise due to climate changes, it becomes increasingly important to grasp the dynamics of coastal hydrology in order to predict the consequences of sea level rise for nature and society.We present a helicopterborne transient electromagnetic survey from a region at the North Sea coast in western Jutland, Denmark, carried out with the purpose of mapping the coastal hydrogeology at a seriously polluted site to assist in the assessment of the extent of the pollution and to provide data for remediation activities. Data are subjected to constrained inversion with onedimensional multi-layer (smooth) models. The results are presented as model sections and as maps of mean conductivity in elevation intervals. The extent of the pollution plume estimated from the survey results is mainly in accordance with results from other investigations, but also points to hitherto unknown directions of seepage. The interleaving of fresh water extending under the offshore shallow sea and the salt water infiltrating under the onshore fresh water aquifer can be clearly discerned and reveals preferential flow channels.