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Accuracy and stability of data from airborne electromagnetic systems is an issue addressed in numerous publications and conference presentations. We have recently done a comprehensive and well documented comparison of data from ground based measurements and repeated SkyTEM lines from different altitudes. the ground based test line was made on the National Danish Test Site and consists in two perpendicular lines each of a length of 1.0 km. On each of these lines 25 soundings with a 40x40 m2 TEM system was made. the TEM system measures the first time gate in 8 micro sec from begin of ramp and was prior to the measurements calibrated at the test site. Comparing Inversion results form the SkyTEM and the groundbased data shows that the model repeatability of the SkyTEM system is excellent in both the same altitude and in different altitudes. Also, there are no directional problems, i.e. it does not matter in which direction the lines are flown. the agreement to the groundbased reference sections is also very good, showing that the SkyTEM-system yields data of same high quality as the groundbased system. A validation of the SkyTEM system at the data level was set up to examine the responses gate by gate, which, among other things, rules out the equivalent model issue. the different comparisons in the data space show that the SkyTEM system repeats the test lines equally good in different altitudes, and the agreement to the ground based responses is very good and in general within the data standard deviation of 5%. the validation serves as a quality assurance of the high-precision modeling of the SkyTEM system, together with the robust processing and Inversion scheme developed for the data. the validation of the SkyTEM system now set the standard for the data quality expected for any airborne system to qualify for the Danish national groundwater mapping effort.

During the last decades the scope of Time Domain Induced Polarization (TDIP) has considerably broadened from mineral exploration to environmental geophysics, mainly to clay identification and landfill characterization. Furthermore modern instruments allow multi-channel acquisition of IP data with multi-core cables and steel electrodes. for TDIP surveys the acquisition time is shorter and the logistic simpler in comparison with Spectral Induced polarization (SIP) surveys. Despite this the Inversion techniques have not undertaken consequential enhancements: TDIP data are usually inverted using only integral chargeability, without considering the effective shape of the transmitter pulse or the system transfer function of the receiver. for these reasons a new Inversion algorithm has been developed using the full time decay of the IP response to reconstruct the distribution of the Cole-Cole parameters of the Earth. the forward response is computed modeling the full current waveform (in terms of sequence of positive and negative pulses) and instrumental low-pass filters. Even when stacking, needed for noise reduction and self potential removal, the signal changes significantly during the process and the finite numbers of pulses have to be modeled to obtain accurate time decays. the new approach allows moving from a qualitative Interpretation of TDIP data for recognition of anomaly patterns to a quantitative analysis, able to discriminate soil lithotypes and, if present, the type/grade of contamination: a case history from a landfill-stream interaction supports this conclusion, correlating pollution and Inversion parameters. the forward response has been implemented in the 1D Laterally Constrained Inversion scheme (1D-LCI), producing layered sections of the subsurface with lateral smooth variations. Currently a 2D and 3D implementation are being developed.

The spatially constrained inversion (SCI) is a robust methodology for quasi-3D modeling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. It can be implemented with airborne or ground-based data, both in frequency and time domain. The airborne EM data here presented show how the SCI produces laterally smooth results with sharp layer boundaries that respect the 3D geological variations of layered settings. Information migrate horizontally through spatial constraints applied between nearest neighboring soundings, and allow to resolve layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. Data sets, models and spatial constraints are inverted as one system, producing layered sections with smooth horizontal variations. The SCI suppresses the elongated artifact commonly seen in horizontal maps (i.e., average resistivity, or saltwater boundary elevation maps) resulting from profile oriented data sets. Being an over-determined parameterized inversion problem, it produces a full sensitivity analysis of the output models, an essential tool for the evaluation of the results.

In this presentation, we discuss geophysical methods for general vulnerability mapping of aquifers. The vulnerability of aquifers is to a large degree determined by the content of clay minerals in the near surface geology. Clay minerals have a large cation exchange capacity (CEC) and clayey sediments reduce the infiltration speed thus reducing the aquifer vulnerability. In sedimentary areas, the near surface geology often shows large variations, and therefore the requirements for dense detailed geophysical data are required in order to achieve a satisfying geological resolution. The DC resistivity method is a good choice as it has a high near surface resolution. Alternatively, very early transient electromagnetic measurements can be used. The latter are cost effective and can often be made from the air. We discuss advantages and drawbacks of the two methods and show an example from a 100 km survey where we have used a newly developed variation of the helicopter-borne SkyTEM system for vulnerability mapping. Both the Zand the X-component of the decaying field were measured and the data were inverted using a laterally constrained inversion method.

Since 1995, we have collected electromagnetic (EM) field data using a rotating azimuthal geometry analogous to that described for resistivity surveys by other workers. These EM azimuthal resistivity data are relatively easy to acquire and can be collected in areas with high electrode contact resistance. As an aid to interpreting field results, including apparent paradox of anisotropy conditions, we have used physical-scale modeling, and have run a suite of numerical models. Numerical and physical-scale modeling results indicate that the primary factor affecting the orientation of the apparent resistivity ellipse relative to a macroanisotropic linear conductor is the depth to the target (relative to the transmitter-receiver coil separation). At target depths less than about 10% of the coil separation, an electromagnetic paradox of anisotropy is observed in the modeling tank, in the numerical model, and in our field data (i.e., high values of apparent resistivity along the strike of the target). While physical-scale modeling is limited by the availability of materials of appropriate conductivity, numerical modeling provides a sufficiently extended parameter space for these analyses. The numerical model also indicates that the conductance of the target does not effect the orientation of the apparent resistivity ellipse.

Time domain electromagnetic soundings have been calculated over 2D / 3D structures in a layered halfspace and hereafter inverted using a 1D least squares time domain code. The forward calculations are done in the frequency domain and the subsequent transform to time domain is performed using fast digital filters. The study focuses on typical hydrogeological models. The investigated models include a resistive dyke, a resistive beam and resistive block structure with edges with varying slopes in a conductive layered halfspace with an overburden. The 1D interpretations of the 2D / 3D responses show that the vertical resolution is reduced and the resistivities are smeared out. The 2D / 3D responses over models with a resistive body in a conductive host is easily fitted with the 1D models whereas it is not possible to fit the responses from good conductors in a resistive host very well.

By the integrated use of geophysical methods a ltydrogeological investigation with a very high r~~solution of the geology has heen carried out in Denmark. llsing newly developed methods such as auger Ellog drilling with levrl specific Water sampling and pulled array contitttlous electric profiling together along with more Ouniliar techniques such as transient electromagnetic sounding we have perfortned a general exantinatiott of the vulnerability, the volumetric extent and the chemical properties of the aquifer itt the area. The protection of the aquifer was less than expected. There was a rorrelation between the water quality and the lithology of the aquifer.