Steadily growing abstraction of ground water together with the impact of pollution<br>from surface infiltration have increased the need for hydrogeological investigations of<br>aquifers. The mapping of subsurface resistivity structures using electrical and<br>electromagnetic methods has gained a central role in the set-up of hydrogeological<br>models of catchments in Denmark.<br>In this context the transient electromagnetic method (TEM) has proven to be a<br>powerful tool. Regional investigations with TEM have produced results, which leave no<br>doubt as to the ability of the method to delineate fresh-water sandy aquifers in more well<br>conducting surroundings of clay or salt water. These results have been obtained using a<br>dense grid of measurements and demonstrate the necessity to estimate the data quality by<br>correlating nearby data sets. The fact that these investigations are often carried out in<br>densely populated areas, where the cultural noise level is high, accentuates this even more.<br>Investigations meeting these demands require much field work thus increasing the cost of<br>the survey.<br>A new technique, Pulled Array Transient Electromagnetic Method (PA-TEM), is<br>under development. With this method a transmitter coil and a receiver coil are towed<br>across the surface along profile lines while measuring. With this technique the cost of the<br>field work is drastically reduced. Furthermore, as the measurements are recorded very<br>densely, nearby data sets can be correlated to ensure good data quality, and distortion<br>from cultural effect from hidden powerlines, fences, etc. in the data sets is more easily<br>discovered and reduced.


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