SSSAJ: Volume 75: Number 6 • November–December 2011 1 Soil Sci. Soc. Am. J. 75:2011 Posted online 8 Sept. 2011 doi:10.2136/sssaj2011.0063 Received 17 Feb. 2011. *Corresponding author (eef.meerschman@ugent.be). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Imaging a Polygonal Network of Ice-Wedge Casts with an Electromagnetic Induction Sensor Soil Physics I n many parts of the mid-latitudes of the northern hemisphere, the past existence of peri-glacial conditions is evidenced by the presence of ice-wedge casts and relic sand wedges (French, 2007). Tese cryogenic structures are the remnants of thermal contraction cracks formed in permafrost-afected soils (Kolstrup, 1986). Progressive inflling of these cracks with ice, sand or both, resulted in wedge-shaped bodies of ice, sand or sand-ice (French, 2007; Ghysels and Heyse, 2006; Murton and French, 1993; Vandenberghe and Pissart, 1993). When changing climatic con- ditions caused the permafrost to thaw, the wedge-shaped cavities were flled with wind- and water-transported sediments resulting in their preservation as ice-wedge casts or ice-wedge pseudomorphs (Harry and Gozdzik, 1988). Consequently, the wedge flling has a diferent composition than the host material. Te surface expression of thermal contraction cracks is generally a network of polygons, still observable in modern periglacial environments at high latitudes (French, 2007). In central Europe and North America, polygonal networks of ice-wedge casts were ofen covered by eolian or fuviatile loess or sand, so their pattern is rarely directly observable. However, the morphology of these polygo- nal networks provides valuable information about past environmental and cli- matic conditions, since their formation depend on many factors such as soil tem- perature gradients, mineral composition of the soil, moisture content and varia- tions in air temperature (Dutilleul et al., 2009; Mackay and Burn, 2002; Plug and Werner, 2002, 2008; Romanovskij, 1973). Apart from imaging the ice-wedge casts for paleoclimatological reconstructions, characterizing their abrupt chang- Eef Meerschman* Marc Van Meirvenne Philippe De Smedt Timothy Saey Mohammad Monirul Islam Fun Meeuws Ellen Van De Vijver Gunther Ghysels Res. Group Soil Spatial Inventory Techniques Dep. of Soil Management Faculty of Bioscience Engineering Ghent Univ. Coupure 653 9000 Gent, Belgium Images of the morphology of a polygonal network of ice-wedge casts are a valuable aid to paleoclimatological reconstructions. Usually such images are obtained by aerial photography showing polygonal crop marks refecting textural diferences between wedge flling and host material. Our objective was to investigate an alternative method by measuring the soil apparent electrical conductivity (EC a ) with an electromagnetic induction (EMI) sensor. Based on an aerial photograph showing polygonal crop marks in an agricultural feld in Belgium, a test area of 0.63 ha was selected. A small part of the test area (6 by 6 m) was excavated revealing a clear pattern of ice-wedge casts. Te wedges penetrated clay-rich Tertiary marine sediments, covered by a 0.6-m layer of eolian sandy sediments, and were associated with the permafrost during the last glacial period. We took 94 subsoil (0.6–0.8 m) samples distrib- uted over the test area and analyzed their texture. Te results showed a clear diference between the Eocene host material (on average 21% clay) and the Quaternary wedge flling (on average 6% clay). Te test area was surveyed with an EMI sensor (we used an EM38DD) which resulted in an accurate image of the polygonal network. We concluded that an EMI survey is an appropriate technique to image the morphology of a polygonal network of subsoil ice-wedge casts. A fnal perspective comprises the strong heterogeneity of the subsoil, since nearly half of the subsoil consists of ice-wedge material. Tis might open perspectives for precision agriculture in such landscapes. Abbreviations: DOE, depth of exploration; EC a , apparent electrical conductivity; EMI, electromagnetic induction.