SUMMARY The in-phase response collected by portable loop–loop electromagnetic induction (EMI) sensors operating at low and moderate induction numbers (≤1) is typically used for sensing the magnetic permeability (or susceptibility) of the subsurface. This is due to the fact that the in-phase response contains a small induction fraction and a preponderant induced magnetization fraction. The magnetization fraction follows the magneto-static equations similarly to the magnetic method but with an active magnetic source. The use of an active source offers the possibility to collect data with several loop–loop configurations, which illuminate the subsurface with different sensitivity patterns. Such multiconfiguration soundings thereby allows the imaging of subsurface magnetic permeability/susceptibility variations through an inversion procedure. This method is not affected by the remnant magnetization and theoretically overcomes the classical depth ambiguity generally encountered with passive geomagnetic data. To invert multiconfiguration in-phase data sets, we propose a novel methodology based on a full-grid 3-D multichannel deconvolution (MCD) procedure. This method allows us to invert large data sets (e.g. consisting of more than a hundred thousand of data points) for a dense voxel-based 3-D model of magnetic susceptibility subject to smoothness constraints. In this study, we first present and discuss synthetic examples of our imaging procedure, which aim at simulating realistic conditions. Finally, we demonstrate the applicability of our method to field data collected across an archaeological site in Auvergne (France) to image the foundations of a Gallo-Roman villa built with basalt rock material. Our synthetic and field data examples demonstrate the potential of the proposed inversion procedure offering new and complementary ways to interpret data sets collected with modern EMI instruments.

Geophysical techniques are clearly beneficial for archaeology, but limits exist. A good knowledge of these limitations is important in order to estimate precisely the reliability of these remote sensing techniques. The Canal Seine‐Nord Europe project provides a good opportunity to compare geophysical results with those from trial trenching with a mechanical digger. In 2009, magnetic and resistivity surveys were undertaken over a surface of approximately 60 ha, independently of archaeological evaluation by trial trenching. Thirteen archaeological sites, mainly constituted in pits, post‐holes and ditches, were discovered in this area. Twelve of them were detected by trial trenching and only three by geophysical survey. This case study based on archaeological feedback shows that geophysics has to be used with caution for evaluation of this type of archaeological and pedological context (luvisols on decarbonated loess) very common for the north of France.

Die durch große Infrastrukturprojekte bedingten Präventivgrabungen sind oft ein Ort, an dem neue Arbeitsmetoden eingesetzt werden. Bei dem vom INRAP seit 2008 durchgeführten Projekt Seine-Nord Europe wurden mehrere wissenschaftliche Methoden, insbesondere die Geophysik, angewandt, um bestimmte archäologische Fragestellungen zu beantworten. Die Haupteigenschaft dieses 2500 ha umfassenden Projektes besteht darin, die Geophysik in der Ausgrabungsphase und nicht in der Diagnosephase mit einzubeziehen wie dies bei anderen Projekten der Fall war. Die Ergebnisse waren besonders bedeutend, da die Geophysik nur in Fällen von klar definierten archäologischen Fragestellungen angewandt wurde. Im Endeffekt wurden zwischen 2009 und 2013 einunddreißig Studien durchgeführt. Drei Ansätze sind zu unterscheiden : Der erste betrifft die Prospektion der Fundstellen außerhalb des Ausgrabungsareals, um die Ausdehnung der archäologischen Reste zu bestimmen. Die Ergänzung der Grabungsdaten durch die geophysikalische Kartographie liefert einen Überblick der archäologischen Stätte und ermöglicht eine einwandfreie Bestimmung der Strukturen. Der zweite Ansatz betrifft die Integration einer archäologischen Stätte in ihren geomorphologischen Kontext und erfordert die Ausweitung der Untersuchung auf wesentlich größere Flächen. Er wurde insbesondere im Tal der Oise für die Charakterisierung der Ablagerungsmorphologie im Zusammenhang mit den vor-und frühgeschichtlichen Siedlungen angewandt. Der dritte und letzte Ansatz ist neuartiger, da es darum ging, neue Denkanstöße für die Phase nach dem Abtragen der oberen Fundhorizonte einzubringen. So konnten anthropogene Phänomene herausgestellt werden, die mit dem bloßen Auge oft schwer oder gar unmöglich zu erkennen sind. Für jeden dieser Ansätze werden die Beiträge und deren Durchsetzungsbedingungen im Rahmen des Projektes Seine-Nord Europe diskutiert. Diese Synthese zeigt die Vielfalt der möglichen Anwendungen der Geophysik bei klar definierten archäologischen Fragestellungen und in einem bekannten archäologischen und pedologischen Umfeld. Die Geophysik hat in diesem Fall ihren Platz in der Palette der Hilfsmittel des Archäologen.

The site of Goulet “ Le Mont” was discoverd during the archaeological survey preceding the construction of A88 highway between Caen and Sées. It is located near Argentan on the first plates dominating the Orne, which flows a few hundred meters away, and surmounts one of its affluent, the Houay. The site is established on the slope of this valley. It is marked by sections of ditches of 10 to 50 metres long, which draw a vast enclosure, whose diameter is gauged to be 550 metres, according to results of the excavation, aerial photographies and geophysical prospecting. This sums up to 1,7 kilometre of ditch and an internal surface of around 20 hectares. Within the excavation, only the western part of this structure could be observed. The ditches were dug in the calcareous substrate. Parts of the ditch located at the western end are no more than thirty centimetres under the level of scouring (0,70 metre under the surface of the ground) while the others can be as deep as 1,30 or 2 metres under this same scouring. The furniture discovered within the ditch is not very common. It is attached to middle Neolithic II, between 4400 and 3600 BC according to the absolute datings. The filling of the structure allowed the construction of a pollinic transept which revealed a significant impact of cereal growing in the close surroundings of the enclosure.

The site of Moulins-sur-Céphons is located close to Levroux (Indre, France). This publication deals with the geophysical survey undertaken in 2007. It brings another vision of the Neolithic site which covers an area of about 6 ha. Results are detailed and clearly show benefits of magnetic survey as a detection method and also as a characterization method of some archaeological phenomena. Hypothesis concerning the lack of archaeological evidence in several places will be discussed. Thanks to these results, this site can be compared with other similar sites of the western part of France.