Institut du Littoral et de l'Environnement

This paper presents the results of high resolution magnetic cartography realized with a caesium vapor magnetometer on marshland plots in Western France. This site is interesting because of the clayey substratum homogeneity and the historical knowledge of plot exploitation. A wooden truck running on PVC rails allowed acquisition of accurate signals with dynamics of about 2 nT. Natural grassland presents weak decametric variations correlated to a micro-toposequence. Conversely, plots recently tilled show linear magnetic anomalies. These anomalies are interpreted as the effect of tractor tyre soil compaction which modify soil properties. In addition, the study of a drained plot, by a wavelet transform, reveals the presence of a frequency band close to the drain intervals. Moreover, the magnitude of the linear magnetic anomalies is higher above drains. This suggests that the compaction influence on the magnetic carriers is more important in drained, hence aerated, zones.

Caesium magnetometers are new tools for soil property mapping with a decimetric resolution [Mathé, V., Lévêque, F., 2003. High resolution magnetic survey for soil monitoring: detection of drainage and soil tillage effects. Earth and Planetary Science Letters 212 (1–2), 241–251]. However, when the magnetic anomalies are only a few nanoteslas (nT), the geologic and pedogenic signal must first be isolated from magnetic disturbances for this method to be useful. This paper investigates the instrumental artifacts and environmental disturbances to adapt the survey protocol to slightly magnetic soils.Among the possible instrumental sources of disturbances listed and quantified, the most significant are: 1) The battery effect upon sensors 2 m away (classic protocol, about ± 0.15 nT) while increasing this distance up to 10 m cancelled it; 2) The noise level of magnetometers and sensors, which, according to tests on two magnetometers and three sensors, rarely and randomly exceeds 0.1 nT, but seems to increase with the electronic component age.Among the environmental disturbances, temporal variations such as diurnal variation or fluctuations linked to the moving of metallic masses play a major role, although the pseudogradient or base-station methods have commonly cancelled them. The efficiency of the latter is strongly dependent on the source nature. However, the ground currents and electromagnetic fields propagating in soils cause more problems. As a first step to better understand such disturbance sources, uncommon magnetic signal variations supposedly due to electromagnetic wave conversions and likely linked to the railway traffic are presented.Based on previous results, an adapted protocol using one magnetometer and two caesium sensors (0.3 and 1.6 m above the surface) is proposed to increase the signal / noise ratio. At first, to maintain an accurate horizontal and vertical location of the sensors, the latter are affixed to a wooden handcart running on plastic rails. Rails adapt to micro-topography, thereby decreasing strongly the soil–sensors distance variations. Anomalies due to topography rarely exceed 0.1 nT. Finally, a method to remove diurnal variations from high-resolution magnetic maps is proposed. Parallel profiles performed successively are adjusted by a cross-profile. Assuming that the temporal variations during each profile are negligible (less than 0.05 nT), this technique, contrary to the pseudogradient, preserves both the decimetric and the metric anomalies (gain of more than 1 nT).

... 1 Centre Littoral de Géophysique, avenue M. Crépeau, 17042 La Rochelle cedex 01, France, and 2 Laboratoire HydrASA, CNRS-UMR ... XRD patterns were decomposed into elementary curves, Gaussian or Lorentzian functions, using the DECOMPXR software of Lanson (1997 ...

The marshlands of western France provide the opportunity to relate the magnetic properties of a recent sedimentary clay soil to pedogenesis, including the impact of agriculture and drainage on magnetic mineralogy. We studied a plot of drained land that had been ploughed up to 1998 and under grass since. A new thermomagnetic method was used to identify and to quantify roughly the magnetic minerals, which represent less than 1 g kg−1. The most abundant Fe oxides are haematite (45%) and goethite (45%). However, trace amounts of the ferrimagnetic minerals maghemite (4%), magnetite (3%) and, to a lesser extent, iron sulphides (1%) provide most of the magnetic signal. This signal allowed us to identify magnetic horizons that relate to the soil horizons. The topsoil is characterized by a strong magnetic enhancement (4-fold), and the thickness of this layer increases close to the drains. Relative contents of ferrimagnetic phase also increase laterally with decreasing distance to the drains. Magnetic enhancement coincides with the most aerated and developed layer, in which water circulation, root colonization and hence soil fertility are greater. Ferromagnetic minerals sensu lato, especially maghemite and magnetite, can be considered as mineralogical tracers partly and indirectly reflecting soil fertility of these clay-rich marshland soils.

"The question of function of hearths in prehistorical caves is fundamental to characterize the nature of human occupation. The hearths could be used for lightning, cooking or heating. Their spatial repartition, as their paleotemperature, should allow to identify their function. The localization can be visual by the presence of ash and charcoal or by identication of rubefaction. The accumulation of ash and charcoal can also correspond to the emptying of hearths and the rubefaction can be chemical than thermal. The accumulation of ash and charcoal can correspond to the emptying of hearths and the rubefaction can be chemical instead of thermal.
The thermoremanent magnetization and the neoformation of magnetic minerals, together induced by heating, produce an importante deformation of the local magnetic field. Thus, the local variations of the magnetic field may allow to localize and to count the hearths without mistake even if the hearths are masked by a draping of clayed deposit.
The walls caves morphology are also sources of local deformations of the magnetic field. For this reason it is important to have a 3D information on the deformation of the magnetic field. Moreover, the geolocalisation is difficult in the caves. To resolve these different problems for such kind of prospect, the 3D mapping was realized by tracking the sensor position with a laser tacheometer during the deplacement of the sensor in the space.
The protocol was developed in the cave of “les Fraux” (Saint-Martin-de-Fressengeas, Dordogne, France), accidentally discovered in 1989 (Carozza et al., 2009). This cave is a rare example of a Bronze Age site (1300 BC) with archaeological vestiges (ceramics, bones, hearths) and rock art (incision, engraving, finger plot). The conservation of this site is exceptional as the cave entrance just collapsed after the last occupation by man, 3300 years ago.
The experimental contraint in this recent prehistorical cave are low compare to the older ones. This allows us to developpe a new approach to estimate the paléotemperatures of hearths (Brodard et al., 2012).
"

The cave of Les Fraux (Dordogne, France) is an exceptionally preserved Bronze Age site with both archaeological vestiges and rock art. Numerous traces of fires were observed and we were particularly interested in studying their effects on sediment to better understand their use centuries ago. Our present objective is the evaluation of the intensity of the fires by determining past temperature (palaeotemperature) attained by the sediment. The principle of palaeotemperature determination is based on thermoluminescence (TL) properties of quartz and magnetic susceptibility of the sediment. First, a cartography of magnetic susceptibility was made in situ. On the basis of both susceptibility cartography and visual patterns of sediment, small samples (1 - 2 grams) were taken out. Quartz grains were extracted from these sediment samples. Besides, fourteen thermal references were processed by heating fractions of the pristine sediment of the cave (unfired) that was also sampled. They were heated in air in an electric oven according to an accurately designed thermal protocol. Each fraction was heated at a maximum temperature varying from 200°C to 650°C, keeping an unheated sample as a witness. The TL signal shows significant variation allowing the palaeotemperature determination. By comparison of TL signal of anciently heated quartz to the reference ones, we were able to obtain the maximum equivalent temperature attained for each samples extracted from ancient fires. Thanks to that we could estimate magnetic susceptibility as a function of temperature and draw palaeotemperature cartography of the studied areas. The next step will be archaeological experimentations of fires that will allow us to validate our methodology and model fires by their intensity, issued energy (light and heat), quantity of wood consumed in a given duration. That project will contribute to a better identification of the function of the site.