Environment

Between September 7th and October 7th 2015 BAAC Vlaanderen conducted an interdisciplinary archaeological research in an area situated near the Schoorhavenweg in Verrebroek (East Flanders, Belgium); the reason for this being the construction of a logistic park. Earlier investigations by GATE resulted in the discovery of a number of Stone Age find spots in a buried and predominantly intact podzol soil, and this over an area of at least 25 ha. In the initial research phase six small and probably Early Mesolithic artefact scatters, identified during previous research, have been excavated. Additionally, the low-lying zones of the research area were investigated by geological trenches, situated at right angles through them. The latter was done to gain insight into the genesis and infill of these depressions, which have been interpreted as erosion gullies.

Tussen 25 augustus en 4 september 2014 is, naar aanleiding van een verkavelingsplan van het terrein (afd. 2, sectie B, perceel 931z) gelegen aan Kerkelare/Larestraat in Lovendegem, provincie Oost-Vlaanderen, een archeologisch onderzoek uitgevoerd. De opgraving is uitgevoerd door BAAC Vlaanderen bvba in opdracht van de Vlaamse Maatschappij voor Sociaal Wonen (VMSW) met als doel het bodemarchief en eventuele aanwezige archeologische waarden die door de graafwerken zullen worden verstoord in kaart te brengen en te onderzoeken. Op basis van het proefsleuvenonderzoek dat in mei 2013 werd uitgevoerd door BAAC (Krekelbergh & De Cleer 2013) is 2400 m2 van het 0,40 ha grote plangebied geselecteerd voor verder archeologisch onderzoek. Uit deze prospectie is namelijk gebleken dat er zich sporen van landelijke bewoning uit de middeleeuwen in het geselecteerde gebied bevinden.

Het plangebied is onderzocht in zeven aaneengesloten werkputten en brengt sporen uit de volle middeleeuwen en nieuwe tijd aan het licht. Het gaat om een complex greppelsysteem, drie waterkuilen, enkele kuilen, een wegtracé en een paar bakstenen structuren. Hoewel het natuurwetenschappelijk onderzoek nog moet worden uitgevoerd, zal dit artikel de eerste resultaten van het onderzoek belichten.

In the spring and summer of 2015, the " Bureau voor Archeologie, Architectuurhistorie en Cultuurhistorie " (BAAC) performed an archaeological excavation in the municipality of Roeselare (province of West-Flanders, Belgium). Prospective research by means of trenches had revealed the presence of traces from prehistoric, Roman and Medieval times. West of these occupation sites, however, a palaeovalley of the " Krommebeek " was situated, a small obsequent stream that flows from the steep front of the cuesta of Tielt. During the excavation, large soil sections were dug and registered in order to reconstruct the original course of the Krommebeek in historical and prehistoric times. It showed that several small Holocene channels existed within a broad Late-Glacial river valley. The infill of those channels consists of organic peat and alluvial clay. The finds of hazelnuts, prehistoric pottery and a flint tool within the infill of those channels seem to indicate that they were already exploited in prehistoric times, possibly from the Mesolithic period on. Dumps of anthropogenic wooden finds may indicate that there was also interaction between the channels and occupation sites nearby in later times.

Multiple apparent electrical conductivity (ECa) measurements with an electromagnetic induction (EMI) sensor frequently reveal analogue patterns caused by conductive features in the soil. A procedure was proposed to highlight different archaeological anomalies based on combinations of the simultaneous ECa measurements with the DUALEM-21S instrument. After selection of a 3.5 ha study site, 0.79 ha has been recorded by archaeological excavation. Since the majority of the archaeological features were found between the plough layer and 1.0 m below the soil surface, a set of four equations were developed to model the EC within that predefined depth interval. This set of four equations employed the four depth response curves specific to the four DUALEM-21S coil configurations. The modelled conductivity between 0.5 and 1.0 m (inline image) showed a larger variability across the archaeological features than the raw EC data. To quantify the added value of this modelled conductivity, inline image and measured ECa were compared with the rasterized map of the archaeological traces. Finally, the inline image map proved to be better able to distinguish between the archaeological features and the ‘empty’ background. This technique allowed the highlighting of vague anomalies in the simultaneous DUALEM-21S ECa measurements. Copyright © 2012 John Wiley & Sons, Ltd.

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 reflecting textural differences between wedge filling and host material. Our objective was to investigate an alternative method by measuring the soil apparent electrical conductivity (ECa) with an electromagnetic induction (EMI) sensor. Based on an aerial photograph showing polygonal crop marks in an agricultural field 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. The 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 distributed over the test area and analyzed their texture. The results showed a clear difference between the Eocene host material (on average 21% clay) and the Quaternary wedge filling (on average 6% clay). The 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 final perspective comprises the strong heterogeneity of the subsoil, since nearly half of the subsoil consists of ice-wedge material. This might open perspectives for precision agriculture in such landscapes.

Paddy rice fields are kept inundated during most of the growing period. This requirement is challenging to achieve because of the lack of suitable technologies to detect rapidly percolation prone zones within these fields. The objective of this study was to evaluate a methodology to identify water leakage areas to support precision soil–water management at a within-field level. Therefore, a Floating Sensing System (FloSSy) was designed to record the soil apparent electrical conductivity (ECa) of a paddy field both under dry and inundated conditions using the electromagnetic induction sensor EM38. Comparison of ECa data sets obtained under inundated and dry conditions showed that the ECa measurements under inundated condition (ECa-i) were more strongly related to soil properties due to the absence of variability in soil moisture and the increased stability of the floating sensing platform. Therefore, we proceeded with the ECa-i measurements and grouped them into two classes using a fuzzy k-means classification method. These classes showed significant differences in water infiltration: lower ECa values represented a higher infiltration rate and vice versa. This effect was attributed to differences in soil texture, more specifically the sand content, and its effect on water retention. It was concluded that an ECa-i survey with FloSSy allowed the detection of soil heterogeneity linked to downward water fluxes which has a potential to support precision soil–water management in inundated fields.

Most geological and soil maps are not detailed enough to represent the high lateral and vertical textural variability in the subsoil of coastal lowlands. Intensive sampling campaigns need to be carried out to quantify this variability. As an alternative, a proximal soil sensing procedure based on a single survey with an electromagnetic induction instrument was used to map a 6.5 ha Holocenetidalarea in Belgium. We investigated the effectiveness of amulti-receiverapparentelectricalconductivity (ECa) survey for mapping the trace of tidal paleochannels. From a limited number of augerings, a three-layered soil was observed composed of a clayey top layer, a clayey infilling of the tidalchannel above a subsoil consisting of coarse sandy material. A fitting procedure allowed modelling the conductivities of both subsurface layers, after which the four simultaneous ECa measurements were combined to model the depth of the interfaces between the three layers. The predictions were validated by 16 depth observations along a 150 m transect. A correlation coefficient of 0.91, with an average error of 0.23 m, was found between the predicted and measured depths of the clay-sand interface. We concluded that the dense ECa measurements (2 by 2 m resolution) allowed reconstructing a precise three-dimensional representation of the tidalchannels.

The archaeological evaluation of landscapes in the framework of developer-led archaeology is often based on extensive trenching programmes complemented with auger surveys in wetland environments. During the archaeological evaluation of a 90 ha polder site in the north-west of Belgium, a mobile multi-receiver electromagnetic induction (EMI) survey was used as a main prospecting technique. The use of a mobile survey allowed to map the entire study area at a very fine resolution (over 25 measurements per m2). Incorporating a multi-receiver EMI instrument enabled measuring the apparent electrical conductivity (ECa) and the apparent magnetic susceptibility (MSa) of four different soil volumes simultaneously at each location. The detailed maps provided insight into the archaeological and geomorphological features of the site. Among the detected structures were a large medieval farmstead, a palaeoriver system and a number of military remains from World War I (WWI). The vertical discrimination potential added insight into the vertical facies changes, which allowed modelling the palaeolandscape and helped determining the depth of detected medieval features. The different MSa measurements gave additional insight into the WWI structures. In this paper, we give an overview of the possibilities of combining multiple ECa measurements for interpreting vertical soil variability together with an example of the added information from simultaneously gathered MSa data. More generally, the diverse potential of multi-receiver EMI survey for geoarchaeological research is demonstrated.

ABSTRACT Most geological and soil maps are not detailed enough to represent the high lateral and vertical textural variability in the subsoil of coastal lowlands. Intensive sampling campaigns need to be carried out to quantify this variability. As an alternative, a proximal soil sensing procedure based on a single survey with an electromagnetic induction instrument was used to map a 6.5 ha Holocene tidal area in Belgium. We investigated the effectiveness of a multi-receiver apparent electrical conductivity (ECa) survey for mapping the trace of tidal paleochannels. From a limited number of augerings, a three-layered soil was observed composed of a clayey top layer, a clayey infilling of the tidal channel above a subsoil consisting of coarse sandy material. A fitting procedure allowed modelling the conductivities of both subsurface layers, after which the four simultaneous ECa measurements were combined to model the depth of the interfaces between the three layers. The predictions were validated by 16 depth observations along a 150 m transect. A correlation coefficient of 0.91, with an average error of 0.23 m, was found between the predicted and measured depths of the clay-sand interface. We concluded that the dense ECa measurements (2 by 2 m resolution) allowed reconstructing a precise three-dimensional representation of the tidal channels.

ABSTRACT Mobile multi-receiver electromagnetic induction sensors provide simultaneous readings of the apparent electrical conductivity (ECa) from overlapping soil volumes. Therefore, small contrasting features can be difficult to identify because they have a limited contribution to the bulk measurement, especially if they are present in the subsoil (i.e. beneath the topsoil). Integrating ECa data from simultaneous measurements with multiple coil configurations has the potential to elucidate the variability within the soil profile as it enables modelling the electrical conductivity (EC) for distinct depth intervals. Therefore, it was our objective to develop a methodology to enhance the delineation of contrasting subsoil features, such as in-filled gullies and archaeological features. We selected a 3.5 ha study site where contrasting features were expected. A three-layered build-up was taken as the initial EC-slicing model. After varying the interface depths, the shallowest and deepest EC-depth slice showed a clear minimum of their combined variances at interface depths of 0.36 m, which corresponded to the ploughing depth, and 0.86 m. This implies that the EC-depth slice in-between these depths, contained a demonstrably higher variability. A sub-area of 0.85 ha was completely excavated to a depth of 0.7 m, revealing the subsoil features and the host material. An automated edge detection algorithm showed that the EC-depth slice was superior to any individual ECa measurement for delineating the contrasting subsoil features. Therefore, we concluded that depth slicing by integrating simultaneous ECa signals from a multi-receiver EMI sensor clearly improved the identification of subsoil features.

ABSTRACT A key characteristic of flooded paddy fields is the plough pan. This is a sub-soil layer of greater compaction and bulk density, which restricts water losses through percolation. However, the thickness of this compacted layer can be inconsistent, with consequences such as variable percolation and leaching losses of nutrients, which therefore requires precision management of soil water. Our objective was to evaluate a methodology to model the thickness of the compacted soil layer using a non-invasive electromagnetic induction sensor (EM38-MK2). A 2.7 ha alluvial non-saline paddy rice field was measured with a proximal soil sensing system using the EM38-MK2 and the apparent electrical conductivity (ECa) of the wet paddy soil was recorded at a high-resolution (1.0 × 0.5 m). Soil bulk density (n = 10) was measured using undisturbed soil cores, which covered locations with large and small ECa values. At the same locations (within 1 m2) the depth of the different soil layers was determined by penetrometer. Then a fitting procedure was used to model the ECa – depth response functions of the EM38-MK2, which involved solving a system of non-linear equations and a R2 value of 0.89 was found. These predictions were evaluated using independent observations (n = 18) where a Pearson correlation coefficient of 0.87 with an RMSEE value of 0.03 m was found. The ECa measurements allowed the detail estimation of the compacted layer thickness. The link between water percolation losses and thickness of the compacted layer was confirmed by independent observations with an inverse relationship having a Pearson correlation coefficient of 0.89. This rapid, non-invasive and cost-effective technique offers new opportunities to measure differences in the thickness of compacted layers in water-saturated soils. This has potential for site-specific soil management in paddy rice fields.