Ground‐penetrating radar survey over bronze age circular monuments on a sandy soil, complemented with electromagnetic induction and fluxgate gradiometer data

Abstract

AbstractThis paper presents a ground‐penetrating radar (GPR) survey over two circular structures originally surrounding Bronze Age burial mounds at the site of Koekelare (western Belgium). The region is characterized by sandy soils. Their low water storage capacity and the consequent moisture contrasts in dry summers played an important role in the detection of over 1000 Bronze Age funeral monuments through aerial archaeology in the past few decades. At Koekelare, low attenuation of GPR waves resulted in the detection of a double and single circular ditch. A fluxgate gradiometer survey and electromagnetic induction (EMI) measurements did not clearly reveal the ditches. For the GPR wave velocity analysis, constant velocity migration tests were combined with time‐domain reflectometry (TDR). The TDR measurements were made at different depths within the ditches and in the adjacent undisturbed soil, so that the differences in the physical soil parameters could be assessed.At a depth of approximately 0.45 to 0.8 m, the relatively homogeneous ditch fill produces few GPR reflections compared with the undisturbed soil, and is visible as a weak negative anomaly on the horizontal slices. At this depth, the grey or brownish black ditch fill was found in augering samples, clearly distinguishable from the yellowish brown sandy soil outside the ditches. The transition between the ditch and the underlying soil caused a gradual reflection of radar energy at a depth of approximately 0.8 to 1.2 m, although TDR showed no marked differences in relative permittivity between the ditches and the surrounding soil, and no clear steps as a function of depth. Copyright © 2009 John Wiley & Sons, Ltd.