Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: a case study from NW Spain

Abstract

Geophysical exploration methods allow the detection of archaeological features before any excavation is carried out on these sites. This is due to the contrast of properties between the buried archaeological structures and objects and the surrounding soil, sediment, or rock. Although Gamma-ray spectrometry (GRS) is [widely] used for geological exploration and mapping, it has been scarcely used in archaeology so far, despite the successful results of previous studies on the matter. In situ GRS is a non-destructive method that allows direct assessment of uranium-238 (238U) and thorium-232 (232Th) from daughter radionuclides of their decay chains, as well as potassium-40 (40K), on soils and rock outcrops. The technique documents the concentration of these isotope concentrations in the topsoil by surface measurements and this enhances its potential for archaeological exploration. However, two assumptions must be made: the archaeological objects must contain a different concentration of radionuclides than the surrounding sediment or soil, and they must be buried in the terrain less than 25–30 cm deep. In this work, we present the results of the use of in situ GRS for the study of a buried structure in the archaeological site of Cidadela (Galicia, NW Spain). Firstly, we have tested in situ spot GRS measurements to detect rock-built structures buried in the sediments; secondly, we have excavated the surveyed area. The results are reliable despite the low radioactive content of the rocks used as building materials, given that the burying and sediments also have low amounts of radioactive isotopes. Although the direct use of the estimates of K, U and Th has not proved successful, the use of U/Th, Th/K and U/K ratios provided reliable results