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Th/K and Th/U Ratios from Spectral Gamma-Ray Surveys Improve the Mapped Definition of Subsurface Structures

School of Geography, Queen's University, Belfast, N. Ireland

* Corresponding author. E-mail: a.ruffell{at}qub.ac.uk

Spectral gamma-ray data can be obtained by non-destructive, automated, rapid and inexpensive survey methods. Previous studies utilise only total count, or total count as well as K, U and Th data. This work examines the use of Th/K and Th/U ratios to define the subsurface extent of partially-buried features at the centimeter to meter scale. On-site results are presented from two case studies as two-dimensional cross-sections. Changes in Th/K and Th/U ratios coincide with the known location of buried structures to within 10 cm horizontal resolution. Gradual changes in total count, K, U and Th measurements give a lower horizontal accuracy of 30 cm to 1 m. Grids of data were manipulated in ArcGIS^TM using a thin plate spline function to maximise information use and provide ‘easy to interpret’ maps of the survey areas. Unlike total count or individual element maps, Th/K and Th/U ratio maps can be compared to the known location of subsurface structures, vindicating the use of ratio cross-sections and maps in archaeological, geotechnical and forensic applications. It is concluded that the capacity to observe sub-surface features is enhanced through the use of Th/K and Th/U ratios.