Gamma ray attenuation in the soils of Northern Ireland, with special reference to peat

Abstract

This study considers gamma ray attenuation in relation to the soils and bedrock of Northern Ireland using simple theory and data from a high resolution airborne survey. The bedrock is considered as a source of radiogenic material acting as parent to the soil. Attenuation in the near-surface is then controlled by water content in conjunction with the porosity and density of the soil cover. The Total Count radiometric data together with 1:250k mapping of the soils and bedrock of Northern Ireland are used to perform statistical analyses emphasising the nature of the low count behaviour. Estimations of the bedrock response characteristics are improved by excluding areas covered by low count soils (organic/humic). Equally, estimations of soil response characteristics are improved by excluding areas underlain by low count bedrock (basalt). When the spatial characteristics of the soilclassified data are examined in detail, the low values form spatially-coherent zones (natural clusters) that can potentially be interpreted as areas of increased water content for each soil type. As predicted by theory, the highest attenuation factors are associated with the three organic soil types studied here. Peat, in particular, is remarkably skewed to low count behaviour in its radiometric response. Two detailed studies of blanket bogs reveal the extent to which peat may be mapped by its radiometric response while the intra-peat variations in the observed response may indicate areas of thin cover together with areas of increased water content.

Peat Mapping Associations of Airborne Radiometric Survey Data

“…This level is then potentially subject to attenuation by the cover material (soils and superficial deposits). When considering low count attenuation behaviour across soils, the potential lateral variations in bedrock radiogenic responses (changes in geology) also need to be addressed [7]. The majority of detailed case studies presented here, do not involve any significant changes in bedrock response.…”

“…Within the data sets acquired, it has become evident that the gamma ray attenuation considered here, displays a very evident association with organic soils, and with peat, in particular. Beamish [7] noted that that attenuation across the soil profile is controlled by moisture content in conjunction with the density and porosity of the materials. The theory of attenuation is reprised here with special regard to peat and organic soils.…”

“…In the case of such mixtures it is necessary to replace the attenuation coefficient (µ) in Equation (1) by a summation over the three phases. In the case of a uniform surface layer (a half-space), each phase becomes a fractional component and the material may be described in terms of its porosity and degree of saturation [7]. Following Beamish [7], Figure 2a shows the attenuation curves obtained for two mineral soils (dry bulk densities of 1.1 to 1.6 g/cm 3 ).…”

“…Again following Beamish [7], Figure 2b compares attenuation with degree of saturation for a range of near-surface materials. The behaviour of an intact bedrock material with bulk density of 2.7 g/cm 3 (e.g., a limestone) with a porosity of 10% is shown; the curve displays the lowest sensitivity to degree of saturation and would represent relatively intact bedrock at outcrop.…”

“…Radiometric attenuation across the two largest blanket bogs in NI (i.e., the Garron Plateau and Cuilcagh Mountain) was studied by Beamish [7] using a Total Count (TC) measure of the radiometric response. Beamish [7] identifies a linear relationship between TC and air absorbed dose rate (DOSE) for the NI Tellus data set. Here a new analysis across the extensive cross-border Slieve Beagh blanket bog is conducted using the merged DOSE data set from both surveys.…”