Relationships between gamma-ray attenuation and soils in SW England

Soil studies using radiometric data typically employ one or more of the main, naturally occurring radioelement estimates (potassium, thorium and uranium) to undertake a variety of soil property assessments. This study concerns an attenuation assessment of high-resolution radiometric data obtained by a recent airborne geophysical survey in SW England. These data provide continuous measurements over a wide-range of region-specific soils and their parent bedrock materials. A prime motivation for this study is the observed complexity of the spatial variance in the radiometric signal level. Although such data may be jointly classified according to soil and bedrock types, variable attenuation levels in the signal levels remain to be explained. The data appear to carry information on soil properties additional to that of texture or other available soil descriptors. Existing gamma-ray theory indicates that the attenuation behaviour of radiometric data is jointly controlled by soil density and wetness in the upper ~ 60 cm of the soil profile. Low density, highly organic soils (e.g. peat) produce readily identifiable and variable attenuation zones. All soil types are predicted to attenuate radiometric signal levels but at lower density-wetness sensitivities. The broad radiometric response level is, as expected, found to be controlled by bedrock. Clay mineral soils provide the most uniform response behaviour with respect to bedrock type. Peat soils display the lowest amplitude and most variable signal levels. The data from similar bedrock formations, even with the same lithological descriptor (e.g. argillaceous), can display distinctly different geostatistical behaviour when the same soil type is considered. A variety of inferred attenuation zones are discussed in relation to supporting information on soil property and soil and land-use classifications. Spatial inconsistencies in existing database descriptors of organic rich zones are demonstrated and it is evident that the radiometric data can assist in resolving such ambiguities. The supporting control information has been found to be often ambiguous or unavailable at a scale appropriate to the field-of-view of the airborne measurements. Within this wider context, it is suggested that an observational database, such as that supplied by the radiometric data, may assist in providing enhanced spatial assessments of the soils and soil properties encountered