USGS Open-File Report 2005-1413: Terrestrial Radioactivity and Gamma-ray Exposure in the United States and Canada

Introduction

Naturally occurring radioactive isotopes can be found in most rocks and soils. The most abundant of these are potassium (K⁴⁰), uranium (U²³⁸), and thorium (Th²³²). The radioactive decay of these elements results in the emission of gamma-rays of sufficient intensity that they can be measured using instruments in aircraft flying close to the ground. The measurement technique is known as airborne gamma-ray spectrometry (AGRS) and quantitative methods were developed 30 years ago (Grasty and Darnley, 1971) that allow data to be expressed in terms of element concentrations and dose rate.

In the 1970s, following the 1973 OPEC oil embargo, the governments of Canada and the USA took steps to stimulate uranium exploration. The Uranium Reconnaissance Program (URP) in Canada, and the National Uranium Resource Evaluation (NURE) program in the USA, included AGRS surveys with similar technical specifications. The NURE program covered almost all the USA but the Canadian program ended before much of Canada could be surveyed. Small areas were added during the 1980s and 90s, bringing coverage of Canada to about 35%. The data from each country were published in a variety of formats at different scales. The work presented here is a unified compilation of the US and Canadian datasets and is intended to facilitate regional comparisons on a continental scale. Maps of the data show many-fold variations in the surface abundances of potassium, thorium and uranium, and also in their relative abundances. A dose rate map illustrates the resulting variation of background radiation with location. These data provide a regional picture of the natural radioactivity across 60% of North America and demonstrate the importance of standardised measurement procedures.

An early application of airborne gamma ray spectrometry is described by Foote (1970) and Darnley and Grasty (1971) discuss using airborne surveys for mapping. Methods for quantitative calibration of spectrometer systems are discussed by Grasty and Darnley (1971) and Grasty (1975) presents calibration and data processing issues specific to uranium. Calibrated airborne gamma-ray surveys provide estimates of the mean surface concentrations of potassium (K), equivalent uranium (eU) and equivalent thorium (eTh). The prefix "equivalent" indicates that determinations are based on gamma rays emitted by daughter products of U238 and Th232; such determinations are accurate provided the radioactive decay series are in equilibrium. Each airborne ''measurement" or data point samples an area of several thousand square meters to a depth of about 30 cm (Duval, Cook, and Adams, 1971) and equilibrium is normally assumed in samples of this size. At a nominal terrain clearance of 120 meters a continuous strip approximately 250 meters wide is sampled (Grasty, Kosanke, and Foote, 1979; Pitkin and Duval, 1980). Typically there are about 18 overlapping data points per km flown. The North American map is based on approximately 100 million data points. Flightline spacing determines the percentage of a survey block that is sampled. The line spacings employed in the North American compilation range from 25 km (equal to 1% sampling of the surface) to 1km (equal to 25% sampling) in limited areas. In the United States (US), the surveys in Alaska were flown with 10 km line spacing. Most of the western part of the conterminous US was surveyed with 5 km line spacing, and most of the eastern part with 10 km spacing. In Canada, the line spacings ranged from less than 1 km to more than 25 km and most of the areas were surveyed with a 5 km line spacing.