GAMMA-RAY SPECTROMETRY Cristine Morgan, Texas A&M University Jim Thompson, West Virginia University

Outline

• Gamma radiometrics? • Examples of applications in soil and earth science • Proximal sensors (field-scale examples) • Airborne sensors (regional-scale examples)

Gamma Radiation

• Emitted by naturally-occurring elements in rocks and soils

• Gamma rays have variable energies • Energy is characteristic of the radioactive isotope

• Measures from the upper 30 cm of soil or rock • Travel for up to 300 m through air • Little attenuation by vegetation

Gamma Radiation

• Potassium, Uranium, Thorium (and Cesium) • Relatively long half-lives • Relatively abundant in the environment • Produce gamma radiation of measurable intensity

40K 238U, 235U 232Th 137Cs

Gamma Radiation

Applications

• Related to mineralogy and geochemistry • Soil parent material • Weathering and pedogenesis • Geomorphic activity (source, age) • Erosion and deposition

What Properties Can Be Mapped?

• Parent materials • Soil texture (sand content, clay content) • Nutrients and other elements (Fe, K) • Soil organic matter, O horizon thickness • pH

• Bulk density • Water retention • Saturated hydraulic conductivity

The Mole The Soil Company Netherlands

Proximal Mobile Sensors

RS-700 Radiation Solutions

Canada

700950 701000 701050 701100 701150 701200 701250

3483850

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Alt

158.5159159.5160160.5161161.5162162.5163163.5164164.5165165.5166166.5167167.5168168.5169169.5170

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Elevation Cs

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170.55170.6170.65170.7170.75170.8170.85170.9170.95171171.05171.1171.15171.2171.25171.3171.35171.4171.45171.5171.55171.6

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Elevation Cs

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Airborne Sensors

• Available data • USGS Aeroradiometric surveys of North America

• Product of the National Uranium Resource Evaluation (NURE) surveys (1975-1983)

• 2-km resolution raster data • Modeled from widely-spaced (5-10 km) flightlines

mrdata.usgs.gov/radiometric

Potassium

Equivalent Uranium

Equivalent Thorium

Dose

Sample Points (NCSS + FIA)

Preliminary Model

3.68 9.23

kg C m-2

SOC (0 - 20 cm)

Summary

• Positive • Fast • Quasi-continuous • Useful for large areas • Nearly non-destructive

• Negative • Only indirect

measurements • Need precise spatial and

non-spatial transfer functions

• Interpretation of geophysical results can be non-unique in terms of soil properties

Summary

• Combined with other environmental data, gamma has great potential as a tool for soils survey • Harmonization • Disaggregation

• Need a national strategy • How is gamma responding to soil variability in different

parent materials? • What scale of gamma data is needed to provide

effective information? • Best practices for harmonization and updating?