3.2 - Soil Surveying - Gamma Radiometrics

CHECKLIST FOR COLLECTING GAMMA RADIOMETRIC SURVEYS (OR GAMMA RADIOMETRIC DATA)

• Gamma data need only be collected once unless massive soil renovation (such as claying or spading) has been undertaken post-survey.

• Ground-truthing of major gamma radiation zones should include soil laboratory testing for sand, silt and clay, gravel content, exchangeable cations, pH, EC and phosphorus absorption. Potassium and sulphur are also usually measured.

• For detailed financial analysis, gamma radiometric data should be analysed with yield data.

Gamma radiation is high-frequency electromagnetic radiation which is used as a soil-sensing technique in agriculture. Gamma surveys measure the radiation emissions from the decay of naturally occurring radioisotopes in the topsoil to predict soil properties such as texture and mineralogy.

In PA three channels are typically measured which relate to the decay chains of potassium, thorium, and uranium. Some instruments also provide a total count reading, which is the sum of all gamma radiation, measured in counts/second.

Gamma radiometers are an effective instrument when used in conjunction with EM surveys. Combining gamma radiation with EM data has been demonstrated to improve the accuracy of predicting soil properties, particularly in areas of low electrical conductivity.

In Western Australia, where there are widespread sandy and gravelly duplex soils, most geophysical soil surveys use a combination of EM and gamma radiometric surveys.

Similar to EM surveys, gamma radiometric data needs to be ground-truthed. Soil cores are generally taken to a depth of 30 cm as most gamma radiation detected at the soil surface is emitted from the top 30–40 cm of soil. On loose, deep sands (sand depth >60 cm), deeper soil cores can be of value as gamma emissions can travel from a greater soil depth.

Gamma radiometry can be used to complement EM and provide a more comprehensive definition of soil types in the following situations:

• In areas of very low soil conductivity gamma radiometrics are used to distinguish between deep sand and gravel profiles.

• In areas of high soil conductivity, gamma radiometrics can be used to distinguish between clay profiles and saline soils.

• On the northern sand-plains of WA the total gamma count has helped to identify soil profiles with better water-holding capacity.

• Gamma radiometrics is effective in delineating sand profiles from decomposed granite loams.

Soil sensors are mounted on a vehicle with RTK GPS for simultaneous collection of EM, gamma radiometric data, and elevation simultaneously. Shallow EM surveys frequently fail to capture the true level of field variability as shown here.

The thorium radiometrics survey (b) however, highlighted significant changes in soil structure on a gravelly duplex soil in WA. At both locations one (1) and two (2) in Figure 3, the EM survey indicated a low electromagnetic currency (shown in red). The thorium channel detected contrasting values for these locations.

Soil cores extracted to 60 cm and subsequent laboratory testing revealed significantly different soil profiles at each location. Site 1 was gravel-dominated soil (low EM and/ or high thorium), while site 2 was a deep sand (low EM and/or low thorium). The thorium channel is recognised for its ability to help distinguish between gravel and sand soil types.

INTERPRETATION OF GAMMA RADIOMETRICS

• Ensure landscape and soil formation processes are taken into account when interpreting gamma radiation data.

• Gamma radiometrics is a specialised soil sensing technique which is best utilised and interpreted using the services of an experienced data consulting group.