The gamma-ray data for digital soil mapping in Thailand

The gammaA geo-referenced airborne gamma-ray image of the study area at nominal scale 1:250,000, produced for the Thai Department of Mineral Resources (Kenting Earth Science International limited (KESIL), 1982; Wisedsind et al., 1994) was acquired. The AGRI data was collected at the beginning of the winter season in November 1985 by aircraft with flight line spacing of 2 km, terrain clearance of 400 m and a flight line direction west to east, flown at a constant height above the ground of 400 ft (MTC). The production of radiation and ternary radiation maps of Thailand were produced using IAEA method (Angsuwathana and Chotikanatis, 1997; IAEA, 2003), which resulted in an image with 400 x 400 m pixels. The gamma-ray spectrometer, developed by KESIL, contained 12 crystals in a 50.34 litre Harshaw NaI (Tl) crystal scintillator and recorded gamma-rays in 256 channels. The measured energy spectrum ranges from 0 to 3 MeV (wavelengths between 0.03 x 10-4 to 4.13 x 10-4 nanometres). The sensor measures the natural radiation from decay series of potassium (K), thorium (Th) and uranium (U) in the upper 45 cm of the Earth’s surface. The following energy windows were used to measure the total count (TC) and three radioelements: TC = 0.40-2.82 Mev., K = 1.36-1.56 Mev., U = 1.66-1.86 Mev., and Th = 2.42-2.82Mev. Potassium is measured directly from the decay of 40K and is expressed as a percentage. Thorium and Uranium are inferred from daughter elements associated with distinctive isotopic emissions from 208Tl and 214Bi in their respective decay chains and are expressed in equivalent parts per million and coded as eU and eTh. A complication is that 214Bi is also a decay product of radon gas, 222Rn, itself a decay product of radium, 226Ra. Radon concentration is highly dependent on soil moisture, being practically absent near the surface in dry soil and abundant in saturated soil (Grasty, 1997). Another complication is that the signals for 40K and 208Tl (i.e., eU) are attenuated in wet soil; this has been used to map soil moisture in homogeneous soil materials using K/eTh ratios (Carroll 1981). Atmospheric Rd is also affected by changes in air density due to temperature and pressure, thus data acquired in cool high-pressure conditions may have up to 30% enhanced Rd compared to warm low-pressure, thereby distorting the eU signal (IAEA, 2003). For these reasons the eU signal is considered less reliable than those for K and eTh.-ray imager