Testing mobile ground-based gamma-ray spectroscopy for measuring 40K in an agricultural field (Spain)

Proximal gamma-ray spectroscopy is an effective technique for monitoring the spatial and temporal distribution of terrestrial radioelements like ⁴⁰K, which is inversely proportional to the volumetric soil water content SWC (m³/m³). In recent years, PGRS has become a promising sensor to infer topsoil water content at an intermediate field scale supported by adequate calibration and corrections, but to date, it has not yet been used in Spain. The aim of this contribution is to test the response of mobile ground-based gamma measurements conducted while walking over an agricultural plot of 400 m² of bare soil. Two surveys were conducted, a day before and a day after a 16-liter rain episode, allowing us to i) evaluate the information obtained when using the mobile mode, ii) test the response of the PGRS to dry and wet soil conditions, and iii) compare mobile measurements with stationary records taken during one hour at the 4 vertices of the study plot. A scintillator detector of 0.3 L sodium iodide (NaI) crystal was used to evaluate the region of interest for total counts of ⁴⁰Potassium (1461 KeV). The mobile measurements were conducted 0.5 m above the soil surface and taken in stop-and-go mode (instead on-the-go mode) with two sets of 5 transects spaced 5 m apart placed in a perpendicular direction to cover the study plot. While walking we stopped every 1m for 10 sec. to collect a total of 21 measurements per transect, obtaining 210 data for geostatistical interpolation. These preliminary results show higher content of ⁴⁰K (cps) during the dry compared to the wet survey, and some differences in the spatial distribution of ⁴⁰K for both surveys. Similarities and parallel trends were observed when comparing mobile and stationary measurements, supporting the promising use of PGRS technique.