Methods of surveying the thickness of humous horizons using ground penetrating radar (GPR): An example from the Garmisch-Partenkirchen area of the Northern Alps

Ecological studies commonly demand elaborate knowledge of the distribution pattern of humous horizons, especially in forest ecosystems with high humus variability. Conventional investigative methods are time-consuming and surface-destructive. Ground penetrating radar (GPR), in contrast, offers a fast and non-destructive method of portraying the shallow subsurface. Against this background, the reliability of GPR-derived metric data on terrestrial humous horizons was tested at a study site in the altimontane zone of the Bavarian Alps (1,260 m a.s.l.). The GPR survey, using a shielded bistatic antenna device at 800 MHz, is based on eight parallel lines within a 30 × 30 m plot. The metric interpretation of the GPR results is validated by a trench, which records the thickness of humous horizons at a 10-cm interval, and is compared to conventional sampling methods, represented by 18 soil monolith profiles of both random and grid-based distribution. The results show that it is not possible to resolve the borders of single humous horizons. The delineation of humous against non-humous horizons, however, shows correlation (R = 0.553) with the reference data and delivers reliable data on the average thickness and variation of humous horizons. The observed thickness deviations between GPR and the trench line range between -9.5 and 7.5 cm, with remarkably narrow deviations of -2 to 2 cm or less in 50% of the cases. In summary, supported by the high number of data records (n = 1,727), GPR creates a much better and none destructive image of the actual humus distribution pattern compared to conventional methods.