Peat mapping

Peat, which is composed of partially decomposed organic matter, plays a crucial role in carbon sequestration and storage. Peatlands store an estimated 5–20% of terrestrial soil carbon while covering only around 3% of the global land surface. Accurate mapping of peat thickness is therefore essential for assessing carbon stocks, evaluating climate risks from peatland degradation, and planning construction works in peat-bearing terrain.

Traditionally, peat thickness is measured by manual coring or push-probing: slow, labour-intensive methods that provide only discrete point measurements. Gamma-ray spectrometry (GRS) has emerged as a promising complementary approach, with the particular advantage that it can be deployed on drones or vehicles, enabling rapid, continuous coverage of difficult terrain.

Physical mechanism: gamma-ray attenuation by peat

A gamma-ray spectrometer measures natural background radiation emitted by radionuclides (⁴⁰K, ²³⁸U and ²³²Th) present in mineral soils and bedrock. When these materials are covered by peat, the emitted radiation is partially absorbed before reaching the detector. A thicker peat layer absorbs more radiation, so a lower count rate indicates greater peat depth.

Attenuation follows the Beer-Lambert law:

I = I₀ · e^(−μ · ρ · x)

where I₀ is the initial radiation intensity, I the detected intensity, μ the mass attenuation coefficient (cm²/g), and x the peat thickness. The mass attenuation coefficient is approximately constant for all natural materials. Peat has a much lower bulk density than mineral soil or water, which is why gamma radiation can penetrate several metres of peat while sensing depth in water-saturated mineral soils is limited to about 0.3–0.6 m.

Calibration and mapping

The count-rate-to-thickness conversion must be calibrated against corings at each site because substrate radioactivity and peat density are site-specific.

Case studies

A drone-borne survey over 118 hectares at the Carrigeen Wind Farm site in Ireland demonstrated that 25 flights over two days provided peat thickness maps that closely matched 110 existing corings (R² = 0.81). Thicker peat was concentrated in low-lying geomorphological features consistent with known landscape controls on peat accumulation. The drone survey required two days of fieldwork against the three weeks needed for the original coring campaign (Koomans et al., 2026).

A ground-based survey using a vehicle-mounted detector over a 10 ha fen peatland in Denmark (peat depth 0.1–7.3 m) combining gamma-ray and terrain elevation data achieved near-perfect agreement with validation corings (Lin's CCC = 0.97, RMSE = 0.63 m) using as few as 30 calibration samples (Koganti et al., 2023).

Summary

Gamma-ray spectrometry provides a physically well-founded and efficient method for mapping peat thickness. The method requires site-specific calibration against corings, and accuracy decreases where peat moisture content is highly variable or where peat exceeds approximately 3–4 m depth. When used in the screening phase of a project, drone or vehicle-based surveys reduce the need for intensive coring campaigns while providing spatially continuous information that guides more targeted ground investigation. Beyond thickness mapping, the physical basis of the method offers the additional possibility of estimating peat bulk density and, from that, carbon content.

References

Koomans, R., de Vries, K., Ceulemans, S., Whiteford, J., 2026. Peat mapping with drone-borne gamma spectrometry. GIM International, Issue 2 2026, 9–11.

Koganti, T., Vigah Adetsu, D., Triantafilis, J., Greve, M.H., Beucher, A.M., 2023. Mapping peat depth using a portable gamma-ray sensor and terrain attributes. Geoderma 439, 116672. https://doi.org/10.1016/j.geoderma.2023.116672

Beamish, D., 2013. Gamma ray attenuation in the soils of Northern Ireland, with special reference to peat. Journal of Environmental Radioactivity 115, 13–27. https://doi.org/10.1016/j.jenvrad.2012.05.031