Peat mapping with drone-borne gamma spectrometry

Information about peat thickness is not only relevant in the context of peatland conservation and climate adaptation, but also for engineering applications and infrastructure planning. A drone-borne survey of 118 hectares of terrain under inspection for a wind farm in Ireland has demonstrated that airborne gamma-ray spectrometry is a useful and accurate method for mapping peat in various terrains. The radiometric data revealed a clear relationship between gamma count rates and peat thickness, enabling the creation of predictive maps. This case study shows that spatial variations in peat thickness can be assessed reliably from airborne as well as ground-based surveys.

In line with national and EU climate targets, Ireland is committed to reducing greenhouse gas emissions and increasing its share of renewable energy, including by significantly increasing the nation’s onshore wind capacity. The Carrigeen Wind Farm is a land-based site that involves the potential development of about 11 wind turbines across two clusters. In the prospective phase of the project, information on the soil and underground is crucial – not only for the installation of the turbines, but also for planning roads and creating a power grid connection to a local substation.

The region around Carrigeen is underlain by Lower Carboniferous limestone covered with Quaternary glacial and post-glacial deposits. These deposits are locally covered with peat accumulations. From a geotechnical perspective, information on the spatial distribution and thickness of these peat accumulations is crucial. However, mapping the spatial extent of peat is a challenge. Traditionally, peat thickness is measured with methods such as coring, which is slow and can be difficult when the terrain is difficult to access. A comparative test was therefore set up to investigate whether gamma-ray spectrometry can provide information on peat thickness at the accuracies needed for spatial planning.

Use of a drone

For this case study, an MS-350 CsI gamma-ray spectrometer weighing 2.7kg was mounted under a DJI Matrice 350 drone (see Figure 1). The detector was linked to the drone’s differential GNSS stream via the SPH Skyhub, which integrates sensor data, positioning and flight control within a single platform.

Flight lines covered a total length of approximately 40km, flown at altitudes of 20-30m above ground level. At these heights, the sensor footprint was sufficiently large to provide continuous coverage while maintaining the spatial resolution required for peat thickness mapping. In total, 25 individual flights were required to complete the 118-hectare survey, which was executed over a period of two days.

Reference observations from a total of 110 corings were available for calibration of the gamma-ray signal measurements. These corings had already been placed in an earlier phase of the project which required approximately three weeks to complete.

From gamma-ray data to peat thickness

At each coring location, the gamma-ray signal was sampled from the total count grid produced after the survey. The data showed a clear relationship between gamma count rate and peat thickness. The attenuation of the gamma-ray signal by peat can be described by an exponential decay model. To estimate peat thickness using gamma-ray measurements collected by the drone, this model was fitted to the data for the western and eastern areas separately (see Figure 2).

Results

The peat thickness map (see Figure 3) clearly demonstrates a strong spatial correspondence between the peat thickness derived from gamma-ray measurements and the thickness observed in coring data. Thicker peat deposits are mainly found in the low-lying, river-related geomorphological features, while shallower peat occurs towards the margins, reflecting the underlying landscape controls on peat accumulation.

A cross-section along the black line in Figure 2 shows how the depth measurements with the gamma-ray sensor correspond with the corings that were placed within a region of 50m along that line. This data illustrates the agreement between both measurement approaches. Along this profile, peat thickness derived from gamma-ray measurements closely follows the depth trends observed in the corings. Both datasets identify the same major thickness maxima and minima, as well as rapid transitions over short distances, indicating that the gamma-ray sensor is sensitive to changes in peat thickness at the sub-field scale.

An underestimation of peat thickness by the gamma-ray measurements is observed at the third peak around 2,000m. This local underestimation is likely related to heterogeneity in peat moisture content, which can lead to an underprediction of thicker peat packages. Minor discrepancies between the two methods can further be attributed to the point-scale nature of corings versus the footprint-averaged response of the gamma-ray measurements, as well as small-scale variability in peat properties. Nevertheless, the results demonstrate that gamma-ray measurements provide a spatially detailed method for mapping peat thickness.

Conclusions

Thanks to a large number of corings already being available at the site, there was a strong basis for calibration and validation in this pilot study. The close agreement between peat thickness derived from gamma-ray measurements and the coring data confirms that radiometric attenuation can be used to reliably estimate peat thickness at site scale.

This project shows that the drone-based setup allows surveys to be carried out efficiently in terrain that is otherwise difficult to access, making it suitable for a wide range of site investigations. Knowledge of the thickness of peat in soil layers is not only important for peatland conservation and climate adaptation studies, but is also relevant when planning construction works. This study demonstrates the potential of drone-borne gamma-ray spectrometry as a practical method for mapping peat thickness for such applications.

When applied in the screening phase of a project, this method reduces the need for intensive coring campaigns while providing spatial information and supporting informed planning decisions. Moreover, the spatial variation in peat thickness identified with the gamma-ray spectrometer enables coring locations to be planned much more efficiently and targeted to key zones of interest.

About gamma-ray spectrometry

Various geophysical techniques for mapping the thickness and spatial extent of peat layers in an efficient and non-invasive manner have been studied with growing interest in recent years. One of these methods is gamma-ray spectrometry. A gamma-ray spectrometer measures the natural background radiation emitted by radionuclides (such as potassium, thorium and uranium) present in mineral soils and bedrock. When these materials are covered by peat, the emitted gamma radiation is partially absorbed before reaching the detector. A thicker layer of peat will reduce the measured gamma ray intensity accordingly.

Further reading
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/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