Focus 6, 2023 - Hyperspectal imaging of drill cores at NGU

Hyperspectral mineral classification superimposed on a core box photo shows colours of yellow, orange and blue. — Hyperspectral mineral classification superimposed on a core box photo. Tobias Kurz, NGU.

The sixth focus sheet of 2023 is about hyperspectal imaging of drill cores at NGU. It`s written by Tobias Kurz and Nolwen Coint.

Innovative mineral mapping

Mobile hyperspectral laboratory at NGU is lifted by a truck with a crane. — Figure 1: Mobile hyperspectral laboratory at NGU.

Hyperspectral imaging - also termed imaging spectrometry - is an emerging method for mineral identification and mapping and is increasingly applied in rock laboratories and for mapping in geosciences.

This non-destructive and semi-quantitative method is based on the diagnostic reflectance and absorption properties of minerals and other materials within the visible and infrared light.

Hyperspectral imaging, in contrast to photographs and multispectral imaging, measures the reflected light with a higher number, commonly hundreds of very narrow spectral bands, resulting in a continuous reflectance curve per image pixel.

The pixel spectra can be analysed for diagnostic mineral properties or by comparing with reference spectra from spectral libraries. The image nature of these data also enables data exploitation with various image processing methods including pixel or object-based image classifications or machine learning methods.

Diagnostic reflectance properties of minerals occur within different spectral ranges and include beside the visible also the infrared and thermal-infrared spectral range.

Portable hyperspectual laboratory at NGU

Hyperspectral core laboratory at NGU equipped with a SiSuRock core scanning system and a VNIR and SWIR hyperspectral camera. — Figure 2: Hyperspectral core laboratory at NGU equipped with a SiSuRock core scanning system
and a VNIR and SWIR hyperspectral camera.

NGU operates a mobile hyperspectral laboratory using a SisuRock core scanning system from Specim
Spectral Imaging Ltd (Oulu, Finland) equipped with two high performance hyperspectral cameras as well as a high resolution full-frame photo-camera.

The hyperspectral cameras measure within the wavelength ranges 400-1000 nm (visible to near-infrared - VNIR) and 1000-2500 nm (shortwave-infrared - SWIR). These spectral ranges are suitable to identify and map many rock- and ore-forming minerals or key minerals related to alteration processes.

The SisuRock system is designed to scan entire core boxes to enable fast turnarounds and scan capacities of couple of hundreds of core-metres per day. With this imaging method, the mineral composition can be efficiently determined, and thus mineral variations and trends can be disclosed over the entire core lengths with a spatial resolution of approximately 1 mm.

The laboratory at NGU is installed in a container and can transported and temporarily set up at core repositories or drill sites when shipment of the drill cores is not feasible.

Innovative core logging with hyperspectual image enables:

Hyperspectral core imagery showing core box photos. — Figure 3: Hyperspectral core imagery showing core box photos, hyperspectral auto-cluster images, hyperspectral mineral classification and derived hyperspectral core logs (form left to right: kaolinite, montmorillonite, chlorite) showing mineral trends over the entire core length.

Mineral identification and distribution over entire core length with high spatial resolution
• Non-destructive, non-contact, semi-quantitative
mineral mapping
• Guiding representative sampling
• Analysis of spatial mineral distributions at the
core surface
• Framework for integrative core analysis
• Fast turnaround time
• Valuable information for many different
applications and geological settings

Table 1: Main specifications of the hyperspectral cameras from Specim used at NGU’s mobile hyperspectral laboratory.

	VNIR CAMERA	SWIR CAMERA
Wavelength range	400-1000 nm	1000-2500 nm
Number of max spectral bands	946	288
Band sampling	0.6-5.0 nm	6 nm
Number of spatial pixels in scan line	2184	384
Spatial pixel size @ rage of c.50 cm	c. 0.2 mm	c. 1 mm
Digital resolution	16 bit	16 bit

References

Ongoing projects at NGU:
• Application of hyperspectral imaging for logging carbonatites and assess the distribution of Rare
Earth Elements mineralization in cores from the Fen alkaline and carbonatite complex.
• Basement Fracturing and Weathering On- and Offshore Norway projects (BASE 2 and 3)
• Clay mapping in drill cores