Estimations have awakened the hope to find large amounts of precious minerals in the oceans. But how can we distinguish these remotely from other materials on the seafloor?
Minerals from the deep sea
The digital age has made the need for minerals explode. These are traditionally extracted from land-based mines, often in developing countries where the impact on the natural environment and on social structures gives raise to ethical questions.
The need for Europe to be more self-sufficient in minerals and the development of new automated technologies are key drivers to look for opportunities to exploit marine-based minerals. However, understanding the potential of seafloor mineral resources, and the environmental impact of related exploration and exploitation activities, depends critically on an understanding of their location, abundance and characteristics. Currently, the critical information to support this understanding is lacking because roughly 99% of the ocean floor – more than 360 million km2 – are virtually unexplored.
Why underwater hyperspectral imaging?
Detailed exploration and seabed mapping is required in order to find target areas suitable for deep-sea mining. Typical seabed mapping methods are of acoustic nature, involving echosounders or sonar systems. These methods allow the detection of features on the seafloor, but additional sampling is required to properly identify these features and differentiate between, for example, simple rocks and valuable polymetallic nodules. However, taking seabed samples in the deep sea is challenging and time-consuming. Therefore, an alternative mapping method is being investigated that allows to characterize seabed features without the help of seabed samples.
Today, hyperspectral images from satellites are commonly used to classify terrestrial minerals on a large scale based on their individual spectral signatures. This method requires a light source, normally the sun, but sunlight does not penetrate beyond water depths of 200 m. A hyperspectral imager was therefore developed that can be applied underwater using its own light source. This underwater hyperspectral imaging (UHI) sensor has been successfully used in shallow waters. In 2015, the first deep-sea UHI sensor was developed, which is now tested for mineral classification purposes in the deep sea.
My research activities at NTNU
The EU-funded project “Blue Mining” aims to provide breakthrough solutions for sustainable deep-sea mining and involves both the exploration and exploitation phases. The main targets for deep-sea mining are seafloor massive sulfides and manganese nodules. Data are obtained from the Peru Basin in the Pacific (survey in summer 2015), from the Mid Atlantic Ridge (two cruises in summer 2016), and from the Arctic Mid Ocean Ridge (MarMine cruise in summer 2016). Water depths in these field studies vary from 500 m to 4200 m.
For data acquisition, the UHI sensor is mounted on an underwater vehicle such as an ROV (remotely operated vehicle) or AUV (autonomous underwater vehicle) that surveys the seafloor at an altitude of 1-2 m. The UHI sensor is looking vertically downwards, with two lamps for seabed illumination mounted on each side of it. The sensor records the intensity of the light that is reflected back. In order to detect only the mineral resources, the influence of a range of other factors, such as the optical properties of the water column, the lamps, vehicle motion, and the sensor itself, needs to be removed during post-processing.
Through optical fingerprints, the final images allow the extraction of reference spectra for features that have already been identified by other means (e.g. video data, samples). These reference spectra, combined with additional spectra from laboratory experiments, are then collected in a database that will be the basis for automated classification of UHI data. With this new methodology and UHI technololgy, new data can be classified quickly and analyzed for the desired seabed mineral targets, thus sparing the need to recover samples from the seafloor.
If you want to know more about this, sign up for Ocean Week in Trondheim 9.-12.May
This blog entry was written by postdoctor Ines Dumke,
Department of Marine Technology