Woodsmith's Hunt for Red October

Over the past few weeks there has been an unusual sight in the bright blue skies above North Yorkshire. A Basler BT-67 (converted DC-3) aircraft has been flying at an average height of one hundred metres over the Woodsmith Project and surrounding area. The project is located just south of Whitby in the North York Moors National Park, an area of forest and moorland within the UK. The aircraft is called Mia and belongs to Bell Geospace who have been carrying out an airborne Full Tensor Gravity (FTG) Gradiometry and Magnetic Geophysical survey across the project area, commissioned by Anglo American.

The Woodsmith Project philosophy has always been to minimise our environmental footprint, and the passive airborne geophysical methods used will allow us to better understand the orebody and host stratigraphy without disrupting the local environment. The aeroplane has a 29 m wingspan, enabling maximum stability while flying to gather high accuracy data, and turbine engines pointing upwards to reduce the noise at low-flying elevations. The noise from 100 m away is equivalent to a blender.

The instrumentation on board has been passively measuring the Earth’s ambient magnetic and gravitational fields across the project area. The Earth’s gravitational field is directly related to density and can be measured on land using a weight on a spring. The tension in the spring length is affected by the force of attraction from the mass of the ground below. This conventional gravity technique can’t be directly used in an aircraft due to the turbulent environment, which would generate different orders of magnitude greater than the small gravitational variation we are trying to measure. The innovation of an FTG survey is in the ability to collect extremely sensitive measurements from a moving platform, which enables coverage of a large area quickly and accurately.

The technology itself was initially developed by Lockheed Martin for the US Navy to enable nuclear submarines to navigate the ocean floor without using sonar and risking their discovery - think Sean Connery in the film ‘The Hunt for Red October’. The technique uses military-grade accelerometers (like we find in our smartphones to count steps but far more sensitive) working in pairs. As both accelerometers are affected by the same inertial movements of the aircraft, subtracting the response of one accelerometer from the other effectively cancels any aircraft motion-induced ‘noise’, leaving only the very tiny, real variations in the Earth’s gravitational field, which correspond to density variations in the ground below. FTG builds a 3D picture of subsurface density variations through the implementation of 6 pairs of these accelerometers on 3 spinning discs which are mutually perpendicular.

The Woodsmith polyhalite orebody formed from evaporation of brines on the basin margin of the ancient Zechstein Sea, some 250 million years ago. The FTG survey will pick up subtle differences in density between evaporite minerals, namely sulphate and chloride minerals, which predominate in separate areas of the basin. This newly acquired information is being integrated with complementary pre-existing 2D seismic data and drill hole information to further map and model the coastline at the time of the polyhalite deposit formation. Improving our understanding of the overall basin morphology will allow us to better understand the smaller scale geometry of the orebody itself, enabling us to plan and mine more effectively and safely in the future.

Now that the survey is complete the data is being processed. Interpretation will involve sophisticated science and maths, integrating physics-constrained neural networks to generate synthetic density models. It will involve collaboration across multiple disciplines in the wider Anglo American group, with the local geologists at Woodsmith working closely with the global Discovery and Geosciences team, and utilising cutting-edge Machine Learning insight from the global Data Analytics team, to ensure we will get the most value from the information.