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Geological maps define the Earth’s surface and the underlying geological structure; they show the dispersal of various types of bedrock in an area and can reveal clues about its geological past. Sometimes the best way to map the geology of the land is to get up in the air to study it from above, but how can planes do this?
Geological surveys aim to determine what rock types occur at or near the surface and how they are linked to each other. Aerial photography can provide a broad overview of the geological relationships within an area but doesn’t offer any detailed information about the mineral composition or the fabric of the rocks. Likewise, planes equipped with LiDAR – a remote sensing method that uses pulsed light to measure the distance to Earth – can be used to generate precise 3D maps giving details about the shape of the Earth and its surface characteristics, but it doesn’t offer detailed knowledge about rock types.
Small planes carry various equipment to obtain different measurements that can be combined to provide a detailed picture of the geological makeup of the rock and soils. Low-level geophysical surveys take measurements of the magnetic field, as well as the radiometric and electrical conductivity of the Earth’s near-surface.
Magnetism
A magnometer is mounted on the tail stinger at the back of the plane to measure variations in the Earth’s magnetic field. Different rock types contain different levels of magnetic minerals, which can be plotted on top of 3D magnetic maps to reveal the geological structure of the upper crust in the subsurface, particularly the spatial geometry of bodies of rock and whether there are any faults and folds. This can be especially useful if the bedrock is hidden by surface sand, soil, or water.
Sophisticated processing methods are utilized to build 3D models of the magnetic properties of rocks from the surface down to depths of several kilometers. This classic method is used to complement other information used to model geology and geological structures down to those very deep layers.
Spectral Radiometry or Gamma Spectrometry
A gamma-ray detector is positioned inside the plane to measure the natural radioactivity from elements such as potassium, uranium, and thorium in shallow soil and rocks. All rocks naturally contain isotopes that emit low levels of harmless radioactivity. The highly sensitive spectrometers take measurements of gamma rays released from the soil, the amount of which varies depending on the composition of the rock.
Data obtained using spectral radiometry or gamma spectrometry can enhance geological and soil maps and can be analyzed to refine the surface and near-surface geological mapping, providing a more precise picture of the geochemical status of the environment.
Electromagnetism
Electromagnetic systems mounted in pods at the end of each wing are used to measure variations in conductivity between different soils and rocks. This active system emits electromagnetic waves that travel through the air into the soil before being partially re-emitted and collected by a detector. The signal varies depending on the conductivity of the subsoil.
This method allows scientists to collect nearly continuous data over large areas, including hard-to-reach places. The data collected produces 3D images of variation in electrical conductivity of the subsurface, from the immediate surface down to a few hundred meters depending on the material.
Case Study
Between November 2018 and March 2019, the US Geological Survey used a plane to conduct an airborne survey of the rock layers under a region of southeastern Missouri and western Illinois. It was the largest magnetic and radiometric survey that the USGS has flown in its history of collecting magnetic survey data. It flew between 80 and 140m above ground in a grid pattern to measure background levels of natural radioactivity that can be used to map different types of surface rocks and soil. The data will be used to provide state of the art subsurface maps that will contribute to a wide range of 3D representations of both exposed and concealed geology.
Further reading
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