The interior of Earth in 3D

Special to the Reporter from the European Space Agency

As a child, Tommaso Santagata, speleologist and expert in 3D cave mapping, would have never imagined that he would test some of the newest and most innovative 3D-scanning technologies for future space exploration. In 2017, he spent five days of intense mapping in Lanzarote, Spain, during the PANGEA-X (Planetary ANalogue Geological and Astrobiological Exercise for Astronauts) an European Space Agency (ESA) Spaceflight Analog field campaign. While the first results from that pioneering course keep flowing in, the adventurous speleologist has produced the largest 3D scan of a lava tube on Earth.

Along with geologists Umberto Del Vecchio and Marta Lazzaroni, he mapped most of the lava tube system as part of a project supported by the Cabildo of Lanzarote and the University of Padova, Italy. The resulting map comes alive in great detail, helping local institutions to protect this subterranean environment. It also provides scientific data to study the origins of the tube and its peculiar formations.

The terrain

The PANGEA-X expedition ventured into the “La Cueva de los Verdes” lava tube in the Spanish island of Lanzarote, one of the world’s largest volcanic cave complexes with a total length of about 8 kilometres. The cave has both dry and water-filled sections.

The 6-km dry portion of the lava tube has natural open skylights, or jameos as the locals call them, that are aligned along the cave pathway. Some of the caves are large enough to accommodate residential streets and houses.

These formations are similar to those found on Mars and on the moon. Being underground structures, they offer good shelters from radiation. This similarity makes Lanzarote a great environment to train astronauts and simulate space exploration.

Why 3D mapping inside a cave?

When a new environment is discovered, mapping the area is always a first starting point of exploration. This also applies to missions to other planets, where one of the main objectives will be to choose places to setup base camp.

Lava tubes are environments with a constant temperature, shielded from cosmic radiation and protected from micrometeorites, providing safe habitats for humans.

Precisely measuring the geometry of lava caves will allow scientists to improve their models and better understand their evolution on other celestial bodies.

For these reasons, learning how to map lava tubes on Earth is helping exploration off Earth. ESA astronaut, Matthias Maurer, joined the expedition to test two different instruments developed by Leica Geosystems, the Leica Pegasus:Backpack and the Leica BLK360.

Mobile mapping

Leica Geosystems mobile mapping team trained Maurer on how to operate the Pegasus:Backpack in just 20 minutes.

The astronaut walked through the difficult terrain and checked the results on the spot through a tablet. He performed his cave-mapping mission by walking along the tube and back to compare the accuracy of the data.

“Hiking and performing geological mapping with the high-tech backpack was easy and efficient. I can perfectly see it integrated in our spacesuits for future exploration missions to the Moon or Mars,” said Maurer.

The Pegasus:Backpack synchronises images collected by five cameras and two 3D imaging LIDAR profilers, the laser equivalent of radar. It enables accurate mapping where satellite navigation is unavailable, such as in caves.

The missions

The team did two different acquisitions with the Pegasus:Backpack to test all the positioning technologies embedded in this solution. Both missions were processed with the Leica Pegasus Manager software.

Starting from the outdoor with good GNSS conditions then going indoor in challenging GNSS conditions with very low or zero satellites coverage and finishing the mission outdoor with good GNSS conditions. For this type of mission, the team used multiple positioning technologies: GNSS + Inertial Measurement Unit (IMU) + SLAM. The processing software recognised automatically the different phases of the mission.  

The Pegasus:Backpack, the first position-agnostic solution, could track Maurer’s movements during the data acquisition, and the IMU recorded them 125 times per second. This way, the team obtains a first good trajectory with greatest accuracy at both the beginning and the end of the mission. The team needed to re-enforce the calculation for the part with zero satellites coverage using SLAM. At this stage, no pictures or point clouds are created. The part of the mission without any GNSS information used the trajectory obtained in the previous step as an input value to process the SLAM algorithm. The result is an improved trajectory with an estimation of the positioning error where point clouds, pictures orientation and spherical views are generated.

A pure SLAM mission is typically a mission in GNSS-denied environments, like buildings, caves and tunnels.  The main positioning sensors used for this type of mission are the compass, the IMU and SLAM Only LiDAR (So LiDAR). Putting the parameters correctly, the complete mission could be processed in one single click. A basic trajectory of the Pegasus:Backpack was processed using information from the compass and the IMU. The complete mission uses this first trajectory as an input value to process the SLAM algorithm. Point cloud and pictures orientation and spherical views are generated with this trajectory.

3D laser scanning

In Lanzarote, lava tubes generally develop along tunnels on different levels due to lava flowing over multiple eruptions and following cracks and crevasses left from previous eruptions. It is not always possible to access the upper levels without climbing equipment.

As part of the CAVES 2016 training course, the team used photogrammetry – getting precise measurements and 3D data from at least two photographs – as a good alternative. However, photogrammetry cannot always guarantee good results, especially without the right light conditions.

To solve these problems, the PANGEA-X campaign tested the BLK360, the smallest and lightest imaging scanner on the market. The Leica Geosystems team operated it in set positions, obtaining 360° images of the environment in just three minutes by pressing one button and aligning the scans directly through a tablet app.

In less than three hours, data from both instruments obtained a complete 3D model of a 1.3 km section of the lava tube.

The PANGEA-X campaign used two of the latest Leica Geosystems technologies for a demanding mission. Both technologies provided valuable information and accurate data to map areas in a short period of time where satellite navigation was unavailable.

A version of this story first appeared in the European Space Agency blog.