Magnetic Surveys With Unmanned Aerial Systems: Software for Assessing and Comparing the Accuracy of Different Sensor Systems, Suspension Designs and Compensation Methods
Creators: name: Kaub, Leon
name: Keller, Gordon
name: Bouligand, Claire
name: Glen, Jonathan M. G.
Publisher: name: FID GEO
Year: 2021

description: A typical problem for magnetic surveys with small Unmanned Aerial Systems (sUAS) is the heading error caused by undesired magnetic signals that originate from the aircraft. This can be addressed by suspending the magnetometers on sufficiently long tethers. However, tethered payloads require skilled pilots and are difficult to fly safely. Alternatively, the magnetometer can be fixed on the aircraft. In this case, aircraft magnetic signals are removed from the recordings with a process referred to as magnetic compensation, which requires parameters estimated from calibration flights flown in an area with magnetically low‐gradients prior to the survey. We present open‐source software fully written in Python to process data and compute compensations for two fundamentally different magnetometer systems (scalar and vector). We used the software to compare the precision of two commercially available systems by flying dense grid patterns over a 135 × 150 m area using different suspension configurations. The accuracy of the magnetic recordings is assessed using both standard deviations of the calibration pattern and tie‐line cross‐over differences from the survey. After compensation, the vector magnetometer provides the lowest heading error. However, the magnetic field intensity recovered with this system is relative and needs to be adjusted with absolute data if absolute intensity values are needed. Overall, the highest accuracy of all suspension configurations tested was obtained by fixing the magnetometer 0.5 m below the sUAS onto a self‐built carbon‐fiber frame, which also offered greater stability and allowed fully autonomous flights in a wide range of conditions.
description: Plain Language Summary: Mapping the strength of the Earth's magnetic field is widely used for imaging the subsurface in geophysical exploration, detecting unexploded ordnance, and investigating archaeological sites. Small Unmanned Aerial Systems (sUAS), commonly referred to as drones, offer fast, flexible, and affordable aeromagnetic surveys. The main challenge in using sUAS in magnetic surveys is the magnetic signals generated by the sUAS itself, which may swamp out natural geologic signals. In some setups the sensors are attached to the sUAS with long tethers, which are difficult to fly safely. We developed a compact system in which the sensor is attached to the aircraft with a 0.5 m carbon‐fiber frame. We correct the magnetic signals caused by the sUAS using a process called magnetic compensation. This requires specific calibration maneuvers performed at the beginning of a survey. We developed software for processing the magnetic data and computing the magnetic compensation. We tested this configuration with two magnetometer systems that are commercially available and compared it to a tethered configuration and a setup where the sensor systems were fixed to the landing gear of the sUAS. The fixed‐frame system combined with compensation offers higher flight stability and more accurate magnetic recordings than the other configurations.
description: Key Points: This study compares the accuracy of Unmanned Aerial Systems (UAS) scalar and vector magnetometer systems with different suspension designs and compensation methods. We provide software to process, perform magnetic compensation, and assess the accuracy of UAS scalar and vector magnetometers data. A fixed frame suspension method combined with magnetic compensation offers flight stability and highly accurate magnetic recordings.
description: DOI, U.S. Geological Survey (USGS) http://dx.doi.org/10.13039/100000203