Picture credit: Geophysique TMC
Introduction
The integration of cutting-edge technology has transformed geophysical exploration. Drone magnetic surveys have emerged as a game-changer, offering unparalleled precision, enhanced efficiency and cost-effectiveness. Compared to traditional methods (ground-based surveys and airborne techniques using helicopters/aircraft), drone technology overcomes limitations in accessibility, accuracy and even cost. This breakthrough technology reshapes the landscape of geophysical exploration, enabling better mapping of subsurface structures and insights into the Earth’s composition.
What is Drone Magnetic Survey?
Aeromagnetic/airborne/drone magnetic geophysical surveys are the determination of the intensity of the geomagnetic field to detect local magnetic variations in rocks. The resulting airborne magnetic data is often used to support the mapping of geological formations, their influencing anomalies and faults to directly detect any magnetic mineral concentrations.
There are many different types of airborne magnetic geophysical surveys: magnetic, radiometric, electromagnetic, gravimetric sensors are fixed to helicopter, airplane or drone platforms to take measurements at a given meter. These surveys make it possible to effectively cover large areas that may be inaccessible or even hazardous without the need for any line-cutting, thus significantly reducing overhead costs, and the costs are lower than any geophysical survey land survey for a large area. However, the resolution they provide is many times higher than terrestrial geophysical surveys.
This method can be applied in mineral exploration, archaeology, environmental assessments and infrastructure monitoring.
What are the advantages of Drone magnetic survey?
Drone magnetic survey, compared to terrestrial geophysics offers several benefits:
Accessibility: Drones can easily navigate challenging terrains such as dense forests, mountainous regions, and bodies of water, allowing access to areas that would be difficult or impossible for ground crews.
Precision and Efficiency: Drones fly at lower altitudes (often 5-50 meters above ground level) compared to traditional airborne surveys, resulting in higher resolution data collection that is five times clearer than conventional methods.
Cost-Effectiveness: The operational costs of drone surveys are significantly lower than those of manned aircraft or ground-based surveys due to reduced labor requirements and faster data collection times.
Environmental Impact: Drone surveys minimize disruption to ecosystems by requiring less ground clearance and leaving a smaller footprint compared to traditional methods.
Safety: Utilizing drones reduces the need for personnel to work in hazardous environments, thereby enhancing safety during data collection.
What are the Challenges related to drone magnetic survey?
Site Challenges
Hilly Terrain
Magnetic surveys are rarely conducted over completely flat terrain. An important consideration here is to understand the topology of the target site beforehand and acquire high-resolution elevation data to study the site. This elevation data can be procured in the form of Digital Elevation Models (DEMs) or Digital Surface Models (DSMs). These are typically raster images representing a site or an area and allow you to visualise the site elevations in a GIS visualisation utility such as QGIS or ArcGIS. The same dataset can also be used to automate the UAV’s flight path so that it follows the terrain at the target site, maintaining a safe distance from the ground whilst ensuring the highest quality data.
Takeoff Locations
Another challenge in pre-flight planning is to accurately pick the most optimal takeoff locations for the drone. This is particularly important if you are flying in hilly terrain as it can be hard to maintain visibility of the drone at all times if you don’t pick a high enough location to takeoff. Conversely, if you choose a location too high, your drone may not be able to fly more than 200m below the takeoff point. So choosing the optimal takeoff location is key. These locations also govern how efficient your flight will be – particularly if you are surveying a large area that has been split into multiple smaller blocks.
Operational Challenges
When you are on-site, though, the streamlined operations will bring you to cost savings. Armed with the right equipment, terrain data and carefully planning, it’s time to consider the main operational challenges.
Drone Configuration
Magnetic surveys with a drone can be particularly challenging because of 1 key feature – the integration of the drone and the magnetometer. Being a special purpose magnetic survey, it is often not possible to rigidly attach the magnetometer to the body of the drone (as is often the case with other payloads). Attaching the magnetometer close to the drone would mean introducing the drone’s magnetic signature into the captured dataset, thereby introducing noise and reducing the data quality significantly.
Takeoff and Landing
Since the magnetometer is in a suspended configuration, the only way to safely take off and land the drone is under manual control. This would typically involve landing the sensor first, moving the drone to the side and then landing the drone safely away from the sensor. It is important to understand that manual control is only required during takeoff and landing. The survey itself should be conducted with flight software that automates the flight path for high-precision data capture.
Suspended Configuration
The suspended configuration also comes with a whole host of flight challenges. Flying in too windy conditions is a no-go. It is important to use flight automation software that can smoothen out any sharp corners in the flight path to reduce the pendulum swing of the magnetometer. This is something hammer can help with. Simply press a button in the mission planning settings and the generated flight path will automatically be smoothened out for the best results.
Mission Planning Challenges
Terrain Awareness
When it comes to mission planning, one of the first questions to ask is – does your mission planning software support terrain awareness? More importantly, does it support terrain awareness with imported Digital Elevation Models (DEMs) or Digital Surface Models (DSMs).
With magnetic surveys, it is inevitable to find targets in hilly terrain. In these scenarios, the best approach is to purchase digital elevation models or digital surface models of the target area and import them into your mission planning software. The software needs to work completely offline and correctly calculate the flight path by taking the terrain into account.
Area Splitting
In the case of surveying a large area (1sq. km+), you would find that due to battery constraints on the drone and the regulatory constraints on the flight envelope (500mx500m in most countries), you would have to split the large area into smaller blocks and survey them separately. Over here, it’s important to have enough overlap between the smaller blocks and identical flight parameters so that the results can be later stitched together into a single picture. You can do this very easily in hammer by selecting the total area and splitting it into small squares of the same size.
Hammer also ensures that when you change the flight parameters in one block, they are automatically copied over to all blocks so that all flights have the same flight parameters (line spacing e.g.) for easy stitching later.
Flight Line & Tie Line Spacing
As with a traditional magnetic survey, it is important to understand the geophysical signature of the target, the capabilities of the magnetic sensor and therefore calculate the best line spacing for your project. The same applies to tie line spacing. We recommend working with a geophysicist to understand the best spacing for your project. Once you have an understanding of the line spacing, you can input the spacing as a parameter into the flight software.
Onboard Waypoints
An important consideration while flying large scale magnetic surveys is to understand the limitations of the drone used. For instance, the DJI Matrice 600, popularly used in magnetic surveys has an onboard limitation of 99 waypoints. A waypoint is just a 3D point used to create the flight path. This means that after travelling through 99 waypoints, the drone will stop, and wait to receive the next batch of 99 waypoints from the flight software. How the flight software handles this transition between waypoints is critical to the overall success of the mission. In Hammer, you can set pre-configured options on what to do once 99 waypoints have been executed by the drone. For hilly areas, we recommend setting the drone to come back home so that the next batch of 99 waypoints can then be uploaded.
Conclusion
To sum up, drone magnetic survey is a great tool for geophysical exploration as it enhances accuracy and is cost-effective. Compared to terrestrial geophysics, drone magnetic survey helps with precision. Some challenges in applying drone magnetic survey can be carried out and they are related to site, operational and even mission planning so you must take care of those parameters when planning a magnetic survey using drones.
Sources:
https://www.sagtechgeophysics.com/en/drone-magnetic-surveys/
