Magnetic Surveys with Drones (UAVs) — Key Considerations
Overview
Magnetic and other geophysical surveys have always been challenging. They have traditionally involved the use of heavy, expensive equipment in hilly & inaccessible areas. Traditionally, the options were limited to walking with a ground based magnetometer (slow) OR flying a de-magnetised helicopter (expensive).
Drones have changed the Game
In the last 5 years, drones and airborne magnetometers have changed the game. Now a traditional magnetic survey can be conducted 5x faster and 10x cheaper using a magnetometer-mounted drone. Not only is the survey faster and more affordable, it results in higher resolution data collection as the drone (UAV) can be flown much closer to the surface as compared to a helicopter.
Above all, drone magnetic surveys offer a safe and reliable way of acquiring geophysical data even in the most inaccessible terrain.
Now that we understand the benefits of using a drone over a helicopter or ground surveys, let’s have a look at some of the key considerations to bear in mind while conducting these surveys.
Choosing the Right Equipment
As with any survey, it’s important to commence project planning with the expected outcomes in mind. The data you needed for your specific application will determine a number of hardware and software choices for the survey:
1. Equipment: This typically involves choosing the correct drones, magnetometers, suspension systems, mobile tablets and more. We recommend starting off with the magnetometer here and building a system around that. Two commercially available high quality magnetometers are the Geometric MagArrow and the Sensys MagDrone R4.
With regards to drones, the DJI M600 remains popular however other heavy lift drones such the DJI M300, Skyfront Perimiter or Alta X Free Fly can also be used.
2. Flight Parameters & Software: Another important consideration is the choice of flight parameters and subsequently the mission planning software. A mineral exploration survey is typically flown 20–30m from the ground level with 50m flight lines and 150–200m tie lines. These would change if you’re flying a UXO survey close to the ground (1–2m AGL), and therefore choosing a flight software that can account for this is important. Our flight software, Hammer has mission mode specifically designed for magnetic surveys.
3. Processing Software: Lastly, it’s also important to consider how you’d want to process the data. Traditional geophysical processing tools such as Geosoft Oasis Montaj should work fine here, but bear in mind that the processing may have to account for the drone noise and the unique flight characteristics of the drone survey.
Once you’ve selected the right equipment for the job, it’s important to consider the operational and flight challenges of the field, and have a plan to combat them ahead of time. Let’s try to understand these challenges better.
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 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 to high, your drone may not be able to fly more than 200m below the takeoff point. So choosing the optimal takeoff location is absolutely 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.
Planning for terrain and takeoff locations ahead of time based on the target site will save multiple days of time on site.
Operational Challenges
When you are on site though, it’s the streamlined operations that will bring you the 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 1 key feature — 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.
Instead, the magnetometer is suspended from 2–3m away from the drone using cables as shown below.
Takeoff and Landing
Since the magnetometer is in a suspended configuration, the only way to safely takeoff 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 and it is important to use flight automation software that can smoothen out any sharp corners in the flight path to reduce 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 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 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 a 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 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 for 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.
Note — This limitation only applies to the DJI 600, 200 and 210 series and has been removed in the new DJI Matrice 300 series.
Data Processing & Analysis
Once the data has been captured in the best way possible, we recommend using a tool such as Geosoft’s Oasis Montaj to process and visualise the dataset. We also recommend working with a Geophysicist to eliminate noise from the dataset during processing. If you’d like to get in touch with our Geophysics partners, please feel free to contact us and we’d be happy to assist.
In Action & Results
We hope this guide provided you with an exhaustive overview of what to expect in undertaking drone-enabled magnetic surveys.
Here’s a recent magnetic survey in action conducted using the DJI M600, Geometrics MagArrow and Hammer Missions, followed by analysed results.
Summary
And that’s all for this post! Now you know how to best prepare yourself for drone magnetic survey. If you haven’t got a Hammer account as yet, and would like try Hammer, you can get started on our free trial.
To learn more about our enterprise solutions, including mission collaboration, data processing and AI solutions, please contact us at team@hammermissions.com.
We look forward to hearing from you.
- The Hammer team
