Historically, magnetic and other geophysical surveys posed significant challenges, requiring the deployment of cumbersome and costly equipment, especially in rugged and hard-to-reach terrains. Traditionally, the options were limited to the laborious process of walking with a ground-based magnetometer (slow) or the extravagant choice of employing a demagnetized helicopter (expensive).
Transformation with Drones
Over the past five years, a paradigm shift has occurred with the advent of drones and airborne magnetometers. This technological advancement has revolutionized drone survey magnetic, allowing for a 5x increase in speed and a 10x reduction in costs by utilizing magnetometer-equipped drones. Beyond the efficiency gains, these surveys now yield higher resolution data, as drones (UAVs) can be flown much closer to the surface compared to helicopters.
The paramount advantage of drone magnetic surveys lies in their ability to provide a secure and dependable means of acquiring geophysical data, even in the most challenging and remote terrains.
Having established the superiority of drones over helicopters and ground surveys, let’s delve into some key considerations to bear in mind when conducting these surveys.
Choosing the Right Equipment
When embarking on a survey, it is crucial to initiate project planning with the desired outcomes in mind. The specific data requirements for your application will influence the selection of hardware and software for the survey:
- Equipment: This phase involves the careful selection of drones, magnetometers, suspension systems, mobile tablets, and more. We recommend starting the planning process with the magnetometer and then building a comprehensive system around it. Notable high-quality magnetometers in the market include the Geometric MagArrow and the Sensys MagDrone R4.
Concerning drones, the DJI M350 RTK remains a popular choice as a heavy-lift DJI drone and can serve the purpose effectively.
- Flight Parameters & Software:
An additional crucial factor to consider is the selection of flight parameters, coupled with the subsequent choice of mission planning software. For a mineral exploration survey, the standard practice involves flying at an altitude of 20-30m above ground level, utilizing 50m flight lines and 150-200m tie lines. However, these parameters would need adjustment for a close-to-the-ground UXO survey (1-2m AGL). Hence, it is vital to opt for flight software capable of accommodating such variations. Our proprietary flight software, Hammer, features a dedicated mission mode specifically tailored for magnetic surveys.
- Processing Software:
Finally, the choice of processing software is a critical aspect to contemplate. While traditional geophysical processing tools like Geosoft Oasis Montaj can be effective, it’s essential to note that processing may need to address specific challenges posed by drone noise and the distinctive flight characteristics of drone surveys.
Once the appropriate equipment has been selected, it becomes imperative to take into account the operational and flight challenges inherent in the field. Developing a proactive plan to overcome these challenges in advance is crucial. Let’s delve into a deeper understanding of these challenges.
Magnetic surveys seldom take place over entirely flat landscapes. An essential consideration in such scenarios is to thoroughly comprehend the topography of the designated site beforehand and procure high-resolution elevation data for a comprehensive site analysis. This elevation data can be obtained in the form of Digital Elevation Models (DEMs) or Digital Surface Models (DSMs), typically presented as raster images representing the site or area. These models enable visualization of site elevations using GIS visualization utilities like QGIS or ArcGIS. Additionally, the same dataset can be utilized to automate the UAV’s flight path, ensuring it closely follows the terrain contours at the target site, maintaining a safe distance from the ground while guaranteeing the highest quality data collection.
Another challenge during pre-flight planning is the precise selection of optimal takeoff locations for the drone. This becomes particularly crucial in hilly terrain, where maintaining continuous visibility of the drone can be challenging without choosing a sufficiently elevated takeoff point. Conversely, opting for a location that is too elevated may restrict the drone from flying more than 200m below the takeoff point. Therefore, the careful selection of the optimal takeoff location is paramount. These chosen locations also significantly influence the efficiency of your flight, especially when surveying a vast area divided into multiple smaller blocks.
Proactively planning for terrain considerations and selecting takeoff locations based on the characteristics of the target site can result in substantial time savings during the on-site operations.
While streamlined operations on-site lead to cost savings, it’s essential to address key operational challenges when armed with the right equipment, terrain data, and meticulous planning.
Conducting magnetic surveys with a drone presents a unique challenge—the integration of the drone and the magnetometer. Due to the specialized nature of magnetic surveys, rigidly attaching the magnetometer to the drone’s body (as commonly done with other payloads) is often impractical. Placing the magnetometer close to the drone introduces the drone’s magnetic signature into the captured dataset, resulting in noise and compromising data quality. Instead, the magnetometer is suspended 2-3 meters away from the drone using cables.
Takeoff and Landing
Given the suspended configuration of the magnetometer, safe takeoff and landing require manual control. This typically involves landing the sensor first, moving the drone to the side, and then safely landing the drone away from the sensor. It’s crucial to note that manual control is only necessary during takeoff and landing; the survey itself should be conducted with flight software automating the flight path for high-precision data capture.
Suspended Configuration Challenges
The suspended configuration introduces additional flight challenges, particularly in windy conditions. Using flight automation software becomes crucial to smooth out any sharp corners in the flight path, minimizing the pendulum swing of the magnetometer. Hammer, for instance, offers a solution – a simple button press in the mission planning settings automatically smoothens the generated flight path for optimal results.
Flight Line & Tie Line Spacing
Similar to a traditional magnetic survey, it is crucial to comprehend the geophysical signature of the target, understand the magnetic sensor’s capabilities, and consequently calculate the optimal line spacing for your project. This principle also extends to tie line spacing. Collaborating with a geophysicist is recommended to determine the most suitable spacing for your specific project. Once the line spacing is determined, you can input this parameter into the flight software.
When conducting extensive magnetic surveys, it is imperative to be mindful of the drone’s limitations. For example, the DJI Matrice 600, a commonly used drone in magnetic surveys, has an onboard limitation of 99 waypoints. Waypoints are 3D points used to establish the flight path. After reaching 99 waypoints, the drone will pause, awaiting the next set of 99 waypoints from the flight software. How the flight software manages this transition between waypoints is crucial for the mission’s overall success. In Hammer, predefined options can be set to determine the drone’s actions once it has executed 99 waypoints. In hilly terrains, it is advisable to configure the drone to return home, allowing for the upload of the next batch of 99 waypoints.
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.