Magnetic surveying is a widely used and cost-efficient remote sensing method for the detection of subsurface structures at all scales. Traditionally, magnetic surveying has been conducted as ground or airborne surveys, which are cheap and provide large-scale consistent data coverage, respectively. However, ground surveys are often incomplete and slow, whereas airborne surveys suffer from being inflexible, expensive and characterized by a reduced signal-to-noise ratio, due to increased sensor-to-source distance. With the rise of reliable and affordable survey-grade Unmanned Aerial Vehicles (UAVs), and the developments of light-weight magnetometers, the shortcomings of traditional magnetic surveying systems may be bypassed by a carefully designed UAV-borne magnetometer system. Here, we present a study on the development and testing of a light-weight scalar field UAV-integrated magnetometer bird system (the CMAGTRES-S100). The idea behind the CMAGTRES-S100 is the need for a high-speed and flexible system that is easily transported in the field without a car, deployable in most terrain and weather conditions, and provides high-quality scalar data in an operationally efficient manner and at ranges comparable to sub-regional scale helicopter-borne magnetic surveys. We discuss various steps in the development, including (i) choice of sensor based on sensor specifications and sensor stability tests, (ii) design considerations of the bird, (iii) operational efficiency and flexibility and (iv) output data quality. The current CMAGTRES-S100 system weighs ∼5.9 kg (including the UAV) and has an optimal surveying speed of 50 km/h. The system was tested along a complex coastal setting in Brittany, France, targeting mafic dykes and fault contacts with magnetite infill and magnetite nuggets (skarns). A 2.0 × 0.3 km area was mapped with a 10 m line-spacing by four sub-surveys (due to regulatory restrictions). The sub-surveys were completed in 3.5 h, including >2 h for remobilisation and the safety clearance of the area. A noise-level of ±0.02 nT was obtained and several of the key geological structures were mapped by the system.

Using Uncrewed Aerial vehicles (UAVs) to rapidly scan areas for potential unexploded ordnance (UXO) can provide an efficiency increase while minimizing detonation risks. We present a complete overview of how such mappings can be performed using scalar magnetometers, including initial sensor testing, time stamping validation, data positioning, noise removal, and source model inversion. A test survey was performed across disarmed UXO targets, during which three scalar magnetometers were towed in an airframe (“bird”) 10 m below a small (<25 kg) high speed (∼10 m/s) UAV to avoid magnetic disturbances from the UAV itself. Data were collected across ∼58 min of flight, with each sensor traversing ∼31.7 km to acquire dense data coverage across a 600 m × 100 m area. By using three individual magnetometers in the bird, UXO detection results across single-sensor data and several different multi-sensor configurations can be compared. The data obtained exhibited low apparent noise floors (on the order of tens of picoTesla) and retained a precision that enabled targeted modelling and removal of high-frequency noise with amplitudes of ±5 picoTesla. All of the different gradiometer configurations tested enabled recovery of most targets (including all major targets), although the horizontal configuration performed significantly worse in comparison.

Taking advantage of a multi-sensor (multispectral and magnetic) drone survey, we address the detailed geological mapping and modeling of a mineralization in its geological environment. We stress that these high-resolution data allow us to bridge the gap between field observations and a regional aeromagnetic survey. On the one hand, the combination of multispectral imagery with field geological observations enhances detailed geological mapping. On the other hand, the combination of field magnetic susceptibility measurement and their use in detailed to regional magnetic modeling, constrained respectively by UAV-borne and airborne magnetic surveys, allows deriving a model of the mineralization consistent across the scales. This is demonstrated in a case study in a complex polyphased magmatic-metamorphic environment on the coast of French Brittany. The target area hosts a pseudo-skarn mineralization, exhibiting an outstanding magnetic anomaly. The combination of remotely sensed and field data allows deriving a realistic conceptual and geometrical model of the magnetic mineralization in its geological environment, tightly constrained by field observations and measurements.

Abstract. Airborne magnetic surveys are an important and efficient tool for mapping the subsurface, providing insights e.g. into mineral deposits. Compared to traditional ground methods, airborne magnetic surveys offer great advantages with improved access and rapid sampling. But the cost and hassle of transporting and operating a conventional manned airborne magnetic survey system are strong impediments for its wider use. In addition, the conventional airborne systems are challenged by the need for low-altitude (&amp;leq; 80 m) surveying to detect small-scale subsurface features evident in ground surveys. Portable and compact airborne magnetic survey systems using unmanned aerial vehicles (UAVs) can not only bridge the gap between conventional airborne magnetic surveys and ground magnetic surveys but also complement magnetic surveys to fit broader geophysical applications. Therefore, developing high-quality, stable, and portable UAV-borne survey systems is of high interest to the geophysical exploration community. However, developing such a system is challenging owing to strong magnetic interference introduced by onboard electric engines and other onboard electronic devices. As a result, tests concerning the static and dynamic magnetic interference of a UAV are critical to assess the severity of the interference and can help to improve the design of the system at the early stage of development. A static experiment and two dynamic experiments were conducted to understand the characterization of the magnetic interference of our hybrid vertical take-off and landing (VTOL) UAV. The results of the static experiment show that the wing area is highly magnetic due to the proximity to servomotors and motors, but the area along the longitudinal axis of the UAV is relatively magnetically quiet. To reduce the magnetic signature, the highly-magnetic servomotors on the wings were replaced with less magnetic servomotors of a brush-less type. Assisted by aerodynamic simulations, we further designed a front-mounting solution for two compact magnetometers. Two dynamic experiments were conducted with this setup to understand the dynamic interference of the UAV in operation. The results of the dynamic experiments reveal that the strongest source of in-flight magnetic interference is mainly due to the cables connecting the battery to the flight controller and that this effect is most influential during pitch maneuvers of the aircraft.

Abstract. Using unmanned aerial vehicles (UAVs) for airborne magnetometry offers not only improved access and rapid sampling but also reduced logistics costs. More importantly, the UAV-borne aeromagnetometry can be performed at low altitudes, which makes it possible to resolve fine features otherwise only evident in ground surveys. Developing such a UAV-borne aeromagnetometry system is challenging owing to strong magnetic interference introduced by onboard electric and electronic components. An experiment concerning the static magnetic interference of the UAV was conducted to assess the severity of the interference of a hybrid vertical take-off and landing (VTOL) UAV. The results of the static experiment show that the wing area is highly magnetic due to the proximity to servomotors and motors, whereas the area along the longitudinal axis of the UAV has a relatively smaller magnetic signature. Assisted by the static experiment and aerodynamic simulations, we first proposed a front-mounting solution with two compact magnetometers. Subsequently, two dynamic experiments were conducted with the setup to assess the dynamic interference of the system. The results of the dynamic experiments reveal that the strongest source of in-flight magnetic interference is the current-carrying cables connecting the battery to the flight controller and that this effect is most influential during pitch maneuvers of the aircraft.