## 2 Methods  
In this study, magnetic data were collected approximately 1 km offshore from Chilpo Harbor in Pohang, southeastern Korean Peninsula (Figure 1). We mounted the MagDrone R3 fluxgate magnetometer system (Sensys, Bad Saarow, Germany) on the ship, consisting of two fluxgate magnetometers originally used in drones, and capable of acquiring a wide range of magnetic field (\(-\)75,000 nT to 75,000 nT) with a high sampling frequency (up to 250 Hz) and high resolution of 0.15 nT. We target geomagnetic field data (typically ranging from 30,000 to 60,000 nT) and magnetic anomaly (\(-\)2000 to 2000 nT). The measurement platform was the PARANG-5500 Unmanned Surface Vehicle (USV) manufactured by Ocean Tech (Goyang, Republic of Korea) (Figure 2). This USV features an autonomous navigation system capable of obstacle detection and employs multiple communication methods, including 2.4 GHz Wi-Fi, 900 MHz ISM band radio, and Iridium satellite communication.  
The vessel's hull is primarily aluminum alloy, offering several advantages for magnetic surveys. First, as a paramagnetic material, aluminum exhibits weak magnetic susceptibility, significantly reducing hull-induced interference in magnetic measurements. Second, aluminum provides excellent corrosion resistance and an optimal strength-to-weight ratio for marine operations. The stable magnetic properties of aluminum ensure long-term consistency in magnetic measurements, as the hull's magnetization can be mitigated and remains stable throughout operations, unlike ferromagnetic materials that show both strong induced and permanent magnetization (see Figure S6 in Supporting Information). However, essential ferromagnetic components such as engines, generators and other equipment still contribute to the vessel's magnetic signals. These magnetic influences cannot be ignored when correcting the USV-mounted magnetic data.  
To correct for the USV's magnetization effect, we collected five circular maneuver datasets with a 10 m radius, considering the vessel size and shallow depth (20-30 m) of the geomagnetic source. We also acquired data from the same track-line eight times to confirm the consistency of the geomagnetic anomalies after the correction (Figure 1b,c).  
In this experiment, sensor 1 is positioned at the center of the mast to mitigate electromagnetic perturbations, using the mutual cancellation of magnetic fields at the bow and stern [3] and sensor 2 is positioned closer to the vessel's electric motor (see Figure 2). This setup allowed us to compare the influence of magnetic field interference by the electric motor of the USV. The data is acquired with a 250 Hz sampling rate, while vessel attitude and position were recorded using onboard IMU sensors and DGPS at 50 Hz and 10 Hz, respectively.  
Figure 1: Survey area and USV (Unmanned Surface Vehicle) track line correcting for the ship’s magnetization. The USV conducted specific rotational patterns to correct the onboard fluxgate magnetometer and compensate for the vessel’s magnetizations. (**a**) A Satellite image [27] in the study area with the survey trackline of the USV. The yellow box indicates the area where the USV conducted the figure of rotation maneuvers for the ship’s magnetization correction (**b**). The green box depicts repeated tracklines in (**c**). (**b**) Detailed view of the figure of rotation maneuvers for the correction. The black solid line shows the complete track, and blue, red and green solid lines display each figure of rotation track (correction track 1, correction track 2, correction track 3). (**c**) Map view of the repeated survey tracklines run by the USV along the same path. Each color represents a different trackline.