## 5 Results and Discussion
Focusing on the corrected USV data (Equation (3) applied), analysis along the entire track reveals significant variations due to residual high-frequency noise, even after the main correction (Figure 7a). This noise is particularly prominent during the vessel's turningmaneuvers and is thought to originate from vessel vibrations or attitude errors occurring during these turns. To compute the final geomagnetic anomalies suitable for interpretation, we subsequently removed spiky noise and applied an additional low-pass filter. This filtering resulted in final anomaly ranges of \(-310\) to \(340\) nT, \(-1216\) to \(1827\) nT, and \(-324\) to \(171\) nT for the X, Y, and Z components, respectively (Figure 7a, red solid line). Despite filtering, potential low-frequency discrepancies remain, particularly associated with maneuvers. To investigate consistency and residual effects, data from repeated tracklines (eight passes total) were analyzed (Figure 7b). Plotting the corrected data against latitude exhibits the generally consistent geomagnetic field throughout the transit path, confirming strong correction. However, track-line (e.g., latitudes \(36.153\)-\(36.1535^{\circ}\) N and \(36.1565\)-\(36.157^{\circ}\) N), different anomalies of the X and Y components were still observed. These residual effects are due to VM and are likely caused by eddy currents from the ambient magnetic field when the vessel turns rapidly. To prevent the generation of eddy currents, it is possible to use non-conductive materials for the hull, such as Glass Fiber Reinforced Polymer (GFRP) [21]. However, GFRP has no electromagnetic shielding effect, and it can perturb measuring magnetic fields highly susceptible to EMI from ferromagnetic parts or various equipment installed inside the hull, resulting in increased noise when measuring magnetic fields. Therefore, more complex shielding and grounding designs are required to resolve it. On the other hand, aluminum is a conductive material and it can effectively block EMI by the Faraday cage effect, resulting in simplifying the grounding design of research vessels that are equipped with a large number of precision measuring devices and increasing the stability of magnetic field measurements. Meanwhile, the magnetometer's inherent DC offset and drift are implicitly removed together during the correction process of the figure of rotation maneuvers. This is achieved by compensating for the vessel's major magnetic effects (i.e., permanent and induced magnetization), which in turn converges the overall measured values to the background magnetic field. The resulting magnetic field cannot be fully corrected by the applied method (Equation (3)) and thus remains as a low-frequency signal. Notably, the VM effect on the paramagnetic USV disappears rapidly within about 50-100 m after a turn, corresponding to a relatively short temporal relaxation (approx. 0.5-1 min at 3 knots). This contrasts sharply with conventional ferromagnetic R/Vs (e.g., [31]), where eddy current effects persist much longer after direction changes (cf. Figure 56b). To understand the accuracy of the corrected and filtered fluxgate data along track segments unaffected by VM, we compared our results with high-resolution data from a crossing towed scalar magnetometer survey (Figure S8a). The comparison shows good agreement in the overall trend and shape of the magnetic anomaly between the corrected fluxgate data and the towed scalar data (Figure S8b), confirming that relative magnetic variations are reliably captured. However, a consistent offset of approximately 200 nT exists between the absolute anomaly values from the two systems, likely reflecting differences in absolute calibration or baseline levels between the distinct measurement systems and platforms. The residual VM effect highlights that VM remains a significant challenge in shipborne fluxgate magnetic data processing even for paramagnetic platforms.