Dense vertical profiling of the natural magnetic field with a suspended QuSpin QTFM Gen 2 under azimuth-balanced acquisition

Andrew Musinov , Eugene Podgorbuntsev, Roman Gornov

Sensor height is part of the observation geometry of a magnetic survey. We quantified how processed, azimuth-balanced total magnetic field changed with sensor height above ground level (AGL) for a suspended QuSpin QTFM Gen 2 at four agricultural-field sites in Xã Bác Ái Tây, Khánh Hòa, Vietnam. The sensor was suspended 20 m below a geophysical drone helicopter. At each nominal condition from 25.0 to 50.0 m AGL in 0.5 m increments, the freely rotating tubular bird completed 8-12 slow yaw turns. Bird-mounted range LiDAR measured sensor AGL; a stationary QTFM Gen 2 monitored temporal variation; acquisition was nominally 60 Hz; and wind did not exceed 2 m/s. The analysed dataset contains four vertical profiles, each with 51 dependent height conditions. Relative to the nominal 35 m condition, the processed field differed by +15.8 to +26.7 nT at nominal 25 m and by -6.2 to -10.3 nT at nominal 50 m. The nominal 25-50 m response span was 22.0-37.0 nT, and the local gradient near 35 m was -0.78 to -1.42 nT/m. Linear interpolation to actual LiDAR AGL showed a maximum absolute departure of 0.8 nT within the full physical corridor 34.5-35.5 m. A deterministic inverse-cube benchmark, calculated as A35(35/h)^3 rather than regression-fitted, matched the processed anomaly with RMSE 0.043-0.100 nT. The response magnitude differed among sites, excluding one universal scalar nT/m height correction. Operationally, uncontrolled AGL variation can introduce non-geological field differences large enough to affect line acceptance, levelling and subsequent modelling, whereas physical corridor control reduces this risk at acquisition. The experiment does not directly quantify spatial continuation, map displacement or drill-target error; those require coincident multi-height survey lines or grids.