  Small-loop frequency-domain electromagnetic (FDEM) devices measure a
secondary magnetic field caused by the application of a stronger primary
magnetic field. Both the in-phase and quadrature component of the secondary
field commonly suffer from systematic measurement errors, which would result in
a non-zero response in free space. The in-phase response is typically strongly
correlated to subsurface magnetic susceptibility. Considering common
applications on weakly to moderately susceptible grounds, the in-phase
component of the secondary field is usually weaker than the quadrature
component, making it relatively more prone to systematic errors. Incorporating
coil-specific offset parameters in a probabilistic inversion framework, we show
how systematic errors in FDEM measurements can be estimated jointly with
electrical conductivity and magnetic susceptibility. Including FDEM
measurements from more than one height, the offset estimate becomes closer to
the true offset, allowing an improved inversion result for the subsurface
magnetic susceptibility.
