The depth of investigation (DI) in electromagnetic (EM) soundings is a maximum depth at which a given target in a given host can be detected by a given sensor. The factors affected the DI are: the sensor sensitivity, precision, operating frequencies, the techniques used in data processing and interpretation, noise level and target and host properties. In this paper, the depth of investigation for frequency domain airborne electromagnetic surveys is calculated. Simple relations are derived for the DI over layered half-space models. Results show that the depth of investigation increases if the skin depth increases, the threshold value decreases, the separation distance increases, and the resistivity contrast between host rock and target increases.