Asymptotic Expansions for the Calculation of the Transient Electromagnetic Fields Induced by a Vertical Magnetic Dipole Source Above a Conductive Halfspace

We have derived asymptotic expansions that can be used to estimate the impulse response of a conductive halfspace when excited by an airborne electromagnetic system. Two expressions are required to calculate the response, one is applicable in the late-time regime and a second is used for the early-time regime. Fortunately, there is a small overlap in the late- and early-time regimes. Asymptotic expansions are commonly used for interpreting ground EM data, but our expressions are the first ones applicable to the airborne situation. The expansions are simple to program, can be calculated extremely rapidly and will be useful for understanding and interpreting airborne electromagnetic data. As an example of the use of the expressions, we have calculated the response of seawater and compared this with the airborne electromagnetic response measured off the coast of Perth, Western Australia.