2.5D electromagnetic (EM) modelling computes the response of a 3D source from an arbitrary 2D geoelectrical model. As such, it is practical for airborne EM (AEM) data to be inverted using 2.5D modelling provided that the geoelectrical crosssection is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries, and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic and practical frequency and time-domain AEM surveys for which inversion run-times are on the order of hours.Witherly, K. E., I~i n e . R. J., and Raiche A., 2003, The application of airborne Zhdanov, M.S.. and Tartaras, E., 2002, Inversion of multi-transmitter 3D electrornagnetics to the search for high conductance targets: 16Ih Geophj~sical electromagnetic data based on the localized quasi-linear approximation: Confemnce and Exhihition, Australian Sociey of Explorution Geophysicists,Geophysical Journal Internatio~l, 148,506-5 19. Expanded Abstracts.Zhdanov, M.S., and Chernyavskiy, A,, 2004, Rapid three-dimensional inversion of multi-transmitter electromagnetic data using the spectral Lanczos decomposition method: Inverse Pmhlems, 20, S233-S256.

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We develop an algorithm for modelling the time-domain EM response of a general 2-D geometry excited by a 3-D source. A frontal solution, isoparametric finite-element method is used to solve for the y-components of the magnetic and electric fields in the frequencywavenumber (x, ky) z, w) domain where y is the direction of strike.Step response solutions are computed for 31 frequencies over the range 1 Hz to 100 kHz and transformed into the time-domain with a digital cosine filter.The response of a 2-D simple block target in a uniform half-space is compared to 3-D models of similar cross section with strike lengths of 0.5, 1, and 2 km, computed using the compact finite-element method. Further comparisons were made with the results of a 3-D time-stepping finite-difference solution for a block on the boundary of a quarter-space. In both cases the agreement between the 2.5-D and the 3-D results is close at early delay times with the 3-D target response decaying more rapidly than the 2-D target response at later times as expected. Although the late time-amplitudes differ, the profile shapes are very similar indicating that the 2.5-D model is useful for interpreting practical exploration problems.