b'2.5D AEM inversionFeature2.5D Airborne Electromagnetic inversion: A review of the benefits of moving to a higher dimensionimprovements in resolution, performance and to add new features. For example, the recent introduction of a variable finite element mesh resolution in the X direction has allowed better definition of narrow conductors and improved productivity for surveys requiring higher mesh resolution.Forward modelling and joint inversion of induced polarisation (IP) has also been added in an effort to manage commonly encountered IP effects in surveys flown with the more powerful suspended loop systems such as VTEM, SkyTEM and Xcite.The softwares advantages over industry standard Conductivity Depth Imaging (CDI) and 1D inversions are:Rod Paterson Intrepid Geophysics1. The ability to handle topography and thereby remove rod@intrepid-geophysics.com topographic artefacts e.g. resistive anomalies on hills.2. The ability to model resistivity contrasts up to 1 million to 1.3. The elimination of non-geological pant-leg conductors by Introduction being able to handle strong vertical/lateral discontinuities. Pant-legs are typical of 2D effects in a 1D inversion. For Intrepid Geophysics began operating a 2.5D Airborneexample, a resistor is imaged beneath a conductive pant-leg Electromagnetic (AEM) inversion service in early 2016, followingcentred above a vertical conductor. The 1D assumption is two years of software development by Intrepid Geophysics andinadequate for imaging the 3D structure as the horizontal-Jovan Silic, the primary EM software modelling and inversiondipole behaviour cannot be explained by a 1D conductivity algorithms developer. The software was developed to facilitatestructure (Oldenburg et al. 2019).targeting of ore bodies, mapping of geology and geological4. Prediction of geologically reasonable fold structures that structures, and the location of water aquifers for explorationare accurate for dips greater than 20. 1D is only reasonably and development purposes. accurate for dips up to about 20 over extensive (large The 2.5D AEM inversion software makes use of Intrepids 3Dconductors) without significant lateral conductivity GeoModeller as a user interface, a visualisation tool and forvariations. 1D inversions over an approximate 1D horizontal creating forward models, but runs as a separate MPI applicationlayer with significant lateral conductivity variations will not using control and batch files. The inversion results are loadedonly produce artefacts, but can place conductive features at into GeoModeller for visualisation and QC. In addition, thedepth where there are none.results add value to 3D GeoModellers implicit geological5. Joint inversion of X and Z components linked by a full vector modelling package, which already contains a stochastictreatment of Maxwells equations.plugin for magnetic and gravity inversion. As such, resistivity/ 6. Joint inductive and IP (chargeability, time constant and conductivity and chargeability join magnetic susceptibilityfrequency effect) inversion.and density as physical rock properties that can be modelled,7. The ability to constrain inversions using a resistivity reference conforming with Intrepids overall philosophy of derivingmodel based on known or hypothesised geology.rational geology from geophysics. In a non 1D geological scenario, it would be preferable to invert The 2.5D software is a substantial rewrite and parallelisation ofthe data in 3D and remove the assumption of geological strike the original CSIRO/AMIRA project P223, ArjunAir code, and iswith respect to flight direction. However, this is not always a now named Moksha. The rewrite implemented a new adaptivepractical option for the following reasons:solver and forward model (Silic et al. 2015). For reference,1. 3D at a survey scale will not be valid at wider line spacing AMIRA P223 ran with strong industry support from 1981 untilwhen lateral continuity is not assured.2008, a period of 27 years. The partially finished software was2. 3D is invariably an under-determined inversion problem released into the public domain in 2010. with more unknowns than data points. Thus, it requires Advantages of the 2.5D application compared to the 1D are thatthe imposition of extra conditions or assumptions on the it can model topography and irregular subsurface structuresinverted model.where the structure along strike is assumed to be constant for a3. 3D solves for millions of unknowns requiring large compute geo-electric distance greater than the AEM 3D source footprint.resources so that inverting at high spatial resolutions The computation is based on the response of a 2D model to abecomes very expensive in both time and cost.3D source (hence the 2.5D descriptor) and can be applied to2.5D is a good compromise as it uses a numerical 3D structures whose conductivity precludes the spread of theimplementation of Maxwells equation in an over-determined source wave beyond the 2D region during the time range of thesystem. This is in contrast to the 3D system of equations data (Raiche et al. 2008). being under-determined as described in the second point While the 2.5D module has been fully operational sinceabove. The 1D class of solvers largely ignores the horizontal 2016, the software is being continually refined to delivervariations.33 PREVIEW AUGUST 2020'