Introduction

Welcome to the Exploration Geophysics Special Edition for the 7th International Workshop in Airborne Electromagnetics (AEM 2018) held at the historic Hotel KoldingFjord, southern Jutland, Denmark. The conference followed on from the previous workshop on AEM held in South Africa (AEM 2013). The event was organised by the Hydrogeophysics Group of Aarhus University and chaired by the Organising Committee consisting of Professor Esben Auken, Associate Professor Anders Christiansen, Jesper Pedersen, and Karen Dalsgaard. Assisted by the Scientific Committee (Esben Auken, Anders Vest Christiansen, Gianluca Fiandaca, Hongzhu Cai, Andreas Aspmo Pfaffhuber, Jared Abraham, Aaron Davis, Paul Bedrosian, Mehrdad Bastani, Jan Gunnink, Pierre-Alexandre Reninger, Bernhard Siemon, Yusen Cooper, and Vikas Baranwal), the Committee selected over 90 papers and many posters for presentation over a 3-day period in June 2018. As is traditional at the closing of the AEM conferences, the location of the next International Workshop will be held in Australia. We hope to see you there.

Based on the success of the previous conference on airborne electromagnetics, I decided to punish myself once again by approaching the Organising Committee of AEM 2018 and offer my service as an organiser for another special issue in Exploration Geophysics, the journal of the Australian Society of Exploration Geophysicists. A Special Edition Committee was formed, headed by Dr Mark Lackie (Senior Lecturer in Geophysics, Department of Earth and Planetary Sciences, Macquarie University Australia; and Exploration Geophysics Editor-in-Chief), Dr Aaron Davis (CSIRO, Australia), Dr Anders Vest Christiansen (Aarhus University, Denmark), Dr Andi Pfaffhuber (NGI, Norway), and Camilla Sørensen (SkyTEM, Denmark). Together, the Committee handled reviews and revisions for the 17 submitted papers.

The edition has been a long-time coming but is hopefully worth the wait. Many of the papers have already been published online, but this is the first time they are collected together in print. We have papers covering recent advances in software and inversion, data processing, induced polarisation (IP), and case histories of applying airborne electromagnetics to geophysical problems.

Doug Oldenburg, Lindsey Heagy, Seogi Kang, and Rowan Crockett present a compelling case for open-source 3D modelling software and how sharing codes and ideas will promote the industry, make it more efficient, and allow us to solve more complicated problems (Oldenburg et al. 2019). This is supported in a companion paper by the same authors, but in a different order in Lindsey Heagy’s paper on simulations and inversions (Heagy et al. 2019).

Modelling in three dimensions will become more important, as shown by Xiuyan Ren, Jim Macnae, and Lachlan Hennesey in their paper that compares voxelated 3D inversions to stitched 1D modelling codes (Ren, Macnae, and Hennessy 2019). The complexity of 3D models can be daunting, and choices need to be made for the algorithm and meshing method. Carsten Scholl and Frederico Miorelli choose to use vertically unstructured grids in the Otze inversion codes (Scholl and Miorelli 2019).

Hossein Ali Ghari, Malte Voge, Mehrdad Bastani, Andi Pfaffhuber, and Behrooz Oskool show there is still merit in applying one- and two-dimensional models for inversion of AEM data by presenting finite element modelling on unstructured triangular meshes (Ghari et al. 2019). Modelling in one dimension is not obsolete as is shown in Evgeny Karshakov’s paper on modelling using extended Kalman filters. His results show that Kalman inversion modelling can converge rapidly to realistic results (Karshakov 2019).

Even though inversions may be the final tool that a geophysicist uses at the end of the survey, it is sometimes appropriate to use faster techniques to give insight into AEM data. This is shown by José Martínez, Richard Smith, and Danilo Díaz in their paper that discusses time constants in mineral exploration survey data (Martínez, Smith, and Vázquez 2019). After investigating AEM data for tau maps, geophysicists may wish to process their data prior to further inspection and inversion. Pierre-Alaxandre Reninger, Guillaume Martelet, José Perrin, and Marc Dumont discuss processing methods and improvements that can be made to data when local tuning and processing is introduced, particularly for the near surface (Reninger et al. 2019).

Near-surface resolution can be further improved when methods are used to incorporate time gates that are either closer to the transmitter current shut-off or are in the on-time of the transmitting waveform. Guro Skurdal, Andi Pfaffhuber, Aaron Davis, and Sara Bezin show that early time gates can improve near-surface resolution in AEM data (Skurdal et al. 2019); and Esben Auken, Nikolaj Fogel, Kristoffer Andersen, Niklas Nyboe, and Anders Christiansen demonstrate improved resolution using on-time data (Auken et al. 2019).

With advances in science come new developments and a notable one in the AEM community is polarisation induced (IP) by electromagnetic fields. Andrea Viezzoli and Giovanni Manca investigate the detection of IP in AEM systems using parameter sweeps of the Cole–Cole parameters and transfer functions of modern AEM systems (Viezzoli and Manca 2019). Seogi Kang, Doug Oldenburg, and Lindsey Heagy use stretched exponentials rather than Cole–Cole parameters to investigate IP detection in time-domain coincident-loop systems (Kang, Oldenburg, and Heagy 2019). Finally, Marco Couto, Gianluca Fiandaca, Pradip Maurya, Anders Christiansen, Jorge Porsani, and Esben Auken reparametrize the Cole–Cole model using maximum phase angle due to the correlation in the parameters (Junior et al. 2019). They demonstrate their method with a study of the Lamego gold mine in Quadrilátero Ferrífero, Brazil.

On the topic of case histories, there are papers in this edition that show the unique and expanding investigative ability of AEM to explore the subsurface. Yusen Ley-Cooper, Ross Brodie, and Murray Richardson present on AusAEM: airborne electromagnetic data collected on an unprecedented scale for the provision of pre-competitive conductivity-depth data across the Top End of the entire continent of Australia (Ley-Cooper, Brodie, and Richardson 2019). Neil Foley, Slawomir Tulaczyk, Esben Auken, Denys Grombacher, Jill Mikucki, Nikolaj Foged, Krista Myers, Hilary Dugan, Peter Doran, and Ross Virginia measure the thickness of permafrost in Antarctica and develop a heat-flow model to validate their results (Foley et al. 2019). Less exotic, perhaps, but no less importantly, Subash Chandra, Joy Choudhury, Pradip K Maurya, Shakeel Ahmed, Esben Auken, and Saurabh K. Verma apply AEM to map palaeovalleys in India (Chandra et al. 2019), while Andrea Viezzoli, Antonio Menghini, Nick Ebner, and Paul Hilliard take us back to the roots of AEM development in mineral exploration by looking for volcanic-hosted massive sulphides in Western Australia (Viezzoli et al. 2019).

Well that about covers it! In total, there are 17 papers for you to read, think about and learn from. I am glad to be part of this special edition, and happy to write the introduction to a volume that, I hope, will be useful and informative. Enjoy.

Disclosure statement

No potential conflict of interest was reported by the author.