First European Airborne Electromagnetics Conference

This paper focuses on the sampling of the electromagnetic transient as acquired by airborne TDEM systems.

Typically, the sampling of the electromagnetic transient is done using a fixed number of gates whose width grows logarithmically (log-gating). The log-gating has two main benefits: improving the S/N ratio at late times, when the electromagnetic signal has amplitudes equal or lower than the natural background noise, and at same time ensuring a good resolution at the early times.

We propose a new sampling method, adapting the gating on the base of the slope variations in the EM transient.

Using synthetic models, we show that the adaptive-gating scheme improves both accuracy and precision of the estimated parameters of the inverse models.

The advantages of 2.5D airborne electromagnetic inversion in 3D geological mapping applications compared to the more commonly used CDI transforms or simple 1D inversions are described using an example from the Bryah Basin in Western Australia.

We demonstrate this using a substantially rewritten version of ArjunAir (Wilson et al, 2006), a product of the CSIRO/AMIRA consortium (project P223F).

The ArjunAir inversion solver has been replaced with a new GSVD (Paige et al, 1981) solver, with adaptive regularisation which also incorporates a misfit to the reference model and a model smoothness function.

The ArjunAir forward modelling code has been revised to fix two errors which manifest at late times around high resistivity discontinuities and in steep topography.

We allow the use of a starting or reference geology/resistivity model to influence the inversion. The software has been parallelised using Intel MPI.

The software is implemented in a commercial 3D geological modelling package with an intelligent graphical user interface for inversion setup, for introduction of geological reference models and for visualising results.

Apparent Resistivity, 2.5D Forward and 1D and 2.5D Inversion methods are integrated in a single 3D geological, electromagnetic and potential field (gravity and magnetics) forward and inverse modelling environment.

Tellus airborne geophysical data clearly maps structural complexities and assists in the identification of mineral potential areas in Ireland. Complex detail is mapped particularly well when both EM and magnetic data are interpreted together. The EM lineaments often relate to faulting and / or shear zones running parallel to the bounding faults with gold and base metal occurrences commonly associated with these structures. These conductive structures are oriented dominantly in NE-SW and some in N-S. A comparison of gold and base metal occurrences with these lineaments at known places suggest that they are controlled by these lineaments. Mapping of these lineaments based on the geophysical data will help to identify new target areas for mineral potential

The presented study is part of a series of international research cooperations, started in the year 2007 and still ongoing. The study area is located at the east coast of the Yucatan Peninsula, Mexico, and comprises part of the Sian Ka’an biosphere reserve and the town of Tulum, Quintana Roo. Three airborne surveys, conducted in 2007, 2008, and 2015 by the Geological Survey of Austria, covered an area of some 300 square kilometres including cave systems already mapped by exploration divers. First processing of airborne electromagnetic data revealed a clear signature from known caves but furthermore the picture of a vast unexplored hidden conduit network. However, lateral and depth resolution was limited due to measurement drift and noise as well as due to specific behaviour of the applied inversion technique. Newly developed algorithms for careful post processing of the AEM data and inversion results improve the signal to noise ratio significantly and enable the imaging of well defined structures in the underground. So, the AEM-method is proved its ability to quickly deliver crucial structural information of karst water regimes over a difficult accessible area and with unique depth information compared with previous results.