Comparing resistivity models from 2D and 1D inversion of frequency domain HEM data over rough terrains : cases study from Iran and Norway

• Frequency domain helicopter-borne electromagnetic (FHEM) surveys have been used as aneffective tool for the exploration of underground resources for as long as airborne electromagnetics(AEM) has existed. Large FHEM data sets are commonly interpreted with very fast 1Dinversion algorithms that often numerically adequately fit the data sets, even though they yieldincorrect results near 2D/3D geological structures. The present study aims to compare 1D and2D inversion algorithms when applied to the reconstruction of geologically complex regions.Wehave developed a 2D inversion algorithm incorporating the Levenberg–Marquardt least-squaresapproach regularised through spatial constraints to retrieve 2D electrical resistivity models associatedwith arbitrary surface topography. The approach uses a 2D finite element frequencydomain solution and a tailored triangular meshing algorithm based on the Ruppert’s Delaunayrefinement for the forward modelling. We illustrate how rough topographic effects obscure theFHEM response and affect recovered resistivity models through numerical experiments. We alsodemonstrate the influence of acquisition frequency and resistivity structure on the topographiceffect. In the Appendix, we discuss the FHEM footprint concept from a 2D perspective to assesshow 2D effects affect and bias 1D inversion results. Complex 2D synthetic scenarios are presentedto compare 1D and 2D inversion in various settings. Two field cases from Norway andIran are presented to show the model improvements with 2D inversion. For the Norwegian case,the 2D FHEM inversion aligns well with a model retrieved from ground-based electrical resistivitytomography. We show the bias imposed on the 2D inversion of the data set from Iran byimproper system calibration.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geofysik (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geophysics (hsv//eng)

Nyckelord

AEM

HEM

frequency domain

2D

numerical modelling

inverse theory

electrical properties

Geofysik med inriktning mot fasta jordens fysik

Geophysics with specialization in Solid Earth Physics

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