An efficient 2D inversion scheme for airborne frequency-domain data

Abstract

In many cases, inversion in 2D gives a better description of the subsurface compared with 1D inversion, but, computationally, 2D inversion is expensive, and it can be hard to use for large-scale surveys. We have developed an efficient hybrid 2D airborne frequency-domain electromagnetic inversion algorithm. Our hybrid scheme combines 1D and 2D inversions in a three-stage process, in which each step is progressively more accurate and computationally more expensive than the previous one. This results in an approximately 2x − 6x speedup compared with full 2D inversions, and with only minor changes to the inversion results. Our inversion structure is based on a regular grid, in which each sounding is discretized individually. The 1D modeling code uses layered models with derivatives derived through the finite-difference method, whereas our 2D modeling code uses an adaptive finiteelement mesh, and it uses the adjoint-state method to calculate the derivatives. By incorporating the inversion grid structure into the 2D finite-element mesh, interpolation between the different meshes becomes trivial. Large surveys are handled by using local meshing to split large surveys into small sections, which retains the 2D information. The algorithm is heavily optimized and parallelized over the frequencies and sections, with good scalability even on nonuniform memory architecture systems, on which it is generally hard to achieve a satisfactory scaling. The algorithm has been tested successfully with various synthetic studies as well as field examples, of which results from two synthetic studies and a field example are shown.

Quantifying Geophysical Inversion Uncertainty Using Airborne Frequency Domain Electromagnetic Data—Applied at the Province of Zeeland, the Netherlands

“…As hydrogeophysical investigations are often conducted in areas with young, horizontal or subhorizontal substrata, physical property contrasts are generally horizontally smoothly varying. Furthermore, depending on system altitude, system frequency, and subsurface EC (Tølbøll & Christensen, 2007), HEM systems have a relatively small footprint (~100-200 m); therefore, the benefits of 2-D (e.g., Boesen et al, 2018;Li et al, 2016) and 3-D (e.g., Cox et al, 2012;Scheunert et al, 2016) inversions are limited and considered to be impractical compared to the less computationally expensive 1-D techniques. As a result, this study will focus on 1-D inversion methods only.…”

Airborne electromagnetic mapping of coastal groundwater salinity

“…As hydrogeophysical investigations are oft en conducted in areas with young, horizontal or subhorizontal substrata, physical property contrasts are generally horizontally smoothly varying. Furthermore, depending on system altitude, system frequency, and subsurface EC (Tølbøll and Christensen, 2007), HEM systems have a relatively small footprint (~100-200 m); therefore, the benefi ts of 2-D (e.g., Boesen et al, 2018;Li et al, 2016) and 3-D (e.g., Cox et al, 2012;Scheunert et al, 2016) inversions are limited and considered to be impractical compared to the less computationally expensive 1-D techniques. As a result, this study will focus on 1-D inversion methods only.…”