3D inversion of an integrated ground-based and waterborne transient electromagnetic survey

Abstract

Recent instrument advancements in the transient electromagnetic (TEM) method enable waterborne applications as well as traditional ground-based surveys. We investigate a common framework to handle combined data sets from ground-based and waterborne TEM surveys under one model domain. The modeling complexity increases for two main reasons: (1) multidimensionality effects are unavoidable in data from settings with strong conductivity contrasts and (2) different systems have different sensitivity footprints which are challenging to integrate into a common domain. We address these challenges using a previously developed 3D inversion scheme: first, octree-based forward modeling is used to describe the multidimensional environment for more accurate field simulations. Second, a decoupling between the forward and inversion mesh offers the flexibility of modeling individual soundings to minimize computational costs, while allowing a commonly shared model domain for the inversion. We validate the method through synthetic and field case studies. The synthetic studies indicate that: (1) a careful forward mesh refinement is required for models with thin and highly conductive top layers; (2) compared with 3D forward responses, 1D modeling has an approximate 300% error directly over the coastline decreasing to 10% error 50 m away; and (3) the 3D inversion outperforms the 1D inversion by a lower data misfit and more accurate model reconstruction. The field case further underlines the better consistency of the 3D inversion, which delineates the lithologic transition from sand to clay and is verified by a better agreement with existing borehole data. Based on these experiments, we conclude that (1) 3D inversion is preferred over strong resistivity contrasts arising along a coastline, (2) careful mesh refinement and decoupling of the forward and inversion mesh is an efficient approach to handling computational challenges on forward while maintaining a common inversion mesh, and (3) more focus on optimization is required to realize a full-scale 3D inversion for integrated surveys in a coastal area.