Application of 3-D Electromagnetic Inversion in Practice: Challenges, Pitfalls and Solution Approaches

In recent years, three-dimensional (3-D) inversion of electromagnetic (EM) data gained a lot of attention and the number of 3-D EM case studies has increased. Many publications discuss the challenges and advancements of 3-D inversion with respect to the numerical aspects and often show synthetic studies to prove their assumptions. On the other hand, field data have other/additional demands than synthetic data sets. There are challenges and requirements to fulfil along the entire sequence from survey planning to interpretation. To obtain a meaningful and reliable interpretation it is not sufficient to only be aware of and address issues with respect to one step along this sequence. Ideally one should be concerned with all or at least most of them, because many of these challenges are related to or even consequences of each other. Not all issues when dealing with field data can be solved, but one should at least be aware of the consequences of unavoidable shortcomings as this knowledge may be crucial for interpretation. With the intention to raise awareness, this review comprises a variety of difficulties related to the data acquisition, the numerical part—preparation for and performance of 3-D inversion—and the interpretation itself, when dealing with field data sets. The majority of published work on 3-D EM inversion of field data is related to magnetotellurics; nevertheless, there are also aspects discussed that are specific to other EM methods or illustrate different ideas to deal with challenges (e.g., airborne and controlled-source EM).