Stability Requirements for Long-Term Monitoring Studies Based on TEM

Recent advances in transient electromagnetic (TEM) instrument development now allow the study of long-term dynamics in groundwater systems using semi-permanently deployed instrumentation, automated for regular acquisition overextended periods. This new application shows great promise to inform on highly dynamic groundwater systems. However, the change in the TEM signal due to dynamic hydrogeological processes is generally small (a few percent of the measured signal), which demands a high level of instrument consistency to avoid biasing trends in the measured data. Thus, it is imperative to understand the stability requirements that TEM instruments must have if they are to be deployed in long-term time-lapse studies. Here, a workflow is proposed for monitoring stability in TEM instruments that are meant to be used in monitoring studies. The workflow includes tracking of peak current and waveform variation during data acquisition and estimation of how observed variation migrates into changes in the data and model space. A range of synthetic studies are examined to gain insights into the potential errors in the data space and in the model parameters after inversion based on field data acquired using a fully dedicated monitoring TEM system. A discussion of the impact of the various effects is included based on which we conclude that the changing waveform during the data acquisition mainly impacts the early time gates. However, the errors can be minimized by creating a mean waveform, which is created by measuring a suite of individual waveforms during the data acquisition process.