Geoelectrical techniques are widely used to monitor groundwater
processes, while surprisingly few studies have considered audio (AMT)
and radio (RMT) magnetotellurics for such purposes. In this numerical
investigation, we analyze to what extent inversion results based on AMT
and RMT monitoring data can be improved by (1) time-lapse difference
inversion; (2) incorporation of statistical information about the
expected model update (i.e., the model regularization is based on a
geostatistical model); (3) using alternative model norms to quantify
temporal changes (i.e., approximations of l(1) and Cauchy norms using
iteratively reweighted least-squares), (4) constraining model updates to
predefined ranges (i.e., using Lagrange Multipliers to only allow either
increases or decreases of electrical resistivity with respect to
background conditions). To do so, we consider a simple illustrative
model and a more realistic test case related to seawater intrusion. The
results are encouraging and show significant improvements when using
time-lapse difference inversion with non l(2) model norms. Artifacts
that may arise when imposing compactness of regions with temporal
changes can be suppressed through inequality constraints to yield models
without oscillations outside the true region of temporal changes. Based
on these results, we recommend approximate l(1)-norm solutions as they
can resolve both sharp and smooth interfaces within the same model. (C)
2012 Elsevier B.V. All rights reserved.