Time-lapse resistivity surveys are used to monitor changes in the subsurface. In some situations, it is expected the resistivity will only decrease (or vice versa) with time. The 4-D ERT inversion technique includes a temporal smoothness constraint to ensure that the resistivity changes in a smooth manner with time. However, it does not directly constrain the direction of the temporal changes in the resistivity. In some cases, the time-lapse models might show an increase in the resistivity with time in parts of the inverse model where it is expected to only decrease based on other information. We modify the 4-D ERT inversion method to remove this artefact. We first use the standard 4-D ERT inversion algorithm to generate an initial model. If the resistivity is expected to decrease with time, for the model cells that show a resistivity increase a truncation procedure is used where the resistivities of the different time models are reset to the mean value. The method of transformations is then used to ensure that the resistivities of the later time models are always less than the first model. The constraints can be applied to selected regions in the model in cases where additional information is available.


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