The influence of multidimensional structures on the interpretation of LOTEM data with one-dimensional models and the application to data from Israel

The influence of multidimensional resistivity structures on inversion results of Transient ElectroMagnetic (TEM) data using one-dimensional (1-D) models is examined. The investigation is focused on the Long-Offset TEM (LOTEM) method. Synthetic data for the electric and magnetic field components typically measured at LOTEM receiver sites are calculated for a wide variety of models. These transients are inverted with a 1-D smooth model inversion algorithm. The inversions results do not exactly reproduce the original model. An attempt to reduce the model distortions caused by the multidimensional structures with different regularisation schemes fails. Statistically, the new regularisation schemes reduce the roughness of the models slightly, which will suppress artificial structures. In general, however, the geological interpretation will not be changed. The inversion of synthetic data sets shows that the different LOTEM-components provide different information with respect to the multidimensional structure. The 1-D models resulting from automated inversions thus may be inconsistent for the different components. In most cases meaningful information about the underlying model can be derived from the pseudo-sections of composed 1-D models. However, the inversion results do not only reflect the subsurface close to the receiver (Rx) but also the subsurface close to the transmitter (Tx) and between Rx and Tx. In the best cases, the inversion results thus reflect the gross resistivity structure of the subsurface. Comparing the TEM results to the selective information from a borehole therefore might show some discrepancies. The LOTEM results will also be different to results for the SHort-Offset TEM (SHOTEM) modification, as the volume of the subsurface contributing to the induction process differs significantly from the one for the LOTEM setup. The inconsistencies of the models for the different components causes a problem. The various models derived from different TEM-components at one single position have to be merged into one consistent geophysical model. A common approach would be to invert all data sets jointly in one inversion. However, this approach turned out to be dangerous as the so-called joint-inversions of different LOTEM-components tends to produce artifacts instead of suppressing them. A new joint-inversion approach is presented. Here, variations between the models for the different data sets are allowed to account for inconsistencies in the transients. The regularisation of the inversion favours results, where the variations between the models for the different data sets are small. The resulting pseudo-sections in general show far less spurious structures than for the pseudo-sections derived from the standard joint-inversions. In addition, the different models allow to analyse the inconsistencies and to estimate the reliability of a certain feature. Further it is shown that the data distortion by certain inhomogeneities close to the Tx can be accounted for by introducing additional parameters to the inversion process. This way pseudo-sections can be derived from the data sets which reproduce the gross resistivity structure very well for almost all data sets. The new regularisation scheme, the new joint-inversion algorithm and the additional distortion parameters are used to interpret data sets from a field campaign in northern Israel. The objective of the project is to derive the distribution of saline water in the subsurface. This knowledge would help to determine the source of saline water currently flowing into the Sea of Galilee and degrading its water quality. The new interpretation scheme proves to be very helpful to derive meaningful resistivity models from the measured data sets. The analyses of the data shows that some stations have to be excluded from the interpretation. Nevertheless, a cross-section between the Mediterranean and the Sea of Galilee is derived from the TEM data sets. It supports the hypothesis that saltwater from the Mediterranean plays an active role in the salination of the Sea of Galilee.