Geomagnetic sounding of conductivity anomalies in the lower crust and uppermost mantle

Knowledge of the structure of the lower crust and uppermost mantle is of special importance in understanding plate tectonics. Electrical conductivity of this region has been measured recently in various parts of the world. Transfer functions are still the most widely used quantity in data analysis and model fitting. Anomalies in the horizontal magnetic components in combination with anomalies in the vertical component have been found very useful in locating conductivity contrasts. With these, when the cause of the anomaly is a concentrated line current, both the position of the line current and its depth can be directly located. The method of hypothetical event analysis is another new technique and this is highly suited for areas having complex subsurface geology or areas under non-uniform source fields or both. The anomaly is more suitable for modelling geophysical structures when it is separated into regional and local components. Model calculations still are not very satisfactory and the importance of one dimensional calculations must be emphasized for they give direct information on the variation of conductivity with depth, which is the purpose of GDS. We need more results, especially from tectonically active areas, before the underlying physical processes can be completely understood.