The separation of the anomalous magnetic field with respect to the depth of lateral contrasts of conductivity is discussed in this paper. It is shown that, for most geophysical cases, this separation is very simply performed for periods greater than 1800 s: the anomalous field of an intralithospheric source is the difference between the observed anomalous field and the one related to the lateral contrasts of conductivity in the superficial part of the crust (mostly the coast effect).Electromagnetic data observed in California are analysed using this method. An anomalous field of intralithospheric source is shown to exist. The study of this field provides us with indications about lateral variations of the physical and chemical conditions in the lithosphere; the obtained results are in good agreement with data drawn from other Earth sciences.Finally, we show that such results can be obtained in other tectonically active areas: Japan and Peruvian Andes.

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