Electrical investigation in the Chaîne des Puys (France)

Numerous geologists have studied the structure of the Chaîne des Puys, our knowledge of which has recently been enlarged by geophysical research. Some of the hypotheses concerning its structure at average depth had to be checked, and so in 1975 the National Institute for Astronomy and Geophysics (I.N.A.G.) provided the necessary support for such work.An anomaly of the electrical conductivity was clearly apparent, both from the magnetotelluric method in a wide range of frequencies as well as from more classical methods (electrical sounding) or from newer methods such as differential magnetic sounding.It has been proved that the structure, which is believed to be responsible for this anomaly of conductivity, is quite different from other inferred structures which result from the interpretation of magnetic data only. However, although the coexistence of these different structural elements is not impossible, heat-flow measurements and geochemical studies do not support the assumption of an active magma chamber (a few kilometres deep).