MAGNETIC-FIELD CHANGES ON LAVA FLOW TO DETECT LAVA TUBES

During the 1991-1993 Etna eruption (Italy) seven surveys of the total geomagnetic field intensity were carried out on two traverses, ca. 80 m long, of the active flow, where it displayed evidence for the presence of a lava tube underneath. This experiment aimed at evaluating the efficiency of magnetic surveys in detecting the presence of tubes and at following their evolution in time. The data showed the presence of an intense anomaly corresponding to the lava tube at the surface. The use of a suitable thermomagnetic model allowed the calculation of the tube's magnetic effect at the surface (an inverted dipolar anomaly of ca. 4000 nT). Examination of the space-time evolution of the magnetic field in other parts of the profile revealed the presence of several anomalous variations (up to 1000 nT) whose location was very persistent in time. In particular, the occurrence of relatively rapid magnetic field fluctuations in a well-defined area in both profiles, was attributed to the presence of another lava tube where magnetic effects by thermo-mechanical erosion at the boundaries are likely to take place.