SURFACE AND UNDERGROUND MEASUREMENTS OF GEOMAGNETIC-VARIATIONS IN THEMICROPULSATIONS BAND

Geomagnetic fluctuations in the frequency band 2-70 mHz recorded simul taneously at a depth of about 1200 m in the underground Gran Sasso Lab oratory (central Italy) and on the earth's surface (approximately alon g the Laboratory vertical) are compared using multivariate spectral an alysis. Experimental problems and analytical techniques adopted for th e signal processing are discussed. In particular, a modification of th e standard least-squares method for estimating multiple-input transfer functions is proposed. The results show apparent different skin effec ts for each magnetic field component and a significant coherence betwe en the underground vertical signal and the horizontal signals, suggest ing the presence of lateral inhomogeneities in the underground conduct ivity structure. The results are also consistent with an average resis tivity of the intervening medium of the order of 10-20 m and with the presence of a more conductive layer at greater depth.