Surface and underground measurements of geomagnetic variations in the micropulsations band

Geomagnetic fluctuations in the frequency band 2–70 mHz recorded simultaneously at a depth of about 1200 m in the underground Gran Sasso Laboratory (central Italy) and on the earth's surface (approximately along the Laboratory vertical) are compared using multivariate spectral analysis. Experimental problems and analytical techniques adopted for the signal processing are discussed. In particular, a modification of the standard least‐squares method for estimating multiple‐input transfer functions is proposed. The results show apparent different skin effects for each magnetic field component and a significant coherence between the underground vertical signal and the horizontal signals, suggesting the presence of lateral inhomogeneities in the underground conductivity structure. The results are also consistent with an average resistivity of the intervening medium of the order of 10–20 Ωm and with the presence of a more conductive layer at greater depth.