5Hiromasa Shima, Susumu Sakashita, Tsuyoshi Kobayashi, De-velopments of non - contact data aequisitio Techniques in e-lectrical and electromagnetic explorations, Journal of APPliedGeoPhysies 35( 1996)167 - 173.
6G. Ranieri f L. Sambuelli, A new Procedure to Perform differ-ential underground gravity measurements, Journal of APPliedGeoPhysies 36(1996)123 - 129.