Laboratory and underground trials of geophysical and television logging in a horizontal borehole have been made with the object of assessing their potentiality for use in probing ahead of tunnels. The commercially-available geophysical logging methods tested were gamma-gamma (long and short spacing), natural gamma, neutron-neutron, neutron-gamma, self-potential and resistivity; in addition a commercially-available borehole television camera was tested and a new ultrasonic velocity logger was designed and used. It was concluded that: (1) of the commercially-available geophysical logging methods, only the gamma-gamma method would seem to be useful in probing ahead of tunnels: it provided a good overall measure of density. (2) the borehole television camera was most successful in showing the presence of joints, cavities and other features. (3) the prototype ultrasonic velocity logger provided a general indication of rock strength and could readily be developed for use in routine probing ahead. (Author/publisher)
Abstract