EXPERIMENTS IN SITU OF ELECTRICAL RESISTIVITY CHANGES

Under natural conditions in the field some experiments have been done for electrical resistivity changes with strain in partially saturated soil layers and siliceous limestone layers containing cracks. The major results are as follows:(1) At very low stresses, the resistivity drops (or rises) when rock (or soil) layers are compressed (or extended).(2) The relative change in resistivity of the rock or soil layers, /, is sensitive to strain changes (); this is particularly true for the rock containing cracks. Amplification factor, (/)/, changes with strain, and generally ranges from 100-200 at a strain of 10-4 up to about 103 at a strain of 10-5.(3) With increasing intensity and dimension of the force, the resistivity change increases. When the distance between the point of measurement and the force is increased, the resistivity change decreases and shows a delay.(4) For horizontally isotropic soil layers, resistivity changes due to stress variation exhibit directivity, that is, the changes measured along different profiles are not identical., measured along the profile perpendicular to the pressure, usually has a larger drop. As for ∥, parallel with the pressure, a much smaller drop, or sometimes a little rise, occurs. For 45, the drops are moderate. In contrast, the resistivity change of the siliceous limestone layer containing cracks is not only related to the directions of the pressure but also to that of the cracks.The drop of ∥ usually is larger. has a smaller drop when the pressure is parallel to the cracks, and rises distinctly when the pressure is perpendicular to them.