Method and apparatus for probing and inspecting an underground volume

The sampling process for measuring electrical resistivity by applying an electrical current via two emission electrodes (A,B) and measuring the potential between two measuring electrodes (M) comprises placing a probe containing a number of emission electrodes (A) and a number of measuring electrodes (M) in a drill shaft, with the number of emission electrodes being greater than the number of measuring electrodes. The emission electrodes are spaced at regular intervals and the measuring electrodes are placed centrally between the emission electrodes along the length of the probe. A fixed emission electrode (B) is placed "at infinity". The emission electrodes are supplied in turn and the potential between a pair of measuring electrodes is measured to obtain information representing the soil at the level of that pair of electrodes and at different positions in the soil relative to the probe, each position being related to one emission electrode. The above step is repeated for the different pairs of measuring electrodes to obtain information representing the soil at various levels. Also claimed are: (i) the application of the above process to controlling the injection of material such as concrete or grout into the soil; (ii) a probe for use in the above process; and (iii) a sampling device for carrying out the above process.