| 摘要 |
A percussive-action mole (10) is energised by compressed air supplied through hollow rods (36) in a string (12) connected to the mole. A hydraulic motor (18) rotates the string and mole. The mole head (30) has a slant face (32) and a transverse permanent magnet (34). After each new rod is added to the string, the air supply to the mole is stopped to halt the mole which continues to be rotated. The field fluctuations from the magnet are detected by a magnetometer (24) using its probe at three positions (50), (52), (54) determined by a triangular frame (22) placed flat on the ground. Calculations using the three readings each representing the distance of the magnet from the respective position on the frame enable the position and depth of the magnet to be determined. After completion, the passage (38) can be reamed to larger diameter to receive a gas pipe or other service.
|
