Importance

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Abstract

A percussive-action mole 10 is energized 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 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 determiend 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.

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First Claim