| 摘要 |
Two geophones located above an underground pipeline are spaced from each other and adjacent to both sides of a benchmark location. As a pig travels through the pipeline sound waves are generated. As a pig approaches a benchmark location, sound waves are received first by the geophone which is closest to the source of the sound waves, that is, the upstream geophone and then by the downstream geophone. The geophones generate electrical signals that are fed to a comparator circuit which is connected to upstream and downstream indicators, one of which is activated when a pig passes the benchmark depending on whether the sound was first received by the upstream or downstream geophone. An alternate embodiment adds a magnetometer at the benchmark location capable of sensing changes in the ambient magnetic field caused by the passage of a pig.
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