A Seismic Sensor is an early game item used to track down Oil/$. Unlike most tools, the sensor does not require pressure/$ to operate. When right-clicked/$, it will show whether or not Oil was found somewhere directly below the block the sensor was clicked on.
A Seismic Sensor can have two results:
No Oil found./$
No Oil is found right under this block. Keep looking.
Found Oil <distance>m below. It contains about <amount> buckets of Oil./$
Oil is found! The clicked block would be a suitable place to put a Gas Lift/$ to pump the Oil out.
Crafting a Seismic Sensor