A method of logging an underground formation from within a borehole includes the steps of emitting an electromagnetic signal from a first location in a borehole, determining a time at which the signal is emitted, detecting a direct component of the signal which has passed from the first location to a...http://www.google.com.tw/patents/US5552786?utm_source=gb-gplus-share專利 US5552786 - Method and apparatus for logging underground formations using radar
