A short-pulse electromagnetic transponder for hole-to-hole use

We have made hole-to-hole observations through nearly 20 m of granite using an electromagnetic transponder (an active reflector) in one borehole and a single-hole short-pulse radar in another. We found that the transponder is inexpensive, operationally simple, and effective in extending the capability of a short-pulse borehole radar system to allow hole-to-hole operation without requiring timing cables. A detector in the transponder senses the arrival of each pulse from the radar (which may be millivolts in amplitude); each pulse detection triggers a kilovolt-amplitude pulse for retransmission. The transponder "echo" may be stronger than that of a passive reflector by a factor of as much as 120 dB. The result is an increase in range capability by a factor which depends on attenuation in the medium and hole-to-hole wavepath geometry. Single-hole reflection-mode echoes are still available at times prior to the transponder pulse arrival. The transponder is helpful in yielding velocity information, because the radar-transponder distance is known and the echo time is observed. Field tests have demonstrated that the transponder is a useful alternative to employing timing cables in some short-pulse hole-to-hole measurement situations.