Abstract: A survey electronics package (32) of a survey system (10) includes a transmitter (36) and a receiver (38). The transmitter (36) transmits a survey signal which a radio (12) residing in a detection zone (14) receives and demodulates to generate an audio echo signal. The audio echo signal is magnetically radiated from a speaker (26) of the radio (12). The receiver (38) magnetically senses this magnetically radiated audio echo signal. A bandpass filter (48) in the receiver (38) and a correlation process (86) insure that only a valid audio echo signal is recorded. When the detection of a valid audio echo signal corresponds to a radio (12) tuned to a particular radio station, a record is made of the detection.