Abstract: A transmitter for probing drill holes includes a ground-located conductor loop (1) which is supplied with a current that has a wave shape which includes at least one low frequency (e.g., 200-3000 Hz). The frequency is crystal controlled and normalized to time signals from GPS satellites. A receiver includes coils which function to sense field changes having the same frequencies as the transmitter and is housed in a probe (6). The probe is lowered with the aid of a typical drill pipe (5) and lacks signal connection with the transmitter. Instead, the probe includes an accurate quartz clock which is frequently calibrated and which enables the phase of the received signal to be determined synthetically relative to the phase of the transmitter signal, e.g., by means of a signal processor.