A magnetic anomaly detector includes a magnetically active device which produces analog signals in response to magnetic fields. The analog signals are time quantized and then discrete-time differentiated. The differentiated signals are applied to a low-pass filter. The filtered signals are Fourier transformed into an amplitude-frequency spectrum. An error detector compares at least one reference amplitude-frequency spectrum with the reference spectrum to identify the presence or absence of a magnetic anomaly. The resulting error is displayed on a monitor or operates an alarm.