There is something to it but not the way you put it,
RF can be reflected when they hit a metal object, the lower the frequency the deeper the ground penetration but weaker the reflection,
You will have better results with high frequencies in very shalow depths or on surface or near surface, But have nothing to do with the earth's magnetic field or buried object's magnetic field.
You can detect magnetic field of a buried metalic object with a magnetometer, And you can transmit a signal and also be able to detect reflection if these signal when come in contact with a metalic object.
Hope that helps.
Yanis