The vibroseis method has, for half a century, achieved great success in land seismic exploration. However, some practical issues still arise that have remained theoretically unexplained. For example, on soft ground, the vibrator produces subharmonics and ultra-subharmonics in addition to main harmonics; whereas on hard ground, the vibrator generates harmonics only. Geophones on soft ground also behave abnormally while geophones on hard ground behave normally. This paper analyzes these phenomena and demonstrates that the softness of the ground's top layer is responsible for subharmonics and ultra-subharmonics. This soft ground layer causes the geophone abnormality as well. Unfortunately, quantification for this behavior has not been achieved.