Vibroseis sources are the principal nonimpulsive sources used in land seismic exploration. To achieve an improved image of the subsurface with higher accuracy and better resolution, it is necessary to take the true ground force as the reference in calculating the cross-correlation function with the far-field data instead of using the theoretical reference signal. The true ground force is defined as the total compressive force exerted by the Earth upon the vibrator baseplate. Theoretically, it can be calculated by integrating the compressive stress over the surface area that contacts the baseplate. In practice it is very difficult to quantify the true ground force accurately. As the vibrator shakes on highly variable surfaces such as desert sand dunes, the vibrator will generate more harmonic distortion. At the same time, the baseplate will flex and rock. This results in poor repeatability of source signatures. This means that the true ground force varies from place to place and becomes impossible to determine.