Fig. 2.  Comparison of impact energy from typical active and roadside passive surveys.
(Right) Fig. 1.  Relationship between deformation and impact energy.
Deeper investigation depth requires generation of longer wavelengths (lower frequencies) because the analyzing depth range is directly proportional to the range of the
analyzable wavelengths.  Generation of longer wavelengths, then, requires a larger deformation of the ground surface, requiring a higher impact power (
Fig. 1).  The impact
power is a kinetic energy (E), determined as E=(1/2)m*v*v, where m is mass of the impacting object and v is its vertical velocity at the time of impact.  
Fig. 2 shows the energy
(E) comparison between the typical active source of falling  or accelerated masses of different sizes and typical traffic energy from a truck of a typical weight moving at different
speeds.  In the latter case of moving truck, it was assumed that only 10% of the total kinetic energy is consumed to generate surface waves.  The comparison clearly shows
that energy from moving cars is greater than that from the active source by a few orders of magnitude.
Why Passive?

While the active method using an artificial seismic source, like a sledgehammer, can
often achieve the goal of Vs estimation down to a few tens of meters (for example, 30
m), there are instances where the investigation depth is insufficient due to either elastic
properties of the near-surface materials or the unusually deep investigation depth
sought. Although a more-powerful active source like a heavy-weight drop is sometimes
used to overcome this limitation, the depth range gained often may be trivial. It seems
that the impact power necessary to achieve a distinguished gain in investigation depth
may have to be a few to several orders of magnitudes greater than the power delivered
by most active sources. Such a source, if invented, will not only be expensive but also
inconvenient in field operations, which will discourage its popular use in engineering
projects.  On the other hand, passive surface waves generated from natural (e.g., tidal
motion) or cultural (e.g., traffic) sources are usually of a low-frequency nature, with
wavelengths ranging from a few kilometers (natural sources) to a few tens (or
hundreds) of meters (cultural) (Okada, 2003), providing a wide range of penetration
depths and therefore a strong motivation to utilize them.