Vibratory plate compactor

http://www.arrowmaster.com/cmp.shtml

Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to the machine's static weight. The vibrating mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers). The compactors deliver a rapid sequence of blows (impacts) to the surface, thereby affecting the top layers as well as deeper layers. Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible. Based on the materials being compacted, a certain amount of force must be used to overcome the cohesive nature of particular particles.
Reversible plates and smooth drum vibratory rollers are appropriate for production work. Granular soil particles respond to different frequencies (vibrations) depending on particle size. The smaller the particle, the higher the frequencies and higher compaction forces.
Reversible Vibratory Plates
In addition to some of the standard vibratory plate features, reversible plates have two eccentric weights that allow smooth transition for forward or reverse travel, plus increased compaction force as the result of dual weights. Due to their weight and force, reversible plates are ideal for semi-cohesive soils. A reversible is possible the best compaction buy dollar for dollar. Unlike standard plates, the reversible forward travel may be stopped and the machine will maintain its force for "spot" compaction.
http://www.concrete-catalog.com/soil_compaction.html

Figure 9 illustrates how soil stiffness measurements can be used in controlling road construction. In this example, an actual compacted roadbed has been excavated; a drainage pipe installed; and soil reinstated, but not fully compacted. A medium-size vibrating roller was used to complete the compaction as stiffness measurements were made with SSG.
http://www.tfhrc.gov/pubrds/marapr98/soil.htm

Vibratory compactors now account for the majority of all compaction equipment sold worldwide. And for good reason. The vibratory system first developed in the late 1950's provides excellent results in both soil and asphalt applications.

http://www.hammag.com/english/pages/technology/oszillat.html

If it is a self-propelled machine, it's a "vibrating roller", maybe also a "single-drum vibrating compactor", depending on the type of machine.