A system is provided for detecting the passage of vehicles and the classification thereof by weight using geophone outputs and a unique density measurement in which the number of peaks of the geophone signal above a predetermined threshold over a number of time frames indicates the presence of a vehicle, with the number of time frames in which the density exceeds the threshold indicating whether the vehicle is a heavy vehicle such as a tank, or a light vehicle such as a car, with the threshold eliminating both manmade and natural noise, as well as distinguishing seismic vibrations due to personnel and animals. In one embodiment, various thresholds are utilized to detect the onset of vehicle presence and the end of the event.