Passive seismic are the survey methods that use the environmental noise as source. We can separate methods that use array of sensors and single station methods. Concerning the former, the processing procedures are the same as for the active surface waves methods.
Concerning the single station method, usually known as H/V or HVSR (Horizontal to Vertical Spectral Ratio), is a sperimental assessment of the ratio of spectral amplitude of horizontal (H) and vertical (V) of environmental noise on the surface, that can be measured with a specific 3-components seismometer.
The result of the measurement is an experimental curve representing the value of the ratio of the mean spectral amplitude of the environmental noise of as a function of the vibration frequency. The frequencies where the H/V curve are maximum are related to the site resonance frequency.