Seismic method can be divided into active seismic and passive seismic. Active seismic or reflection seismic use controlled seismic source like dynamite, seismic vibrator, or air gun and make use of reflected wave to discern subsurface properties. Passive seismic, in other hand make use of background noise from naturally occurring event like earthquake, oceanic microseism, or from artificial noise such as urban traffic. Passive seismic usually focuses on a low frequency signals (0 – 20 Hz) and sometimes called low frequency seismology.
Our main interest in passive seismic is in hydrocarbon microtremor, a low cost exploration method for for hydrocarbon indicator. Hydrocarbon microtremor method used spectrum analysis of recorded seismic data for creating map of hydrocarbon probability. If we have velocity information of subsurface, we can do Time Reverse Modeling to get 3D position information that create microtremor anomaly.