Hi, Low Frequency Passive Seismic tries to sense very low frequencies (1-10 Hz) with special seismometers. The source is normally passive, i.e., no induced sources like vibrators, dynamites, airguns, etc are required. The special seismometers are spread over an area (depends on depth and areal extent of investigation) and readings are recorded for hours, days, weeks, etc depending on the objective of the study. The recordings are analysed for spectral anomalies like time-frequency, vertical to horizontal spectral ratio, etc. There is lot of physics and mathematics involved in the analysis. Maps like gravity-magnetic anomalies are created to highlight the areas of interest.
Since you are from Ahmedabad, you can contact Institute of Seismological Research, Raisan, Gandhinagar for further details.