Low frequency in depth?

The depth of microtremors origin can provide an important clue about the propagation path, as to whether it is coming from deep or shallow sources. Most relevant hydrocarbon reservoirs are located at a depth much larger than that one surface waves can penetrate; so only body waves can carry signals whose anomalies might detect oil and gas presence. A field experiment was carried out in a test site acquiring 3-component records continuously for over 3 weeks. This case study seems to support a shallow propagation path for microtremors and definitely highlights the instability in the wave polarization over time.