Almost 20 years ago, Tury Taner and his co-authors Fulton Koehler and Bob Sheriff noted the occurrence of lower apparent frequencies for reflectors on seismic sections beneath gas and condensate reservoirs. As they stated in their article (Geophysics, 1979), there were no good explanations to explain the phenomenon. As it turns out, little research has been published on this topic since then, although at least four different consultancies (that I know of) are offering interpretations based on finding zones of anomalously low frequency. Although the detection of anomalous zones is clearly the first step in applying this possible...
Research Article|August 01, 2004
The low-frequency gas shadow on seismic sections
The Leading Edge (2004) 23 (8): 772.