Existing seismic exploration direct hydrocarbon detection methods primarily focus on the properties of the sound-reflecting boundaries present in the earth. These methods are founded on the theory that the strength of the sound reflection from the boundary itself is determined by certain lithological properties of rock within the layer above and the layer below a given boundary.
However, such reflection-based methods are far from perfect. Reflections at each point on a boundary depend on at least eight variables (P-wave velocity above, S wave velocity above, density above, P wave velocity below, S wave velocity below, density below, angle of the incident ray path and bed thicknesses which may cause tuning effects or the lack thereof). The interplay between these variables makes it difficult to determine any particular one with accuracy. Therefore methods that do not rely on the strength of the reflection boundary for direct detection are desirable.
Recent research indicates that the velocity of the seismic wave may vary with frequency in the presence of permeability and this frequency domain based phenomenon is independent of the strength of the reflection boundary. Measurements of this phenomenon can be used to image hydrocarbon reservoirs where hydrocarbons can move through the pore throats easier than brine.