The key of searching for carbonate reservoirs is to find large solution cavities full of fluids. Using frequency difference analysis can identify the nature of the cavity fillers.When the cavity filler is fluid

it has intense amplitude

low velocity and tuning property;when the filler is quasi viscous fluid

its tuning reflection is 90% of absolute reflection;when the filler is loose deposit

its tuning reflection is 77% of the absolute reflection.These media are most possibly oil preserving bodies.When the master frequency focuses 20～30Hz

and the cavity height is higher than 2～3 m

the natnre of the filler could be easily identified.This technique was used in Tahe Oilfield

four horizons of underground fossil river channels were discovered under the weathered surface in S-48 Well.