TUNING EFFECT AND INTERFERENCE REFLECTIONS FROM THIN BEDS AND COAL SEAMS

The Ricker and Widess criteria of resolving thin beds are established and widely accepted axioms. Analysis of tuning effects from an isolated thinning bed is commonly used in estimating the thickness of a thin bed. However, frequency and amplitude tuning effects may also result from a sequence of thin beds. Depending on the thicknesses, acoustic properties, and the relative location of thin beds in space, tuning effects from interference reflections can sometimes deform the seismic wavelet and cause complex waveforms. Coal seams as thin as 1/20 of the predominant wavelength can produce reflections, and these reflections may be a composite of primary and multiple reflections. Interference reflections from two closely spaced coal seams were recorded and used to interpret a seam anomaly (roll) that may create adverse conditions for longwall mining. The seismic data and computer model results correlated with findings of several boreholes.