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Figure 1. (Right) Generalized curves showing how the acoustic impedances of gas sands, water sands and shales increase with depth. Bright spots occur above depth A, where there is a large contrast in shale and gas-sand impedances but a modest difference between shale and water-sand impedances. Polarity reversals occur between depths A and B, where water-sand impedance is greater than shale impedance but gas-sand impedance is less than shale impedance. Dim spots occur below depth B, where the three impedance curves converge and there are only small impedance contrasts between shale and either type of sand, brine-filled or gas-filled. (Left) Examples of seismic reflectivity for each of the three sand/shale impedance regimes, taken from AAPG Memoir 42 (sixth edition).