Confirmation that a bright spot zone in question is low velocity can sometimes be made by looking at constant velocity stacks or the common-depth-point gathers. When this confirmation does exist, then it is usually possible to do simple ray theory to get a reasonable estimate of the pay thickness, especially if the water-sand velocity and the gas-sand velocity are either known or can be predicted for the area. The confirmation referred to can take the form of under-removal of the primary events or be exhibited by multiple reflections from the bright spot zone. Such under-removals or multiple reflections will not be seen on the stacked sections but are sometimes obvious on the raw data, such as the common-depth-point gathers, or can be implied by looking at constant velocity stacks of the zone in question at different stacking velocities.

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