Method and system for determining normal moveout parameters for long

Abstract

A method and computer system for deriving and applying normal moveout corrections to seismic survey traces is disclosed. In each common midpoint (CMP) gather, a stacking velocity function of travel time is first determined for near-offset traces. For each reflective event, a time window about a depth point is selected. Time anisotropy and velocity anisotropy values are then determined for the depth point, for example by way of a semblance analysis applied to envelopes of the traces over the full offset range. Normal moveout corrections are then determined by a weighted sum expression of travel time (zero-offset time plus moveout correction) The weighted sum adds a first travel time expression corresponding to a Dix equation expression with a second travel time expression corresponding to a skewed hyperbolic expression that includes the time and velocity anisotropy values, with the weighting dependent upon the offset of the particular trace. Near-offset traces receive relatively higher weighting of the Dix equation expression, while far-offset traces receive relatively higher weighting of the skewed hyperbolic expression. Because higher order power expressions are eliminated according to this approach, improved moveout correction accuracy is obtained over near and far offsets, with reduced sensitivity to measurement error.