Seismic travel-time triplication
= Seismic travel-time triplication
A ray family can fold in the mapping from ray parameter or turning depth to surface range, so that three rays arrive at the same range with different <travel times>. A sufficiently strong, thin high-positive-gradient zone can produce this effect. The first-arrival curve is the lower envelope of the branches; other branches represent later arrivals. <Gradient> contrast and thickness determine whether folds exist.