S. N. Domenico
Inferior reflection quality in the Gulf of Suez at the objective depth interval is attributable in part to surficial multiple reflections. An excellent example of the latter is observed on a typical seismic line from the northern Gulf of Suez. An increase in prominence of the multiple reflections appears associated with decreasing depth to a dipping high-velocity layer below the water bottom.
Inversion of a second-order polynomial time-distance function, fitted to the observed refraction onset time and associated distance values, gives the velocity-depth function for sediments between the water bottom and a high-velocity layer. Velocities thus determined increase nonlinearly with depth from a value near water velocity at the water bottom. Depths to the high-velocity layer are obtained from the associated head-wave linear time and distance function and by ray tracing in the overlying sediments.
As the high-velocity layer approaches the water bottom from subwater depths exceeding 0.6 km to a depth of 56 m, intensity of the multiple reflections increases to the extent of completely dominating individual records to a time of at least 3 seconds. The estimated plane-wave normal-incident reflection coefficient at the top of the high-velocity layer increases with decreasing depth to this layer, approaching 0.5 at the shallowest depth. This substantial reflection coefficient further substantiates the existence of multiple reflections between the high-velocity and water layer. However, existence of water-layer multiples cannot be ruled out. The estimated water-bottom reflection coefficient is approximately 0.3, a substantial value. Multiple reflections of considerably less intensity re apparent where the high-velocity layer is deepest, and it is probable that such are water-layer multiple reflections. Unfortunately, water-layer multiple reflections and multiple reflections between the water surface and high-velocity layer cannot be separated by their coincidence with time-distance (normal moveout) curves, the configuration of each visibly matching the curves equally well.
AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California