Multiple Reflections And Head Waves In The Gulf Of Suez

Abstract

ABSTRACT 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 in the northern portion of the Gulf. 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-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-distance function and by ray tracing in the overlying sediments. As the high-velocity layer approaches the water bottom from sub water 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 layer 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 are apparent where the high-velocity layer is deepest, and it is likely 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 move-out) curves, the configuration of each visibly matching the curves equally well. INTRODUCTION Prominent near-surface multiple reflections are encountered over much of the Gulf that effectively mask reflections from strata at depth. An excellent example of progressive degradation attributable to surficial multiple reflections is observed on a typical seismic line in the northern portion of the Gulf of Suez. The purpose of the investigation reported here-is to determine the nature of these multiple reflections that increase in intensity from one to the other extremity of the line. Seismic recordings shown here were obtained along the line shown on the map in Fig. 1. This line, extending in a northeast-southwest direction, is one in an extensive survey covering the northern portion of the Gulf of Suez. Typically, each line is covered in two parts, the marine vessel moving toward the shoreline from the approximate center of the line for each part (Lines 1A and 1B). This procedure provides coverage nearer the shoreline than would the opposite travel direction, since coverage loss near the shoreline is avoided due to turning onto the line.