Presentation of 3-D Seismic Results

Abstract

The processed seismic data of a 3-D grid can be presented using conventional seismic sections along the direction of acquisition and a perpendicular direction. They can also be presented using any chosen direction. Like in 2-D seismic it is possible to use colors to enhance the variations of certain parameters such as amplitude or frequency. Constant time horizontal sections are very helpful for the Interpreter Since they allow a direct isotime contouring. Tomoseis sections, where color is used to represent the instantaneous phase, give the dip of the seismic events and thus help and improve the above contouring. A selected seismic horizon can also be flattened for studying some of its characteristics such as amplitude and frequency. It is then easy to obtain sections similar to the constant time sections, but parallel to this reference level. This process allows direct isopach contouring. Examples of these different displays obtained from actual 3-D offshore seismic surveys will be presented. INTRODUCTION Seismograph prospecting for oil is now a little more than 50 years old. During the first decades, the interpretator had to pick the reflected events on individual records (fig. 1), to check whether they correlate from each record to the next, then to draw a vertical time sections (fig. 2), then to read the times of selected horizons and transfer such readings on a location map, then to contour the isotime - lines, so as to eventually obtain an structural image of the subsurface (fig. 3). In the mid-fifties, with the introduction of the magnetic recording, some of these tedious tasks were taken out from our poor interpretator, who directly disposed of a vertical time sections along the acquisition lines (fig. 4). The digital recording and processing, surprisingly enough, although it launched tremendous possibilities in the field of signal enhancement, dit not appreciably help in the data handling at the interpretation stage. It is only very recently that efforts were made to capitalize on the computers capabilities, in order to supply the interpretation with documents on which he can directly visualize the geological Implications of his seismic Interpretation. Such objective assumes that, first, there is a spatial 'continuity of the seismic data collected; it is what has been designated under the general name of "3D seismic". For offshore surveys, it amounts to shooting a large number of parallel lines, and relying on the sea currents to spread the measurement points on a regular grid (fig. 5) : surface position - (fig. 6) : subsurface position. PRESENTATION OF 3-D SEISMIC RESULTS Another necessary condition is to have relocated the seismic energy where it has been reflected, and not where it has been recorded. This is migration stage of the data processing, the effect of which can be seen by comparison of fig. 7, 8, 9 (7 : grid stack - 8 : 3D migrated grid stack - 9 : 3D migrated grid stack 500 m apart, whereto the deep seismic event of fig. 7 has moved).