MARINE DATA EXAMPLE

I now demonstrate the stacking velocity and migration velocity algorithms on some CMP gathers recorded in the Gulf of Mexico. In addition, I discuss cpu times and I/O behavior of the two algorithms with respect to this marine data example.

Figure is a near offset section displaying a bright spot in the Gulf of Mexico. Figures and are the mid and far offset sections respectively. The offset distances are 241 m, 1391 m, and 2341 m. These three constant offset sections are representative of hydrocarbon reservoirs which exhibit spatially variable reservoir rock properties and/or pore constituents. The lateral variation in relative P impedance in the near offset section could be indicative of spatially variable gas content, and in turn, variable porosity, for example. Of course, some spatial amplitude variation is almost certainly due to near surface ``overburden'' focusing/defocusing effects, and would have to be compensated in any amplitude inversion process. The three sections also indicate a general AVO amplitude increase with offset, which is a common signature of HC gas. A migration-inversion of these data could potentially better resolve the spatial distribution of the reservoir properties which cause lateral variations in seismic amplitude (Lumley and Beydoun, 1991, and Parsons, 1986).

 

near
Figure 1 Near offset section. Constant offset is 241 m.

 

mid
Figure 2 Mid offset section. Constant offset is 1391 m.

 

far
Figure 3 Far offset section. Constant offset is 2341 m.

Figure shows four CMP gathers extracted from midpoints in the 4-5 km distance range of the common offset sections. The strong AVO effect is readily visible in the gathers. Ideally, a high spatial resolution velocity analysis at a fine CMP interval spacing across the section might detect significant lateral variations in velocity. Given some in situ well log/core measurements, it may be possible to correlate such velocity variation to spatial variations of rock properties such as pore gas/fluid content, porosity or clay content (Han, 1986).

 

cmps
Figure 4 Some representative CMP gathers.