Wide- and narrow-azimuth seismic surveys over fault zones and stratigraphic traps

Abstract

Exploration geophysicists have, in recent years, learned that wide-azimuth seismic (WAS) surveys have several advantages. These include relatively small acquisition footprint; the ability to lay out shot or receiver points that "avoid" surface obstacles; higher resolution than narrow-azimuth seismic surveys (NAS); effective attenuation of some cultural and other noise; and multiple suppression (Williams and Jenner, 2002; Cordsen, 2002, and Hall et al., 2002).These advantages are significant. But questions remained concerning the ability of WAS to produce useful data in fault zones or for detecting stratigraphic traps. Since these are key exploration challenges in China, studies were conducted to answer those questions. This paper studies and evaluates the effectiveness of 3D WAS and NAS in a fault zone and over stratigraphic traps in western China.Evaluation of WAS and NAS in a fault zone. The 3D data for the evaluation are from Turpan Basin. Figure 1a is a satellite photo of the 3D geometry. Parameters included 20 active receiver lines; 168 active channels per line; 14 shots/salvo; aspect ratio = 1 (shown in Figure 1b); number of rolls = 2; fold = 100; bin size = 25 ǂ 25 m; source = vibrator.Conventional relative amplitude preservation in the time domain was used in processing. Figure 1c shows the alonghorizon final coherency cube slice, from which we can see that the reverse faults and the strike-slip faults are formed in the area of interest by the stress from the NNW and NNE. The strike of the former is EW and the latter NNW-NNE.The first three parts of Figure 2 show, respectively, the final migrated sections with azimuths of 0°(inline), 90°and 135°. The arrows indicate major differences. A continuous reflection event is present at 1.6 s in Figure 2a (0°) which disappears in Figure 2b (90°), and becomes discontinuous in Figure 2c (135°). At 2.6 s, there is a discontinuity at 0°, an imaged result at 90°for the thrust fault, and two separate imaged results of fractures in Figure 2d. It goes without saying that there is only one geologic structure in the subsurface at a certain depth, and that this is more correctly depicted by a wide-azimuth survey (WAS) than by NAS (although, one exception, NAS may get a better result than WAS when imaging in the vertical direction).The first three parts of Figure 3 show, respectively, the reflection information of the CDP gather (whose position is shown in Figure 1c) with azimuths of 0°(inline), 90°and 135°. The reflection layer at 3 s (NMO datum) in Figure 3 corresponds to that at 2.6 s (final datum) in Figure 2. The difference of CDP reflection data from the above analysis tells us that there is a good reflection on offsets between 1000 and 2000 m, when observed at 0°, and the relatively continuous reflections lie on offsets between 2000 and 3000 m observed at 90°with the disappearance of the continuous reflections on offsets between 1000 and 2000 m. The same observation in terms of reflection continuity can be made at 135°, except for differences in reflection quality and...