NARROW VS WIDE AZIMUTH ON 3D SEISMIC SURVEY

Tri Satyo S.P

Independent Seismic Consultant

For Designing Land 3D Seismic Survey

NARROW VERSUS WIDE AZIMUTH

The distinction between narrow and wide azimuth surveys is made on the basis of the aspect ratio of the recording patch. The aspect ratio is defined as the cross-line dimension of the patch divided by the in-line dimension. Recording patches with an aspect ratio less than 0.5 are considered narrow azimuth, while recording patches with an aspect ratio greater than 0.5 are wide azimuth.

Small-aspect-ratio patches (so-called narrow azimuth) lead to a more even distribution of offsets. However, these patches have, as the name indicates, a limited range of azimuths. Schematically, narrow azimuth surveys have a linear offset distribution with respect to offset similar to 2-D data (Figure-1a); however, when plotted against offset squared, the offset distribution shows bunching at the near offsets (Figure-1b). Narrow azimuth patches are better for AVO and DMO purposes and when significant lateral velocity variations are present (Lansley, 1994).

Fig. 1. Narrow- versus wide-azimuth templates and offset distributions. X offset distance.

Wide-azimuth surveys (i.e., patches that are closer to a square) have a nonlinear offset distribution with respect to x, with a heavy weighting of the far offsets (Figure-1c). However, when plotted against offset squared, the distribution is nearly linear (Figure-1d). Wide-azimuth surveys are better for velocity analysis, multiple attenuation, static solutions, and a more uniform directional sampling of the subsurface. These di-agrams are schematic, and variations in offset distributions may occur in real data.

Comparisons have been made to illustrate differences between narrow and wide azimuth acquisition. Figures-2, 3, and 4 compare a wide-azimuth acquisition consisting of a patch of 12 lines with 60 stations per line in the left column (a, c, e) to a narrow-azimuth acquisition comprised of a patch of 6 lines with 120 stations per line in the right column (b, d, f). The respective aspect ratios are 0.80 and 0.20 based on station spacing of 60 m and receiver line spacing of 240 m.

Since neither the source point density nor the number of receivers in the patch have been changed, the nominal fold for both acquisition strategies (Figures-2a, 2b) is 30 (Figures-2c, 2d). What does change, however, is the configuration of the fold taper, which is much less in the cross-line direction for the narrow azimuth patch (Figure-2d). The fold at an offset limit of 1500 m is significantly lower for the narrow azimuth patch (the fold scale is constant in Figures-2e, 2f). 

The comparison of the offset distribution shows that the wide-azimuth patch has traces closer to the source points than the narrow-azimuth patch (Figures-3a, 3b), assuming that the same number of receivers are utilized in the patch. An average trace count of the narrow patch was copied onto the wide-patch display (Figure-3a) with the corrected scaling. The stick diagrams (Figures-3c, 3d) and the offset variation within a box (Figures-3e, 3f) indicate that the offset distribution is better for the wide patch be-cause of the nonlinearity in the source receiver spacing that results from the azimuthal distribution of the receivers. 

An azimuth-dependent trace count shows that there is a more even distribution of source–receiver pairs for the wide patch (Figures-4a, 4b). The azimuth distribution is far more varied for the wide patch than for the narrow patch (Figures-4c, 4d). The rose diagram in Figures-4e and 4f uses color to indicate the multiplicity of the occurrence of a particular source–receiver pair in offset and azimuth distribution (for the entire survey) and shows the focused nature of the narrow-azimuth patch.

Fig. 2. Wide versus narrow patch — template and fold; a. wide-azimuth template, b. narrow-azimuth template, c. wide-azimuth fold distribution at full offsets, d. narrow-azimuth fold distribution at full offsets, e. wide-azimuth fold distribution at 1500 m offsets, and f. narrow-azimuth fold distribution at 1500 m offsets.

Fig. 3. Wide versus narrow patch — offset distribution; a. wide-azimuth offset distribution — trace count, b. narrow-azimuth offset distribution — trace count, c. wide-azimuth offset distribution — stick diagram, d. narrow-azimuth offset distribution — stick diagram, e. wide-azimuth offset distribution — offset variation within a box, and f. narrow-azimuth offset distribution — offset variation within a box.

Fig. 4. Wide versus narrow patch — azimuth distribution; a. wide-azimuth azimuth distribution — trace count, b. nar-row-azimuth azimuth distribution — trace count, c. wide-azimuth azimuth distribution — spider diagram,d. narrow-az-imuth azimuth distribution — spider diagram, e. wide-azimuth azimuth distribution — rose diagram, and f. narrow-az-imuth azimuth distribution — rose diagram.

85% RULE - ASPECT RATIO

Tri Satyo S.P.

Independent Seismic Consultant

For Designing Land 3D Seismic Survey

85% RULE

Three-dimensional crews often record with more channels on the ground than are necessary. This practice extends the recorded X_max beyond the required X_mute. Equipment availability should be taken into consideration when the patch size is determined.

If a wide-azimuth survey is desired, one must de-cide on the best aspect ratio for the patch. For the moment, this discussion is restricted to square patches with an aspect ratio of 1.0, meaning that the in-line di-mension equals the cross-line dimension. Consider a circle of unit area (i.e., 1.0) with a radius of X_mute representing the mute zone (large red circle in Figure-1). If the patch lies entirely outside this circle, then 27% of the channels in the patch are being used to record data that will most likely be muted out. While these channels may have some value for longer wave-length refraction analysis, using that many extra channels may be expensive. The recorded X_max is a factor of √2 larger than the required X_mute.

On the other hand, one can reduce the patch to lie entirely within the mute zone, as shown by the small blue square. X_max is now measured along the diagonal of the patch, but this patch covers only 64% of the area of the design objective, i.e., the large red circle. This is the other extreme of inefficiency; there are only a few traces that lie at offsets close to the ideal mute distance. The recorded X_max equals the required X_mute in this case. Some companies select a patch size that equals the large red square in order to record offsets of X_mute in all directions. However, the large red square is twice the area of the small blue square, and there is a significant difference in cost and effort be-tween the two patches.

The 85% Rule is a compromise and determines the aspect ratio of the patch relative to the desired X_mute. The 85% Rule is a simple way to optimize the area of usable traces recorded and the number of channels needed. The rule works as follows (Figure-2):

1.    Determine the desirable X_mute.

2.    Choose the in-line offset X_r to be 0.85 x  X_mute.

3.    Choose the cross-line offset X_s to be 0.85 x X_r = 0.72 x X_mute.

For a real example with Xmute  =    2000 m,
Half in-line dimension            Xr  = 85% x Xmute  = 1700 m     Half cross-line dimension           Xs   =  85%  x  Xr     =  1445 m Aspect Ratio           Xs   /   Xr    =  85%                                               Fig-1.   Patch dimension Vs  Xmute The usable area of the patch relative to the circle of Xmute increases from 64% to 78%.  Only a small part of the patch is outside the theoretical maximum offset Xmute (e.g., for  a 6-line patch the traces at offsets greater than Xmute are less than 2.5%).     Additional  receiver lines farther out than indicated by  this  patch  are  mostly  out-side the usable  offset Xmute. Therefore, the longer  dimension  of  the  patch  is  preferred  to be in the  in-line  direction.               The  dimensions  may  need  some  slight  adjustment  to  be  suitable  for  other  considerations  in  the  design  of  the  3-D.       The  recorded  Xmax  is 1.13 times   the required  Xmute;   most likely the mute affects only t the far extremes  of the two farthest receiver lines.                                                Fig-2.  Ideal patch using 85% rule Referring back to Figure-1, the relationships between the different areas are shown graphically in Figure-3.  The  red  circle  of  radius  Xmute  has  unit  area  (100% at  100% Xmute)  and  is  shown as the solid line. The inner blue circle with a radius of 0.71 Xmute  contains  50%  of  the  area  of  the larger circle. If the patch is entirely within Xmute  (inner  blue  square  in  Figure-1),  then  the  curve describing its area  deviates from the  y = x2 form at 0.71 Xmute. The area of this smaller blue patch is  64% of the unit area (red circle).                                                    Fig-3. Percentage of area covered for various choices of Xmute On the other hand, if the patch is entirely outside the red circle of Xmute (larger red  squarein Figure-1), then the curve describing its area deviates from the y = x2 trend  at 100% Xmute.   The area of  this  larger  patch  is  127%  of the unit area, which is  twice  the  area  of  the  smaller patch, with its maximum offsets being 141% of the  mute offset.   Therefore  if  one  lays  out receivers in a square patch that has Xmax  measured  in  the  in-line  direction, then twice the number of receivers are needed in comparison to a patch that has Xmax measured along the diagonal. The patch using the 85% rule covers 78% of the unit area described by the red circle  of Xmute.  The  green  patch covers an area that is 22% larger than the smaller blue  square (compare Figure-2). The patch using the 85% rule is an excellent compromise for the patch  design with the recorded maximum offsets being 113% of the required  mute zone.   The design provides a sufficient number of large offsets, and only a few  traces may have to be deleted in the stack. The author  : Tri Satyo S.P  - Sr. Geophysicist