02 03 04 05 06 07 08 09 10 11 12 I ! I / 1 \ , I, I, FIG. 4. Signature 33 degrees behind array. Scale factor = 0.495. (Figure 2). The scale factors shown in the figures give the relative amplitude scaling between these three figures. Modeled radiation patternsThere are many parameters that may have an effect on the observed radiation patterns. Some of these are the 2-D array geometry, air gun distribution within the array, variations in the firing times of the air guns, and variations in the depths of individual air guns. The radiation pattern due to the array geometry can be modeled using equally weighted ideal dipole sources (point source plus free-surface ghost) at each air gun position within the array, and summing these at desired observation points. Such modeling predicts an isotropic radiation pattern for the square array, symmetrical directionality with more energy in the in-line vertical plane for the wide array, and symmetrical directionality with more energy in the crossline vertical plane for the long array. Other observed radiation pattern properties for these three arrays must therefore be due to other factors.Modeling of the far-field radiation patterns using measured near-field air gun signatures allows the effect of the gun distribution within the arrays to be observed. Because the subarrays are designed with the largest guns in the front and the smallest guns in the rear. some forward directionality results from gun distribution. This elect does not completely account for the strong forward directionality observed for the wide array.The firing times of each individual air gun and its depth can also be incorporated into the model. Because the firing times were closely controlled and monitored during data collection, they will not have a significant effect on the radiation patterns. The depths of the air guns were also monitored during data collection and displayed a rather wide variation within the arrays. Variations of more than 2 m above or below the nominal air gun depth of 10 m were observed. even within a single subarray. These gun depth distributions were also observed to be slowly varying from shot to shot. Including these observations in the radiation pattern modeling scheme provides yet a better prediction for the observed radiation patterns.In conclusion, the observed data confirm our ability to make field measurements of 3-D radiation patterns, allowing us to observe the effectiveness of various array designs. For instance, wide arrays are designed to minimize out-of-line scattered energy and maximize in-line energy. The observed data show that the range of take-off angles for which the energy is less than 6 dB down from the peak energy is nearly twice as wide in the in-line plane for the wide array as it is for the square array. Similarly, long arrays are designed to minimize in-line multiple energy. The observed data show that the radiation pattern, as measured above, is almost onefourth as wide in the in-line plane and nearly twice as wide in the crossline plane ...

No abstract

No abstract