Multiple broadband processing

For the results obtained so far, data from a 10 Hz wide band centred at a spectral peak within 80-110 Hz was chosen. Figure 5.1 shows a set of spectra from all ten hydrophones of the horizontal array over the frequency interval 10 Hz–1 kHz (from shot W127). There are several additional peaks within the frequency interval of interest 40-140 Hz. A logical extension of the work so far is to include data from more bands in the processing.

Figure 5.1 Typical frequency spectra from 10 hydrophones of the horizontal array.

Frequency interval 10 Hz to1 kHz. From shot W127.

Data from a 10 Hz band within 80-110 Hz is combined with data from an adjacent band or with data from a 10 Hz band at a lower frequency. Eight shots from at ranges 3-27 km are treated. In order to reduce computational efforts, the equivalent water depth for the nominal source rangeonlywas used. The seabed model of Table 4.4, developed for the first eight km from the array, and a single sound speed profile in water as measured at the array site was used. Fully bathymetry-optimised ambiguity surfaces using the equivalent depth at each range step are subsequently computed in section 5.6.

5.5.1 Short-range shots

Two short-range shots detonated within 3 km of the array (E115 and W139) are treated. For these shots, results using one band of 10 Hz within 80-100 Hz were poor with low values of Bartlett match, even though range estimates close to the true were obtained. Here data from a second band at a lower frequency is added in processing. Results are shown in Table 5.5.

Label Band

Table 5.5 Source range and depth estimation using incoherent broadband Bartlett

processor with acoustic data recorded endfire to a 10-element horizontal array at the seabed. The processing band(s) and total number of frequency

components (in parenthesis) is listed in the second column.

Both shots are now located correctly in rangeand depth. There is a lower sidelobe level (secondary peaks –1.0 dB and –1.8 dB or more down from the main peak for the two shots respectively) than was achieved using data from a single band.

The low Bartlett energy for the main peak is attributed to two effects: the acoustic field at short range has a higher sensitivity to the seabed model and may require a more accurate forward model for replica field computations.

5.5.2 Medium-range shots

Table 5.6 lists results for shot W127 using two bands at 90 Hz and 110 Hz separately as well as from these two bands combined, and for shots W122 and W125, with data from bands at 40 Hz and 95 Hz used separately and combined. Similar computations were performed for shots E109 and E110 but results are not shown here.

Shot Processing

Table 5.6 Same as Table 5.5. Shots W127, W125 and W122.

For shot W127, combining data from two bands, the peak of the ambiguity surface is at the correct location. For shot W125 using data at the 40 Hz band, the peak is at the correct location. This result is also obtained using the two bands combined, with the second peak -1.11 dB down from the main peak. For shot W122 the peak is at the correct location only when combining data from the two bands. The second peak is -0.62 dB down from the main peak. Selected ambiguity surfaces are plotted in Figure B.3.

5.5.3 Longer-range shots

For shot W116 at range 16.5 km and W112 at nominal range 19.8 km, additional

low-frequency data at 16 Hz is included in the processing. The assumption is that the environment description of Table 4.4 becomes less accurate at longer range westward, but that

low-frequency data will be less sensitive to this uncertainty. On the other hand, a lower resolution in depth and range is expected. Table 5.7 lists results for shot W116.

Shot Processing Table 5.7 Same as Table 5.5. Shot W116.

Best results were obtained when using data from the 16 Hz and 40 Hz bands, the second peak of the ambiguity surface was -0.49 dB down from the main peak with a relatively small number of sidelobes. Processing including the 80 Hz band did not yield good results.

Table 5.8 lists the main peaks of the ambiguity surfaces for shot W112. Six of the ambiguity surfaces are plotted in Figure B.4.

Shot Processing

Table 5.8 Same as Table 5.5. Shot W112.

The range estimate is overall improved from those obtained using a single broadband at 80 Hz;

estimates are now all at 17-20 km in range. Best results were obtained when including data from the 40 Hz band in processing alone, the second peak of the surface is then -1.92 dB down from the main peak. For the ambiguity surface using the 40 Hz and 80 Hz bands, the second peak is -0.43 dB down. Also note that the main peaks are wider when using low-frequency data below 20 Hz; thus, a good depth resolution is not obtained using data from these bands alone. Finally, a shallow shot W104 at nominal range 27 km was selected. Data from three 10 Hz wide frequency bands centred at 20, 40 and 90 Hz was selected for processing. In addition, a low signal level band at 55 Hz (estimated 12-18 dB lower level) was included. Table 5.9 lists the main peaks of the ambiguity surfaces. Ambiguity surfaces are plotted in Figure B.5.

Shot Processing

W104 D BB 20+40+90 -2.87 27100 26440 90 83

Table 5.9 Same as Table 5.6. Shot W104.

Best results were obtained when including data from the 20 Hz and 40 Hz bands, the second peak of the ambiguity surface is then -0.96 dB down from the main peak. The peak is at correct depth but offset 700 m in range. Including also data from the 90 Hz band, the second peak was -0.79 dB down. Processing using the low-level 55 Hz band alone yielded a good range estimate with a very broad main lobe almost 2.0 dB above the secondary peak, but at a low value of Bartlett energy.