Southern Bangladesh Greg Beresford* (Consultant, Geokinetics),
Phil Johnston, Chevron.
Arbitrarily oriented linear arrays (nominal 25m length) are
more effective than bunched phones for reducing ambient noise on seismic lines laid out through populated villages in
southern Bangladesh. This noise reduction is clearly evident on stack data with typical fold of 40. The properties of this noise suggest it’s origin is in many areas dominated
 by cultural activity: village activity, farm machinery and
 people walking on the line sometimes in close proximity to individual geophone elements. Trees grown in these village areas for food and shade are believed to couple a significant amount of wind noise into the ground. Both the
cultural noise and the tree-coupled wind noise have directional properties which are not related to the seismic line orientation.
In these village areas, obstructions often prevent laying out in-line arrays. A practical operational procedure in these areas has been to bunch phones. Field experiments have shown, however, that arbitrarily oriented linear arrays are
more effective at reducing ambient noise than bunched  phones. This technique has been used successfully during  production acquisition through villages in southern Bangladesh.
Introduction
Population density in Southern Bangladesh is high and village activity produces high levels of ambient noise during seismic acquisition. The landscape is partially inundated and scattered with villages and trees situated on higher ground and man-made levee banks. An effective
receiver group for shot-generated noise in many parts of Bangladesh has been shown to be an in-line array of about 25m. Unfortunately, in-line arrays on seismic lines passing through villages are often impossible to lay out due to
obstructions and the traditional compromise has been to use a bunched phone array. In this paper we show that due to the omni-directional nature of the ambient noise, seismic data quality is significantly improved by retaining the 25m
array length wherever possible and avoiding obstructions  by allowing the array to be layed out in any direction.
In December 2005 a set of pre-start up tests was conducted
 by Chevron and Grant Geophysical for a 2D seismic  program in Southern Bangladesh. This survey was to include explosive sources on land and airgun sources across
numerous rivers. The receiver types included geophone arrays and flushed hydrophones on land and OBC phones in the rivers. The start-up experiments included shot depth
tests into 2 parallel receiver lines separated by about 4m
and extending over a distance of about 1km. One line was a noise spread with a 5m group interval deploying bunched
 phones (12 in a 1m diameter circle), the other was a section of the production spread with a 25m group interval
deploying an in-line 12 element 25m array. The two lines ran parallel and were separated by about 4m. Although the idea of slant arrays had not been considered at the time of these tests, the data acquired allows us to compare the noise
recorded by the in-line and bunched phones in what was an area of high village activity with numerous trees coupling wind noise.
Noise amplitudes
By selecting every 5th  receiver on the noise spread close  pairs of receivers can be analysed for similar noise conditions; see Figure 1.
The top traces show higher levels of ambient noise
consisting of wind noise and village activity (footsteps). For analysis, two traces from the production spread had
 bunched phones and were removed; two traces had shortened arrays and were retained. Clearly the in-line
array reduces ambient noise even though it may not arrive along the receiver line.
The spectra in Figure 2 show a noise reduction of at least 9dB for the in-line array at the peak frequency of 11Hz. This is a conservative estimate as two of the traces in the
 production spread are from shortened arrays.
Figure 1: Noise strip recorded on the noise spread (top traces) and the parallel production spread (bottom traces)
offset by only 4m. The top traces show higher levels of ambient noise consisting of wind noise and village
activity (footsteps).
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Nature of the Noise
The shot depth tests carried out in the pre-start up trials were recorded into the noise spread. Figure 3 shows the FK
spectrum of a record with the shot in the centre of the noise spread: a 0.5kg charge at 21m depth. Three dispersive modes can be identified with frequencies from about 5Hz to 20Hz. These are largely un-aliased at a 5m group
interval, but the dotted lines show where this noise would alias with the production group interval of 25m. The buried source excites the faster modes (2,3) traveling in the deeper layers with dominant frequencies 7Hz to 11Hz. High amplitudes align with the 1st notch of the 25m in-line array
 – its response is shown at the bottom of Figure 3. This
explains the effectiveness of the 25m array for attenuating
in-line coherent noise generated by a buried explosive. Without the array and with a 25m group interval, the strongest noise would be un-attenuated at k=0 (red arrow in
Figure 3).
For ambient noise generated by distributed surface sources of relatively low energy (eg. cultural sources: farm machinery, trucks, human foot traffic), it is not surprising that mode 1 is most strongly excited. This can be seen in
Figure 4, which shows human footsteps very close to one of the bunched phone arrays in the noise spread; the record is a noise strip with no shot fired. Close footsteps produce a direct wave and a slower mode which corresponds to mode
1 in Figure 3. It is 12Hz at k=0.1 m-1. Very close sources are effectively in-line. Note the higher frequency content of (9 to 20Hz) compared to that for modes 2,3 in Figure 3. A very high percentage of this noise is beyond the notch for a
25m array – shown on Figure 4 as a dotted line labelled ‘0’ at k=0.04 m-1. A 12 element array with geophone spacing
of 2.08m will not alias this noise until about 22Hz. The noise will not wrap back inside the notch until about 40Hz. Even for noise arriving at an angle of 47 degrees to the in- line direction, frequencies above 10Hz will occur beyond the notch and be attenuated.
For cultural noise sources more distant from the array and
more numerous, the apparent velocities in-fill the wedge in FK defined by the lower limit phase velocity of 120m/s; see Figure 4. (The group velocity is lower still: roughly 80m/s.) This effect is shown for a different section of the noise
spread where more distant human footsteps appear as hyperbolic; see Figure 4. This is the spectrum of all ambient noise sources (cultural and wind) impinging on the receivers together. This is the omni-directional noise which
can effect data so severely in Bangladesh. Clearly, on average this FK spectrum will not depend on array
orientation and we can expect a 25m slant array to exhibit the same spectrum as the in-line array for this type of noise.
Figure 4 shows that 60% of the FK spectrum is outside the notch tramlines. For noise with a uniform power spectral density, one would expect the array attenuation to be considerably less than 8dB (0.4 reduction). The average
attenuation shown in Figure 2 does reach 8dB or more  because the noise has a peak frequency of 11Hz and most energy lies outside the notch tramlines.
An omni-directional array response can be computed for a
12 element 25m array by integrating responses for a fixed wavenumber over all horizontal angles of incidence. The array response which results is shown in Figure 5 with the
location of the in-line array notch labeled in red. Attenuation at this wavenumber and beyond is about 0.4 (- 8dB). If the dominant frequency of the noise is about 11Hz and its velocity 120m/s we expect wavenumbers of about
-13.5
-51dB
Bunched phones
In-l ine array
 A ve ra g e am p li tu d e s p ec tr a
4 0 H z
Figure 2: Average amplitude spectra of the noise strip
traces shown in Figure 1. The in-line spectrum on the right is 9dB less at 11Hz (peak) than the bunched phone
spectrum on the left.
Figure 3: The three modes (1,2,3) of ground roll generated by a buried charge - 0.5kg at 21m - into the
noise spread of bunched geophones at 5m spacing.
17 EG S an Antonio 2007 Annual Meeting
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Slant arrays to reduce ambient noise
0.1m-1. If the waves are distributed uniformly in angle, then a 12 element 25m array will give an average attenuation of 8 to 10dB (see Figure 5).
CMP stacks
A short segment of the seismic line on which the tests were
conducted was selected to compare data quality for  bunched phones versus in-line arrays. Stacks were formed from traces originating exclusively from bunched phones or
exclusively from in-line arrays. This segment of line had about an equal number of both receiver types. This resulted in two stacks with an average fold of 30 to 40. Figure 6 shows the stack with in-line array receivers on the left and
the stack with bunched phones on the right. There is a marked improvement in data quality for the stack with in- line arrays. Deeper reflections in particular are almost lost with bunched phones but are quite clear with in-line arrays.
The stack comparison given in Figures 6 is from a  production line. Arrays and bunched phones are not co- located, so is not a controlled test. The bunched phones
were deployed when an obstruction prevented the use of an in-line array. In this survey area, obstructions tend to occur in villages, where ambient noise sources are in close
 proximity to the line. So the ambient noise level is probably
somewhat higher in those receiver locations where bunched  phones were deployed.
After acquiring the first production line, it was decided that
there was sufficient evidence to support the use of slant arrays whenever possible and to revert to short arrays or
 bunched phones only as a last resort. Analysis of data from many seismic lines across a wide range of conditions in the
survey confirms that slant arrays perform as predicted by the noise analysis present here.
Conclusions
Densely populated areas in Bangladesh give rise to very high levels of omni-directional ambient noise which in the South of the country tend to excite surface waves with
wavelengths well attenuated by 25m linear arrays. When obstructions prevent an in-line array from being deployed, it is better to use a slant array when possible and choose
 bunched phones as a last resort. Slant arrays produce at
least 8dB more attenuation of ambient noise than do  bunched phones and are no worse than bunched phones in
reducing in-line, shot-generated noise.
Acknowledgments
The authors would like to thank Chevron for permission to  publish this material and Geokinetics for their financial support
Figure 4: FK spectra of the noise strip for 2 sections of the
noise spread. Top: close footsteps that are seen to excite mode 1 (Figure 3), and bottom: distant and distributed noise sources which in-fill the FK spectrum with higher apparent velocities and a lower limit of 120m/s. The
dotted lines show k notches for 0 and 47 degree azimuths.
Figure 5: The omni-directional array response of a 12
element 25m array. Beyond the in-line notch at 0.4 m-1,
attenuation exceeds 0.4 (-8dB).
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Slant arrays to reduce ambient noise
Figure 6. Comparison of stack sections over the same CDP range on the pre-start up production spread. Both sections have a fold of about 40. On the left only receivers with an in-line geophone array have been stacked and on the right only receivers with bunched phones have been stacked. Reflections are much clearer on the left confirming the reduced ambient noise on the
in-line arrays.
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EDITED REFERENCES
 Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2007
SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web.
REFERENCES
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