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	Title     OBS-2013: Instrumentation notes
	By O.M.	Date 06 February 2014	Version 0.1

Click to see OBS-2013 photo album.

SURVEY, PROJECT DESCRIPTION

GEOPHYSICAL EQUIPMENT

DATA

Profiles

PRELIMINARY ANALYSIS OF MINI-STREAMER IRREGULARITIES

Description of irregularities

1. First breaks (= direct arrival of air gun signal, following a straight line from source to receiver) arrives much too early.
2. From FFID = 2809 the frequency content of the signal changes considerably. The recording log does not indicate any changes in system configuration - like number of guns firing, air pressure, de-activation of filtering in the GEODE mini-streamer recording system. The sudden change in signal characteristics is easily observable both in the time and frequency domain (see plots below).
3. In shot gather displays, channel 8 - the most remote channel - appears to be nearest to the vessel.
4. The frequency plots show an unusual amount of signal energy in the very low-frequency part of the spectrum.

Conclusions

Based on error analysis below, we have arrived at the following conclusions:

1. The Mini-streamer tail buoy has probably been snagged into airgun floats, causing the streamer to take the shape of an "U" behind the ship. Channel no 8 was thus located nearest to the ship.
2. Before FFID=2809 offset between source and center of nearest group (no. 8, due to the streamer U-shape mentioned above) is approx. 24 meter, and after FFID=2811 it is approx. 9 meter. Offset is of course much too small. It is assumed that hydrophone group no. 8 has been damaged because of this. Normal offset should be approx. 102 meter.
3. The frequency spectrum indicates presence of unusual amount of signal energy in the very low frequency part (0-10 Hz). We believe this is due to the streamer U-shape causing lots of oscillations.
4. After FFID=2811 it seems the streamer positioned itself much deeper in the water. By looking at bottom reflections before and after FFID=2811 the depth difference seems to be 15.6ms, which is two-way travel time; thus increased depth should be (15.6ms * 750m/s) = 11.7 meter. This will increase the streamer's ability to capture low frequency signals.

Mini-streamer layout, source position, expected First Break times

• Mini-streamer:
  • Number of hydrophone groups: 8 ea
  • Group center spacing: 6.25 m
  • Number of hydrophones per group: 12
  • Hydrophone spacing within group: 0.5 m. Thus group length is (11*0.5 m) = 5.5 m
  • Lead-in length: 150 m
  • Lead-in deployed, from stern: 135 m
  • It appears that channel 8 is defective (no reflections).
• Source:
  • Distance from stern to source: 36 meter
• Offset between center of source, and midpoint of first group in streamer should thus be: (135m-36m) + (5.5m/2) = 101.75m. Assuming sound velocity of 1500 m/s, first break should arrive after 0.0678s = 67.8 ms

Offset between source and first group: Before FFID 2809, group 7 (as no. 8 is defective) has First Break after 20ms; which means that ch. 8 should have First Break after (20-4.2) = 15.8ms (see below). That means offset is (15.8ms * 1500m/s) = 23.7m. After FFID=2811, offset is (13.5ms-4.2ms)*1500m/s => (9.3ms*1500m/s) = 9.3m.

• Mini-streamer group center spacing is 6.25 m. With a water sound velocity of 1500 m/s the first break time difference between each channel should be: (6.25 m / 1500 m/s) = 0.0042 s = 4.2 ms

The 8 individual channels in each shot gather shown. Measured First Break time difference between two successive groups: 4.1 ms - expected 4.2 ms - that's a fairly good match. We can thus assume active section is aligned to ship's long axis.

Dimensions:

  Lead-in ............................: 150.00 m
  Active section: (7*6.25m) + 5.5m  ..:  49.25 m   (8 hydrophone groups 6.25 m apart, one half group (group length 5.5 m) at each end.
  Tail ...............................:  12.50 m
                                        --------
                       Total length ..: 211.75 m
                       -------------------------

Viewing data in time domain

Data in time domain - click to enlarge

	Difference between arrival time of bottom refection before and after FFID=2810 measured to 15.6 ms (TWT), which amounts to 11.7 meter.

Frequency domain analysis

Data in frequency domain - click to enlarge