Naxys:
Test of Low Frequency Acoustic Combustion Source (LACS)

May 4. - 6., 2004

University of Bergen (UoB)
Dept. of Earth Science
Allé gt. 41, N-5007 Bergen, Norway
Tel: (+47) 5558 3600
Fax: (+47) 5558 3660
http://www2.geo.uib.no
A line in Byfjorden near Bergen was shot with both LACS and a BOLT PAR airgun 20 cu inch.

 

1. Survey Area

Map of survey area is shown below.


Fig. 1. Overview survey area, click image for PDF file

 

2. Instrumentation

The instrumentation system is shown in fig. 2 and described in table 1.

Fig. 2. Overview of instrumentation system. These components not shown: Power supplies for GPS (Item 10), LACS trigger I/F (Item 7), Naxys Filter/Amp (Item 6).

 

Item Description Supply Comments / Links
1 LACS: Low Frequency Acoustic Combustion Source Naxys  
2 Airgun BOLT Par 1900 20 Cu inch UiB  
3 Mini-streamer, Teledyne Mod. 28420
20 ea B-1 Hydrophones on 0.52 meter centers
 UiB Layout:
http://www2.geo.uib.no/equipment/seismic/marine_streamer/geology_streamer/kobling.pdf

Hydrophone element B-1 datasheet:
http://www2.geo.uib.no/equipment/seismic/marine_streamer/geology_streamer/B-1.pdf
4 LACS Control unit Naxys  
5 6 ea bottles compressed air, 280 Bar, reduction valve to 120 Bar Naxys  
6 Naxys filter / amplifier, 21 or 40 dB BNC O/P Naxys  
7 Naxys trigger I/F UiB Just an integrated circuit, 74HC14 (Hex Schmitt trigger Inverter)
8 Airgun Controller GUNCO UiB  
9 Amplifier / Filter, Mfr: Nortronic Norway, Mod: Universal Filter Type 707 UiB  
10 GPS receiver, Mfr: Garmin, Mod: GPS35-HVS
Setup to output $PGRMF sentence, 1 Hz rate		 UiB http://www2.geo.uib.no/equipment/misc/gps/garmin_gps35/garmin_gps_35.htm
11 LinAcq Data Acquisition System,
using ComputerBoards A/D-converter PCI-DAS1602-16		 UiB http://www2.geo.uib.no/linacq/pci-das1602-16/
12 LinAcq I/O Panel UiB  

Table 1: Instrumentation system components.

 

3. Data

IMPORTANT: Nav data is stored as UTM coordinates (meter resolution) in Seismic Unix trace headers. These header fields are of 32-bit integer type. Raw position data from GPS - with higher resolution - has this format:

  • Latitude: ddmm.mmmm, where
    dd = degrees, mm.mmmm = minute, with fraction of minute
  • Longitude: dddmm.mmmm, where
    ddd = degrees, mm.mmmm = minute, with fraction of minute

Raw GPS data stored in the two last columns in nav data files. You must extract events according to time information in Seismic Unix trace headers!

Test # Description Setup Data file Recording parameters Raw GPS nav data files
1 LACS signature test
  1. Mini-streamer: Vertical, center actve section at 30 m depth
  2. LACS at 5 m depth
  3. Depth approx 300 m
  4. Item 6: 40 dB
  5. Item 9: 0 dB gain, 2Hz - 90 kHz BP filter
  6. Recording triggered 128 ms before airgun fires
 5-May-2004/data/testing-1.su 1000 Hz, 1024 ms, ± 10 V range Here
2 Electronics noise test
  1. Item 4:
    1. 50 ohm termination input
    2. Gain - see next column
  2. Item 9: As test #1
  3. Grounding: Star topology, center = item 4, arms to item 9, item 11 and power supply item 4.
  4. GUNCO, item 8: Set to manual gun tuning, 10 ms delay
 6-May-2004/data/electronics-noice.su

Change in setup during recording:
(fldr = trace header parameter)

  • fldr < 23 : 40 dB gain item 4
  • 23 < fldr < 39: 21 dB gain item 4
  • 39 < fldr < 55: Removed grounding item 11
  • 55 < fldr < 88: Grounding item 11 moved to item 12, A/D channel #2 BNC ground
  • 88 < fldr < 106: Grounding item 11 moved to item 12, chassis connection, screw on rear side
  • 106 < fldr: Grounding item 11 back to chassis connection, screw near power supply filter assy.
 As per trace header values  
3 Shooting line, source = LACS
  1. Item 4: 21 dB gain
  2. Item 9: 20 dB gain, BP filter parameters as above
  3. Item 3, mini-streamer: Offset = Ask Peter S ("10 m shorter then clothing mark")
  4. Item 1: 5 m depth
 6-May-2004/data/testing-5.su

Start of recording: 8:30:30
End of recording: 9:32

 As per trace header values Here
4 Shooting line, source = BOLT airgun 10 cu inch As per test # 3, except that BOLT Airgun at 5 m depth 6-May-2004/data/testing-6.su

Start of recording: 10:19:13
End of recording: 11:16:13

 As per trace header values Here
5 Shooting line, source = LACS (higher pressure?? Ask Peter S) As per test # 3 6-May-2004/data/testing-8.su

Start of recording: 13:29:11

LinAcq screenshots.

 As per trace header values

(new: 2000 Hz sampling rate)

 Here

Table 2: Test descriptions.