Reprocessed Multichannel Seismic Reflection Data
Recorded in Cook Inlet, Offshore Alaska, Line 755

This data set contains multichannel seismic data recorded in the Western Gulf of Alaska, Cook Inlet, in 1975 (98 Channels, 2ms sample interval, 6s record length, 33.5m shot interval, 53.46km processed line length), reprocessed by the USGS in 2003 through the point of Dip Moveout (DMO), post stack migration, and depth conversion. The original data were recorded in by Petty Ray Geophysical under contract to the USGS..

The disc structure of this dataset is contained in file README.1ST.

USGS personnel who contributed to this data set:

Mike Fisher, Western Region, Chief Scientist for the project that collected the data.
John Miller, Central Region:  performed the reprocessing.
Harold Oliver and Nick Zihlman, Central Region:  prepared the data set for release.

The following files are included:

1. Fifteen (15) files in seg-y format containing unprocessed shot records
2. Six (6) files in seg-y format containing stacked data at various stages of processing.
3. Three (3) files in pdf format containing images of the unmigrated, migrated, and depth converted data
4. One (1) file in jpeg format containing a sketch map
5. One (1) file in text format containing latitude and longitude
6. One (1) file in jpeg format containing the observer reports
7. Two (2) files in text format containing RMS velocities used for 1) stacking and 2) migration/depth-conversion.

Navigation: The line location is given in this generalized map (file map.jpg) of the Cook Inlet area.  The line is straight;  latitude and longitude of every 100th shotpoint is given in file navigation.txt.

Reprocessing:  The data were reprocessed by USGS between January and May, 2003 using the ProMAX seismic data processing system, owned by the USGS.  The data were first demultiplexed from the original field tapes, written to ProMAX internal  format and then were then processed as follows:  geometry assigned (17.75m CDP interval), initial velocity analysis, DMO, 2nd velocity analysis, stacking, post-stack enhancement, migration, bandpass filter, and depth conversion.  After processing, the raw data were written to SEG-Y disk images in groups of  100 shots per file and six (6) versions of the stacked data at various stages of processing were written to seg-y files.  Three images of the stacked data: DMO-stack, migrated-time, and migrated-depth, were created and stored in pdf-format files.  The velocities used for stacking were stored in a text file (stackvels.txt).  Those same stacking velocities smoothed used for both migration and depth conversion were stored in another text file (migdepthvels.txt).

Data Recovery:  6.35% of the data could not be recovered from the original field  tapes because the tapes had degraded over time. There should  have been 1559 field records, but only 1,460 records could be recovered  from the tapes. For example, on the first output file, Field Files 25-28,  54, and 56-61 could not be recovered from the original tapes.

The following are the Channel Assignments on the field data records:
Channel Description Offset Range Group Interval 
-1 Auxiliary    
1-36 Data  654m - 1481.5m 33.5m
37-48 Mini-spread 292.25m - 108m 16.75m 
49-84 Data 1448m - 275.5m 33.5m
85-86 Auxiliary  
87-98 Mini-spread 292.25m - 108m 16.75m 
Channels used in the processing:  The following 77 channels were used in Processing: 
1-36, 49-84, 90, 92, 94, 96, 98
The use of these channels resulted in a receiver spread that consisted of source-receiver offsets from 2654.0m to 108m at a group interval of 33.5m 
Dates of Recording:  Line 755, Cook Inlet, offshore Alaska was recorded by Petty Ray Geophysical in 1975. The actual date of recording is ambiguous.
According to paper documentation (file 755obslog.jpg) available, a Production Processing Request" was submitted on July 26, 1975 by a person named COTRONE. Work order number was 4063; User Group IDs were: 1050 and 1031-18 A "Field Reels Recap" form was submitted on August 4, 1975 by crew 1031-18, which corresponds to the User Group ID above. The Analyst's name was Friar. A notation of WO4063 was made, which corresponds to the Work order number above.

Summary of the Six (6) seg-y files containing different versions of the processed data:
  File Name
(located in folder segy)		 Description of Data in File
pdf-format image (located in folder pdf)
1 755cookinlet-unmig-time.sgy Stacked, unmigrated data
2 755cookinlet-dmo-time.sgy Dip Moveout (DMO), veleocity analysis, stack 
3 755cookinlet-dmo-post-stack-time.sgy Post-stack enhancement applied after DMO
dmostack.pdf (pdf-format image)
4 755cookinlet-migrated-time.sgy Post-stack migration of dmo-stack 
5 755cookinlet-migrated-time-filter.sgy Post-migration bandpass filter
migstack.pdf (pdf image)
6 755cookinlet-dmo-migrated-depth.sgy Depth conversion of filtered migration
depth.pdf (pdf image)

Summary of the fifteen (15) seg-y files containing the 1,460 field records:
  File Name
(located in folder segy)		 Shot Numbers Field File Numbers 
1 755cook_1_100.sgy Shots 1-100 1-111 
2 755cook_101_200.sgy  Shots 101-200 112-223 
3 755cook_201_300.sgy  Shots 201-300 224-328 
4 755cook_301_400.sgy  Shots 301-400 329-436 
5 755cook_401_500.sgy  Shots 401-500 437-536 
6 755cook_501_600.sgy  Shots 501-600 537-639 
7 755cook_601_700.sgy  Shots 601-700 640-740 
8 755cook_701_800.sgy Shots 701-800 741-843 
9 755cook_801_900.sgy Shots 801-900 845-947 
10 755cook_901_1000.sgy  Shots 901-1000 948-999;  0-52 
11 755cook_1001_1100.sgy  Shots 1001-1100 53-162 
12

755cook_1101_1200.sgy

  Shots 1101-1200 163-265 
13 755cook_1201_1300.sgy Shots 1201-1300 266-368 
14 755cook_1301_1400.sgy  Shots 1301-1400 369-494 
15 755cook_1401_1460.sgy  Shots 1401-1460 495-559 
Header Words:  A header word named Source was created and was stored in bytes 17-20 of the seg-y trace header. For processing, this number can be considered the consecutive shotpoint number.
The Field File numbers range from 1-999 and 0-559.
The Source numbers range from 1-999 and 1000-1559.

File Numbers:  A complete list of the file numbers in each file can be found in file README.TXT in the segy folder.