Data processing and archive

Seismic reflection data

The seismic reflection data from all three Polar Star cruises were digitally recorded in SEG-B multiplexed format on 9-track tapes using a DFS-V seismograph. The sample interval for the field data was 2 ms and the record length varied from six to eight seconds. A deep-water recording delay of up to four seconds was used in the deep-water basins.

Following each cruise the field data were computer-processed at the USGS data processing facility in Palo Alto, California. The data processing sequence was similar for each of the three data sets. Field data were demultiplexed and resampled to an 8 ms sample interval. Bad shot records and noisy traces were deleted. Deep-water delay was added back to the data to produce a consistent record length of ten seconds for all reflection profiles. Normal move-out (NMO) correction was applied to each shot, although the correction was quite small because the maximum source-receiver offset was approximately 350 meters (150 meter streamer length plus 200 meter leader). The velocities used to calculate the NMO correction were determined from the sonobuoy refraction data. Each shot record was stacked to form one trace per shot. The shot records consisted of two-channels, except for 1993 lines 9301 through 9305, which were 12-channel. Following the shot-stack spiking deconvolution, a 10-40 Hz bandpass filter, a 500 ms AGC operator, and a water column mute were applied to the data. Julian day and GMT for each shot were written in the trace headers and the data were output as 32-bit floating point SEG-Y files. These data files and tiff image files of the seismic profiles can be downloaded from Table 2.

Sonobuoy refraction data

The sonobuoy refraction/wide-angle reflection data were originally recorded on analog tape and later digitized at an 8 ms sample interval using USGS software. Limitations of this software enabled only every second shot to be digitized for the 1988 sonobuoy records, but subsequent software improvements enabled all shots of the 1992 and 1993 sonobuoy data to be digitized. Source/receiver offsets were calculated from direct arrival times and then written in the SEG-Y trace headers in bytes 37-40. The shot numbers in the sonobuoy trace headers (bytes 21-24) are the shot numbers of the reflection line (bytes 181-182) along which each sonobuoy record was acquired. This information can also be found in the Sonobuoy Summary Table. Noisy traces and data with offsets beyond interpretable arrivals were truncated. Trace record length was limited to that needed to record the direct arrival. An 8-32 Hz bandpass filter and a 500 ms AGC operator were applied to the data. Each sonobuoy record was written as a separate file in 16-bit integer SEG-Y format. Offset-based trace spacing for plots of the sonobuoy records is especially important for the 1992 data, where ice conditions caused numerous changes in ship course and speed.

The sonobuoy data files and tiff images files of the seismic profiles can be downloaded from Table 3. The variable trace spacing in the tiff images is a function of offset.