Tracklines of Chirp Seismic Lines 15 Through 27 for USGS Cruise GYRE 99002 (L15_27LN.SHP)

GPS navigation for the ship and slant-range distances between the ship and the vehicle (when they were available) were logged to disk in the field

An acoustic transponder was used to determine range from the ship to the sonar fish; however this system did not always function, particularly in deeper water. Slant range and sonar fish depth below the sea surface were used to calculate the layback distance (horizontal offset) between the ship and sonar fish. The fish was assumed to be directly behind the ship, given the azimuth of the ships track. Different methods were used to calculate or estimate the layback distance for different lines.
Lines 15 and 16 of Mosaic Area Two do not have acoustic ranging data available. The SEG-Y file headers contain ship position (UTM zone 15). The ASCII shotpoint navigation files contain geographic coordinates for sonar fish position obtained by comparing fish depth and layback distance from lines 17 and 18 where acoustic slant ranges were good. Because lines 15 and 16 are in relatively shallow water and fish depths did not exceed 300 meters, this method of estimating layback is reasonable. Based on the layback verses fish depth analysis of lines 15 and 16, the horizontal errors may be as much as 50 meters. Layback depends on other variables other than fish depth. Ship speed was not considered in the analysis.
Lines 17 and 18 of Mosaic Area Two were determined to have good acoustic slant range, thus the calculated layback was used to obtain sonar fish position. The ASCII shotpoint contain geographic coordinates for sonar fish position as well as ship position. The SEG-Y file headers contain ship position (UTM zone 15).
Lines 19-27 of Mosaic Area Two had unreliable slant range information due to the length of fish layback and deeper water. Laybacks were estimated for lines 19 and 20 based on the analysis of fish depth and layback in lines 17 and 18, but when fish depths exceeded 300 meters the error in this method increases. Instead another method of comparing water depth from the sonar fish (fish depth and altitude combined) to NOAA gridded Seabeam bathymetry. Offsets between the bathymetry and the water depth from the fish showed the apparent layback for each line. Using this method, the layback was estimated and the corrected navigation compared to the gridded bathymetry. This showed good correlation between water depths and sea floor features. Furthermore, the estimated layback was used to navigate the sidescan sonar data in the mosaicking process, and features on adjacent sidescan sonar lines matched well. The ASCII navigation files contain ship position as well as layback positions. The SEG-Y file headers contain ship position (UTM zone 15).
The processing was done by David Foster, and is summarized in Foster and others (2000)

The navigation text file was imported to ArcView and converted to a shapefile.

Edits to the metadata were made to fix any errors that MP v 2.9.36 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). Attempted to modify http to https where appropriate. The link to the larger work citation had to be fixed. The links to the browse graphics had to be fixed. An online link was added to the cross-reference. The source information was incomplete and had to be modified to meet the standard. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.