Trackline navigation for Multi-channel streamer seismic-reflection profiles collected by the U.S. Geological Survey along the Delmarva Peninsula, MD and VA, 2014

A SIOSEIS seismic processing software script was used as follows: the raw SEG-D shot files were read with the process SEGDDIN specifying geometrics format; the process HEADER was used to insert time of day into the header based on the start and end times recorded in the HYPACK files and what was entered in the survey log. The process GEOM was used to describe the shot and streamer geometries and to calculate the reflection point (RP) numbers used to gather (sort traces) the seismic line. The process GEOM also set the shot-receiver distance into the trace header of every trace. GEOM type 6 was specified (because positions were absent or bad in the header), which computes a distance from last shot (DFLS) for each shot based on the shot time in the header and the navigation from an ASCII file containing time and position. The ASCII navigation file was derived from the HYPACK data and merged with reflection point files with the Unix join command. The process GATHER was used to sort the shot order traces by the RP numbers (computed by GEOM). A normal move out (NMO) applied a travel time correction to each trace based on time offset and a velocity of 1500 m/s. A script was used to apply layback to the navigation supplied from HYPACK. Using Easting and Northing coordinates (UTM Zone 18 N, WGS84) from the trace headers, the script calculated easting and northing differential values between trace positions. Headings between consecutive traces were calculated using the arctangent function (arctan2(dy,dx)), and reciprocal back bearings were determined using a lookup table. Back bearings were smoothed along track using a moving median function. Layback easting and northing offsets were calculated by multiplying the linear distance between the GeoEel and the shipboard DGPS receiver by the sine and cosine of the smoothed back bearing, respectively. Offset values were then added to the original coordinates to produce layback positions. Lastly, the RP trace gathers were written with the process DISKOX in SEG-Y rev. 1, IEEE floating point format. This step and all subsequent steps were completed by Dave Foster.

A SIOSEIS script was used to stack the RP gathers, apply a bandpass filter and automatic gain control. The process DISKIN read the RP sorted SEG-Y file. The process STACK was used to sum traces, compute the average amplitude for each trace sample, and write the computed samples to one trace. The trace header values of the first trace in the gather were used for the stacked trace. The process FILTER applied a zero-phase bandpass frequency domain filter between 600 and 2000 Hz with a slope of 48 decibel per octave slope. The process AGC applied an automatic gain control (AGC) with a window length of 5 ms. The computed multiplier for each AGC window was reduced by fifty percent. Lastly, the processed stacked traces were written to disk with the process DISKOX in SEG-Y rev. 1 , IEEE floating point format.

A Seismic Unix script was used to read the trace headers of the stacked SEG-Y files and write RP, layback corrected X and Y, year, day, hour minute, seconds only where the original channel number was equal to one (the channel closest to the source). Some CDP gathers did not contain a trace with channel one in the header and were eliminated. An AWK script was used to format the trace navigation with layback-corrected positions in a comma-delimited file (CSV).

Text files created in the previous step were concatenated into a comma-delimited text file, then imported into ArcMap (version 9.3) using 'Create Feature Class from XY Table' and saved as points (UTM Zone 18, WGS 84) in the Esri shapefile format. The point shapefile was converted from points to polylines using VAC Extras Points to Line v2. The 'Line' field was used to define which points were used to generate each polyline feature. Finally, ArcToolbox (9.3) was used to reproject from WGS84 UTM 18N projection to Geographic coordinate system. Additional fields for survey ID, vessel, and seismic device were added as well as a length field, which was populated by using "calculate geometry" in the attribute table based projection WGS84 UTM Zone 18 N.

The online links to the data were updated to reflect the new server hosting the data. Additionally, other small edits could be made to the metadata, such as modifying http to https where appropriate. The metadata date (but not the metadata creator) was edited to reflect the date of these changes.

Added keywords section with USGS persistent identifier as theme keyword.