Chirp seismic reflection and navigation data collected in Chesapeake Bay during USGS Field Activity 2022-020-FA

PROCESS STEP 1:
A C program, jsf2segy, written by Thomas O'Brien (last updated by William Danforth, July 2022), read the raw EdgeTech JSF format files, extracted (-x argument) the real (full waveform) trace data from the analytic traces, and wrote out SEG-Y and CSV files. The CSV files contained ping (shot) number (acquired at a tae of 4 Hz), time, motion data (heave, pitch, and roll) from the motion sensor on the ET3400 tow fish, and tow fish depth below the sea surface from the pressure depth sensor located on the ET3400 tow fish. Heave was also stored in the JSF converted to SEG-Y's extended trace header (bytes 225-226) and tow fish depth in the source depth standard SEG-Y Source Depth trace header (bytes 49-52).
This process step and all subsequent process steps were performed by David S. Foster.

PROCESS STEP 2:
(2.1) A Shearwater Reveal processing flow 0100_ImportSEGY read the SEG-Y files created with jsf2segy and mapped SEG-Y trace header values needed for processing: shot/ping number (FFID); tow fish depth (Source Depth in millimeters (mm)/1000); Navigation Reference Point (NRP) coordinates (both in Source and Receiver coordinates in arcseconds/1000); Date and time (Year, Julian Day, Hour, Minute, Second); and Heave (bytes 225-226 as 16-bit integer in mm). For lines 1 and 2, the Discover acquisition software was set to automatic file changes that resulted in multiple SEG-Y files for each line. These files were concatenated on input to create a Reveal SEIS proprietary file for each line. The trace length was reduced to 100 milliseconds (ms) from the original trace length of 200 ms after it was determined there were no reflections, other than multiples, observed greater than 100 ms within the profiles. The HeaderMath tool in the Reveal flow converted NRP geographic coordinates to decimal degrees, source depth (m) to two-way travel time (TWT) in ms, and heave from meters to TWT ms. Due to acquisition equipment issues, navigation was absent in the Line 3b SEG-Y trace headers, see the horizontal position accuracy report for more details. For this line, Hypack navigation was imported as a CSV file to create Reveal NRP geographic coordinate headers using the SEG-Y time of day trace headers, converted to seconds after midnight, and the Hypack data, also as seconds after midnight. The tool DBWrite wrote a CSV file with the fields NRP_LON, NRP_LAT, FFID, GPS_OFFSET (-19 m), YEAR, DAY, HOUR, MINUTE, SECOND that was used to calculate the layback navigation subsequently described in step 2.4. The tool CTAN converted the real traces to envelope (instantaneous amplitude) and these envelope traces were output to another SEIS format file to be used as input in step 2.2. Both files maintained the imported and calculated headers.
(2.2) A Reveal processing flow, WBTpick, input the envelope trace data and applied static corrections from the Heave header and the tow fish depth (both TWT ms), after the tow fish depth values were smoothed with a 151-point boxcar filter. A custom Python tool (ObspyPicking) was used to automatically pick the peak amplitude within a user defined TWT window from the envelope trace data resulting in consistent picks of the sea floor, which contained some outlier picks, that were saved in a Reveal table file. These files were edited graphically in Reveal by overlaying the picked seafloor on the static-corrected, shifted envelope data and deleting outliers. The edited table was saved, so it could be used later to apply swell filtering (if necessary) and to apply a TVG function hung from the seafloor, rather than starting from TWT 0.
(2.3) A Python script written by Wayne Baldwin was run in Jupyter Notebook to create SQLite tables containing the ET3400 navigation data. Using the Reveal CSV tables created in step 2.1 as input, the script: removed duplicate navigation coordinates caused by the 5 Hz DGPS rate and 4 Hz ping rate and interpolated unique coordinates for each trace, calculated layback (-19 m) tow fish positions relative to the NRP coordinates, and transformed the layback geographic coordinates to Universal Transverse Mercator (UTM) Zone 18 North (N) coordinates. Point navigation data were output to two SQLite tables, one containing point features for every trace, and the other containing point features for the first and last traces, and traces at even 500 shot intervals in between for all lines. The script also created tracklines (polyline vector dataset) from the final navigation for each line. The polyline features were output to an SQLite table with fields containing shot numbers, dates, and times for the start and end of lines, and the length of each line in kilometers. Attribute fields for line name, Survey ID, Vehicle ID, and Device ID were added to records in each table. Tables were saved in the SQLite database for each Julian Day for unique shot fixes, 500 shot intervals, and the tracklines.
(2.4) A Reveal processing flow, named Statics-Layback, input the real trace data SEIS files, truncated trace lengths from 0.2 to 0.1 seconds, merged layback navigation CSV files with seismic headers, and applied static corrections from the Heave header and the tow fish depth (both TWT ms) after the tow fish depth values were smoothed with a 151-point boxcar filter. Due to higher sea states (roll) on Julian Day 206 and early Julian Day 207, lines 1, 2 and 3a required additional static corrections to remove residual heave and tow fish depth variation. The seafloor pick table (from step 2.2) was used to insert the seafloor picks to a Reveal header. The module, HeaderFiltering, applied a 151-point boxcar filter to the picks and the HeaderMath tool subtracted the original picks from the smoothed picks. The difference was used to shift the trace data. The DBMerge tool was used to import the layback navigation coordinates to Reveal headers for all the data. After converting the real trace data to envelope traces with the tool CTAN, processed SEG-Y files (envelope and real) were exported with layback coordinates in the SEG-Y headers. These are the SEG-Y files provided in this data release. Separate envelope trace files were exported after applying TVG for plotting as described in Process Step 4. In the Reveal tool Output-SEG-Y Options a summary of processing and header information are described and added to the EBCDIC header of each SEG-Y file, like the example from Line1_206_env.sgy below.
Example SEG-Y textural file header:
SURVEY_ID: 2022-020-FA Chesapeake Bay; VESSEL: R/V Rachel Carson YEAR: 2023 NAME(GEOMETRY): Line1_206_env.sgy DESCRIPTION: Envelope chirp with static (heave and fish depth) correction
SYSTEM: EdgeTech SB3400, two 2-16 kHz transducer array, PVDF receiver CHIRP PULSE: 3 kHz to 16 kHz, 20 ms pulse length SHOT INTERVAL: 0.25 seconds TRACE LENGTH: 0.1 seconds SAMPLE RATE: 0.02 ms
PROCESSING: Convert EdgeTech JSF format to SEG-Y using jsf2segy C program Input SEG-Y, set trace length from 0.2 to 0.1 seconds, read source depth and heave from SEG-Y headers. Set FFID to start at 1 Apply static shift (heave and source depth), merge layback (-19 m) navigation Output IEEE 32-bit floating point SEG-Y
Shotpoint (FFID) navigation are stored in seconds of arc WGS84 (scaled by -100) in source coordinates, longitude in bytes 73-76 and latitude in bytes 77-80 Coordinate scaler in bytes 71-72, divide by 360,000 for decimal degrees Source depth below the surface (fish depth) is in bytes 49-52 (scaled by -1000) Heave stored in bytes 225-226 (16-bit integer) in millimeters
For additional information concerning these data, refer to the associated data release: https://doi.org/10.5066/P13DCCU3
End of SEG-Y textural file header.

PROCESS STEP 3:
The SQLite database created in Process Step 2.3 was opened with Global Mapper (version 25) and Julian Day tables for tracklines and 500 shot intervals were added to the map view. The trackline Julian Day tables were exported as a single polyline shapefile (2022-020-FA_ET3400_Tracklines.shp). The Julian Day 500 shot interval tables were exported to a single point shapefile (2022-020-FA_ET3400_sht500.shp). These are the shapefiles provided in this data release.

PROCESS STEP 4:
The Seismic Unix Release 42 script Plotchirp created variable density postscript plots of the envelope traces and the envelope traces with TVG applied. The profiles show TWT (seconds) along the y-axis (left margin) and shots (labeled at 500 shot intervals) along the x-axis (top of profile). The postscript images created with Seismic Unix were converted to 150 dpi PNG images using ImageMagick (version 6.9.11-60). The plots are labeled with additional profile details along the bottom of the x-axis. These labels may be truncated for short lines.