Originator: U.S. Geological Survey
Publication_Date: unpublished material
Title: SB-512i SEG-Y trace data
Geospatial_Data_Presentation_Form: digital data
Source_Citation_Abbreviation: SEG-Y SB-512i data
Source_Currentness_Reference: ground condition
Chirp seismic data were collected using an EdgeTech Geo-Star FSSB sub-bottom profiling system and an SB-0512i towfish (0.5-12 kHz), which was towed from the A-frame of the R/V Hugh R. Sharp with the transducer between 10 and 19 m below the water line, and approximately 60 meters astern of the GPS antenna mounted on the aft 01 deck handrail. SonarWiz 6 (version 6.05.001) seismic acquisition software was used to control the Geo-Star FSSB topside unit, digitally log three trace data (envelope, real, and imaginary) in the SEG-Y Rev. 1 format (IEEE floating point), and record GPS navigation coordinates to the SEG-Y trace headers (in arc seconds of Latitude and Longitude, multiplied by a scalar of 100). Data were acquired using 1 - 2 second (ms) shot rates, a 40-ms pulse length, and a 1 to 6 kHz frequency sweep. Traces were recorded with a 46-microsecond sample interval over lengths between approximately 800 and 1600 ms.
PROCESS STEP 1:
SIOSEIS (version 2018.1.2), Shearwater Reveal (version 4.1), and python were used to process SEG-Y data and extract navigation data. The processing flows and scripts used are summarized below and in the following processing steps.
1) The SIOSEIS script sio_renum read the raw SEG-Y files, extracted the envelope-detected trace, renumbered shots starting from one, and wrote out new SEG-Y files. The original shot numbers, which were assigned by SonarWiz sequentially over the duration of an acquisition session despite SEG-Y file changes, are preserved in the raw SEG-Y data.
2) The SIOSEIS script siothresh_pltwbt was used to predict water-bottom times from the envelope SEG-Y traces in each file and write the predicted two-way travel times for the seafloor to a '*.wbt.csv' file.
3) The Shearwater Reveal flow read_segy called the following modules: SegyTapeRead read the traces. HeaderMath and UTMLatLong converted the navigation reference point lat/lon positions from seconds of arc to decimal degrees, projected them to UTM Zone 18N WGS 84 meters, and wrote each to new header words (NRP_LAT, NRP_LON, NRP_X, and NRP_Y). DBWrite wrote the UTM positions for each shot to an internal Reveal database table '*.shots.db'. Finally, Output wrote the traces to a new file '*.seis' in the internal Reveal format.
4) Each Reveal '*.seis' file was opened in the trace viewer, and the corresponding 'wbt.csv' was chosen for overlay on the seismic traces so that the SIOSEIS seafloor picks could be evaluated and adjusted where needed using the Reveal line digitizing tool. New '*.wbtedit.csv' files were saved for each line with corrected picks. In addition, a new Reveal table '*.tfd.tbl' was created to contain estimated two-way travel time towfish depths for each line, which were digitized along the reflection of the outgoing pulse off of the sea surface. For Line-0006 and Line-0007 no seafloor reflection was present due to a 100-ms recording delay applied during acquisition, so a static towfish depth two-way travel time estimate of 16 ms was used instead.
5) The python script swellfilter512i read each '*.wbtedit.csv' file into a numpy array, interpolated picks to the entire range of shots (if needed), and passed the picks through the scipy.ndimage.median_filter algorithm (specifying a window length of 17 and nearest mode) in order to minimize the effects of sea surface heave on the seafloor picks. The median filtered curve was subtracted from the original edited seafloor curve to produce an output series of time shifts approximating the residual heave to be applied by shot to the seismic traces for heave removal. The final swell filtered seafloor picks and static shift values were output to new '*.wbtsfsh.csv' files.
6) The Reveal flow layback_shift called the following modules: Input read the '*.seis' file and sorted the traces to FFID. The custom Python module ShotlineLayback (developed by Nathan Miller of USGS-WHCMSC) defined the horizontal offset between the GPS antenna and the SB-512i transducer (approximately 60 m astern), which was calculated via Pythagorean geometry per file; where the layback astern from the a-frame block is given by the square root of ((average towfish depth (between 10 and 19 m) + measured vertical offset between the a-frame block and water line (7.5 m))^2 + (the tow wire out beyond the block (48 m))^2); and the total layback is provided by adding the offset from the GPS antenna to the a-frame block (16.15 m). The algorithm interpolated a sail line from the navigation reference point positions (NRP_X and NRP_Y), then computed layback positions for the SB-512i shots by translating them back along the sail line by the total offset, and wrote the calculated layback positions to new header words (SRC_X, SRC_Y, REC_X, REC_Y). UTMLatLong converted the layback positions from UTM Zone 18N WGS 84 meters to decimal degrees and wrote each to new header words (NRP_LAT, NRP_LON, SRC_LAT, SRC_LON, REC_LAT, and REC_LON). HeaderMath converted Lat/Lon values from decimal degrees to seconds of arc with a scalar of 100. DBMerge transferred the swell filtered seafloor picks and static shifts stored in '*.wbtsfsh.csv' files to new header words SWFSF and SWFSHIFT, respectively. Table2Header transferred towfish depth two-way travel times stored in '*.tfd.tbl'to the new header word TFDEPTH. ApplyStatic shifted traces first by the inverse of header word SWSHIFT to apply the calculated heave removal static corrections, then by the header word TFDEPTH to apply the towfish depth static corrections. HeaderMath set header words for navigation coordinate units (geographic seconds of arc with scalar), time basis code (UTC), and chirp sweep characteristics (start/end frequencies and length). Output wrote the processed traces to a new SEG-Y files in which the trace header words SRC_X, SRC_Y, REC_X, and REC_Y represent the calculated layback coordinates. DBWrite wrote new CSV text files for each line containing the layback X and Y coordinates (UTM18N m), layback Lat/Lon coordinates (WGS84 dd), shot, year, day, hour, minute, second, navigation reference point X and Y coordinates (UTM18N m), and navigation reference point Lat/Lon coordinates (WGS84 dd). The output SEG-Y files are included in the archive '2017-002-FA_SB512i_SegyData.zip'.
7) The python script 512ilbtoSQL imported the pre-layback and layback navigation, shot, year, day, and UTC time data from CSV files into a SpatiaLite (version 4.3.0) enabled SQLite (version 3.26.0) database, creating two tables containing point geometries. The first contained records for all of the input shots with unique navigation coordinates, and the second maintained records for the first and last shots, and shots at multiples of 100. A 100-shot interval was chosen because it corresponds to the annotation and tick interval provided along the top of the seismic-reflection profile images. End shots and shots at multiples of 100 may not have unique navigation coordinates. The resulting database columns for each table consist of East, North (layback UTM18N m), Lon, Lat (WGS84 dd), LineName, ImageName, Shot, Year, JD_UTC (DDD:HH:MM:SS), SurveyID, VehicleID, DeviceID, East_o, North_o (UTM18N m), Lon_o, and Lat_o (WGS84 dd). A third table was created to contain trackline geometries generated from the unique navigation point geometries for each line (sorted by LineName and Shot), and the line length in kilometers was calculated. The resulting database columns of the line geometry table consist of LineName, ImageName, Shot_init, Shot_end, Year, JDUTC_init, JDUTC_end, SurveyID, VehicleID, DeviceID, and Length_km.
These process steps and all subsequent process steps were conducted by the same person - Wayne Baldwin.
Contact_Organization: U.S. Geological Survey
Contact_Person: Wayne E. Baldwin
Address_Type: mailing and physical address
Contact_Voice_Telephone: (508) 548-8700 x2226
Contact_Facsimile_Telephone: (508) 457-2310
Address: 384 Woods Hole Rd.
City: Woods Hole