Archive of Digital Boomer and Chirp Seismic Reflection Data Collected During USGS Cruises 01RCE05 and 02RCE01 in the Lower Atchafalaya River, Mississippi River Delta, and Offshore Southeastern Louisiana, October 23-30, 2001, and August 18-19, 2002
In October of 2001 and August of 2002, the U.S. Geological Survey conducted geophysical surveys of the Lower Atchafalaya River, the Mississippi River Delta, Barataria Bay, and the Gulf of Mexico south of East Timbalier Island, Louisiana. This report serves as an archive of unprocessed digital marine seismic reflection data, trackline maps, navigation files, observers' logbooks, GIS information, and formal FGDC metadata. In addition, a filtered and gained GIF image of each seismic profile is provided.
The archived trace data are in standard Society of Exploration Geophysicists (SEG) SEG-Y format (Barry and othes, 1975) and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU). Examples of SU processing scripts and in-house (USGS) software for viewing SEG-Y files (Zihlman, 1992) are also provided. Processed profile images, trackline maps, navigation files, and formal metadata may be viewed with a web browser. Scanned handwritten logbooks and Field Activity Collection System (FACS) logs may be viewed with Adobe Reader.
For more information on the seismic surveys see http://walrus.wr.usgs.gov/infobank/g/g601la/html/g-6-01-la.meta.html
These data are also available via GeoMapApp (http://www.geomapapp.org/
) and Virtual Ocean ( http://www.virtualocean.org/
) earth science exploration and visualization applications.
The USGS Florida Integrated Science Center (FISC) - Coastal and Watershed Studies in St. Petersburg, Florida, assigns a unique identifier to each cruise or field activity. The method used to assign the identifier for the activity is as follows: YYPPP##, where YY stands for the last 2 digits of the year in which the fieldwork is conducted, PPP is a 3-letter abbreviation for the project the data are collected for, and ## is a 2-digit event tag that represents a discreet leg or time period of fieldwork. For example, 01RCE05 indicates the data were collected in 2001 for the River Contaminant Evaluation (RCE) Project and the data were collected during the fifth field activity for that project in that calendar year. The naming convention the center uses for each seismic line is as follows: yye###a, where yy is the last 2 digits of the year in which the data were collected, e is a 1-letter abbreviation for the equipment type (i.e., c for chirp and b for boomer), ### is a 3-digit number representing a specific track, and a is a 1-letter abbreviation representing the section of a line if recording was prematurely terminated or rerun for quality or acquisition problems. The boomer is an acoustic energy source that consists of capacitors charged to a high voltage and then discharged through a transducer in the water. The transducer is towed on a sled at the sea surface and emits a short acoustic pulse, or shot, that propagates through the water and sediment column. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), detected by the receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (e.g., 0.5 s) and recorded for specific intervals of time (e.g., 100 ms). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced. For 01RCE05 boomer data collection, an Applied Acoustic Engineering CSP 300 power supply provided 100 joules per shot. Reflected energy was received by a SN Technology NexGen streamer and recorded by Triton Elics Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced every 2 m. Only data received by elements 7 and 8 were summed. The streamer was positioned parallel to the boomer sled and laterally separated from it by about 7 m. The sample frequency of the data was 12 kHz for lines 01b001-01b007 and 24 kHz for lines 01b023-01b028. Trace length for all tracklines was 100 ms. Based on survey speeds of 3.5-4 knots and a shot rate of every 0.5 s, shot spacing was about 1 m. No data were collected for 01RCE05 lines 01b008-01b022. No boomer data were collected during Cruise 02RCE01. The chirp system uses a signal of continuously varying frequency. The sound source and receiver is a towfish, which is typically flown 2-5 m above the seafloor. For each recorded shot of chirp data, 3 channels of trace data are collected. Channel 3 is the "real," or in-phase component of the signal, channel 2 is the "imaginary," or quadrative component of the signal, and channel 1 is the "envelope," or product of channels 2 and 3. Only channel 1 was used to produce the profiles presented here. However, all channels are included in the SEG-Y data files. The seismic source employed for both 01RCE05 and 02RCE01 chirp data collection consisted of an EdgeTech X-Star SB-424 towfish running Triton Elics Delph Seismic FSSB software. For 01RCE05, the sample frequency of the data was 25 kHz, and all tracklines were recorded to 48 ms. Based on survey speeds of 3.5-4 knots and a shot rate of 0.122 s, shot spacing was about 0.25 m. For 02RCE01 chirp data collection, the sample frequency of the data was 24 kHz, and all tracklines were recorded to 40 ms. Based on survey speeds of 3.5-4 knots and a shot rate of 0.125 s, shot spacing was also about 0.25 m. The frequency range for both cruises was 4-24 kHz. No data were collected for 01RCE05 lines 01c001-01c004. Digital data were not recorded for 01RCE05 lines 01c005-01c006 and 01c013-01c022. The original trace files for 01RCE05 lines 01c008-01c010 and 02RCE01 lines 02c07 and 02c09 were divided into two or more trace files (e.g., 01c008 became 01c008a and 01c008b) because the original total number of traces exceeded the maximum allowed by the processing system. Water depths shown on the chirp profiles are relative to the depth of the towfish and not to sea level. Furthermore, the raising or lowering of the towfish during a survey (to avoid obstacles or follow relief) produces a relative shift in the multiple reflections seen in the profile, which may be confused as a geologic feature. Any elevation change of the towfish is recorded in the science logbooks and is evident on the seismic profiles by steep, abrupt shifts in the seafloor return. The unprocessed digital seismic data were stored in SEG-Y format, which is a standard digital format that can be read and manipulated by most seismic processing software packages (Barry and others, 1975). The SEG-Y file format includes a 3,200-byte descriptive header that contains detailed information regarding the data acquisition and processing parameters. All data presented here are stored in SEG-Y, integer, Motorola format. The SEG-Y formatted trace files have a .TRA extension. Additional recording parameters for each trace file can also be found in the .PAR file associated with each .TRA file. However, the .PAR and .PLN files included here are only needed to process or display the data with Triton Elics Delph Seismic software. Also included on these discs are example Seismic Unix processing scripts that allow the user to strip off navigation fixes from the SEG-Y headers, along with a fix for every 500 shots (boomer) or every 1,000 shots (chirp), and produce a filtered and gained GIF image of each profile. These images can then be displayed using a variety of PC-based shareware programs such as ImageMagick (Unix, Linux) or a web browser. USGS software for viewing the SEG-Y headers is also provided. For both cruises, DGPS navigation was provided to the acquisition system about every 1 s by a CSI Wireless DGPS Max WAAS/Beacon DGPS receiver. The accuracy of this receiver is within 5 m. The navigation data, which have not been corrected to reflect the 20-m offset between the boomer shotpoint and the GPS antenna or the roughly 10-m offset between the chirp shotpoint and the GPS antenna, were used to generate the trackline maps presented here. Position fixes for every 500 shots (boomer) or every 1,000 shots (chirp) and for the start of lines are also provided as an aid for registering of the data after plotting. All navigation files are stored as ASCII text files. The trackline maps provided in this archive are set in geographic coordinates, NAD83 (unprojected). They were created using ESRI GIS software ArcView 3.2 and 8.1, exported to Adobe Illustrator for further editing, and saved in JPEG format. These JPEG images are viewable with a web browser. Also included on this disc are the ArcView map documents and shapefiles used to create the trackline maps. The map documents are compatible with ArcGIS 8.1 (Windows). The shapefiles may also be viewed using other versions of ArcView or public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Mac OS X, Unix, Linux), available from the ESRI website at <http://www.esri.com/software/arcexplorer/index.html
>. Field Activity Collection System (FACS) logs and scanned versions of the handwritten logbooks are provided as PDF files.