U.S. Geological Survey Open-File Report 2009–1003
Digital Seismic-Reflection Data from Eastern Rhode Island Sound and Vicinity, 1975–1980
High-resolution seismic-reflection data were collected aboard the R/V Asterias during two cruises in Rhode Island Sound: ASTR75–June in eastern RIS and AST–80–6B in southern RIS (fig. 1; O'Hara and Oldale, 1980; U.S. Geological Survey, 1980). Seismic data were obtained by using a towed EG&G Uniboom system with a Boomer acoustic source. The seismic data were printed on EPC Laboratories, Inc. (EPC), recorders at a quarter-second sweep rate. The Uniboom system operates by emitting acoustic pulses with frequencies of 400 Hz to 8 kHz. Acoustic energy reflected to the ship from the sea floor and subsurface is detected by a single-channel hydrophone array and is converted to an electric signal. The signal is then amplified and filtered to a 400- to 4,000-Hz bandpass (set from 200 to 4,000 Hz on cruise ASTR75–June). The seismic data have 1- to 2-m vertical resolution. Seismic data were collected along tracklines totaling 658 km in length in eastern RIS and 224 km in length in southern RIS.
Ship positioning was determined by using Long Range Aids to Navigation (LORAN-C) navigation, which has an absolute accuracy of 185 to 463 m (U.S. Coast Guard, 1992). Navigation fixes were recorded manually every 15 minutes, at major course changes, and at the start and end points of lines. Ship speed averaged about 9 km/h during the seismic surveys.
LORAN-C time delays were converted to latitudes and longitudes by using LoranGPS software. The cruise in southern RIS used the Group Repetition Interval (GRI) 9960 Northeast U.S. chain and recorded time delays from Caribou, Maine (W), and Carolina Beach, North Carolina (Y), slave transmitters. The cruise in eastern RIS used the GRI 9930 U.S. East Coast chain and recorded time delays from Cape Race, Newfoundland (X), and Dana, Indiana (Z). Time delays from the navigation data based on chain 9960 were automatically corrected in LoranGPS when they were converted to latitudes and longitudes; however, no corrections were available for the 9930 chain, so corrections were calculated and applied manually. Latitudes, longitudes, and time delays for 10 points in eastern RIS were plotted on the original trackline chart. The differences between the plots of latitude/longitude and time delays were measured and averaged. The average correction, a distance of about 1.3 km, was applied to the time delays, which were then run though LoranGPS to generate corrected latitudes and longitudes. The corrected latitudes and longitudes were checked to ensure that they plotted accurately on the original trackline chart.
Seismic-reflection profiles from each survey were digitized and are provided in TIFF, GIF, PNG, and SEG-Y formats. The seismic lines were scanned at 300-dpi resolution and 8-bit grayscale and saved as TIFFs. They were scanned at the USGS Woods Hole Coastal and Marine Science Center with an Ideal Contex Crystal TX 40 scanner using IdealScanOS version 2.5.4 software. TIFFs of the seismic lines in this report are 300-dpi black-and-white images that are in a format consistent with images in previous reports and have a resolution great enough so users can print them on plotters at their original scale (approximately 52 cm wide) with little loss of quality. Also provided in this report are 200-dpi grayscale PNG images that more closely represent the original seismic data; however, these files are much larger in size. Low-resolution GIF images were created from 200-dpi grayscale TIFFs to serve as preview images for rapid viewing. SEG-Y files were created from 200-dpi grayscale TIFFs that were cropped and resampled in Adobe Photoshop CS2, so that each pixel width was equal to a trace and each pixel height was equal to a time sample. These TIFFs were converted to SEG-Y files by tif2segy, a script that utilizes Seismic Unix (http://www.cwp.mines.edu/cwpcodes/) and Netpbm image tools by Andrew MacRae, Saint Mary's University, Halifax, NS. The images used to make the SEG-Y, PNG, and GIF files were clipped to known start and end locations and were cut in some places during processing; as a result, some of the line segments differ from those of the black and white TIFF images.
Fifteen-minute navigation point and trackline shapefiles were made from the latitudes and longitudes generated from the LoranGPS software. One-minute navigation shapefiles that complement the SEG-Y files were created by using Linear Referencing Tools and XToolsPro in ArcGIS 9.2 to make a route from the 15-minute trackline and point shapefiles, hatches every minute, convert the hatches to a shapefile, and add attributes to the shapefile. Shot-point navigation text files that could be imported into the seismic-data interpretation package LandMark Graphics Corp. (LGC) Seisworks were made from the 1-minute navigation shapefiles.
Cruise information can be accessed in the data table below or through the navigation bar on the upper left side of this page. Seismic data can be accessed through the Seismics Profiles pages for each cruise, through the GIS Data Catalog, or downloaded directly from the cruise subdirectories in the data/seismics folders on Disk A (PNG images of the seismic lines are on Disk B) if the report is accessed through the DVD-ROMs. The GIF preview images can also be accessed through interactive maps on the Seismic Profiles pages for each cruise. Navigation data can be accessed through the GIS Data Catalog or downloaded directly from the cruise subdirectories in the data/nav folders on Disk A if the report is accessed through the DVD-ROMs. Fifteen-minute trackline navigation data are provided for each cruise in ESRI shapefile format with fields for cruise identification, latitude, longitude, line number, date, time, LORAN-C time delays, comments, and time in minutes after midnight. One-minute navigation data are provided as shapefiles and space-delimited text files. One-minute navigation shapefiles include fields for line number, time in hours and minutes, time in minutes after midnight, date, easting, northing, and shot number. One-minute shot-point text files contain fields for unique line identification, shot number, easting, and northing.
Data files are named by cruise identifier, line number, and line section. For example, a75_625Ab.tif signifies the cruise as R/V Asterias in 1975, during June (in this case "6" stands for June; the leg number, 6, is used for the 1980 cruise), line 25A, section b, and TIFF format. Many lines were divided into sections in order to be scanned completely; therefore, their names differ slightly from those in the logbook in that a lowercase letter is appended to the name after the line identification, such as section "b" in the example above. Capital letters are part of the line name as recorded on the seismic records. Several of the SEG-Y files needed to be further divided into sections to ensure that each file contained continuous data; lines that were split into segments by breaks in the data, such as interruptions caused by belt changes, are denoted _1 or _2 after the section label.