U.S. Geological Survey Data Series 428
In April and July of 1981, the U.S. Geological Survey (USGS) conducted geophysical surveys to investigate the shallow geologic framework of the Alabama-Mississippi-Louisiana Shelf in the northern
A standardized naming convention was established to allow for better management of scanned trackline images within the MASH data rescue project. Each cruise received a unique field activity identification (ID) based on the year the data were collected, the first two digits of the survey vessel name, and the number of cruises made (to date) by that vessel that year (for example, 81CA1 represents the first cruise made by the R/V Carancahua in 1981.) The new field activity IDs 81CA1 and 81GY6 presented in this report were originally referred to as Carancahua 81-1 and Gyre 81-6 at the USGS in St. Petersburg, FL, and 81021, and 81007, and 81008 at the USGS in Woods Hole, MA. A table showing the naming convention lineage for cruise IDs in the MASH data rescue series is included as a Portable Document Format (PDF) file. This report serves as an archive of high-resolution scanned Tagged Image File Format (TIFF) and Graphics Interchange Format (GIF) images of the original boomer and minisparker paper records, navigation files, trackline maps, Geographic Information System (GIS) files, cruise logs, and formal Federal Geographic Data Committee (FGDC) metadata for cruises 81CA1 and 81GY6.
The minisparker from the 81GY6 survey was a caged acoustic energy source generated by a high-voltage electrical discharge between multiple electrodes in saltwater. The minisparker cage is usually towed 5 to 10 meters (m) behind the survey vessel and just beneath the sea surface. The heat generated by the electrical discharge rapidly vaporizes the water, resulting in a small explosion which propagates through the water and sediment column. The boomer system from the 81CA1 survey uses an acoustic energy source called a plate, which consists of capacitors charged to a high voltage and discharged through a transducer in the water. The source is towed on a sled, at sea level and when discharged emits a short acoustic pulse, or shot, which propagates through the water and sediment column.
The acoustic energy discharged by both the boomer and the minisparker systems was reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), detected by the hydrophone receiver, and the amplitude of the reflected energy recorded by an Edward P. Curley Lab (EPC) thermal plotter. This process was repeated at timed intervals (for example, 0.5 seconds (s)) and recorded for specific intervals of time (for example, 100 milliseconds (ms)). The timed intervals are also referred to as the shot interval or fire rate. On both minisparker and boomer analog records the recorded interval is referred to as the sweep, which is the amount of time the recorder stylus takes to travel from the top of the record to the bottom of the record, therefore recording the amplitude of the reflected energy of a single shot. In this way consecutive recorded shots produce a two-dimensional (2-D) vertical image of the shallow geologic structure beneath the ship track.
Positions were recorded in latitude and longitude coordinates every 5 min and appear on the seismic profiles as incremental, hand-annotated vertical event marks. Navigation data were obtained as .dat files, opened using Notepad, and saved as tab-deliminated text files.
Raw shotpoint navigation data were formatted for use with ESRI ArcGIS 9.2 software and projected in WGS84 to visually inspect shotpoint accuracy. Point locations were checked against written notes made on the paper records and in the cruise logbooks. Any errors were identified, reviewed, and rectified. Hour and minute values were not altered from the format in the original data files. Attribute information was updated to include the USGS-Woods Hole field activity ID and the new USGS-St. Petersburg MASH cruise ID, and to identify if the survey had been continuous or split into legs.
Process_Date: 2007 and 2008