Subbottom and Sidescan Sonar Data Acquired in 2015 From Grand Bay, Mississippi and Alabama

All chirp systems use a signal of continuously varying frequency; the system used during this survey produces high-resolution, shallow-penetration (typically less than 75-milliseconds [ms]) profile images of sub-seafloor stratigraphy. The towfish contains a transducer that transmits and receives acoustic energy and is towed below the sea's surface. As transmitted acoustic energy intersects density boundaries, such as the seafloor or sub-surface sediment layers, energy is reflected toward the transducer, received, and recorded by a PC-based seismic acquisition system. The transmitted pulse is repeated at regular intervals (several times a second [s]) and returned energy is recorded for a specific duration (for example, 50 ms). In this way, a two-dimensional (2D) vertical image of the shallow geologic structure beneath the towfish is produced. The seismic source utilized during 2015-315-FA consisted of an EdgeTech SB-424 towfish running Discover version 4.0 acquisition software, towed off the port side of a 17’ Twin Vee Catamaran Center Consul Vessel. The layback and offset from the GPS antenna was 70 centimeters (cm) to port of the antenna, and 160 cm aft of the antenna. The data were acquired using a frequency sweep of 4-24 kilohertz (kHz), 1 millisecond pulse length at 100% power, and a recording length of 100 ms. Based on survey speeds of 3.5 knots, the shot spacing was approximately 0.450 m. The binary portion of the unprocessed seismic data is stored in SEG Y rev. 0, IBM float format, which is a standard digital format that can be read and manipulated by most seismic processing software packages; the first 3,200 bytes of the file header are in American Standard Code for Information Interchange (ASCII) format instead of Extended Binary Coded Decimal Interchange Code (EBCDIC) format. The SEG Y formatted trace files have a .sgy extension and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). The printable profiles provided are chirp subbottom profile images that were processed using SU software. Survey Field Activity Collection System (FACS) and geophysical logs are also provided as supplemental information in the download file, 2015-315-FA_logs.zip. The sidescan sonar data was acquired using a Klein 3900 dual frequency (455 and 900 kHz) towfish and Klein SonarPro version 12.1 software that recorded the sidescan data in industry standard XTF format. The 455 kHz frequency data was utilized for this project. The layback and offset from the GPS antenna was 185 cm to starboard of the antenna, and 20 cm forward of the antenna