University of Texas Institute fore Geophysics 20240820 raster digital data Austin, TX University of Texas Institute for Geophysics This archive contains processed chirp data collected by APTIM Inc. under a joint effort between the Bureau of Ocean and Energy Management (BOEM), coop# M21AC00005, and the Texas General Land Office (GL0), CEPRA (Coastal Erosion Planning and Response Act) project 1706. The 2020 APTIM survey, with ~1100 nm of chirp track line at ~1 nm track spacing, focused on the Sabine, Heald and Shepard banks and also covered major portions of the Sabine Paleo-valley. The University of Texas Institute for Geophysics were contracted on this project to apply our UTIG developed advanced chirp processing techniques (Saustrup et al., 2019) to this OCS chirp data set, to produce a level of image quality that will enable detailed stratigraphic interpretations, and to interpret the processed data within the regional context of our existing GLO (Contract #20-164-000-C216) and BOEM (Coop #M16AC00020) funded projects covering the Trinity River Paleovalley, offshore Galveston, Texas, out to the confluence with the Sabine River Paleovalley. To archive all digital chirp subbottom profile data and associated files collected for the OCS2020 data. All Chirp systems use a signal of continuously varying frequency; the system used during this survey produces high-resolution, shallow-penetration (typically less than 50-ms) profile images of sub-seafloor stratigraphy. The towfish contains a transducer that transmits and receives acoustic energy and is typically towed 1 - 2 m below the sea surface. As transmitted acoustic energy intersects density boundaries, such as the seafloor or sub-surface sediment layers, some energy is reflected back toward the transducer, received, and recorded by a PC-based seismic acquisition system. This process is repeated at regular time intervals and returned energy is recorded for a specific duration. In this way, a two-dimensional (2-D) vertical image of the shallow geologic structure beneath the towfish is produced. The source utilized for the OCS2020 survey consisted of an EdgeTech SB-512i towfish running DISCOVER v. 3.51 acquisition software and towed about 5 m behind the GPS antenna. The data were acquired using a frequency sweep that 0.7-12 kHz, a 0.042 ms sample interval, and approximately 135 ms record length. Based on survey speeds of ~4.5 knots and a shot interval of 0.2 s, the shot spacing was about 0.450 m. The binary portion of the unprocessed seismic data are stored in Edgetech’s JSF file format (.jsf file extension), which includes both the full-waveform match filter output, and the envelope conversion of that record. Our usual chirp processing steps are fully described in Saustrup and others (2018), and involve merging files for each line into a single file, removal of towfish heave artifacts, correcting for towfish depth, equalizing trace amplitudes, secondary deconvolution (to sharpen image) on full-waveform records, and applying a layback correction to the navigation. Processed full-waveform and envelope records are stored in SEG Y rev. 0 (Barry and others, 1975), IBM float format (.segy file extension), which is a standard digital format that can be read and manipulated by most seismic processing software packages. The SEG Y files may be downloaded and processed with commercial or public domain. Processed full-waveform data are given the extension "_realproc.segy" Processed envelope data are given the extension "_envproc.segy" Navigation and other ping data are also provided with extension "_nav.txt files". 20240820 publication date Complete None planned -94.8724 -93.5101 29.5065 28.5990 None Geology Coastal Information Marine Subbottom profile Seismic Reflection Chirp Edgetech SEG Y Sand Resource Trinity River Paleovalley Sabine River Paleovalley Heald Bank Sabine Bank Shepard Bank Geographic Names Information System (GNIS) Content United States of America TX Gulf of Mexico Galveston None. These data are held in the public domain. The University of Texas Institute for Geophysics (UTIG) requests to be acknowledged as originator of the data in future products or derivative research. John Goff University of Texas Institute for Geophysics Research Professor mailing and physical

10100 Burnet Rd. (R2200)

Austin TX 78758 USA 512-471-0476 BOEM, GLO None None None The validity or accuracy of marine subbottom profiles is highly qualitative and depends on equipment and operating condition variables. Visual inspection of the images rendered from the data did not show any major anomalies. This dataset is from three field activities with consistent instrument calibrations. These data are collected along tracklines (2-D) and are therefore inherently incomplete. Geologic details between lines must be inferred. As the subbottom data were acquired, the position of the vessel was continuously determined by a differential GPS system with antenna mounted at the stern of the ship, near the tow point. A layback correction for the ~5 m offset between towfish and antenna has been applied to the processed data as well as the ASCII navigation files. These data are not to be used for bathymetry. Two-way travel (TWT) times in the processed data have been corrected for assumed towfish depth and have been filtered to remove heave artifacts. University of Texas Institute for Geophysics 20240820 Raster digital data online 20240820 publication date University of Texas Institute for Geophysics Chirp subbottom data in raw in processed (.segy) form Chirp processing: The following processing description is excerpted from Saustrup and others (Saustrup, S., Goff, J.A., and Gulick, S.P.S. 2019. Recommended “Best Practices” for Chirp Acquisition and Processing. Austin, TX: US Department of the Interior, Bureau of Ocean Energy Management, OCS Report BOEM 2019-039, 15 p.; doi:10.31223/osf.io/7csjh). Processing is conducted using Paradigm Echos software. Edgetech chirp data are recorded as “.jsf”-formatted files, a native Edgetech format that includes 4 different data channels: “real,” “imaginary,” “envelope,” and “spectrum.” The two channels of interest to the following processing scheme are “real”, which are the full waveform record, and “envelope”, which are the envelope-processed data more commonly displayed. Following data archiving (primary recording to top-side main drive and backup to secondary external drive), and prior to processing, these records must be converted to SEGY format files, which can be done with a number of available utilities. If a single survey line consists of multiple individual files, we find it useful to first convert and then concatenate these records into a single SEGY file for processing. The Edgetech acquisition software also provides an option to record directly to SEGY format. However, this format only includes envelope records; full waveform is only retained in the .jsf files. As noted above, we highly recommend acquiring field data in .jsf or an analogous format, such as .keb files for Knudsen systems, that retains both data types. Our chirp processing scheme involves three primary data streams. The first of these streams includes the critical step of picking the seafloor (to within a fraction of a wavelength at ~5000 Hz, or about 0.1ms), which provides the basis for the other two data streams: real and envelope processing. Processing of real and envelope data in turn involves 3 steps: static corrections (heave compensation, towfish depth and tides), signal processing to improved image clarity, and layback correction for navigation. As noted above, only the envelope processing was conducted on these data. The key step to being able to remove heave artifacts from chirp data, as well as for some signal processing, is to generate a precise pick of the seafloor reflection. A fully-automated bottom picker is desirable for ease-of-use but, in our experience, can fail regularly in the presence of high noise, low seafloor signal, or amplitude variability. Our own bottom-picking algorithm involves an iterative process, beginning with a coarse pick using a simple threshold algorithm, and successively refining using both automated methods and, optionally, user interaction in more difficult cases. The details of this algorithm are complex and beyond the scope of this document. Once completed, the bottom pick enables the user’s ability to move individual records up or down (i.e., apply a “static”) in relation to the seafloor arrival. Heave filtering, described below, is one such application. It is also possible to flatten record to the seafloor, which is useful for quality control; i.e., enhance both the user’s ability to visually identify bad bottom picks and the algorithm’s ability to iteratively refine the picks. Flattening is also a prerequisite for some of the processing steps described below. The seafloor flattening step is reversed later in the processing stream to preserve true topographic features at the seafloor. The data are corrected for any recording delay (nonzero start recording time, also called deepwater delay) that may have been used in the field. This is often the case when operating in deep water; a delayed start of the recording time (a simple option in Edgetech systems, for example) can be used skip over large quantities of potentially useless water column returns and thereby keep record lengths and file sizes to manageable values. A time series for towfish depth is recorded in the field, and used to correct to a sea-surface datum. This depth can be estimated using a variety of methods, including cable length/angle, a pressure sensor mounted on or integrated into the towfish, or ascertained with a USBL system. For best results, this step should be performed before the seafloor picking. We use a simple time interpolation between observed or recorded points. The seafloor picks are smoothed using a user-defined (nominally 35-75 pings) low-pass filter that is large enough to average out heave artifacts. The difference between the filtered and unfiltered seafloor picks forms a static correction to correspondingly shift the traces up or down to compensate for heave. Care must be taken during this step, if possible, to not over-smooth the seafloor and remove true topography (although this is not always possible if seafloor features are of similar wavelength to heave artifacts). Heave correction values as calculated on the full waveform data are stored in a database and applied identically to both envelope and full waveform data. An important best practice for processed data is to incorporate values for final picked seafloor time, smoothed seafloor time, and seafloor static into the trace header. This enables heave compensation filtering to be “undone” so that other correction algorithms can be applied (e.g., fitting the picked seafloor to a known bathymetric surface). Processed envelope records are stored in SEG Y rev. 0, IBM float format (.sgy file extension), which is a standard digital format that can be read and manipulated by most seismic processing software packages. Navigation in Edgetech SEGY files is stored in two places. Header bytes 73-76 (lon) and 77-80 (lat) are the standard SEGY locations containing low-resolution navigation in integer arcseconds. Edgetech then stores a high-resolution decimal degrees with a scaling factor of 600000) in trace header locations 81-84 (lon) and 85-88 (lat). These original navigation points are preserved. In addition, we place a high-resolution, filtered, layback-corrected navigation in trace header locations 181-184 (lon) and 185-188 (lat). Layback-corrected navigation are also exported to ascii-formated, comma-separated values (csv) files, where each line indicates line name, ping number, fish latitude, and fish longitude. 20220415 Point 0.000001 0.000001 Decimal degrees WGS84 WGS_1984 6378137.000000 298.25722210100002 John Goff University of Texas Institute for Geophysics Research Professor mailing and physical

10100 Burnet Rd. ()R2200)

Austin TX 78758 512-468-0476 Digital Data https://www.marine-geo.org/collections/#!/collection/Seismic#summary 20240820 None University of Texas Institute for Geophysics John A. Goff mailing and physical

JJ Pickle Research Campus, Bldg. 196; 10100 Burnet Rd. (R2200)

Austin TX 78758 USA 512-471-0476 goff@ig.utexas.edu FGDC Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998 local time