Locations and grain-size analysis results of sediment samples collected in May and June 2023 using the SEABOSS 2.0 in Nantucket Sound, Massachusetts, in the vicinity of Horseshoe Shoal, by the U.S. Geological Survey during field activity 2023-001-FA (point shapefile and CSV file, GCS WGS 84)

STEP 1: COLLECTED DATA. A marine geophysical and sampling survey (field activity 2023-001-FA) was conducted in Nantucket Sound, Massachusetts, in the vicinity of Horseshoe Shoal in May and June 2023. The ground validation data was collected while the R/V Rafael occupied one of the target sites, and the SEABOSS 2.0 was deployed off the vessel's portside. The SEABOSS was equipped with a modified Van Veen grab sampler, a FLIR Blackfly S BFS-PGE-50S5M-C digital camera with a topside feed, an oblique downward-looking DeepSea Power & Light HD Multi SeaCam video camera with a topside feed, and DeepSea Power & Light LED SeaLites to illuminate the sea floor for video and photograph collection. The elements of this particular SEABOSS were held within a stainless-steel frame that measured ~1 x 1 meter. The frame had a stabilizer fin that oriented the system as it drifted over the seabed. Two red lasers were set 20 centimeters apart (both as they were mounted on the SEABOSS frame and as seen in photographs and video on the seabed) for scale measurements. The red laser dots can usually be seen in the sea-floor photos and videos depending on the bottom type, water clarity, and distance to the sea floor. The winch operator lowered the SEABOSS until the sea floor was observed in the topside live video feed. Generally, the vessel and SEABOSS drifted with wind and current for up to a few minutes to ensure a decent photo with a clear view of the sea floor was acquired. A scientist monitored the real-time bottom video and acquired bottom photographs at points of interest by remotely triggering the FLIR camera shutter. The photographed area is most often within 0.5 to 1.25 meters from left to right. Bottom video was also recorded during the drift from the oblique downward-looking DeepSea Power & Light HD Multi SeaCam video camera directly to a solid-state drive using an Odyssey7 video recorder. Due to equipment overheating issues, the FLIR camera stopped working after site 2023-001-050, so still-image frame grabs were captured from the DeepSea Power & Light bottom videos for the last 10 sites (sites 2023-001-051 through 2023-001-060) described in a subsequent process step. After drifting for up to a few minutes, the winch operator lowered the SEABOSS sampler until it rested on the sea floor. When the system was raised, the Van Veen grab sampler closed and collected a sample as it was lifted off the sea floor. The sampler was recovered to the deck of the survey vessel where a subsample was taken for grain-size analysis at the sediment laboratory at the USGS Woods Hole Coastal and Marine Science Center. During the survey, DGPS navigation from a Hemisphere R131 DGPS receiver (see Positional_Accuracy statement above for more info). Dates and times were recorded in UTC. When the FLIR camera shutter was remotely triggered to take a photo, the photo filename and navigation data for that timestamp were saved to a CSV file, which was used to geotag the images in a subsequent process step. The DGPS data were also overlaid onto the video using a Proteus-V Pro video overlay device. A survey log was maintained during the field activity by the technical and scientific staff as an Excel spreadsheet. It contains information about bottom photo and video acquisition, sediment sampling, and other miscellaneous notes from the survey. A total of 59 sediment samples were collected using the SEABOSS 2.0 at 60 sites during field activity 2023-001-FA. Bottom imagery was collected at all 60 sites.

STEP 2: ASSEMBLED SAMPLE INFORMATION FOR SEDIMENT LABORATORY. The sediment sample times (as recorded in the survey log) were checked by viewing the bottom videos to see when the sampler was lifted off the sea floor at each site and updated with more precise times as appropriate. The latitude and longitude from the video overlay at the time of the sample were also noted for each sediment sample. Sites from the survey day in May had no video overlay so the sample time as recorded on that day was correlated to the ship's navigation log to determine the GPS position. Depths for each sample were extracted from a composite bathymetry dataset for coastal Massachusetts. These depths were updated from a bathymetry grid of the newer data from this mapping program (field activities 2022-001-FA and 2023-001-FA (see Process Step 5). This information was then provided to the sediment laboratory at the USGS Woods Hole Coastal and Marine Science Center with the sample analysis request form for the survey.

STEP 3: SAMPLE ANALYSIS Samples submitted to the Sediment Laboratory for grain size analysis using the Horiba laser diffraction unit (LA-960) and sieving of the ≥ -2 phi fraction are assigned unique analysis identifiers (ANALYSIS_ID), and divided into batches of no more than 30 samples. Each batch is entered into a Microsoft Excel data entry spreadsheet (LD Worksheet Template_SA14.xlsx, where SA14 is the identifier assigned to the sample submission) to record the initial and dried sample weights, as well as the sieved coarse fraction weights. Each batch is also is entered into macro-enabled Microsoft Excel data entry spreadsheets (SA14_GrainSizeWorksheet_LD1-30.xlsm or SA14_GrainSizeWorksheet_LD31-60.xlsm, where SA14 is the identifier assigned to the sample submission, “LD1-30” and “LD31-60” refer to the pre-labeled and weighed glass laser diffraction vials the samples will be run in) to record the measurement data coming from the laser diffraction unit and incorporate the initial, dried, and sieved weights.About 5-10 grams of wet sediment or 3-5 grams of dry sediment are placed in a pre-weighed beaker and the gross weight is recorded. The sample is wet sieved through a 4 mm (No. 5) sieve. If there is any coarse fraction remaining in the sieve, the coarse material is oven dried at 100°C in a pre-weighed beaker, and weighed again when dry. This coarse fraction is dry sieved to determine the individual weights of the -2 to -5 phi fractions, and the weights are recorded in the data entry spreadsheet LD Worksheet Template_SA14.xlsx. The fine fraction in water is collected in a pre-labeled and weighed glass laser diffraction vial. If there is any coarse fraction remaining in the sieve from wet sieving, this vial is also oven dried at 100°C and weighed when dry. If there is no coarse fraction remaining from wet sieving, the sample can proceed directly to processing for analyses by the Horiba laser diffraction unit (LA-960).Fine fractions ready for analysis by the Horiba laser diffraction unit are rehydrated with distilled water if they’ve been dry. Fifteen (15) ml of pre-mixed 40 g/l sodium hexametaphosphate [(NaPO3)6] are added to each sample. If the height of the fluid in the laser diffraction vial is less than 5 cm, more distilled water is added to raise the level to no more than 8 cm in the vial. The samples are gently stirred, covered, and allowed to soak for at least 1 hour (for samples that were not dried) up to 24 hours (for samples that were dried). Soaked vials are placed into an ultrasonic bath and run for 10 minutes at a frequency of 37 Hz with a power level of 100. If the samples appear to be fully disaggregated, they are placed into pre-determined autosampler locations, and are run using the Horiba LA-960 for Windows software to get the fine fraction grain size distributions. The fine fraction distribution data are added to the appropriate data entry spreadsheets (SA14_GrainSizeWorksheet_LD1-30 or SA14_GrainSizeWorksheet_LD31-60.xlsm). The spreadsheet is used to calculate a continuous phi class distribution from the original fractions. Data from this submission were processed using version 2.3 of the data entry spreadsheet.

STEP 4: ADDITIONAL SAMPLE CALCULATIONS Continuous phi class distribution from the original fractions are transposed to the "results" tab in the macro-enabled Microsoft Excel data entry workbook (SA14_GrainSizeWorksheet_LD1-30.xlsm or SA14_GrainSizeWorksheet_LD31-60.xlsm, where SA14 is the identifier assigned to the sample submission, “LD1-30” and “LD31-60” refer to the pre-labeled and weighed glass laser diffraction vials the samples will be run in). Macros in the workbook (“GS_MoM_Arithmetic_1”, “GS_MoM_Arithmetic_2”, "GS_statistics", and "sedimentname") are run to calculate grain size classification and statistical analyses and finish processing the data. Sample, navigation, and field identifiers, along with continuous phi class distribution data, grain size classification, and statistical analysis results are copied and pasted into a final Microsoft Excel spreadsheet (SA14_GS-LD_results.xlsx, where SA14 is the batch number assigned to the sample submission). The processed data are quality control checked and assigned a quality grade based on the examination of the analytical data. Processed data are released to the submitter and incorporated into the laboratory's database. All raw analytical data generated by the samples are archived in the Sediment Laboratory.

STEP 5: CONVERT XLSX TO CSV AND SHAPEFILE The sediment laboratory Excel spreadsheet was saved as a CSV file, column headers were modified, and the CSV file was imported into QGIS (version 3.34.7) as a delimited text layer. The resulting spatial dataset was saved as an Esri shapefile format.

STEP 6: UPDATE DEPTHS New depths for each sample were extracted from a composite bathymetry created from the bathymetry grid in this publication and from survey 2022-001-FA (2022_2023_NtktSnd_Merge_MLLW_2m.tif) using the geoprocessing tool "Sample Raster Values" in QGIS 3.34.7. A new final shapefile was exported. Depths were also updated in the XLXS and CSV files. The new depth data were provided back to the Sediment Laboratory to update these sample records in their database.