Figure 2. High-resolution chirp seismic-reflection profile illustrating stratigraphic features and geometries discussed in the text, offshore of Fire Island, New York, 2011. Location of the profile is shown in figure 1. Approximate water depth in meters is based on two-way travel time of 1,500 meters per second. A, The Holocene marine transgressive surface is marked by a yellow dotted line. Glaciofluvial channels and older Pleistocene sediments are marked by orange dotted lines, and the underlying Cretaceous-age coastal plain strata and the coastal plain unconformity are shown as green dotted lines. B, The chirp seismic-reflection profile displayed without interpretations. Figure modified from Schwab and others (2013).

Figure 3. Map showing an interpolated bathymetric surface generated from interferometric sonar offshore of Fire Island, New York, 2011. Regional bathymetric contours are in meters (m) below the North American Vertical Datum of 1988 (NAVD 88). West of Watch Hill the modern sand deposit is organized into a series of shoreface-attached sand ridges oriented at angles oblique to the shoreline. Figure modified from Schwab and others (2013)

Figure 4. Map showing acoustic backscatter collected with interferometric sonar offshore of Fire Island, New York, 2011. High backscatter is displayed as light tones and low backscatter is displayed as dark tones within the backscatter image. In general, high backscatter corresponds to coarser-grained sediments, and low backscatter corresponds to finer-grained sediments. Regional bathymetric contours are in meters (m) below the North American Vertical Datum of 1988 (NAVD 88). Figure modified from Schwab and others (2013)

Figure 5. Map showing modern sediment thickness offshore of Fire Island, New York, 2011. Regional bathymetric contours are in meters (m) below the North American Vertical Datum of 1988 (NAVD 88). The inset map shows the location of the chirp seismic-reflection profiles, overlain on bathymetry (fig. 3), that were used to map the Holocene transgressive surface (fig. 6) and thus the modern sediment thickness. Figure modified from Schwab and others (2013).

Figure 6. Map showing the Holocene marine transgressive surface mapped by using chirp seismic-reflection profiles collected in 2011 (see inset map) offshore of Fire Island, New York. The bathymetric scale used in figure 3 is used to display the Holocene marine transgressive surface for easy comparison between the two surfaces. Regional bathymetric contours are in meters (m) below the North American Datum of 1988 (NAVD 88). The inset map shows the location of the chirp seismic-reflection profiles, overlain on bathymetry (fig. 3), that were used to map the Holocene marine transgressive surface. Figure modified from Schwab and others (2013)

Figure 7. Map showing the coastal plain unconformity offshore of Fire Island, New York, 2011. Regional bathymetric contours are in meters (m) below the North American Vertical Datum of 1988 (NAVD 88). The inset map shows the location of the seismic-reflection profiles where subbottom penetration and resolution allowed identification of the coastal plain unconformity. In addition to chirp profiles collected in 2011, this grid includes reprocessed boomer and sparker data collected by the U.S. Geological Survey in 1996–1997 (Foster and others, 1999). The relatively sparse seismic-reflection data coverage yields a generalized, regional surface, with less resolution than other maps presented.

Methods—Data Collection and Processing

Survey Operations

An interferometric sonar and a chirp seismic-reflection profiler were used to map bathymetry, acoustic backscatter, and subsurface stratigraphy and structure along approximately 336 km2 of the inner continental shelf offshore of Fire Island, N.Y. The mapping was conducted aboard the MV Scarlett Isabella, May 4 to 17, 2011 (USGS field activity 2011–005–FA). Methods used in the collection, processing, and analysis of the data are detailed in the following sections. A full description of the data acquisition parameters, processing steps, and accuracy assessments can be found in the metadata associated with each spatial dataset (Geospatial Data).

Bathymetry and Backscatter

Approximately 2,800 line kilometers of colocated bathymetry and acoustic backscatter data were acquired by using a Systems Engineering and Assessment, Ltd. (SEA) SWATHplus-M 234-kilohertz (kHz) interferometric sonar system. The sonar transducers were mounted on a rigid pole deployed from the port side of the ship approximately 2.17 m below the water line. Survey lines were run at an average speed of 5 nautical miles per hour (knots), and soundings were recorded over swath widths ranging from 50 to 150 m in water depths of 8 to 32 m, resulting in coverage of about 90 percent of the sea floor in the survey area. A Coda Octopus F180 motion reference unit (MRU), mounted directly above the sonar transducers, measured vertical displacement (heave) and attitude (pitch and roll) of the vessel during data acquisition. Sound velocity profiles were collected approximately every 2 hours by using an ODIM MVP30 moving vessel profiler.

SEA SWATHplus acquisition software and the Computer Aided Resource Information System (CARIS) Hydrographic Information Processing System (HIPS) were used to collect and process, respectively, the raw bathymetric soundings. Navigation data were inspected and edited, MRU and sound velocity data were used to reduce vessel motion and refraction artifacts, and bathymetric filters were used to remove spurious soundings. A Differential Global Positioning System (DGPS) was used for horizontal positioning during data acquisition. Real Time Kinematic (RTK) Global Positioning System height corrections, broadcast from a continuously operated reference station (CORS) at Central Islip, N.Y. (station NTCI), were used to reference soundings to the North American Vertical Datum of 1988 (NAVD 88) and remove water-depth variations caused by tides. A 5-m bathymetric grid was generated within CARIS HIPS. Due to the lack of 100-percent coverage of the sea floor, between-line data gaps are present within the 5-m bathymetric grid. In order to generate a continuous bathymetric surface of the survey area, ArcGIS Spatial Analyst was used to create an interpolated bathymetric grid at a 10-meter-per-pixel (mmp) resolution (fig. 3).

SWATHplus acoustic backscatter data were processed by using Chesapeake Technology’s SonarWiz. Altitudes were selected by using an automated bottom tracker with minor adjustments with manual digitizing. An empirical gain normalization function was applied to the backscatter to optimize the dynamic range and enhance the quality of the mosaic. A 24-bit gray-scale GeoTIFF image of backscatter was then generated at a 5.0-mpp resolution and imported into ArcGIS (fig. 4).

Seismic-Reflection Profiling

Chirp seismic-reflection data were collected by using an EdgeTech GeoStar FSSB subbottom profiling system and an SB 512i towfish (FM frequency modulation swept frequency of 0.5 to 12 kHz). Data were acquired by using a 0.25-second (s) shot rate, a 5-millisecond (ms) pulse length, and a 0.5- to 8-kHz frequency sweep. Data were recorded over two-way travel time travel lengths of 200 ms (4,328 samples/trace and 0.000046-s sample interval) and logged in the SEG-Y Rev. 1 format. SEIOSEIS (Henkart, 2011) seismic processing software was used to shift traces vertically to remove the effects of sea-surface heave, mute water column portions of the traces, and apply time-varying gain and automatic gain control.

Processed seismic-reflection data were loaded into the seismic interpretation software package Landmark SeisWorks 2D, through which erosional unconformities (coastal plain unconformity and Holocene marine transgressive surface) were identified and digitized (fig. 2). The two-way travel times between the sea floor and the unconformities were calculated and converted to thicknesses in meters by using a constant velocity of 1,500 meters per second to produce an isopach of the modern sediment seismostratigraphic unit. ArcGIS Spatial Analyst was used to create interpolated grids of the modern sediment isopach (50 mpp; fig. 5) as well as the Holocene marine transgressive surface (50 mpp) and coastal plain unconformity (100 mpp) relative to NAVD 88 (figs. 6 and 7). Modern sediment less than 50 centimeters (cm) thick is assumed to be zero for sediment-volume calculations because of a conservative estimate of the limits in the vertical resolution of the seismic-reflection data.