MAPPING OYSTER REEFS USING SIDESCAN SONAR AND SUBBOTTOM PROFILING: CAPE FEAR RIVER, SOUTHEASTERN NORTH CAROLINA Kassy A. Rodriguez A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science Department of Geography and Geology University of North Carolina Wilmington 2009 Approved by Advisory committee Troy D. Alphin__________ Martin H. Posey__________ Lewis J. Abrams_________ Co­Chair Nancy R. Grindlay_________ Co­Chair Accepted by DN: cn=Robert D. Roer, o=UNCW, ou=Dean of the
Graduate School & Research, email=roer@uncw.
edu, c=US
Date: 2009.12.16 10:14:10 -05'00'
_____________________________________ Dean, Graduate School
This thesis has been prepared in a style and format consistent with The Journal of Shellfish Research.
ii TABLE OF CONTENTS ABSTRACT....................................................................................................................v ACKNOWLEDGMENTS .............................................................................................vii DEDICATION .............................................................................................................viii LIST OF TABLES .........................................................................................................ix LIST OF FIGURES .........................................................................................................x INTRODUCTION...........................................................................................................1 STUDY AREA................................................................................................................5 METHODS ...................................................................................................................12 Geophysical Survey and Data Processing...................................................................12 Bottom Samples.........................................................................................................15 Geographic Information System (GIS).......................................................................17 OBSERVATIONS AND INTERPRETATIONS............................................................18 Sidescan Sonar Imagery.............................................................................................18 Seismic Facies Summary ...........................................................................................23 Study Areas ...............................................................................................................23 Area I.....................................................................................................................25 Area II ...................................................................................................................31 Area III ..................................................................................................................34 Area IV..................................................................................................................43 Area V ...................................................................................................................48 DISCUSSION ...............................................................................................................52 Oyster reefs and buried oysters in the seismic sub­bottom profiler record ..................52
iii Oyster reefs and oyster cultch in the sidescan sonar record ......................................... 55
Potential factors affecting past oyster reef growth in the Lower Cape Fear River....... 56
Using results to delineate potential oyster cultch placement locations......................... 58
CONCLUSIONS............................................................................................................... 61
LITERATURE CITED ..................................................................................................... 63
APPENDIX A................................................................................................................... 68
APPENDIX B ................................................................................................................... 69
APPENDIX C ................................................................................................................... 72
APPENDIX D................................................................................................................... 95
iv
ABSTRACT The Eastern Oyster, Crassostrea virginica, is a keystone species, useful in assessing of environmental health. Oysters form reefs, which provide habitat for fish, benthic invertebrates and many other species. Oyster populations in the United States have declined due to many different factors. Oyster populations in the Lower Cape Fear River, however, have been negatively impacted by anthropogenic factors including dredging and dredge spoil island creation and the placement of hard structures. There is increasing interest in the restoration of oyster reefs for both ecosystem and fishery function, but due to the limited availability of oyster cultch (shell) for seeding, oyster restoration efforts have been restricted to areas with live oyster populations and/or documented evidence of historic oyster populations. From December 2004 until July 2007 8.93 km 2 of sidescan sonar and seismic sub­bottom profiler data were collected over five areas in the Lower Cape Fear River for the purpose of identifying living and buried oyster reef locations to be the sites of future oyster restoration projects. In addition, a total of 187 bottom samples were collected. Living and buried oyster reefs are identified in the sub­bottom data by a high­amplitude seafloor, signal attenuation below the seafloor, and a consistent seafloor multiple. Sub­bottom profiler data led to the identification of a 1.1 km 2 area of living and buried oyster reefs. Sidescan sonar imagery proved to be most effective in identifying living oyster reefs. Low­relief (<0.5m) reef mounds appear in the sidescan sonar mosaics as a region of speckled texture outlined by an acoustic shadow. Lower density, patchy reefs are also characterized by a speckled texture, but lack an acoustic
v shadow. Taken together the sidescan sonar and sub­bottom profiler data delineated a 0.19 km 2 area as living or potential living oyster reefs.
vi ACKNOWLEDGEMENTS I would like to thank my advisors Dr. Nancy Grindlay, who has been a pleasure to get to know and Dr. Lewis Abrams who has the patience to keep listening to questions without giving the answer, resulting in greater satisfaction from working through a problem. Their guidance, support, and patience were very much appreciated. I would also like to thank my committee members Troy Alphin, always warm, welcoming, and a pleasure to work with in the field, and Dr. Martin Posey for his quick communication and his editorial input on this work. Thanks to Nathan Henry, Bill Hoffman, Chuck Wilson and Stacy Samuelson for their time and providing various data sets. Geophysical data collection was made possible by Captain Gerry Compeau, Russ Barbour, Steven Artabane, and Joe Sonnier. Johnny Murray and Randy Turner are two brilliant people who are happy to make stuff work. Boat captains during bottom sampling were: Steve Brimm, Chip Collier, and Anthony Rodriguez. Yi Chen, Jessica Guido, Dr. Joan Halls, Amanda Maness, Jackie Ott, and Melissa Smith provided technical support. I would like to thank the following people for answering questions, helping in the field, or offering guidance, encouragement, and friendship along the way: Sara Althof, Jeff Andrews, Luke Davis, Chris Dougherty, Simone Gootheusen, Emerson Hasbrouck, Jorge Haylock, Melany Larenas, Jennie Mancinone, Ron Moore, Amanda Maness, Steve Monziel, Dr. Anton Oleinik, Allison Pastor, Natasha Reiger, Dr. Charles Roberts, Anthony, Diane, Nuffy, and Jill Rodriguez, Mary and Josiah Strauss, Roger Shew, Rena Spivey, Beau Suthard, Dr. Paul Thayer, Doug Walker, Dr. Joan Willey, Sirel White, Ken Willson and Dawn York.
vii DEDICATION For my family.
viii LIST OF TABLES Table Page Table 1. Processing steps for single­channel seismic data ............................................. 16 Table 2. Bottom types and seismic facies within the study area..................................... 24 Table 3. Organisms found in Areas IV and V................................................................ 49
ix LIST OF FIGURES Figure Page 1. Study area locater map in southeastern North Carolina ............................................ 3 2. Study area in the Cape Fear River ............................................................................ 8 3. Historic navigational chart 1866 .............................................................................. 9 4. Historic navigational chart 1899 .............................................................................. 11 5. Photographs of sidescan sonar and geopulse boomer................................................ 14 6. Map legend.............................................................................................................. 19 7. Study area sediments and oyster reef summary map................................................. 21 8. Low­relief reef mound and patchy reef in sidescan sonar record .............................. 22 9. Area I sidescan sonar mosaic ................................................................................... 27 10. Areas I and II sidescan sonar mosaic...................................................................... 29 11. Area I dipping and undulating reflectors in seismic and sidescan ........................... 33 12. Area II patchy oyster reef in seismic and sidescan.................................................. 36 13. Area III sidescan sonar mosaic............................................................................... 38 14. Area III oyster photographs.................................................................................... 40 15. Area III low­relief oyster reef mound in seismic and sidescan................................. 42 16. Area IV sidescan sonar mosaic ............................................................................... 45 17. Areas IV and V sidescan sonar mosaic.................................................................... 47 18 Area IV ripple marks in sidescan ............................................................................ 50 19. Area V paleochannel in seismic and sidescan.......................................................... 54 20 Area III proposed oyster cultch placement sites........................................................ 60
x INTRODUCTION Eastern oysters, Crassostrea virginica (Gmelin), are suspension­feeders that form dense reefs and are vital for healthy estuarine systems (Dame 2005, Power and Sanders 2005). Eastern oyster reefs provide a stable habitat for fish and benthic invertebrates and may promote water quality through filtration by enhancing the sedimentation of microscopic particles in the water column. (Kennedy et al. 1996, Posey et al. 2005, Powers et al. 2005). Crassostrea virginica has an extensive presence along the Atlantic coastline of North America, from the Gulf of St. Lawrence in Nova Scotia to the Gulf of Mexico, continuing into Central America and into the West Indian Islands (Yonge 1960). The species’ success is due to the fact that it can survive in a wide range of salinities (5.0–35 ppt), temperatures (15–30 ºC), inundation regimes, and can live in heavily sedimented environments (Kennedy 1989, Yonge 1960). The oyster’s life cycle begins as a trochophore, or free­swimming, ciliated larva (Kennedy et al.1996). When a trochophore lands on a hard surface, it is then called a pediveliger (Kennedy et al. 1996). The pediveliger crawls in a circular path, sensing if the hard surface is suitable for attachment (Kennedy et al. 1996). If the surface is suitable, they attach their left valve and are then called spat (Kennedy et al. 1996). If no suitable substrate is found, oyster larvae will eventually die (Kennedy 1989). Survival of Crassostrea virginica spat depends on the presence of hard bottom for attachment, which provides sufficient growing room and prevents larger spat from overgrowing smaller ones (Kennedy et al. 1996). If there is insufficient hard bottom, oysters become overcrowded, resulting in a long and distorted growth form and a shortened lifespan (Yonge 1960).
Crassostrea virginica is the dominant oyster species in North Carolina and in the Cape Fear River Estuary, which is located in the southeastern portion of the state (Fig.1). Shell middens located along the banks of the Lower Cape Fear River that range in age from the Late Woodland Period, 1200­350 years ago to the late Archaic Period, 6000­ 3000 years ago provide evidence of prehistoric oyster populations in the region (New South Associates 2001). Historical nautical charts show vast expanses of oyster reefs in the mouth of the Cape Fear River (Bolles 1866). Overharvesting, disease, loss of habitat, high silt content and suspended particulate matter due to coastal development and pollution have caused a dramatic decline in oyster populations in the United States over the last century (Smith and Greenhawk 1998, Mallin et al. 2000, Dame 2005). Oyster populations in the Lower Cape Fear River have been similarly impacted by anthropogenic factors. The United States Army Corps of Engineers (USACE) has been dredging the Cape Fear River since 1829 to create navigable waterways. The dredged material was discarded as dredge spoil islands throughout the lower 20 km of the river, adjacent to the main channel. These activities have altered water flow and increased suspended sediments and turbidity, resulting in sedimentation, which in turn has reduced the amount of hard substrate available for oyster spat settlement and growth (Kennedy 1989). Due to these practices, it is assumed that there is a relatively small amount of hard substrate remaining in the lower region of the river (Alphin, T. per. comm.). This area routinely contains a high amount of oyster larvae in the water column, but a lack of hard bottom will limit colonization because of competition, overcrowding, and high post­settlement mortality (Alphin et al. 2006). Currently, oyster reefs are still present in the mouth of the river, but there are no known
2 Fig.2
Figure 1.The study area location within the Lower Cape Fear River in southeastern North Carolina. 3 reefs upstream. There is increasing interest in the restoration of oyster reefs for both ecosystem and fishery function (Mallin et al. 2000). In 2004, 2.4 acres of oyster reef habitat was created in Carteret County, 4.0 acres in Onslow County, and 1.0 acre in New Hanover County. In 2005, 3.5 acres of oyster reef habitat was created in Onslow County (NCCF 2006). In recent years, however, oyster restoration efforts have been restricted to areas with live oyster populations and/or documented evidence of historic oyster populations because of the limited availability of oyster cultch (shell) for seeding. Traditional techniques to identify oyster reefs include poling, tonging, dredging and dragging chains or microphones behind the boat to detect vibrations from a rough bottom (Smith et al. 2001). Underwater videography is also effective when water turbidity is low (Grizzle at al. 2008). Geophysical techniques can also be used to effectively identify and delineate potential restoration sites by mapping living oyster reefs and discovering buried oyster reefs (Smith et al. 2001, Grizzle et al. 2005). Live and buried oyster reefs and other benthic habitats have been successfully identified using high­resolution sidescan sonar and sub­bottom profiling (Roberts et al. 2000; Carbotte et al. 2004). These instruments were used to map the oyster leases in Barataria Bay, Louisiana in water depths of fewer than 2.0 m (Roberts et al. 2000, Allen et al. 2005). Oyster beds imaged in the Hudson River Estuary in New York appear as moderate backscatter on the sidescan sonar mosaic and as regions that are acoustically impenetrable and buried in up to 3 m of sediment on the sub­bottom profiler images (Carbotte et al. 2004). Living oyster reefs were successfully matched to historical oyster reef locations in the Chesapeake Bay using a combination of two types of sidescan sonar, chirp sonar, sub­bottom profiling,
4 mechanical grabs, Global Positioning System (GPS), Geographic Information Systems (GIS), underwater videography, and two types of seafloor characterization systems (Smith and Greenhawk 1998). Subtidal oyster reefs were mapped in Great Bay, New Hampshire using a combination of multibeam and sidescan sonar, underwater videography, and quadrat sampling (Grizzle et al. 2005). The objective of this study was to test the feasibility of detecting living and buried oyster reefs using high­resolution geophysical equipment in the Lower Cape Fear River. If oyster reefs were once present in the Lower Cape Fear River, it may be possible to recolonize the area by placing oyster cultch in locations where buried oyster reefs are found or expanding on currently living oyster reefs. Successful detection of oyster habitat, both living and buried, will demonstrate the applicability of these instruments in shallow water habitat delineation and provide a new method for shellfish restoration projects throughout the southeast region of North Carolina. STUDY AREA The Cape Fear River Estuary is a drowned river valley (Kennedy 1996) and a partially mixed estuary (Becker 2009) with a total area of 100 km 2 (Dame et al. 2000) (Fig. 1). The northern boundary of the estuary is just south of the city of Wilmington, NC where the Cape Fear River and the Northeast Cape Fear River meet. The river widens and spans 25 km at the southern boundary at Southport, North Carolina where it connects with the Atlantic Ocean (NCDNRCD 1983). The Cape Fear River Estuary experiences semi­ diurnal tides of amplitude 1.3 m (Hackney et al. 2003). The tidal amplitude was 0.5 m less in the early 1900’s (Hackney and Yelverton 1990). This change is due to multiple factors including dredging, sea level rise, and storm surges, which in turn has increased
5 the extent to which seawater reaches into the estuary (Hackney and Yelverton 1990). Data from the Lower Cape Fear River Program (LCFRP) stations M35 and M23 during the years 2005–2007 show a temperature range of 8.5–30.9 ºC, a salinity range of 1.7– 35.0 ppt, and a turbidity range of 3.0–20.0 NTU (Mallin et al. 2005, Mallin et al. 2006, Mallin et al. 2007). The northern boundary of the study area is located 2.0 km south of Snow’s Cut, and extends downstream 15 km, just south of Zeke’s Island (Fig. 2). Within this 15 km stretch of river, five areas were selected for mapping. The northernmost areas, Areas I–III, are within 10 km of Snow’s Cut and had no known oyster populations prior to this study. They border dredge spoil islands and potentially contain buried oyster reefs. In the northern areas, water quality is suitable for oysters and there are oyster larvae in the water (Alphin et al. 2006). The presence of oyster larvae in the water column coupled with the evidence of prehistoric oyster populations from shell middens provided the background needed to identify the location of the study area. In 1733 the mouth of the Cape Fear River was the only mapped connection to the Atlantic Ocean (Moseley 1733), but strong storms during the next hundred years would result in the opening, closing, and moving of inlets on the eastern side of the Cape Fear River (Jackson 1996) (Fig. 3). In 1891 a 5 km­long swash defense dam called “The Rocks” was completed, resulting in a significant reduction in water flow between the Atlantic Ocean and the shipping channel of the Lower Cape Fear River (Fig. 4). One result of constructing The Rocks was the development of an artificial estuary called "The Basin". Area IV borders the perimeter of The Basin, extending into the channel, and Area V borders The Rocks, south of Zeke's Island; neither area contains oyster populations.
6 Figure 2. The study area includes five sites within a 15 km stretch of the Lower Cape Fear River from Snow’s Cut in Carolina Beach (SC) to just south of Zeke's Island in Kure Beach (ZI). The Lower Cape Fear River Program (LCFRP) stations M35 and M23 are located at channel marker 35 near the northern portion of the study area and at channel marker 23 near the southern portion of the study area, respectively. Blue dots represent the approximate locations of core borings in the area (Harris and Haw 2004).The 0.3 m resolution digital orthophotos are MRSID (Multi­Resolution Seamless Imagery Database) images from New Hanover County, North Carolina. Midden site locations are from (New South Associates 2002) obtained at the Underwater Archeology Unit, North Carolina Department of Cultural Resources (NCDCR), Kure Beach, North Carolina.
7 Core Borings
8 Figure 3. Historic navigational chart of southeastern North Carolina that shows a tidal inlet south of Federal Point c. 1866 (NOAA), prior to the construction of the swash dam.
9 Figure 4. This historic navigational chart shows the coastline c. 1889 (NOAA) after the swash dam was built. The Basin was formed and a coastline with New Inlet is present.
10 11
Vibracores taken by the United States Army Corps of Engineers from 1993–1998 indicate that the study area is underlain by the Castle Hayne Formation, an Eocene age fossiliferous limestone (Harris and Haw 2004). Beneath the Castle Hayne Formation is the Bald Head Shoals Formation, a 7 m thick Paleocene mudstone with few fossils (Harris 1996). The Bald Head Shoals Formation thins upstream and may not be present in the sub­surface of Areas I–III (Snyder et al. 1994). The PeeDee Formation is a Late Cretaceous argillaceous and calcareous fine­grained sand that disconformably underlies the Castle Hayne Formation and most of the area of the Cape Fear River (Harris and Haw 2004). The Quaternary overlying deposits are composed of sand, mud, and organic material. The Quaternary sediments are typically less than 2.0 m thick, but in places reach thicknesses of > 7.0 m (Harris and Haw 2004). METHODS Geophysical Survey and Data Processing From December 2004 until July 2007, 85 km of along­track sidescan sonar data and 80 km of sub­bottom profiler data were collected over five areas within a 15 km­long section of the Lower Cape Fear River for the purpose of imaging living and buried oyster reefs. The total area of geophysical data coverage was 8.93 km 2 . Sidescan sonar and seismic sub­bottom profiler data were collected simultaneously when mapping Areas I– IV. When mapping Area V, however, vessel space was limited, so sidescan sonar and seismic sub­bottom profiler data were collected on separate days (Appendix A). The instruments were towed at speeds of 3–5 kts. Surveys took place at high tide and ended as the tide was receding. The water depths during the survey ranged from 0.5–5m.
12 Navigational information was provided by Differential Global Positioning System, which receives differential corrections up to 1 m accuracy from the United States Coast Guard beacon located in New Bern, North Carolina, 145 km NE of the study area. Instrument layback is included in the final navigation. A sidescan sonar Edgetech Model DF­1000 Digital Towfish and TEI Isis® Sonar™ Software were used for mapping and data acquisition. Due to the shallow depths of the Cape Fear River, the towfish was hung from a pontoon made of PVC pipe so that it traveled just below the water’s surface (Fig. 5 A and B). A 50 m swath width was used for the northwestern section of Area I and a 100 m swath width was used for the remainder of the study area. Sidescan sonar frequencies of both 100 kHz and 384 kHz were recorded. The 100 kHz signal of the sidescan sonar can penetrate 2­3 cm, but has less resolving abilities than the 384 kHz signal, which does not penetrate the seafloor (Ryan and Flood 1996). The 384 kHz data were used in this study. Field conditions which introduced noise into the sidescan data included pitching of the sonar fish due to sea surface chop on windy days and turbulent water due to boat wakes. All of the sidescan sonographs were corrected for navigation and processed using the slant range and time varied gain (TVG) corrections. They were then digitally mosaicked into 0.25 m­ grids in TEI Delph Map® where a reverse grey scale was applied with the highest backscatter intensities displayed as 0 (black) and the lowest intensities as 255 (white). Highly reflective surfaces such as course­grained sediments, hard structures, or uneven surfaces produce a high amount of backscatter and are displayed on the sidescan imagery as high intensity. Low reflective surfaces such as fine­grained sand and soft mud produce low amounts of backscatter and are displayed on the sidescan imagery as low intensity.
13 A. A B. C.
B C Figure 5. A. A Sidescan sonar towfish hung on PVC pontoon. B. A Sidescan sonar fish rides just below the water's surface. C. A Geopulse Boomer Sub­bottom profiler. 14 The mosaics were then exported as geotiffs to be used in ESRI's ArcGIS 9.1®. Steps necessary for navigation correction and additional information on sidescan sonar processing can be found in Appendices B and C, respectively. A GeoAcoustics GeoPulse Boomer sub­bottom profiler system was used to gather sub­bottom data (Fig. 5C). Seismic data were collected using Delph Seismic + Plus® in Areas I–IV and SB­Logger™ was used to collect seismic data in Area V. The source repetition rate and energy was 200 ms and 105 Joules respectively. The 200 ms repetition rate resulted in a shot spacing of approximately 0.5 m. The theoretical vertical resolution is 0.25 m, but studies using this system in an area of known thickness indicate a practical vertical resolution of ~1.0 m (Grindlay et al., 2005). A 150 Hz low­cut analog filter was applied to the data prior to digitization to eliminate ship noise that can saturate the signal. All seismic data acquired and shown in this document were processed using the steps listed in Table 1. A detailed description of seismic data navigation correction can be found in Appendix D. Seismic processing details can be found in Appendices E and F. Maps with sidescan sonar mosaics, bathymetry contours, and corresponding seismic shot points for interpretation and selection of ground truthing sites were plotted at a 1:1500 scale. Bottom Samples A total of 187 bottom samples, 173 grab and poling samples and 14 dredges, were taken at predetermined coordinates, predetermined transects using GPS navigation, and at random locations within the five study areas. Sampling sites targeted places where the amplitude of sub­seafloor reflectors changed abruptly in the seismic data and locations of high backscatter in the sidescan imagery. Additional sites with backscatter characteristics
15 Table 1 Processing steps for single­channel seismic data. Areas I­III and IV Area V Smooth navigation Smooth navigation Convert files from SEG­Y format Convert files from SEG­Y format Spherical Divergence Gain ( t 2 ) Spherical Divergence Gain (t 2 ) Trace Killing/Surgical Mute Trace Killing/Surgical Mute Bandpass Filter: 1000–1100–2100–2700 Bandpass Filter: 1000–1200–2100–2200 Hz Hz Calculate seafloor location based on a 1500 Calculate seafloor location based on a 1500 m/s sound velocity in water m/s sound velocity in water Seafloor Mute Seafloor Mute Trace Mixing: 3 traces Trace Mixing: 3 traces Automatic Gain Control: 10 ms Automatic Gain Control: 10 ms
16 of extremely low intensity, or mixed intensity were also chosen to sample. Sampling priority was given to any chosen sites that showed the potential for correlation between the sidescan sonar and seismic systems. One poling technique used was to physically push a pole into the sediment, recording sediment type and penetration depth, and recovering a sample by creating a vacuum at the top opening of the pole. In shallow areas this technique was used to determine the bottom type and if there are any buried oyster reefs within the first 2 m of sediment. Another poling technique used was to drag the end of the pole along the river bottom at very slow speeds. This technique was used to get an idea of the extent of an oyster patch. Dragging the pole over an oyster reef creates vibration and noise allowing the person sampling to feel the difference between an oyster reef and a muddy or sandy bottom. In areas where it was too deep to pole, or not possible to retrieve a sample using the pole, the ponar grab sampler was used. The benthic dredge was used for sampling of deeper areas and re­sampling locations with oysters. Geographic Information System (GIS) A geographic information system (GIS) containing historic data, existing data and images, all processed geophysical data, and ground truthing data was compiled in ArcGIS 9.2® and will serve as a tool for future oyster restoration projects in the Lower Cape Fear River Estuary. Contour lines at 0.5 m spacing were created from a 1 arc second Digital Elevation Model (DEM) using the Spatial Analyst tool in ArcGIS® and are used in figures throughout this thesis. The DEM was downloaded from the NOAA National Ocean Service (NOS) Estuarine Bathymetry website. This bathymetry data was collected using the Mean Low Low Water (MLLW) datum, resulting in some survey tracklines
17 crossing the 0 m contour. A detailed listing of the other components of the GIS is located in Appendix G. A legend showing symbols used in the GIS and on the figures is shown in Fig. 6. OBSERVATIONS AND INTERPRETATIONS A combination of sidescan sonar and sub­bottom profiler imagery, and various bottom sampling (within 2 mbsf) techniques were used to interpret surface and shallow subsurface sediments and locate living and buried oyster reefs throughout the five study areas (Fig. 7). Sidescan Sonar Imagery The sidescan sonar imagery is mostly low intensity backscatter throughout the entire study area except where live oysters are found. Initially, locations with a moderate backscatter on the 100 kHz sidescan sonar imagery and low backscatter on the 384 kHz imagery were identified as potential buried oyster reefs and locations with high backscatter in both frequencies were selected as potential living oyster reefs. After identifying oyster reefs in the field and re­examining the data, potential living reef mounds were targeted based on a speckled texture with an acoustic shadow in the 384 kHz sidescan sonar imagery (Fig. 8). The acoustic shadow appears as a faint white outline of the oyster reef, indicating a slight relief (<0.5 m) above the river bottom and that acoustic signal did not hit the seafloor at the edge of the reef. Areas of lower density, patchy oyster reefs have a speckled texture and no acoustic shadow (Fig. 8). Buried oyster reefs could not be identified on the sidescan sonar mosaics.
18 Paleochannel Seismic Transect
Figure 6. A legend showing symbols used in Figures 7–16 and 18–23. Shell midden sites are shown as white mollusk shells and locations with living oysters are shown as green mollusk shells. Mud and sand are brown and light yellow circles, respectively. Brown and light yellow combination circles are mud and sand mixes. Brown and green combination circles represent oysters buried in 10–50 cm of mud. Light yellow and orange combination circles are oyster cultch and sand mixes or sandy bottoms with patchy oysters. Orange circles with a black bivalve shell represent sites with oyster cultch. Dark green lines are dredge transects and are labeled with an arrow and the dredge number. National Oceanic and Atmospheric Administration (NOAA) bathymetric data were collected using the mean low low water (MLLW) datum. On the following figures, depths are represented as negative numbers and elevations are positive numbers. Light green contours represent the intertidal zones which include bathymetry up to 1.5 m, and elevations up to 1.0 m contoured at a 0.5 m interval. The subtidal zone is shown in 1 m cyan and blue contours with bathymetry ranging from 2 m–9 m and from 10 m–14 m, respectively. Labels SS and SB refer to sidescan sonar and seismic sub­bottom images, respectively. 19 Figure 7. This map summarizes the predominant surficial sediments, living oyster reefs and discovered buried oyster reefs in the study area. Blue dots represent the approximate locations of core borings in the area (Harris and Haw 2004).
20 Core Borings
21 B Nadir Acoustic Shadow A Low­Relief Oyster Reef Mound Patchy Oyster Reef Poor Data
Nadir Nadir Fig 8. A. A 384 kHz sidescan sonar mosaic showing a low­relief, oyster reef mound. B. A 384 kHz sidescan sonar mosaic showing a patchy oyster reef. 22 Seismic Facies Summary This section focuses on the correlation of lithofacies and seismic facies within the top 2 m of the subsurface. Through analysis of 80 km of along­track seismic data and the integration of field data, three representative seismic facies, corresponding to four bottom types were interpreted and are shown in Table 2. Seismic facies types are distinguished by a high­ or low­amplitude seafloor. High­amplitude seafloors indicate a higher impedance contrast and are seen in seismic sections collected over oyster reefs on the surface or buried within 0.5 m of mud. Low­amplitude seafloors indicate a lower impedance contrast and are seen in seismic sections collected over muddy or sandy bottom sediments. Facies 1 includes a low­amplitude seafloor reflection with no seafloor multiple and low­amplitude, semi­continuous sub­bottom reflections. This seismic facies is associated with at least 2.0 m of mud, fine­grained, unconsolidated sand or mud/sand mixes (Table 2A). Facies 2 is characterized by a continuous high­amplitude seafloor reflection followed by low­amplitude discontinuous sub­bottom reflections. This seismic facies is associated with oysters on the surface (Table 2B) and oysters buried in 10–50 cm of mud (Table 2C) which is consistent with the observed large seafloor impedance contrast, a strong seafloor multiple and sub­bottom signal attenuation. Facies 3 is characterized by a low­amplitude continuous seafloor reflection, no seafloor multiple, followed by higher amplitude, continuous and undulating subsurface reflections. This seismic facies is characteristic of sand, oyster cultch/sand mixes, and large amounts of oyster cultch (Table 2D). Study Areas Areas I–V have various types of sediment and organisms, are bounded by
23 Table 2. Bottom Types and seismic facies within the study area. 150 m Seismic Line Sections High­ or Low­ Amplitude Low­amplitude seafloor reflection. Low­ amplitude, semi­ continuous sub­ bottom reflections, no seafloor multiple High­amplitude seafloor reflection, signal is attenuated after the initial seafloor return, seafloor multiple Poling Depth 2.0 m A.
2.0 m or more of Unconsolidated Mud or fine­ grained Sand B. Oyster Shell C.
0.5 m Mud over Oyster Shell D. Well­consolidated Low­amplitude 2.0 m Sand seafloor reflection, clear, high­relief acoustic basement, no seafloor multiple Facies 1 Facies 2 Facies 3 Bottom sediment 24 0.0 m (No penetration through oyster reef.) High­amplitude 0.5 m seafloor reflection, signal is attenuated after the initial seafloor return, seafloor multiple anthropogenic structures and natural structures and have different underlying geology. In the following pages, physiological characteristics, geophysical data, and ground truthing results are presented for Areas I–V. Area I The northern boundary of Area I is located 2 km south of Snow's Cut and extends 3.9 km to the south (Figs. 9–10), making this the largest area. It is bound by the Cape Fear River Bank (CFRB) on the eastern side and by three dredge spoil islands on the western side. Area I varies in width from 1.25 km in the northernmost section to 0.8 km in the southern section. There are shell midden sites along the CFRB of Area I: three on the north end and one on the south end. There are also four more shell midden sites 0.5–1.0 km inland, east of the CFRB. Area I ranges from subtidal to supratidal at MLLW with a range of 1.0–1.5 m bathymetry in the northwestern tip to 1.0 m elevations in the eastern part towards the CFRB and 0.5 m elevations in the western part toward the dredge spoil islands. The middle of the northern section is 0.5 m deep while the southern section is supratidal at MLLW. Geophysical data were collected in a north­south orientation. Along­track sidescan sonar data totaled 36.6 km and sub­bottom profiler data collected totaled 29.6 km. Sub­bottom profiler data corresponds with the 100 m swath width sidescan sonar data; no sub­bottom profiler data were collected along with the 50 m swath sidescan sonar data. The total area of geophysical data coverage in Area I is 3.8 km 2 . Eighty­three bottom samples (ranging from 0–2 m deep penetration) were recorded in Area I indicating sandy and muddy bottoms along with some transitional areas that alternate between sand over mud and mud over sand. Twenty­two bottom samples were
25 Figure 9. A 384 kHz sidescan sonar mosaic of the north and middle sections of Area I. The shorter, thinner lines on the northwest side were collected using a 50 m swath­width. Dredges are green lines, labeled with a D. Orange transect lines correspond to seismic profiles. The inset box represents data shown in Figure 11.
26 SS1 Waste Water Treatment Plant SS2 Dredge Lagoon D1 SS3
SS4 Dredge Spoil Island Fig. 11 Line SB3a 27 Figure 10. A 384 kHz sidescan sonar mosaic of the south section of Area I and all of Area II. Dredges are green lines, labeled with a D. Orange transect lines correspond to seismic profiles. The inset box represents data shown in Figure 11.
28 D2 Area I Area II D3 Dredge Spoil Islands
Fig. 12 Line SB7 D4 Peter's Point 29 very fine­grained silty sand and four samples were consolidated sand. Forty­four samples were muddy bottoms and 14 samples were transitional areas. Additionally, two benthic dredges were cast, bringing up mud in Dredge 1 (Fig. 9) and mostly mud with some isolated clams and wood fragments in Dredge 2 (Fig. 10). In general, sand is found on the eastern side of the area, closer to the CFRB and mud is found in the center of the area and on the western side closer to the dredge spoil islands. The mud and sand transitional areas are found in the center of the study area. No living oyster reefs and no buried oyster reefs were identified within the first 2 m of sediment while field checking this area. The sidescan sonar mosaic for Area I indicates a river bottom of generally uniform and low intensity backscatter (Figs. 9 and 10). Sampling in this area indicates bottom sediments of mud and unconsolidated, fine­grained silty sand. There are distinct patches of higher intensity backscatter located in the northwest corner and center of the northern section of Area I (SS1 & SS2 Fig. 9). Nearby bottom sampling suggests these may be abrupt, but small changes in depth and sediment from sand to mud. Ripple marks in both sand and mud are seen in the sidescan sonar mosaic where Area I transitions from the wider northern section to the narrower southern section (SS3­ Fig. 9). Long, curving, low intensity backscatter marks on the eastern side of the study area are interpreted as an anchor drag close to the shoreline (SS4­ Fig. 9). A 200 m­long by 30 m­wide section of extremely low intensity backscatter next to Dredge 2 was determined to be fluid mud during ground truthing. Sub­bottom profiler lines indicate a flat river bottom with no sudden changes in bathymetry. Quaternary sediments range from 4.5 m to 6.0 m thick. Facies I is representative of seismic facies in Area I (Table 2). In the southernmost portion of Area I,
30 seismic profiles show a 1–4° southward dipping reflector at 6.5, 10, and 18 mbsf interpreted to be the PeeDee formation and undulating reflectors located at 3–6 mbsf interpreted to be the Castle Hayne Formation (Figs. 10 & 11). Area II Area II is bound by Peter's Point to the south (Fig. 10) and is tightly bound by the CFRB on the east and a large dredge spoil island to the west. Area II has two shell midden sites, one on the CFRB and one 0.25 km inland. Most of this section has less than 0.5 m of water at MLLW and becomes supratidal near Peter’s Point. Area II has the least amount of geophysical data coverage due to the shallow depths; it is1.8 km long and only 0.25 km wide. Along­track sidescan sonar data collected totals 2.9 km and sub­bottom profiler data collected totals 3.4 km. Geophysical data were collected along north­south trending tracks resulting in a total of 0.39 km 2 of data in Area II. Eleven bottom samples were collected in Area II, identifying mud and oyster shell on the surface (Fig. 9). Seven samples were mud, all located north of the oyster reef. Four samples were oyster shell taken from the oyster reef. Dredge 3 recovered mud from the northern side of Area II. Dredge 4 was pulled over a low­density oyster reef and recovered oysters. The sidescan sonar mosaic for Area II indicates a river bottom of low reflectivity (Fig. 9), and muddy bottom samples were recovered in most of the area. A low­density, patchy oyster reef was found by poling and was then re­visited by Dredge 4 (Fig. 9). There are sporadic clusters of 4 or 5 oysters surrounded by mud making it difficult to distinguish a mud­oyster reef boundary by poling, dredging or an acoustic boundary in the sidescan sonar mosaic.
31 Figure 11. A. The 384 kHz sidescan sonar mosaic shows a continuous low backscatter due to the surface sediments being mud and fine­grained sand. The orange transect corresponds to the seismic profile. B. The seismic profile shows undulating reflectors at 4–5 m below the seafloor and 0.5–1° south dipping beds at 12 m below the seafloor.
32 A B
33 Sub­bottom profiler lines indicate a flat river bottom with no sudden changes in bathymetry. Quaternary sediments are generally about 3.0 m thick. Facies 1 is representative of seismic facies in the northern part of Area II (Table 2). Facies 2 is representative of seismic facies in the southern part of Area II (Table 2). Seismic data recorded over the oyster reef in the southern part of Area II show a strong impedance contrast at the seafloor followed by signal attenuation (Fig. 12). An undulating reflector is observed at 3 mbsf and is interpreted to be the Castle Hayne Formation. Area III Area III is bound by Peter's Point to the north and marsh to the south (Figs. 2 and 13). The Fort Fisher Air Force Recreation Area is located adjacent to Area III on the eastern side of the river, and Sunny Point is across the river to the west. Depths in Area III are ­ 0.5 m in most of the area. Exposure at MLLW occurs on the eastern side, approaching the eastern CFRB, and in the northwest corner where the area approaches the large dredge spoil island that borders Area II. A total of 15.7 km of along­track sidescan sonar data and 15.4 km of sub­bottom profiler data were collected. Geophysical data were collected in a north­south orientation resulting in a total of 1.77 km 2 of data in Area III. Fifty­seven bottom samples (ranging from 0–2 m deep penetration) collected in Area III indicated a muddy bottom, extensive oyster reefs on the surface and oyster reefs buried in 10–50 cm of mud (Fig. 13). Twelve bottom samples were composed of mud and eleven were mud overlying oyster reef. Twenty­seven bottom samples were taken on oyster reefs, one was oysters growing on a crab trap, and six bottom samples were taken on a sandy bottom. The sandy sediments are located in the middle of the north section of Area III and the muddy sediments are located in the middle and southern portions of Area
34 Figure 12. A. The sidescan sonar image shows a relatively uniform low backscatter on the north side that corresponds to a muddy river bottom and a faint textural change to the south, which corresponds to a patchy oyster reef with live oysters and cultch and is indicated by the dashed orange line. The solid orange line corresponds to the seismic profile. B. The seismic image shows a strong, undulating reflector 3 mbsf on the north side and strong signal attenuation below the oyster reef on the south side. A seafloor multiple is seen below the oyster reef. The PeeDee Formation is interpreted at 18 mbsf.
35 A B Patchy Oyster Reef
36 Figure 13. A 384 kHz sidescan sonar mosaic of Area III. The smaller inset box represents data shown in Fig. 15. The large inset box represents data shown in Fig. 20. Dredges are green lines, labeled with a D. Orange transect lines correspond to seismic profiles.
37 Fig. 15 Fig. 20 Ft. Fisher Air Force Recreation Area Line 6
D5 38 III. Dredge 5, sampled in the southeastern part of Area III, was cast near an unsampled location and was chosen because the sidescan sonar record resembled that of records where oysters had been identified (Fig. 13). Dredge 5 recovered live oysters, stout mussels, and live spat (Fig. 14). The oysters on the surface are a single layer built over oysters within a mud matrix and are primarily located in the middle of the western side of the area. Oysters are also found in the middle and southeastern portions of the study area but are less densely populated than on the reef on the western side of the area. The buried reefs are below 15–50 cm of mud and are found in the vicinity of the oyster reefs. On the Area III sidescan sonar mosaics, a faint, low backscatter outline around a speckled texture was ground truthed to be a reef mound (Fig. 15). When ground truthing, it was noted that the mud content within the oyster reef is high. Also, the oyster reefs in Area III are not large bar­type reefs. The structure of these reefs is oyster shell in a mud matrix with a single layer of live oysters on top. This structure paired with the high mud content in the area may have caused less backscatter of the sonar signal, which resulted in this speckled texture in the sidescan sonar mosaic. Sub­bottom profiler lines indicate a flat river bottom with no sudden changes in bathymetry. Quaternary sediments range from 0.0 m to 3.0 m thick. Facies 1 is representative of seismic facies in the northern part of Area III (Table 2). Facies 2 is representative of seismic facies in the southern part of Area III (Table 2). Area III sub­ bottom data have two distinct reflection characteristics. In the northwest portion of Area III (Fig. 13, Table 2­ Facies 1) there are consistent undulating reflectors that occur at 3 mbsf. In the portions of Area III where oysters are present, the seismic profiles are characterized by a high­amplitude seafloor reflection, a seafloor multiple and sub­bottom
39 A Live Spat
B C D Figure 14. A, C, and D show live oysters and B shows live spat on an oyster shell recovered in Area III, D5. 40 Figure 15. A. A 384 kHz sidescan sonar mosaic of a mapped low­relief reef mound in Area III. Dashed orange lines show the outline of the oyster reef. Solid orange transect corresponds to the seismic line. B. The corresponding seismic line over the low­relief reef mound.
41 A B Signal Attenuation
42 signal attenuation all consistent with a strong seafloor impedance contrast produced by the oyster reefs (Fig. 15, Table 2­ Facies 2). Area IV Area IV is located along the western perimeter of The Basin, bound by Federal Point to the north side and Zeke's Island to the south (Fig. 16). It extends 400–600 m west of The Basin wall into the main channel of the Lower Cape Fear River. Area IV also includes a 2.15 km­long survey line with a NE­SW orientation that is between the north end of Zeke's Island and roughly the midpoint of The Rocks (Figs. 16 and17). Area IV is inter­tidal to sub­tidal at MLLW with depths ranging from 1.0 m along The Basin wall to 3.0 m for much of the area. The northwest corner becomes significantly deeper, changing from 3.0 m to 9.0 m as it moves into the shipping channel. The southeast corner of Area IV is inter­tidal with depths as low as 0.0 m that increase in elevation to 0.5 m approaching Zeke's Island. The line collected NE­SW corresponds to a water depth of 1.5 m. The along­track sidescan sonar data collected in this area totals 9.3 km and sub­bottom profiler data collected totals 10.2 km. Geophysical data were collected in a north­south orientation, with the exception of one line, which was collected NE­SW, resulting in a total of 1.15 km 2 of data in Area IV. Nine bottom samples were collected in Area IV indicating primarily sand and little mud in the area (Figs. 16 and 17). Eight of the samples were sand, located around the perimeter of The Basin and in the area close to Zeke's Island. The single bottom sample that recovered mud was taken 0.27 km west of Zeke's Island. Two dredges were cast in Area IV. Dredge 6, adjacent to The Basin, had no recovery, which is consistent with a sandy bottom in Area IV (Fig. 16). Dredge 7, located southwest of Zeke's Island on the
43 Figure 16. A 384 kHz sidescan sonar mosaic of Area IV. Dredges are green lines, labeled with a D and correspond to the numbers in Table 3.
44 Federal Point D6 The Basin Fig. 18
Zeke's Island D7 45 Figure 17. A 384 kHz sidescan sonar mosaic of the SW oriented line of Area IV and all of Area V. Dredges are green lines, labeled with a D; some correspond to Table 3. Orange transect lines correspond to seismic profiles. Pink dashed lines indicate the location of the paleochannel.
46 D7 D8 Area IV
Area V D9 D10 D11 D12 D13 Fig. 19 D14 Line SB13 47 single survey line, yielded 0.25 gallons (1.1 dm 3 ) of oyster cultch and other organisms (Figs. 16 and 17, Table 3), but no live oysters or live spat were found. Dredge 8, also located southwest of Zeke's Island on the single survey line, had no recovery, consistent with a sandy bottom or sparse marine life (Fig. 17). The sidescan sonar mosaic for Area IV shows low backscatter returns, which is consistent with bottom sample data (Fig.16). Sand ripples can be seen throughout the sidescan sonar mosaic in Area IV (Figs. 16 and 18). Backscatter from The Basin wall can be seen in the sidescan sonar mosaic as a narrow, linear zone of strong reflectivity and appears along the perimeter of The Basin (Figs. 16 and 18). Sub­bottom profiler lines indicate a flat river bottom with no sudden changes in bathymetry on the eastern and southern sections of Area IV. In the northwestern section of Area IV, seismic data were collected in the Lower Cape Fear River shipping channel and can be seen in the seismic record as changes in bathymetry of up to 6 m. Quaternary sediments range from 5.0 m to 7.0 m thick. Facies 3 is representative of seismic facies in Area IV (Table 2). Area V Area V is oriented NE­SW along the east side of The Rocks (Fig. 17). Its northern boundary is at the midpoint of the rock wall and its southern boundary is at the end of the rock wall. Area V is entirely intertidal with depths ranging from 1.0–1.5 m at MLLW and become slightly shallower only in the southernmost portion as it approaches the islands. Along­track sidescan sonar data collected in this area totals 20.9 km and sub­bottom profiler data collected totals 21.5 km. Geophysical data were collected in a northeast­
48 Table 3. Organisms found in Areas IV and V. Benthic Dredge Number 7 Organism 9 Half gallon (2.2 dm 3 ) of oyster cultch, juvenile stone crab (photo), stout razor clam Tagelus plebius, Mercenaria sp., gorgonian (soft coral sea fans). 10 Half gallon (2.2 dm 3 ) of oyster cultch with gorgonians. 11 Three gallons (13.2 dm 3 ) of oyster cultch, pinshell with dead spat (left), starfish (right). 12 Photo Quarter gallon (1.1 dm 3 ) of oyster cultch, mussels (top), polycheate sand tubes (bottom), crabs, worms, tunicate Mogula sp., razor clams and a slipper shell. Oyster cultch and gorgonians. 49 Dead Spat
Figure 18. A sidescan sonar mosaic showing the rock wall and sand ripples in Area IV.
50 southwest orientation parallel to the rock wall, resulting in a total of 1.82 km 2 of data in Area V. Twenty­seven bottom samples (ranging from 0–2 m deep penetration) were collected in Area V, indicating a well­consolidated sandy bottom in most of the area except the northwest quadrant, where oyster cultch and benthic organisms were found (Fig. 17, Table 3). Twenty samples indicated a sandy bottom, comprising the majority of Area V. Well­consolidated sands that were difficult to penetrate with the pole were found in the southern portion of Area V and consolidated sands that could be penetrated up to 2 m were found in the northern portion of Area V near the rock wall. Seven samples from the northwest quadrant of Area V were composed of oyster cultch with attached gorgonians and tubeworms and were overlying a sandy bottom. Dredges 9–12 were cast in the northwest quadrant of Area V where oyster cultch was found. The contents of the dredges are summarized in Table 3. Dredge 11 brought up three gallons (13.2 dm 3 ) of oyster cultch, which was significantly more than the other dredges. Dredges 13 and 14 had no recovery, which is consistent with a sandy bottom indicated by the bottom samples. The sidescan sonar mosaic for Area V shows mostly low backscatter returns with some isolated areas of higher backscatter (Fig. 17). Samples recovered in the majority of the area indicate a sandy bottom. In the northwest corner of Area V, large amounts of oyster cultch and benthic organisms were found. The sidescan sonar mosaic for Area V indicates sand ripples in the middle of the line furthest from the rock wall. There are isolated areas of high backscatter ranging from 10–25 m long by 5–10 m wide. These locations were targeted for ground truthing. They were impenetrable with the corer and a sample could not be retrieved with the grab sampler. Oyster cultch with attached
51 gorgonians was recovered in the vicinity of these locations within the northeastern portion of Area V, but not from the other locations in the sandy portions of Area V. Sub­bottom profiler lines in Area V indicate a flat river bottom with no sudden changes in bathymetry. Quaternary sediments range from 3.0 m to 7.0 m thick. Facies 3 is representative of seismic facies in Area V (Table 2).Undulating reflectors from 3–10 mbsf appear in much of seismic profiles across Area V (Fig. 19). On the southwestern side of line SB13 there is an undulating reflector that changes in depth from 2.5 mbsf to 6.0 mbsf and to 2.5 mbsf in 75 m horizontal distance (Fig. 19). This reflector can be traced as continuous features across several adjacent seismic lines in Area V and has a NW­SE trend. Similar subsurface features have been found offshore southern New Hanover County, North Carolina and are described as 50–250 m wide and 7–30 mbsf Plio­Pleistocene paleo­fluvial channels (Snyder et al. 1994). DISCUSSION Oyster reefs and buried oysters in the seismic sub­bottom profiler record Living and buried oyster reefs were found in Areas II and III only. Seventy percent of confirmed surface oyster locations or locations where oysters are buried between 10–50 cm of mud create a strong initial impedance contrast followed by signal attenuation, and a seafloor multiple. Together, these indicate a strong impedance contrast between the water column and the seafloor that is consistent with the direct observation of living or thinly sedimented oyster reefs at the seafloor. Signal attenuation below the oyster reefs greatly minimizes reflections below the seafloor, which is consistent with findings in previous studies such as Carbotte et al. (2004). Seafloor multiples are a known issue when using high­resolution seismic in shallow water, as discussed in Roberts et al.
52 Figure 19. A. The sidescan sonar 384 kHz mosaic shows the sandy river bottom. The orange transect corresponds to the seismic profile. Pink dashed lines correspond to the paleochannel on the seismic profile. B. The seismic profile shows an undulating surface at 4 m mbsf that is interpreted to be a paleochannel.
53 A B Line SB13
Quaternary Sediments Top of Castle Hayne 54 (2000), but in this study seafloor multiples were a consistent observation, and became useful in delineating oyster reefs. Seismic data appeared the same for oyster reefs on the surface and oysters buried within the mud. This is because the vertical resolution of the seismic data is 1.0 m, so it will not resolve a 0.5 m layer of mud over an oyster reef, making the two scenarios appear the same. Oyster reefs and oyster cultch in the sidescan sonar record On the sidescan sonar mosaics, high density oyster reefs with slight relief above the seafloor can be distinguished by a speckled texture with a white acoustic shadow outlining the reef. This was seen in Area III where living reefs were found. Low­density, patchy reefs with no relief above the seafloor can also be distinguished on the sidescan sonar mosaic. These also have a speckled texture, but do not have an acoustic shadow. This was seen in the southern part of Area II. Oysters buried in mud could not be distinguished with sidescan sonar imagery due to the rapid attenuation of both the 384 and 100 kHz sidescan sonar signals. The along­track sidescan sonar data for the majority of Areas IV and V were consistent with ground truthing results of a mostly sandy bottom. Patches of oyster cultch were found in the northwest corner of Area V and would be expected to produce a high backscatter; however, there was no distinctive acoustic signature in the sidescan sonar data. This could be because oyster cultch is extremely weathered and has soft corals and other organisms attached (Table 3­ Benthic Dredge 12). The cultch may have been more dispersed and flat­lying along the river bottom instead of being consolidated and sticking out vertically from the sediments. There also may have been enough sedimentation over
55 the cultch to affect the acoustic backscatter, though none was observed with benthic dredging. Potential factors affecting past oyster reef growth in the Lower Cape Fear River Areas I and II were considered the most likely to contain buried oyster reefs because the eastern CFRB in these areas had documented shell middens. Area I, however, had no oysters present, and Area II had oysters only in the southern part closest to Area III. Shell middens are not limited to the river bank. There are shell middens further inland throughout New Hanover County (Classen 1986, Lawrence et al. 1988) and also in intertidal areas in the Cape Fear River and in the Intracoastal Waterway (New South Associates 2002. This vast distribution of shell middens may be due to one or more factors. Sea level has fluctuated over time, causing the shallow Cape Fear River to undergo significant changes. About 700 years ago, there was a low stand sea level of 1.2 m below the present (Brooks et al. 1989), causing Areas I, II, and III to be at or above sea level. This exposure could have eroded any existing oyster beds. A lower sea level would push the coastline seaward and explain why some shell middens are found in what are now intertidal areas. The shell middens serve as evidence for oysters in the region, but not necessarily in the area adjacent to the middens. Oysters can survive in salinities ranging from 5 ppt–35 ppt. Salinity values from LCFRP Station M35 located near Area I (Fig. 2) indicate that in six years of monitoring from the years 2000–2003 and 2005–2007, the average monthly salinity values dropped below 5 ppt during 11 of those months. Oysters are not able to survive in low salinity environments (Kennedy et al. 1996) and in a study by Davis (1958) it was observed that oyster larvae has a 100% mortality rate within two weeks in water 10 ppt salinity or less.
56 If the salinity is currently too low to support oyster growth in Area I and the northern part of Area II, then the salinity would not have been high enough in these areas prior to the dredging of Snow’s Cut in 1929, which brings brackish water from the Intracoastal Waterway into the Cape Fear River about 2 km north of this station. In Area III, oyster reefs both buried and living on the surface were successfully located. Area III is 8 km south of the M35 station and 8 km northeast of the M23 LCFRP monitoring stations. In six years of monitoring from the years 2000–2003 and 2005–2007 the lowest average monthly salinity value was 6.3 ppt and dropped below 10.0 ppt for only five months. Area III is further south and closer to the river mouth than Areas I and II and is more shallow than Area I, which may make salinities somewhat higher than in Areas I and II and suitable for present oyster growth. Area III is located just north of Federal Point and is separated from the Atlantic Ocean by a 0.68 km barrier island (Kure Beach) with a maritime forest and a 1.5 km 2 marsh to the south. This gives Area III more protection from storms, which can be devastating to oyster reefs. Areas IV and V were initially the proposed location for an oyster restoration project because they are near historic oyster reef locations (Bolles 1866), and presently oysters are growing on The Rocks adjacent to these study areas. Oyster cultch has collected in parts of Areas IV and V, but is extremely eroded. Other marine invertebrates have grown on this cultch (Table 3), however it is much too eroded and may have a thin layer of sediment making it unsuitable for oyster spat settlement. The marine invertebrates found in these areas were starfish and crabs, which are natural oyster predators and gorgonians, which can settle on top of an existing spat and kill it. Areas IV and V have a turbulent history with multiple inlet openings and closings along the Atlantic Ocean (Jackson
57 1996), made evident by the interpretation of one paleochannel within the seismic data and thick accumulations of Quaternary deposits. The combination of this dynamic environment with dredging and hard structure placement over the last 120 years may explain why there are no oyster reefs at the surface or buried within 2 mbsf in these areas. Using results to delineate potential oyster cultch placement locations Oyster cultch placement is recommended in Area III. Potential or confirmed living or buried oyster reefs, based on the seismic sub­bottom profiler data are shaded blue on Figure 20. Potential or confirmed living oyster populations from the sidescan sonar data are shaded in green. If the green areas from the sidescan sonar data (living reefs) are clipped (an ArcGIS function) from the blue areas from the seismic data (buried and living reefs), the most likely locations of buried oyster reefs remain. The remaining area is still too large for an oyster restoration project, so can be narrowed down further with ground­ truth data. One potential site (purple oval shading in Fig. 20) is next to confirmed, low­ relief reef mound of living oysters and an additional site (round purple shading in Fig. 20) is a predominantly buried oyster reef area. The green areas in the middle to southern part of Area III are patchy oyster reef areas. The southern part of Area II where a small, patchy oyster reef was found may also be a possible location for oyster restoration. Oyster cultch placement is not recommended for Areas I, IV, V, or the northern part of Area II. River sediments in Areas I and the northern part of Area II are characterized by very fine­grained silty sand and mud with no evidence of hard substrate for oyster settlement or growth. In addition, the sidescan sonar mosaics for Areas I, IV, and V show evidence of sand ripples, which indicate a high energy environment. Both areas IV and V have predominantly sandy bottoms, each with a small area where oyster cultch has
58 Figure 20. Potential oyster restoration areas. Blue is a 1.1 km 2 area from seismic lines that indicate potential or confirmed living or buried oysters. Green is a 0.19 km 2 total area from sidescan sonar lines that indicate potential or confirmed living oysters. Purple is 0.07 km 2 (18.3 acres) total of possible repopulation areas where buried oysters have either been identified or inferred from the geophysical data.
59 60
collected and provide a habitat to various organisms, but no new successful oyster growth was observed here. CONCLUSIONS The purpose of this study was to use sidescan sonar and sub­bottom profiler to map buried and living oyster reefs in five areas over a 15 km­long section of the Lower Cape Fear River, southeastern North Carolina. This combination of geophysical instruments had not yet been used to map oyster reefs in the Cape Fear River and the goal was to efficiently find an area suitable for oyster restoration. Pairing the sidescan sonar and the sub­bottom profiler data proves to be a useful technique in finding living and buried oyster reefs. In this study it has been shown that when sidescan sonar imagery shows uniform low backscatter and the sub­bottom profiler shows a high­amplitude seafloor reflector and sub­bottom signal attenuation, this is strong evidence for oyster reef buried in less than 1m of sediment. When the sidescan sonar imagery shows a speckled texture outlined by a white acoustic shadow and the sub­ bottom profiler shows a high­amplitude seafloor reflector and sub­bottom signal attenuation, this is strong evidence for a potential living, low­relief reef mound on the surface. When the sidescan sonar imagery shows a speckled texture without a white acoustic shadow and the sub­bottom profiler shows a high­amplitude seafloor reflector and sub­bottom signal attenuation, this is strong evidence for a potential living, relatively low­density, patchy reef on the surface. The combination of sidescan sonar and seismic sub­bottom profiler data were successful identifying living and buried oyster reef locations in two out of the five study
61 areas. A 1.1 km 2 area that has a combination of living oyster reefs and buried oyster reefs was delineation as a potential site for future oyster restoration. Further studies in this area could involve monitoring of the current oyster reefs using geophysical instrumentation, tracking their growth over time. If there is a restoration project, new oyster reef growth could also be monitored using these instruments.
62 LITERATURE CITED Allen, Y., C. Wilson, H. Roberts, & J. Supan. 2005. High resolution mapping and classification of oyster habitats in nearshore Louisiana using sidescan sonar. Estuaries 28: 435–446. Alphin, T., A. Wilbur, J. Swartzenburg, & M. Posey. 2006. Evaluation of Spatfall in the Cape Fear River Estuary. University of North Carolina Wilmington, Center for Marine Science, Department of Biology and Marine Biology, and J&B Aquafood. Final Report­ Fisheries Research Grant 02­AM­09. Becker, M. L., R. A. Luettich, & H. Seim. 2009. Effects of intratidal and tidal range variability on circulation and salinity structure in the Cape Fear Estuary, North Carolina. Journal of Geophysical Research 114: 1–20. Bolles, C.P. & J.S. Bradford. 1866. Cape Fear River, North Carolina. Coast and Geodetic Survey: Nautical Chart 424. Retrieved on 27 October 2004 from World Wide Web:http://dc.lib.unc.edu/cdm4/item_viewer.php?CISOROOT=/ncmaps&CISOP TR=1036. Brooks, M.J., P.A. Stone, D.J. Colquhoun, & J.G. Brown. 1989. Sea level change, estuarine development and temporal variability in woodland period subsistence settlement patterning on the lower coastal plain of South Carolina. In Studies in South Carolina archaeology: essays in honor of Robert L. Stephenson., edited by Albert C. Goodyear, III and Glen T. Hanson. Columbia, South Carolina: 91–100. Carbotte, S., R. Bell, W. Ryan, C. McHugh, A. Slagle, F. Nitsche, & J. Rubenstone. 2004. Environmental Change and oyster colonization within the Hudson River estuary linked to Holocene climate. Geo­Marine Letters 24: 212–224. Classen, C. 1986. Temporal patterns in marine shellfish species use along the Atlantic Coast in the southeastern United States. Southeastern Archeology 5: 120–137. Dame, R., M. Alber, D. Allen, M. Mallin, C. Montague, A. Lewitus, A. Chalmers, R. Gardner, C. Gilman, B. Kjerfve, J. Pickney, & N. Smith. 2000. Estuaries of the South Atlantic Coast of North America: Their Geographical Signatures. Estuaries 23: 793–819. Dame, R. 2005. Oyster reef restoration: a complex systems problem. Journal of Shellfish Research 24: 320. Davis, H. C. 1958. Survival and growth of clam and oyster larvae at different salinities. Biological Bulletin 114: 296–307.
GeoAcoustics Limited. Operation and Maintenance Manual for the Model 5813B GeoPulse High Resolution Sound Source. Norfolk, England. 24 pp. Grindlay, N., L. Abrams, & L. Del Greco. 2005. Toward and integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high­resolution seismic and sidescan sonar data. Geologic Society of America, Special Paper 385: 139–160. Grizzle, R., L. Ward, J. Adams, S. Dukstra, & B. Smith. 2005. Mapping and characterizing subtidal oyster reefs using acoustic techniques, underwater videography, and quadrat counts. American Fisheries Society Symposium 41: 143–159. Grizzle, R., M. Brodeur, H. Abeels, & J. Greene. 2008. Bottom habitat mapping using towed underwater videography: subtidal oyster reefs as an example application. Journal of Coastal Research 24: 103–109. Hackney, C., M. Posey, L. Leonard, T. Alphin, G. Avery. 2003. Monitoring effects of a potential increased tidal range in the Cape Fear River ecosystem due to deepening Wilmington Harbor, North Carolina. Prepared for the United States Army Corps of Engineers, Wilmington District, North Carolina. 200 pp. Hackney, C. and G. F. Yelverton. 1990. Effects of human activities and sea level rise on wetland ecosystems in the Cape Fear River Estuary, North Carolina, USA. In Wetland Ecology and Management: Case Studies., edited by D.F. Whingham et al. 55–61. Netherlands. Harris, W.B., 1996. An overview of the marine Tertiary and Quaternary deposits between Cape Fear and Cape Lookout, North Carolina. In ed. W. Cleary, Environmental Coastal Geology Cape Lookout to Cape Fear, NC, Carolina Geological Society Fieldtrip Guidebook 1996: 1­10. Harris, W.B. & T. Haw, 2004. Wilmington harbor deepening, Cape Fear River, southeastern North Carolina, geotechnical considerations. Southeastern Geology 42: 279–294. Jackson III, C.V. 1996. The Cape Fear — Northeast Cape Fear Rivers comprehensive study: A maritime history and survey of the Cape Fear and Northeast Cape Fear Rivers, Wilmington Harbor, North Carolina. V. 1 Maritime History. North Carolina Department of Cultural Resources: Underwater Archeological Unit, and the United States Army Corps of Engineers. 433 pp. Kennedy, V. S. 1989. The Chesapeake Bay oyster industry: traditional management practices. In Marine Invertebrate Fisheries: Their assessment and management, edited by M.F. Caddy, 455–477. John Wiley and Sons, New York.
64 Kennedy, V.S., R.I.E. Newell, A.F. Eble. 1996. The Eastern Oyster Crassostrea virginica. College Park, Maryland: Maryland Sea Grant College. 734 pp. Lawrence, D. 1988. Oysters as geoarcheologic objects. Geoarcheology: An International Journal 3: 267–274. Lower Cape Fear River Program (LCFRP). 2004. University of North Carolina Wilmington, Retrieved on 19 th April 2006 from World Wide Web: http://www.uncwil.edu/cmsr/aquaticecology/lcfrp/station%20list.htm. Mallin, M.A., M.R. McIver, & J.F. Merritt. 2006. Environmental Assessment of the Lower Cape Fear River System, 2005. Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina. CMS Report No. 06­ 02. 89 pp. Mallin, M.A., M.R. McIver, & J.F. Merritt. 2007. Environmental Assessment of the Lower Cape Fear River System, 2006. Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina. CMS Report No. 07­ 02. 92 pp. Mallin, M.A., M.R. McIver, & J.F. Merritt. 2008. Environmental Assessment of the Lower Cape Fear River System, 2007. Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina. 87 pp. Mallin, M.A., J.M. Burkholder, L.B. Cahoon, & M.H. Posey. 2000. North and South Carolina coasts. In Seas at the millennium: An environmental evaluation, edited by C. Sheppard, 351–371. Pergamon, New York. Marshall, J. 2004. Event Driven Sediment Mobility on the Inner Continental Shelf of Onslow Bay, NC. Unpublished Masters Thesis, University of North Carolina Wilmington, Wilmington, North Carolina. 86 pp. Moseley, Edward 1733. A new and correct map of the province of North Carolina in the southeast. In Early Maps, William P. Cumming, Chapel Hill, NC: UNC Press, 1962. Retrieved on 2 nd January 2008 from World Wide Web: http://www.ah.dcr.state.nc.us/sections/hp/colonial/Maps/Moseley/default.htm. National Oceanographic and Atmospheric Administration (NOAA), Office of Coast Survey Historical Map and Chart Project, Nautical Chart 424 c. 1889. National Oceanographic and Atmospheric Administration (NOAA), Office of Coast Survey Historical Map and Chart Project, Nautical Chart 31 c. 1866.
65 New South Associates. 2001. Integrated cultural resources management plan 1: Installation overview and historic content. Technical Report 908, Stone Mountain, Georgia, Report for the United States Army Corps of Engineers (USACE). 63 pp. New South Associates. 2002. Integrated cultural resources management plan, Military Ocean Terminal Suny Point 3: Map Series (4). Brunswick and New Hanover Counties. North Carolina Coastal Federation (NCCF). 2006. “Oyster Habitat Restoration.” Retrieved on 19 th April 2006 from the World Wide Web: http://www.nccoast.org/Restoration/oysterhabitat/index_html. North Carolina Department of Natural Resources and Community Development (NCDNRCD). 1983. Status of Water Resources, Cape Fear River Basin Study 1981­1983. Raleigh, N.C. Posey, M. H. T.D. Alphin, & H.D. Harwell. 2005. Restoration of intertidal oyster reefs for ecological function: design considerations. Journal of Shellfish Research 24: 332. Power, A. & D. Sanders. 2005. G.E.O.R.G.I.A. (Generating enhanced oyster reefs in Georgia’s inshore areas): A community­based oyster restoration program. Journal of Shellfish Research 24: 333. Powers, S. P. & K. L. Heck. 2005. Restoration of oyster reefs in Mobile Bay, Alabama: Evaluating expectations of fishery benefits across an environmental gradient. Journal of Shellfish Research 24: 333. Roberts, H. H., C. Wilson, & J. Supan. 2000. Acoustic surveying of ultra­shallow water bottoms (<2.0 m) for both engineering, and environmental applications. Proceedings from the Offshore Technology Conference. Offshore Technology Conference, USA. 571–580. Ryan, W.B. & R.D. Flood. 1996. Side­looking sonar backscatter response at dual frequencies. Marine Geophysical Researches 18: 689–705. Smith, G. & K. Greenhawk. 1998. Shellfish benthic habitat assessment in the Chesapeake Bay: Progress towards integrated technologies for mapping and analysis. Journal of Shellfish Research 17: 1433–1437. Smith, G., K. Greenhawk, D. Bruce, E. Roach, & S. Jordan. 2001. A digital presentation of the Maryland oyster habitat and associated bottom types in the Chesapeake Bay (1974­1983). Journal of Shellfish Research 20(1): 197–206. Snyder, S., C. Hoffman, & S. Riggs. 1994. Seismic stratigraphic framework of the inner continental shelf: Mason Inlet to New Inlet, North Carolina. Department of
66 Environment, Health and Natural Resources. Department of Land Resources. North Carolina Geological Survey Bulletin 96: 59 pp. Yonge, C. 1960. Oysters. St. James' Place, London. 209 pp.
67 APPENDIX A Appendix A describes the design and implementation of the geophysical surveys done to collect data for this study. Maptech National Oceanic and Atmospheric Administration (NOAA) nautical charts of the study area were used in the Maxsea program to create proposed tracklines and tielines that were used as field guides. The beginning and ending coordinates for each line were entered into the Trimble Ag132 DGPS (Differential Global Positioning System) and used to navigate the Carolina Skiff R/V Orca while mapping the western section of Area I, Area III, Area IV, and Area V when the sub­bottom profiler data was collected. The eastern section of Area I, Area II, and Area III were mapped using the RV Seahawk, a 35 ft Catamaran, with the Northstar 951 XD DGPS navigation system. When the sidescan sonar data were collected in Area V, a Furuno dGPS system was used to navigate the Carolina Skiff, R/V Orca. Sidescan sonar and seismic subbottom profiler data were not collected simultaneously in Area V. Sidescan sonar data was collected first. Tracklines from Area V were exported from Isis® Sonar™ and Delphmap™, imported into ArcMap as line files, and used as guides to lay out beginning and end points for seismic tracklines in Area IV. These points were then imported into Hypack Max™ software, where tracklines were created. This software has the ability to show the vessel navigator how far off the trackline they are in order to remain as close to the desired course as possible.
APPENDIX B Appendix B contains information and directions on processing sidescan sonar data. Instructions for smoothing sidescan sonar navigation using Fixheadx from the Command Prompt (Fig. 1): 1. From the programs menu, go to accessories, then command prompt. 2. The command prompt will open. Change to the dos utilities directory by typing: cd c:\dos utilities 3. To call the fixheadx program, type: fixheadx “yourfilepathnamehere” 4. The program will run and will say DONE when finished. 5. Hint: To paste text into the command prompt window, right click on the command prompt icon on the upper left hand side of the window and select paste.
69 Figure 1. Directions for running fixheadx from the command prompt.
70 The Slant Range correction removes the water column from the image. The TVG corrects for the attenuation of the acoustic signal as it leaves the transducers and travels through the water column. The further away from the towfish the signal travels, the more it looses its strength. The TVG tool corrects for this problem by amplifying the data further away from the towfish. The resolution of the sidescan sonar depends on the sonar pulse width, beam spreading, and the speed of the fish. The resolution of each pixel is a function of the range and the number of pixels in the display range. The range setting is determined by taking into account the total area to be covered and the minimum size of the features. Choosing a larger range will result in a greater coverage, reducing ship time and shorter ranges are used for finding small objects. The pulse length is the length of the beam as it leaves the towfish. A shorter pulse length yields higher the resolution. A pulse length of 1.5 m will not be able to resolve two objects 0.5 m apart. Additional sidescan sonar processing steps for Isis® Sonar™ can be found in Jeff Marshall, 2004.
71 APPENDIX C Appendix C contains information and directions on processing seismic data. Instructions for Smoothing Seismic File Navigation Using Delph Navigation Processing 1. Create a new folder for converted data, for example: for Area III lines create a folder called Area III­smooth­ the program will put converted data here and will not alter original files. 2. Open Delph Navigation Processing (on cd called d24prcw.exe) In Programsà Triton Elics Internationalà Delph Seismicà Navigation Processing 3. Click Edit to Begin 4. In source and destination file setting, set source to:
72 Media: Disk à File Format: SEGY 1, Data Format: Motorola Set destination to (same as source): Media: Disk à File Format: SEGY 1, Data Format: Motorola 5. In File name/ File Number: source file nameà browse for input file à select a .tra file from recorded seismic data destination file nameà browse (keep same name as original, but put in the smooth folder) 6. In Optionsà check Filter position and enter 6 à for along track offset, enter the measured layback for the seismic source à for confidence radius enter 5, this is the distance between each navigation point. If it is greater than 5 m, it removes the point. à Overwrite N/Sà click the circle to choose N (this changes –lat to +lat) 7. Check generate processed Nav. File (this outputs a text file as a .nav­ it can be opened in notepad) 8. click edit again, then click start­ this will begin the navigation correction process. 9. this will output .tra, .par, and .nav files in the new smooth folder 10. Open the .nav file in notepad.
73 Triton Imaing,® Inc.’s Delph Nav™ and SB­Interpreter™, FixPing.exe, and Parallel Geoscience's Seismic Processing Workshop (SPW) were used for seismic data processing. Delph Nav™ was used to correct the navigation for seismic data from Areas I–IV. SB­Interpreter™ and FixPing.exe were used to correct and export the seismic navigation for Area V. Instructions on processing sub­bottom data in Sub­Bottom Interpreter are provided by Triton Imaing,® Inc. and can be found at: http://www.tritonimaginginc.com/site/content/public/downloads/Guides/SBInterpreter_1 5/triton_sbi_1_5.htm. Processing seismic data in Seismic Processing Workshop (SPW) SPW was used to process the sub­bottom profiler data. Various processing techniques were used such as trace killing and surgical mutes to remove noisy shots and turns, respectively. A sea­floor mute was applied to the data in order to eliminate the water column and the direct wave. Seismic lines in which the direct wave interfered with the river bottom were not muted or were muted using Adobe® Photoshop®. In order to find the best configuration in which to view the seismic images, a filter panel was created. This figure uses a section of a seismic line where different filters and gains are applied to the data and then compared. The first seismic data processed was from Area I. A filter panel was created with a 400 m section of line 3 in which the automatic gain control (AGC), pre­raster gain, amplitude setting, and trace mixing was held constant, while a comparison of typical band­pass filters and band­pass filters based on the power spectrum of that line were incorporated. The power spectrum is a graph of power (amplitude 2 ) versus frequency. Some of the seismic lines had a significant amount of noise resulting in two power peaks at 150–1000 Hz and 1100–2100 Hz. By testing different filters and
74 comparing these filters it was possible to determine which peak represented the seismic signal and which one to filter out as noise. Figure 1 shows the power spectrum for Area I, line 3 and Figure 2 shows the filter panel created to test different filters. A bandpass filter of 1000–1100 Hz to 2100–2700 Hz was applied to areas I–IV; a bandpass filter of 1000– 1200 Hz to 2100–2200 Hz was applied to Area V. the entire study area. Once the seismic lines were processed, they were printed at a large scale for interpretation. The peak power of this data fell between 1100 and 2100 Hz at about 1500 Hz. The theoretical resolutions can be calculated from the high frequency value. l = wavelength , f = frequency , v = velocity , l=
l
4 v , f = resolution Theoretical resolution: l =
1500 m / s 1. 0 m = 1 . 0 m , = 0 . 25 m 1500 Hz 4 75 Figure 1. Power spectrum generated by SPW.
76 Figure 2. Filter panel created to compare filters and decide which filter best suits the data.
77 Seismic Processing Workshop Tutorial Part I­ Import seismic data 1. File à New Processing Flow or +N. 2. Flow Items à Input/Output Devices à Seg­Y Diskfiles 3. Highlight Seg­Y Disk File in the flowchart 4. Navigate to the .tra file and select it. box and press +R or Flowchart à Rename 5. Add Seg­Y input
78 6. Double click on Seg­Y to open the Seg­Y Format Data Input Specification box. Check Override Seg­Y Default Definition Click Set Traceheader Field Definitions 7. Check Override Seg­Y Defaults. 8. Add Seismic File
Check Source Point Number. Type 73 in Start byte and select 4 bytes. Check receiver Number. Type 77 in 79 Start Byte and Select 4 bytes Press ok, ok. 9. Rename the seismic file by pressing +R or Flowchart à Rename. A dialog box appears. Press new, type in a file name, and press save.
80 10. Press the ↑ button on the flowchart window and draw arrows to connect the three boxes. When connecting Seg­Y Input to the output, a dialog box will pop up. Select output seismic data à seismic traces. 11. Select the cross­hair button in the flowchart window. Select Seg­Y Input and press +E or FlowchartàExecute. 12. Highlight the output file and select FlowchartàParameters. 13. Copy the number in the total number box and paste it into the Traces per record box. Change the Total Number box to 1.
81 14. Click on More Info. In the Sort Type box select Stack. 15. Double click on the output file and the Seismic Data Display (SDD) window opens. In traces/unit, type 50 and in units/sec type 300. Select trace under plot every and select 300% for the overlap. The plot direction and trace annotation can be determined based on the data.
82 16. In the SDD window, click Condition, choose True Amplitude in the Wiggle Trace Assignment box and check the Automatic Gain Control (AGC) box. The AGC is data dependent. Press OK to close boxes.
83 17. Double click on the output file and the seismic line will open. Part II­ Spreadsheet 1. In the Flow Chart Window, highlight the output file. 2. Choose FlowChart à Spreadsheet. 3. Rx and Tx are the Latitude and Longitude, respectively, in arcseconds format. Check to make sure there is a different XY coordinate for each seismic trace. 4. The Dead Column will be empty, but "true" will fill cell of traces that are killed in part III. 5. The offset column indicates the distance between each shot in meters, which must be calculated from the data. 6. Calculate the average speed over ground and divide by the number of shots per seismic line. Highlight the offset column and click Card Data à Clear Cells.
84 Enter 0 into the first cell and the number calculated for the distance between each shot point into the second cell. Click Card Data à Interpolate Cells and the offset will be interpolated for the entire seismic line. 7. Close the spreadsheet and save changes. Part III­ Kill Noisy Traces 1. Open seismic line by clicking on output box. 2. Click on A in upper left hand corner of seismic line, which opens the Seismic Data display (SDD) 3. Change the traces/unit to a number from 10 to 20 and the units/sec to 500. 4. Open the Trace Condition Control (TCC) window by clicking Condition. Turn off the AGC and press ok, press ok in the SDD. 5. If this caused the seismic line to go blank, go back into the TCC window and select Relative Amplitude in the wiggle assignment box. 6. Click the seisviewer dropdown and select trace selection/editing.
85 7. The cursor will turn into a cross­hair. Click on a trace to select and press Seis Data à Kill Trace. + K or Part IV Creating a Power Spectrum 1. Open the seismic line and the SDD box. 2. Change trace/unit to 50 and units/sec to 300. Click plot every 2 nd trace. 3. Click Seisviewer à Trace Selection/Editing. The cursor turns to crosshairs. Drag the cursor to select multiple traces. 4. Press + C to copy or edit à copy. 5. Go to File à New Calculator Worksheet
86 6. Paste the copied traces into the bottom box by pressing pastes the line number and selected trace numbers. + V or Edit à Paste. This Power Plot
7. Highlight the pasted data and press Power on the right column of the Calculator Worksheet, then press Plot, which is just above the bottom window. 8. A new window opens showing the power spectrum for that line, which can help to create a good filter for the data. Part V Choosing a Filter 87 1. The power spectrum from Part IV has a peak power from 1500­2000 Hz, so a bandpass filter of 1200­1500­2000­2300 can be tested on the data. 2. In the FlowChart dialog box, add a Spherical Divergence Correction. 3. Add a Time Variant Band Pass Filter. 4. Double click on the Time Variant BandPass. In row 1 type 1200­1500­2000­2300 across the row.
88 5. 6. 7. 8. Add a seismic file and rename. Connect the new boxes with arrows. Highlight SDC and execute. Repeat steps 3­6, experimenting with different filters. Part VI Muting 1. Open the seismic line and open the SDD box. 2. Change the traces/unit to a number from 10–20 and the units/sec to 500. Choose plot every trace. 3. Still in the SDD box, choose condition and select True Amplitude for the Wiggle Trace Assignment and select a high AGC for the data. Close the boxes by pressing ok and close the seismic line. 4. Go to Flow Items à Application Steps à Apply Mute.
89 5. Go to Flow Items à Card Data à Mute Definition. 6. Double click on Mute Definition, click Sort Order, and change Sort Type to Stack. Press OK, close box and save changes. 7. Draw and arrow from the output data to Apply Mute. Draw an arrow from Mute Definition to Apply Mute. Draw an arrow from Mute Definition to the output data.
90 8. Open the Seismic Line. Click Seisviewer and choose Mute Picking. 9. Press OK and the Mute Definition Spreadsheet opens. 10. Make the seismic line active and use the cursor to digitize the seafloor. Double click when finished. 11. Click Mute Pick à Pick early Mute. 12. A box will fill in the water column and mute selection points fill in the mute definition spreadsheet. 13. Close the mute definition spreadsheet and save changes. Close the seismic line.
91 14. Double click on Apply Mute, check Early Mute and enter a Mute Taper Length. 15. Create an output seismic file and make an arrow pointing to it from the Apply Mute. 16. Execute the Apply Mute. Part VII AGC and Trace Mixing 1. Open the Seismic Line 2. Change traces/unit to 50 and units/sec to 300 in SDD, plot every 2 nd trace. Check Clip and enter 0. Uncheck the AGC in the TCC and plot True Amplitude as the wiggle trace assignment. Close the seismic line. 3. Go to Flow Items à Signal Enhancement à Trace Mixing. 4. Double click on Trace Mixing and enter a number from 1­3 to mix and press OK.
92 5. Go to Flow ItemsàAmplitude Adjustment à AGC. 6. Double click on AGC and enter a number (data dependent) under operator length in row 1, press OK.
93 7. Add and rename a new seismic file, connect the three new boxes with arrows, press OK to output Seisfileà Seistraces. 8. Highlight Trace Mixing and execute.
94 APPENDIX D Appendix D contains a description of the information included in the Geographic Information System. Historic Data A nautical chart (Bolles 1866) showing locations of historic locations of oyster reefs in the mouth of the Cape Fear River was imported into the GIS using the Georeferencing tool in ArcGIS®. Indian midden sites found near the study area by the Underwater Archeology Unit in Kure Beach, NC were also incorporated into the GIS. Existing Data The base layer of the GIS consists of both Maptech National Oceanic and Atmospheric Administration (NOAA) nautical charts and 2 ft resolution Multi­ Resolution Seamless Imagery Database (MRSID) aerial digital orthophotos downloaded from the New Hanover County, NC GIS Data website. Contour lines at 0.5 m spacing were created from a 1 arc second Digital Elevation Model (DEM) using the Spatial Analyst tool in ArcGIS®. The DEM was downloaded from the NOAA National Ocean Service (NOS) Estuarine Bathymetry website. Geophysical Data All data gathered during this project was included in the GIS. Processed sidescan mosaics of both frequencies in raster format and their respective tracklines as line files were included. The seismic shot point navigation files were sorted in MATLAB®, keeping every 50th shot point, brought into the GIS as point files, and labeled so that the seismic line could be compared to the sidescan sonar mosaic.
95