NS24A-01 INVITED
Integrating high-resolution mapping of the seafloor with sediment-transport measurements to understand coastal erosion in northern South Carolina
Shoreline behavior along the coast of Long Bay, South Carolina is dictated by waves, tidal currents, and sediment supply that act within the overall constraints of the regional geologic setting. This study examined the influence of the geologic framework on coastal evolution through the interpretation of high-resolution geophysical data (swath bathymetry, sidescan-sonar imagery, seismic-reflection profiles), bottom samples and cores. Interpreted geophysical data were used to form conceptual models of sediment flux in the nearshore area, which are being tested by conducting site-specific sediment transport and oceanographic measurements and modeling. The inner shelf of Long Bay extends from the shoreface to about 10 km offshore (5-15 m water depth). It is underlain by coastal-plain strata of Cretaceous/Tertiary age that are incised by large fluvial channels formed during the Pliocene and Pleistocene. The indurated coastal-plain and channel-fill deposits are exposed as hardgrounds over large expanses of the inner shelf, and locally overlain by a discontinuous veneer of sandy Holocene sediment generally less than 1-m thick. A regional unconformity, thought to represent erosion during the most recent marine transgression, coincides with the seafloor in these areas of sparse sediment. Minor bathymetric highs occur where relatively thicker accumulations of Holocene sediment lie above the low-relief, transgressive unconformity. One of the thickest accumulations of Holocene sediment is contained within an anomalous, shore-oblique sand body that lies 3 km offshore of Myrtle Beach and is not associated with a modern tidal inlet. The lobate deposit is approximately 11-km long, 3-km wide, and up to 3-m thick. Cores show that the shoal is a marine deposit less than 5000 years old with a gravelly lag at the base representing the transgressive surface. It contains an estimated volume of 26 million m$^{3}$ of sediment, largely consisting of fine to medium, well sorted quartz sand and is a potential source of material for planned beach nourishment projects in the Myrtle Beach area. Experiments are presently underway to determine the influence of modern hydrodynamic processes in maintaining and/or modifying the sand body. This study represents an interaction of geologists, geophysicists, modelers, and physical oceanographers that is necessary to increase our basic understanding of inner-shelf processes and coastal evolution/behavior. Such an interdisciplinary approach advances the development of a predictive capability (prediction of coastal change) that can be used by the coastal planning/engineering community.
http://woodshole.er.usgs.gov/project-pages/scarolina/index.html
NS24A-02 INVITED
Monitoring Submarine Ground-Water Discharge Using a Distributed Temperature Sensor, Waquoit Bay, Massachusetts
Fresh submarine ground-water discharge (SGD) represents an important pathway for nutrients and contaminants to reach coastal ecosystems, yet the spatial and temporal distributions of SGD remain poorly understood. New fiber-optic distributed temperature sensor (DTS) technology represents a promising approach for high-resolution mapping of SGD. DTS instruments transmit laser light through fiber-optic cables and analyze one of two forms of backscatter (Raman or Brillouin) to produce temperature estimates at distributed points along the cable. Commercially available systems based on Raman scatter offer spatial, temporal, and thermal resolutions of 1 m, 1 minute, and 0.1 degree Celsius, respectively. We present results from a field demonstration of a DTS system at the Waquoit Bay National Estuarine Research Reserve (WBNERR), East Falmouth, Massachusetts. A 1.3-km fiber-optic cable was deployed on a grid extending 80 m parallel and 60 m perpendicular to shore. The cable was weighted and submerged into the bay sediment to a depth of several cm near shore; further offshore, the cable sank into mud under its own weight. During the study period in May-June 2006, the bay was about 5-10 degrees Celsius warmer than fresh ground water at the site; hence cold anomalies in the DTS data are indicative of fresh SGD. DTS temperature measurements were interpolated to produce time-lapse maps of bay-floor temperature at about 1 minute intervals for a 2-week period. The DTS results were interpreted in the context of additional information: local meteorological data, including precipitation and air temperature; bay temperature; bay salinity; and ground-water levels in coastal wells. The DTS data indicate: (1) the development of a cold region at low tide extending from shore to 5 m offshore; (2) a positive correlation between bay-floor temperature and tidal level, with the strength of correlation decreasing with distance offshore; and (3) a positive correlation between bay-floor temperature and bay water temperature, with increasing strength of correlation with distance offshore. From the DTS data, we infer a zone of tide-driven, near- shore, fresh SGD, which is consistent with direct measurements of seepage and pore-fluid salinity. Our results demonstrate that DTS measurements can provide high-resolution information to help infer the extent and timing of SGD.
NS24A-03 INVITED
Use of Seafloor Electromagnetic and Acoustic Backscatter Data for Seafloor Classification: An Example From Martha's Vineyard, Massachusetts.
Near surface offshore geophysical data allow sediment classification in coastal settings at high levels of spatial detail. We present data from offshore Martha's Vineyard, Massachusetts collected as part of the Office of Naval Research's Mine Burial Prediction program. Seafloor electromagnetic data provide estimates of near surface porosity at approximately 10m intervals along each tow-line. In addition, the area has undergone repeat surveys with high resolution acoustic backscatter and bathymetry. In some locations, the geophysical data has been groundtruthed by grab sampling and coring. We examine the spatial variability in near surface sediment properties on the basis of the geophysical data. The EM data are particularly well suited to constructing semi- variograms to display length scales of variability. Preliminary examination does not show any obvious correlation between the EM data and acoustic backscatter, however, further processing of the backscatter is being carried out and so this result is tentative.
NS24A-04
Marine Electromagnetic System Development in the Shallow Water Environment for Radioactive Waste Repository Site Investigation
Radioactive Waste Management Funding and Research Center of Japan has recently conducted a program to develop an electromagnetic (EM) technology for investigating the subsurface to the depths of 1,000m below the seafloor in the near-shore environment. Potential applications include structural studies for geological disposal of radioactive wastes. The system includes both natural field by magnetotellurics and controlled source EM data was collected to evaluate the feasibility of the methods and instrumentation. The shallow water environment is challenging because of high water currents and wave motion effects contaminating the data. We demonstrate the performance test of the new type of instrument, and the field experiment that was carried out in the Monterey Bay of California, USA, in 2003 and 2004. In this paper we describe the instrumentation developed, shows some examples from field trial and finally provide some inversion results using collected and simulated data. The system consists of EM transmitter deployed on the beach in combination with a series of offshore based multicomponent receivers. Field data collected near Monterey California revealed some of the challenges associated with this type of system. Collected data showed the influence of wave and cultural noise as well. In site of these difficulties we were able to accumulate a sufficient quantity of good quality records to interpret results. We show 2-D inversion results which image the �gNavy Fault zone�h which strikes NW-SE offshore Monterey bay in water depths of 10 to 40m.
NS24A-05
The St. Clair Delta: examining depositional history using geophysics and well cores
The St. Clair River Delta, on the border between Michigan and Ontario is a prime example of a turbulent bed friction-dominated delta with a classic birdfoot shape. As a result of late-glacial advances and retreats, isostacy, and changes in outlet points and lake connectivity of the Great Lakes system during the Wisconsin deglaciation, the history of Lake St. Clair is complex. Onset of recent delta formation is associated with a shift in drainage of the Huron/Michigan basins to the Port Huron outlet during the Nipissing-I highstand around 5,500 yBP, prior to which Lake St. Clair was a separate basin. This shift resulted in Lake Huron/Michigan draining through the St. Clair and Detroit Rivers to Lake Erie. In Lake St. Clair, mid-way between Lake Huron and Erie, loss of stream competence led to the formation of a deltaic system with a surface area of approximately 230 km2. Compared to the other Great Lakes, the post glacial history of Lake St. Clair in general and the depositional history of the St. Clair Delta in particular are poorly studied. We used hand augers and a vibrocorer, on- and offshore ground-penetrating radar (GPR), and electrical resistivity methods to characterize this deltaic system. Our results show a stratigraphic section that consists of loamy to sandy deltaic sediments with a thickness of approximately 3 meter, superimposed on lacustrine clays (0.5 - 1 meter thickness), and sand. Datable material in the delta sediments and in the lacustrine clays will be used to improve constraints of Great Lakes' glacial history. Both on- and off- shore GPR have proven successful for identifying sedimentary structures and stratigraphic units in the St. Clair delta. In addition to detection of paleochannels and lateral variation in delta thickness, GPR was used to characterize the modes of deposition and fill of interdistributary bays.
NS24A-06
Lithological Anomalies in Coastal Dunes: GPR Signal Response And Paleoenvironmental Significance
Coastal dunefields have developed through phases of stability and reactivation, both natural and anthropogenic in origin. In many areas, continuous ground-penetrating radar (GPR) images help to reconstruct landscape change, particularly where older periods of aeolian activity are in question. Here we present examples from coastal dunefields along the U.S. Atlantic Coast and Pacific Northwest, Great Lakes, and the Baltic Sea to illustrate the various lithological causes of GPR reflections and discuss their implications for reconstructing climate and landscape change along sandy coasts. Although considered some of the most well-sorted sandy depositional environments, coastal dune lithosomes may still exhibit dielectrically distinct textural variations resulting from changes in sediment source and wind-flow patterns and intensity. Some sedimentological changes (e.g., grain fabric, packing, grading, and water retention) are sufficient enough to produce distinct signal responses, but may be too subtle to be resolved by standard analyses of sediment cores and outcrops. In addition to textural heterogeneity, changes in sediment composition may result in prominent subsurface reflections. GPR has been used successfully to identify and map organic-rich horizons of variable thickness, especially well-developed regional paleosols indicative of landscape stability during relatively mild climatic conditions. In some areas, laterally extensive horizons enriched in heavy minerals produce prominent subsurface reflections. These layers range from sub-centimeter to >5 cm in thickness and are likely due to increased near-surface flow stresses during periods of increased wind action. Semi-lithified or encrusted sand lenses within coastal dunes can be used to map areas of increased diagenetic alteration or changes in groundwater table. The GPR reflections produced by lithological and textural variations highlight the geometry and orientation of aeolian units, as well as show changes in wind direction, mode of slipface deposition (down-wind dune migration) and up-wind accretion. Point-source reflections within stratified dune sequences can be used to determine depth (in addition to CMP surveys), and sometimes dimensions, of buried trees and man-made structures.
NS24A-07
Imaging coastal permafrost conditions and sedimentary structures with GPR
Extensive multi-frequency (50, 100, 250, and 500 MHz) ground penetrating radar surveys were conducted in the Mackenzie Delta, Canada to map the character of near-shore permafrost conditions and sedimentary structures. The data were collected over the sea ice and frozen ground conditions in the winters of 2005 and 2006 using multiple GPR systems. Verification of the interpretation was accomplished through direct measurement of ice thickness and water depth, sub-bottom coring and installation of ground temperature cables. Differential GPS data were used to provide position control on the GPR data and enable effective spatial correlation of the interpreted structures. When the system was towed by a snowmobile, up to 20 line kilometres per day could be acquired with 20 cm trace spacing at 250 MHz and 1 m tracing spacing at 50 MHz. Parameters measured included ice thickness, water depth, sedimentary structure, and the location of thermal interfaces. The presence of floating versus bottom-fast ice had a dramatic effect on the radar propagation and character of the reflections. Reverberations were one of the largest sources of �noise�, however, they were usually restricted to certain frequencies and could be extracted using the coincident multi-frequency data. This form of surveying enabled very effective bathymetric mapping in water depths 0-10 m, quantitative measurement of sub-bottom seasonally frozen and permafrost layers and the character of bottom and sedimentary structures (including sub-aqueous channels).