NS31B-1572
Strengths and Limitations of Marine Resistivity Studies Targeting Submarine Groundwater Discharge (SGD) on the Gulf Coast of Florida
Examples are shown from time series and streaming marine resistivity surveys in Sarasota Bay, the Suwannee River Estuary, and Tampa Bay. In Sarasota Bay, on both regional (kms to tens of kms) and local scales (100s of meters) the relation between resistivity and tracer-based SGD estimates is not obvious. It is possible that much of the SGD is recirculated brackish water. Seafloor resistivities are strongly influenced by stratigraphy, particularly the presence of a clay layer at ~10-15 m depth in the southern part of the bay. On the local scale, the subtler lateral variations in resistivities derived from inversions of resistivity data were not easily reproducible; nearly- coincident lines collected 30 minutes apart in time show differences in local features. This apparent lateral variability in the resistivity profiles may be a consequence of inversion of noisy streaming resistivity data or of inexact positioning of the 100-meter long streamer. In the Suwannee River Estuary, streaming resistivity profiles show dramatic resistivity gradients that clearly correspond to the freshwater/saltwater interface in sub-bottom porewaters and surface waters. Within the estuary there is a positive correlation between resistivity and elevated radon tracer levels that may reflect enhanced SGD. However, there are also plausible mechanisms for these regional scale relationships that do not involve local SGD. For example, a strong downriver gradient in Rn activity that coincides with the freshwater/saltwater interface probably reflects in large part Rn carried downriver from a high-volume spring. On local scales (hundreds of meters) systematic correlations between resistivity and tracer concentrations are found only in the offshore portions of the estuary. In Tampa Bay, high resolution time series resistivity measurements using 56 electrodes spaced 2-m apart provide detailed information on near shore ground water /surface water mixing phenomena. Streaming resistivity results show that much fresher ground water resides below more saline water masses and argues for the strong geologic control on coastal hydrology.
NS31B-1573
Complex Holocene Sedimentation and Erosion in Deep Basins of Lake Superior
Little sediment accumulates on the lake floor of most of the Laurentian Great Lakes in water less than about 100 m deep. Such sediment is thought to be resuspended by waves and currents and "focused" onto the deeper lake floor. New high-resolution CHIRP seismic-reflection data in central Lake Superior suggests that Holocene sedimentation has been considerably more complex there. The late- and post- glacial stratigraphy in Lake Superior consists of glacial-lacustrine red varves overlain by gray varves. The glacial-lacustrine section is capped by poorly laminated, fine-grained, gray Holocene muds. In many areas, the entire post-glacial section is cut by polygonal fractures and faults related to dewatering or syneresis. Our new seismic-reflection data from water depths of 150-250 m indicate that the upper surface of the varved section is extensively eroded, both by planation of varves draped over bathymetric highs and by widespread channeling. The cause of this pervasive erosion is not known, but it may be related to the sudden opening of a low outlet from the lake as the continental ice sheet retreated. Within the Holocene section, small to medium sized (2-4 m deep, 100-300 m wide) channels are formed, in some cases overlying the older channels in the varved section. Commonly, the Holocene channels cut directly into the underlying varved section. Both of these types of channels are partially to fully filled with Holocene sediments. Dipping reflections within the Holocene section suggest considerable complexity in Holocene sedimentation. Large parts of the study area contain only thin ( < 1 m) Holocene section and large areas contain none at all. All of these observations indicated a much more complex set of Holocene erosional and depositional processes in deep water than those implied by the simple focusing mechanism.
NS31B-1574
Advances in Shallow-Water, High-Resolution Seafloor Mapping: Integrating an Autonomous Surface Vessel (ASV) Into Nearshore Geophysical Studies
The U.S. Geological Survey (USGS) has been heavily involved in geological mapping of the seafloor since the 1970s. Early mapping efforts such as GLORIA provided broad-scale imagery of deep waters (depths > 400 meters) within the Exclusive Economic Zone (EEZ). In the early 1990's, the USGS research emphasis shifted from deep- to shallow-water environments (inner continental shelf, nearshore, estuaries) to address pertinent coastal issues such as erosion, sediment availability, sediment transport, vulnerability of coastal areas to natural and anthropogenic hazards, and resource management. Geologic framework mapping in these shallow-water environments has provided valuable data used to 1) define modern sediment distribution and thickness, 2) determine underlying stratigraphic and structural controls on shoreline behavior, and 3) enable onshore-to- offshore geologic mapping within the coastal zone when coupled with subaerial techniques such as GPR and topographic LIDAR. Research in nearshore areas presents technological challenges due to the dynamics of the environment, high volume of data collected, and the geophysical limitations of operating in very shallow water. In 2004, the USGS, in collaboration with NOAA's Coastal Services Center, began a multi-year seafloor mapping effort to better define oyster habitats within Apalachicola Bay, Florida, a shallow water estuary along the northern Gulf of Mexico. The bay poses a technological challenge due to its shallow depths ( < 4-m) and high turbidity that prohibits the use of bathymetric LIDAR. To address this extreme shallow water setting, the USGS incorporated an Autonomous Surface Vessel (ASV) into seafloor mapping operations, in June 2006. The ASV is configured with a chirp sub- bottom profiler (4 � 24 kHz), dual-frequency chirp sidescan-sonar (100/500 kHz), single-beam echosounder (235 kHz), and forward-looking digital camera, and will be used to delineate the distribution and thickness of surficial sediment, presence of oyster beds, and sea bed morphology in water depths less than 5-m. The ASV is a catamaran-based platform, 10 feet in length, 4 feet in width, and approximately 260 lbs in weight. The vehicle is operated remotely through a wireless modem network enabling real-time monitoring of data acquisition. The ASV is navigated using RTK, and heave, pitch and roll are recorded with onboard motion sensors. Additional sensors, such as ADCPs, can also be housed within the vehicle. The ASV is able to operate in previously inaccessible areas, and will not only augment existing shallow-water research capabilities, but will also improve our understanding of the geologic controls to modern beach behavior and coastal evolution.
http://woodshole.er.usgs.gov/operations/sf mapping/default.htm
NS31B-1575
Integrating Seismic Reflection and Ground Penetrating Radar Data at the Marine Corps Air Station, Beaufort, South Carolina
Extensive work has been performed at the Marine Corps Air Station (MCAS) in Beaufort South Carolina to characterize the subsurface and generate a groundwater flow model for an assessment of MCAS aquifer vulnerability and contamination potential using various hydrogeological and geophysical techniques such as slug tests, vertical seismic profiling (VSP), borehole geophysics, vertical electrical soundings, and seismic reflection surveys. The goal of the additional research is to integrate the existing seismic reflection data with newly acquired GPR data in order to: (1) demonstrate the effectiveness of the GPR as a tool for site characterization and (2) obtain hydrogeophysical parameters in a heterogeneous environment. The primary focus of the MCAS Beaufort groundwater flow model is to understand the interaction between the unconsolidated clastic Pleistocene sediments and the underlying Eocene Ocala Limestone, which is the primary aquifer. GPR data were collected along the transect of seismic reflection line MCAS-2 which is a total length of approximately 1700 m with a recording length of 500 ms. There are several wells along the profile that were utilized for hydrogeological and geophysical calibration. Preliminary GPR data were collected adjacent to ground water monitor well BFT-2368, using the 50 and 100 MHz frequency antennas, and consisted of three reflection profiles that vary in length from approximately 210 to 240 m. Comparison of the GPR and seismic data illustrate that the two applications complement one another with respect to resolution. Where GPR data contain better detail (more continuous reflectors) in the shallow subsurface, the seismic data contain less detail but deeper reflections. We are using the seismic attributes such as acoustic impedance, and the dielectric properties estimated from the GPR data in order to make assessments on hydrogeological parameters such as porosity and further, hydraulic conductivity.
NS31B-1576
GPR Studies of South Carolina Barrier Islands
Understanding Holocene sediment dynamics and volumes along coastal margins is of paramount importance for short- and long-term management of coastal zones. This study examines the Holocene stratigraphy, sediment thickness, and sediment volumes within the northeastern South Carolina barrier complexes located along 90 km of populated or pristine coastline. A combination of ground penetrating radar (GPR) data paired with vibracore and deeper core data has been used to determine the formational and evolutional history of these barrier systems. Where intersecting with or adjacent to the topographically higher paleo-Pleistocene shoreline(s), the coastal barrier islands are perched on or extend from a Pleistocene core. GPR reflections of this subsurface barrier core exhibit planar and parabolic reflectors. Alongshore expansion of the barrier is the result of multiple periods of sandy recurved spit shoreline and inlet fill, as indicated by dipping reflectors of <10 m. Further Holocene expansion resulted from the deposition of overwash sand (identified from planar terminated by dipping refectors), eolian sand (identified by steeply dipping reflectors), and cats-eye pond/upper intertidal muddy sediments (identified by high amplitude reflectors). GPR surveys combined with detailed and deliberate coring provide a means to study the subtle complexities of the composite developmental history of these islands.
NS31B-1577
High--Resolution 3D Seismic Chirp Volumes: A Case Study in Small Object Detection
The 3D Chirp high--resolution sub--bottom profiler provides unparalleled imaging of coastal and in--shore seabed and sub--seabed structure by combining the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around four Chirp II transducers, 3D Chirp permits the acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing the perfect base for shallow-water engineering, archaeological, and geological studies. Here we present the results of a survey in an a--tidal basin on the south coast of England. With dimensions of 150 by 200 metres, the study area provided a series of unique challenges. These include a large number of anomalous objects (ranging from 10s centimetres to several metres in size) buried in a thin (0.5 to 1.5 metres) veneer of unconsolidated silt/sediment, overlaying a bedrock surface showing a high acoustic contrast, short-- wavelength roughness, and two distinct changes in topography. Surface and isopach maps, along with extracted seismic attributes, are used to confirm that the detector is acquiring a true 3D volume and illuminating aspects of the subsurface. Particular attention is paid to the properties and distribution of unidentified objects, including detailed analysis of the size, shape, and amplitude of their diffraction hyperbola.