Browsing by Subject "GPR"
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Item Changes in channel geometry through the Holocene in the Le Sueur River, south-central Minnesota, USA(2017-03) Targos, CourtneyPaleochannels preserved on terraces via meander cutoffs during an incisional period record the channel geometry and thus discharge at distinct points in time throughout a river’s history. We measured paleochannel geometry on terraces throughout the Le Sueur River in south-central Minnesota, to track how channel geometry has changed over the last 13,400 years. A rapid drop in base level 13,400 yr B.P. triggered knickpoint migration and valley incision that is ongoing today. Since the 1800’s, the area has developed rapidly with an increase in agriculture and associated drainage, directly impacting river discharge by increasing water input to the river. Five paleochannels were identified on terraces along the Le Sueur River from 1m-resolution lidar data. Ground Penetrating Radar (GPR) was used to obtain a subsurface image across paleomeanders to estimate the geometry of paleochannels. Paleochannel geometry and estimated discharge were then compared to modern conditions to assess how much change has occurred. Three lines were run across each paleochannel perpendicular to the historic water flow. Each of the 15 lines were processed using the EKKO Project 2 software supplied by Sensors and Software to sharpen the images, making it easier to identify the paleochannel geometry. Paleodischarge was determined using the Law of the Wall and Manning's Equation, using modern slope and roughness conditions. OSL samples were collected from overbank deposits on terraces to determine the time of channel abandonment, and supplemented with terrace ages obtained from a numerical model of valley incision. Paleodischarge coupled with depositional ages provide a history of flow conditions on the Le Sueur River. Results show an increase in channel widths from the time paleochannels were occupied to modern channel dimensions from an average of 20 meters to 35 meters. The change was not constant through time, as all paleochannels analyzed on terraces had similar-sized channels. The best way to determine paleogeometry was using the 'best interpretation' of GPR data couple with coring data; and paleodischarge was best estimated using Manning's equation with an n value of 0.035. Results show an increase in discharge compared to paleochannels of a factor of two. Uncertainty estimates in GPR-based paleogeometry can change paleodischarge calculations by 50 %. Incremental flood frequency analyses, based on data obtained from the Red Jacket stream gage at the outlet of the Le Sueur, suggest a 1.5- and 2-year flood of 102 m3/s and 154 m3/s, respectively, which is comparable to estimations of bankfull based on current channel geometry at the Red Jacket gage, validating the methodology. Problems associated with paleogeometry estimations are primarily due to meander bend preservation in the subsurface, challenging GPR interpretation. The increase in channel geometry and discharge implies that the increase in flow associated with drainage and climate change since the area’s development has greatly impacted the Le Sueur River. This resulted in a change in channel morphometry through increased erosion along the bluffs and banks, widening channels. This increase in erosion has directly impacted the amount of sediment delivered to the rivers from banks and bluffs, increasing the fine sediment load in this turbidity-impaired river system.Item Implementation of Ground Penetrating Radar(Local Road Research Board, Minnesota Department of Transportation, 2007-08) Cao, Yuejian; Dai, Shongtao; Labuz, Joseph; Pantelis, JohnThe objective of this project was to demonstrate the capabilities and limitations of ground penetrating radar (GPR) for use in local road applications. The effectiveness of a GPR survey is a function of site conditions, the equipment used, and experience of personnel interpreting the results. In addition, not all site conditions are appropriate for GPR applications. GPR is a nondestructive field test that can provide a continuous profile of existing road conditions. GPR utilizes high-speed data collection at speeds up to 50 mph, thus requiring less traffic control and resulting in greater safety. GPR has the potential to be used for a variety of pavement applications, including measuring the thickness of asphalt pavement, base and sub-grade; assisting in the analysis of rutting mechanisms; calculating and verifying material properties; locating subsurface objects; detecting stripping and/or layer separation; detecting subsurface moisture; and determining depth to near-surface bedrock and peat deposits. These applications are discussed in reference to 22 projects completed throughout the State of Minnesota. Three reports were produced. (1) A technical summary report provides an overview of the project. (2) A comprehensive review of GPR applications for use on local roads is also available. (3) The final report describes the results of the GPR surveys.Item Reconstructing the bay-side geomorphic evolution of a freshwater baymouth bar in response to lake level change using three dimensional (3D) ground-penetrating radar (GPR) data(2019-11) Kremmin, ToddSituated at the southwestern tip of Lake Superior, Minnesota and Wisconsin Points' form a 16-kilometer baymouth bar between Duluth, Minnesota and Superior, Wisconsin, providing the breakwater for the largest and farthest inland freshwater seaport in North America. Comprised of sandy sediment, this baymouth bar's formation is attributed largely to littoral drift from the Wisconsin South Shore and minor sediments from the outflows of the St. Louis and Nemadji River's. Due to continuing differential isostatic rebound of the basin, local lake level at Duluth is presently rising at approximately 25 centimeters/century. The objective of this thesis was to understand the geomorphic evolution of the bay-side of the baymouth bar in response to lake level change in relation to the system overall. Using an approach akin to energy industry seismic studies, the geomorphic expression of the bay-side baymouth bar’s response to lake level change was investigated with 39.62 kilometers of Three-Dimensional (3D) Ground-Penetrating Radar (GPR) data. 8 vibracores’, 11.24 meters’ in total, were advanced within the 3D GPR data to supplement subsurface understanding both spatially and temporally. Radiocarbon material taken from the vibracores were analyzed, dated, and used to establish a chronology for the evolution of the study area (~520.50 Cal yrs. BP to 181.25 Cal yrs. BP). Whole core logger data (including p-wave amplitude and velocity, gamma ray density, acoustic impedance, and fractional porosity) along with Loss on Ignition (LOI) samples taken from each core were used in conjunction with the GPR data to supplement the subsurface stratigraphic and facies identification. In total, 5 facies units were interpreted on the bay side of the baymouth bar, exhibiting a thick clay layer, shoreface ridge and shallow offshore sediments, overwash deposition, fluvial-flood like deposition, and man-made dredge deposits. Historical information and photographs dating back to the 1930’s provide evidence supporting interpretations of dredge material versus natural material. Although this baymouth bar is a young, non-marine system, reconstructing its geomorphic evolution in response to lake level change has become a useful analogue for similar, larger systems involved with base level change. In addition, stratigraphic findings of how such a system’s architecture is configured yield insightful clues towards vintage conventional exploration reservoirs. Finally, these data support a stronger understanding of how such a system geomorphically evolved in the context of the Lake Superior region post glaciation and aid in reshaping knowledge of how other geomorphic features and processes have developed throughout the region, perhaps providing tangible framework for future engineering and environmental management undertakings.