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Ground Penetrating Radar Characterization of Geologic Structure beneath the Aberjona Wetlandby Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on March 24, 1999 in partial fulfillment of the requirements for the degree of Doctor of Philosophy ABSTRACT
Ground penetrating radar (GPR) surveys are performed to determine geologic
structure and to assist in the characterization and eventual modeling of
ground water flow beneath the Well-H region of the Aberjona River in Woburn,
Massachusetts. Even though cone penetrometer and coring studies provide
point source "ground truth" data about the stratigraphy, soil chemistry,
and composition of the first seven meters of peat, sand and silt they are
time-consuming and labor intensive. The advantage of GPR is its ability
to extend point-source ground truth information into two and three dimensions.
This dissertation provides an accurate geologic model of one section of
the Aberjona wetland by improving the state of three-dimensional GPR imaging.
Data fusion, target implantation, and a novel 3-D migration technique are
combined to render the subsurface volume accurately to resolutions of about
30 centimeters in all directions. The technique was tested using various
synthetically generated models as well as carefully constructed test pits
to verify its accuracy and estimate error. For our surveys in the wetland,
we verify the accuracy of our method using the large database of "ground
truth" information about subsurface structure and soil properties. We use
this information along with data collected for this project to assemble
what 3-D geological information we can. For example, direct measurements
of soil bulk properties are made at various depths in several locations.
Borehole conductivity surveys not only confirm core sample measurements,
but also give deep conductivity information not otherwise available. Cone
penetrometer surveys provide high vertical resolution to map stratigraphic
variation. A 3-D resistivity survey provides general information about the
conductivity of the site.
In addition to ground truth measurements, several different types of radar
surveys were employed to estimate soil electrical properties. A new method
of Vertical Radar Profiling (VRP) was developed to provide velocity information
at different locations and various depths throughout the wetland. Morphing
methods were applied as an interpolation tool for surveys with broadly spaced
profiles. A fast three dimensional migration algorithm was developed specifically
for GPR imaging that incorporates layered velocity information and soil
electrical conductivity. Ultimately, this technique is shown to offer a
means of mapping larger regions of wetland stratigraphy more accurately
than was currently feasible.
Return to Theses Return to ERL Home Updated: April, 1999
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