New geophysical method for monitoring emplacement of subsurface barriers Page: 4 of 9
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The inversion algorithm solves both the forward and inverse problems. The forward problem is
solved using a finite element technique. The objective of the inverse routine is to minimize the
misfit between the forward modeling data and the field data, and a stabilizing functional of the
parameters. The stabilizing functional is the solution's roughness. This means that the inverse
procedure tries to find the smoothest resistivity model which fits the field data to a prescribed
tolerance. For additional details, the reader is referred to LaBrecque et al. (1996).
DNAPL field experiment
The experiment was conducted in a double wall steel tank 10 meters square and 5 m deep at the
Oregon Graduate Institute of Science and Technology in Beaverton, Oregon. This tank allowed
for a safe release of perchloroethylene (PCE) into a soil section constructed of sand and clay.
Two layers of powdered bentonite were included as barriers. The tank was saturated with 4 ohm
m pore water to within about 25 cm of the surface. Four electrode arrays were used to generate
two dimensional (2D) images in three planes; L1, L2, L3 and L4. Each array contained 10 lead
electrodes spaced evenly between 50 cm and 275 cm depth. One hundred eighty nine liters (50
gallons) of PCE was released at a single point on the surface approximately midway between
arrays L2 and L3. The release rate averaged about 2.0 liters/hr.
Both bentonite layers are clearly imaged (see lower right panel in Figure 1), the upper layer
extending only part way across the tank. They are also clearly not uniform; the blochyness of the
layers reflects the difficulties installing these structures in the tank as well as imperfections in the
inverse process. However, ERT gives a detailed sectional view of variability in these confining
layers which is impossible to achieve with conventional logging or even surface radar methods.
This result is an example of the capabilities of ERT to delineate in cross section the shape and
extent of a hydraulic barrier.
The other panels in Figure 1 show the reconstructions after a pixel by pixel subtraction of the
background image and therefore show only changes in resistivity distribution. At 3.5 hours the
changes were quite small but the most significant feature is located below the release point--a
resistive anomaly arching around the end of the upper clay. This anomaly probably forms as the
resistive PCE displaces the more conductive pore water. Other small anomalies in this image are
probably from unrelated but natural changes in the pore water conductivity and from data noise.
About 21 hours and 42 liters into the release the anomaly has grown large enough to extend from
the release point, arch around the upper clay and reach down to just touch the lower confining
layer. Apparently, most of the PCE continues to spill over the edge of the upper layer with little
residing on top. Reflecting the dynamic nature of the system, the same small unrelated anomalies
are still present and now a few others are forming. The weak disconnected features forming along
the top of the lower clay may be small accumulations of PCE even at this early stage.
By August 18, at 45 hours, the anomaly begins to spread horizontally along the top of the lower
clay as might be expected of the plume as more PCE reaches that layer. The appendage pointing
up to the right in plane L2-L3 is not expected and we cannot explain its presence.
The release ended Sunday at noon, August, 20 after 189 liters (50 gallons) was released. On
Monday, August 21 the tomographs show a different picture from that 3 days earlier. Only a small
remnant is left of the arching anomaly from the surface and around the upper clay. This is
understandable if it represented the downward moving PCE which by this time has mostly drained
and been replaced by water. However, now a resistive anomaly almost 2 m long sits at the top of
the lower clay layer, centered directly below the edge of the upper layer. From this tomograph we
conclude that the bulk of the PCE has drained from the sands and is pooled on the lower layer. A
2 m diameter pool of 6 cm uniform thickness would accommodate the volume released and so the
anomalies are consistent with a PCE pool at this location. Imaging the PCE movement and
eventual fate is an example of how ERT might be used to verify the integrity of a hydraulic barrier,
the PCE acting as a fluid tracer added to the system to see if it can breech the barrier.2
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Dailey, W. & Ramirez, A. New geophysical method for monitoring emplacement of subsurface barriers, article, October 1, 1996; California. (https://digital.library.unt.edu/ark:/67531/metadc679992/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.