Ground Penetrating Radar (GPR)
For this project the pulseEKKOTM 100 and 1000 GPR systems manufactured by Sensors and Software Inc., will be used to accurately locate the industrial waste. High-resolution cross-sectional images of the stratigraphy of the lakebed and subbottom can be obtained using GPR. GPR is a fairly noninvasive method of collecting data for Half Moon Lake. With different antennae the GPR can effectively “see” several meters through materials deposited on the lake bottom. Different antennae of different frequencies can “reveal” different layering and reflectivity of underlying materials (Jol, 1995). Also a Trimble ProXR Differential GPS, with submeter accuracy, will be utilized to correlate GPR data with GPS data to provide GPR transects that are georeferenced. GPR data is then downloaded and “rubber sheeted” to fit the exact distances recorded by the GPS along many points of the GPR line. The rubber sheeting effect allows for accurate and “real time” processing of the GPR data effectively pinpointing areas of large concentrations of industrial waste.
How does GPR work?
When the transmitter antennae sends electromagnetic (EM) waves into the ground (1) the waves reflect off underground layers which are collected by the receiver antenna (2). Information travels to a computer (3) via fiber-optic cables (4). The computer then displays the data in real time.
By using GPR in the winter, one can set an exact course across the lake for GPR investigation. The line has been marked by flags on the ice the course is then recorded using the GPS for later reference. The GPR is then attached to a toboggan using shipping Velcro to secure the GPR. When collecting data the GPR is slowly dragged along the given path at a constant speed. The data from the GPR is recorded onto a small laptop computer in the field.
Data collection and analysis process;
- After the collection process is complete the data is then downloaded onto several computers and is ready to be compiled.
- GPS data is exported in order to find exact distances between points in the GPR survey. This distance data is then recorded in a spread sheet for further reference.
In the photgraph above the GPR was set up to effectively collect data in winter conditions. Currently work is being done to streamline the set up of the tool and make the set up more permanent. This will aid in the ease of set up and repeatability of data, allowing the spacing of the antennae to be exactly the same on different days of collection.
The image above displays a 50 MHz GPR profile acquired from the southeast thumb of Half Moon Lake. Analyzing the GPR data starts by looking at the raw image for patterns and correlating the patterns with known information. The reflection patterns for this profile penetrated to a depth of roughly 4.5 meters.
The 50 MHz GPR profile above shows our analysis of the spatial reflection patterns. The absence of
reflection patterns in the top blue layer is interpreted as water depth. The horizontal continuous layering
in the yellow section can be interpreted as organic sediment roughly 1/2 meter thick. These horizontal
lines appear to mimic the topography of the lakebed. The green dipping layers located beneath
the yellow sediment layers are interpreted as bed rock or a former riverbed channel. There also appears to be fluvial deposits (red) overlain by organic sediment. The fluvial bedding is non-continuous.
GPR Coverage Map
The above map represents the 114 colleceted GPR lines on HML.