Mapping of Contaminant Plumes
by the Induced Polarization Method

 

 

 

 

 

 

 

 

Figure: Chargeability tomogram

Induced Polarization surveys are closely related to resistivity in instrumentation but measure a delayed voltage response of a polarizable earth. The anomalous response is smaller in magnitude than typical resistivity responses and arises from currents that flow by virtue of polarization properties of the earth. It can be quantified by an electrical property called the chargeability, which is a measure of the polarizability or charge storing ability of the earth material. Resistivity of the earth is mainly a volume property while chargeability arises from interface capacitive properties of rock grains—each provide complementary information about the earth, which frequently serves to improve interpretation. As an example of the application of IP to imaging of contaminant plumes the figure above shows a chargeability tomogram derived from the inversion of 2D time domain IP data, collected at the FS-12 plume site, Massachusetts Military Reservation (MMR). The source of FS-12 was a pipeline leak in the early 1970’s of over 70,000 gallons of JP-4 and aviation gasoline. The contaminants of greatest interest are Ethylene dibromide and Benzene. The data acquisition uses a 2D pseudo-section array and the data was processed by our 2D induced polarization algorithm. Indicated on this figure are the observation wells and zone of contamination in each well. The methodology is that the chargeability in the contaminated areas should be higher compared to the background and should increase with the concentration of contaminants. The figure shows a good match with the monitoring well data. The tomogram shows the contaminant distribution along the entire 1400 feet to a depth of 300 feet, which provide more information than from point samples of wells.

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