SAP Addendum Specific to Field Sampling Plan for AOC-55 Remedial Actions, January 2003

Section 2 - Geophysical Testing

Geophysical testing conducted at AOC-55 and the surrounding area will assist in identifying lateral extents of the area of concern and locations of anomalies which will need removal actions.

2.1 - Electromagnetic Induction

The surface geophysical surveys will be conducted using a Geonics� EM-31 electromagnetic instrument and will be implemented with the goal of defining the lateral extent of buried waste at AOC-55. In the electromagnetic induction (EMI) method, the electrical conductivity of geologic materiel is measured by transmitting a high-frequency electromagnetic field into the earth, producing eddy currents that generate secondary electromagnetic fields that can be detected by a receiver. The eddy currents are induced in the earth by an aboveground transmitter coil, and the resulting secondary electromagnetic fields are coupled to an aboveground receiver coil. Thus, EMI measurements do not require direct ground contact, as is the case for resistivity measurement, allowing surveys along traverse or specific areas to be performed rapidly.

Electromagnetic conductivity measurements are collected along a grid system. The area covered by the grid and the spacing between grid nodes is site-specific and depends on the project objectives. The maximum grid spacing will be no larger than 100 feet by 100 feet, with data points spaced every two feet along each grid line. Gridlines will be spaced every 20 feet.

The first step is to establish a base, or background, station to measure the naturally occurring electromagnetic properties in the site vicinity. The base station will be selected to represent naturally existing subsurface conditions at the site.

The most recent portion of the electromagnetic conductivity survey will be repeated if the base station readings vary by more than 20 percent. The base station readings should be stable unless electronic interference is occurring or unless heavy rains increase the soil saturation.

Data will be continuously recorded with a digital data logger (polycorder). For each survey line, the line number, starting point, direction of traverse, and increment of measurement will be entered in the polycorder. This information will also be recorded in the field logbook, as well as the ending point. Cross-checks will be made between the logbook and polycorder for each line to ensure correct identification and settings. If a discrepancy is found, the survey team will return to the last verified grid point or line and continue forward with the survey.

Data will be collected in both quadrature and in-phase modes. The quadrature mode is generally more useful for investigating the limits of disturbed soil as it allows detection of subtle differences in areal ground conductivity. The in-phase mode is less sensitive and generally more adept for use in locating metal objects.

The data will be plotted upon completion of the survey and before demobilizing to determine if the survey data is valid and the coverage of the site is complete. Additional data will be obtained as needed to complete the survey. Geophysical survey procedures are detailed in the FSP for CSSA (Field Sampling Plan, Volume 1-1 Environmental Encyclopedia).

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