Section 2 - Geophysical Testing

Geophysical testing conducted at AOC-65 and the surrounding area will assist in selecting the optimum locations for the Groundwater Recharge Study and the AOC-65 SVE System. The purpose of the geophysical surveys is to identify the location, depth, and orientation of subsurface features such as faults, fractures, and karst features that may be influencing groundwater migration in the area. A variety of geophysical methods may be utilized initially during the study. Initially, a select group of methods will be employed to identify the subsurface features of interest. The geophysical testing is planned so that geophysical methods that are not providing quality results can be removed from further use in favor of those methods that are generating the most useful information. Through this procedure, only those geophysical methods that provide the desired results will be utilized for the entire study.

The geophysical methods that are proposed for completion at AOC-65 include electrical resistivity imaging (ERI), microgravity, and very low frequency electromagnetics (VLF). ERI, microgravity, and VLF were selected based on applicability for detection of specific features of interest. The ERI survey was selected to provide high-resolution 2-dimensional slices of subsurface features including the ability to potentially identify fractures, faults, and voids. The microgravity survey was selected to identity subsurface voids, and the VLF survey was selected to potentially identify subsurface fractures. In addition, ground penetrating radar (GPR) and seismic reflection surveys may be implemented to supplement the ERI results and to obtain better resolution to depths greater than 50 feet below grade. An overview of the planned test methods is provided in the following sections.

2.1 - Electrical Resistivity Imaging

The ERI method involves generating a current field in the ground and then measuring voltage potential within that induced field. The method measures the electrical resistance of the subsurface materials within the current field. The resistance of the materials is a function of the material type, porosity, degree of saturation, and the electrical properties of the pore fluids. With this method, two-dimensional images reflecting changes in the material type, pore fluids or porosity can be generated. 

Equipment utilized for ERI surveys at AOC-65 will include the Sting R1 resistivity meter and the Swift multi-electrode cabling system manufactured by Advanced Geosciences, Incorporated. ERI surveys are performed by placing stainless-steel stakes in the ground at 5-foot intervals along the desired profile lines. The electrodes on the Swift cable system are then connected to the stainless-steel stakes. The Sting meter is then connected to the cabling system and the survey is performed. During the survey, four electrodes on the cable system are automatically chosen, two are current electrodes and the other two are used to measure the voltage potential at specific locations within the current field. After the four electrodes are selected, the meter sends current into the ground through the current electrodes and measures the voltage potential within the current field. The system then selects four more electrodes on the cable system and another reading is taken. The system continues in this fashion, changing the separation between the four electrodes to increase the survey depth, until an entire data set is collected.

At AOC-65, the cabling system will consist of two 28-electrode cable sets, for a total of 56 electrodes. The cables sets will be constructed with six meters (18 feet) of interconnecting cable between each electrode, limiting the maximum separation of the electrodes for a survey to this distance. Surveys will be performed using the dipole-dipole array with 5-foot electrode spacing. The decision to use 5-foot spacing is based on concerns for interference from buried utilities and other cultural features. Increasing the electrode spacing increases the survey depth, but also increases the separation distance that must be maintained from potential sources of interference. In the area immediately west of Building 90, a gas main, water main, metal security/boundary fence, buried power line, and overhead power lines are present and aligned north to south through the area. These items are potential sources for interference in the resistivity measurements aligned north to south. The potential for interference from these items is less for profiles oriented more or less perpendicular to them.

Following collection, the resistivity data will be processed using the RES2DINV inversion software. The software attempts to model the distribution of true resistivities in the ground that would result in the data set that was measured. The output from the software is a contour map of predicted true resistivities in the subsurface to the maximum survey depth. To enhance the presentation of the contour plots, the model results will be contoured using the Surfer contouring software.

2.2 - Microgravity

Microgravity surveys will be performed to assess the potential presence of voids or dissolution features in the bedrock unit. This method involves measuring the vertical component of gravity at a point using a highly sensitive gravity meter. Microgravity has proven to be a very useful tool for detecting the presence of karst related features for environmental and engineering projects. The primary purpose intended under TO 0058 is delineation of karst features and corroboration of ERI results

A LaCoste and Romberg Model G gravity meter has been selected for the survey at AOC-65. This meter was selected because of its capability to measure relative gravity variations to 0.001 mGals (1 Gal = 1 cm/s²). Modeling of potential gravity anomalies associated with dissolution features indicate that an air-filled or water-filled cavity with a 2-foot radius could potentially be detected to a depth of at least 50 feet with this meter.

Four microgravity profiles will be performed near AOC-65 and will coincide with locations of ERI profiles. Microgravity readings will be collected at 10-foot intervals along the survey lines. Prior to conducting the surveys, the land surface elevation at each measurement point will be surveyed to an accuracy of 0.01 feet. In addition, a local base station will be established and gravity measurements will be collected approximately every 30 minutes to monitor the drift of the meter during the day.

A leveling plate will be placed on the ground at each station and the meter is placed on the plate and leveled. The internal horizontal beam is then released and the gravity reading is recorded once the meter stabilizes. The internal horizontal beam is then re-clamped to minimize fluctuations between readings and the system is moved to the next location.

Microgravity data is entered into a spreadsheet for final processing. To arrive at the final residual gravity anomaly, the measured gravity reading at each point is corrected for effects due to elevation and changes in latitude. In addition, a trend correction may be applied to the profiles to remove regional gravity trends, when necessary.

2.3 - Very Low Frequency Electromagnetics

The VLF method utilizes the very low frequency transmitters located throughout the world to identify conductive linear subsurface features such as fractures. The very low frequency transmitters are normally used by the Department of Defense for communication purposes. At distances greater than 500 km from a transmitter the electromagnetic wavefront will essentially be a planar feature and can be used for subsurface characterization. If a fracture is present in the bedrock and the orientation of the fracture is aligned toward a VLF transmitter, the primary electromagnetic field will generate a secondary electromagnetic field in the fracture. The system measures the electromagnetic fields and if a secondary field is present, the location can be determined. The meter also compares the phase shift between the primary field and the secondary field inducted in the ground. This amount of the phase shift is a function of the size and conductivity of the feature. 

To begin the VLF survey, a test survey will be conducted over a known fracture at the site. At AOC-65, one fracture is visible at land surface with an opening of approximately 4 inches. Data will be collected at 10-foot stations along a line-oriented perpendicular to the strike of the known fracture. Initially, the survey will be performed using the VLF transmitter at Cutler, Maine, which is the strongest signal available at the site. If this initial survey proves unsu essful for detecting the fracture, additional test surveys may be performed using different transmitters and measurements spacing.

GPR surveys may be conducted at the AOC-65 site to supplement the ERI results with respect to identification of subsurface features. Procedures for conducting GPR surveys was presented in the SAP for TO 5068, prepared in February 2001. The procedures may include minor modifications for the probable AOC-65 surveys and are restated here for completeness.

GPR surveys will be conducted using a Geophysical Survey Systems, Inc. (GSSI) SIR-2 instrument with a 100, 200, or 300 MHz antenna. GPR is a subsurface-geophysical technique that uses high-frequency (radio frequency) electromagnetic radiation to acquire subsurface information. A radar pulse sent into the ground from an antenna at the ground surface is reflected back to the receiving antenna at the ground surface by an interface between materials with differing dielectric constants. The radiated energy encounters differentials in electrical properties, which are characteristic of the subsurface media through which the signal passes. These differentials cause some energy to be reflected back to the receiving antenna and some to be transmitted downward towards deeper material. The reflected signal is amplified, transformed to the audio-frequency ranges, recorded, processed, and displayed. The record shows the total travel time for a signal to pass through the subsurface, reflect, and return to the surface. If the antenna array is moved along the ground surface, a continuous cross-sectional profile of the subsurface is obtained. The subsurface profile shows features such as bedding, voids, fractures, faults, water table, and buried objects.

Optimal conditions for application of this technique are sandy or rocky soils in the vadose zone or bedrock with low permeability where water is not permanently present; poor results are obtained in clay or conductive soils. For practical purposes, the signal will not penetrate below the water table. This technique gives the highest resolution among the various geophysical techniques but also generally has the poorest penetration. Resolution ranges from inches to several feet, depending on frequency used. Specific antennas cover frequencies of 80-1000 MHz. The lower frequencies provide greater depths of penetration; the higher frequencies provide better resolution.

The complete GPR system contains the following components:

2.4 - Ground Penetrating Radar

During the GPR survey, the antenna array is towed along the ground surface by hand or by vehicle to produce a continuous subsurface cross-section. The depth of a reflecting object can be calculated from the travel time of the signal and the electrical properties of the subsurface materials. In practice the equipment is generally field calibrated against a test pit or boring for each project site. If subsurface conditions are heterogeneous, depth determinations can be difficult and of limited value.

Prior to conducting a GPR survey, a grid system must be established and constructed on the site so that any detected subsurface anomalies can be a urately located in the future. The grid spacing will be established at the discretion of the field investigator. Multiple lines that cross the site in several directions are used to survey the site. Transect lines are selected based upon any anomalies identified during the resistivity or microgravity surveys, areas of obvious of suspected subsurface disturbance, and overall a essibility for the recording equipment.

The initial setup and operation of the GPR system will include the procedural steps outlined in the manufacturer�s operation manual. The initial settings are determined on a site-by-site basis and must be correct in order to collect a urate and usable data.

Equipment function checks and instrument calibration are described in detail in the operation manual. �Real time� data are displayed on the Digital Control Unit and shall be monitored during survey operations to ensure a urate data collection. 

GPR data are recorded and stored in the Digital Control Unit. Each survey line is recorded as an individual file. Logbook entries must be made at the time of data collection. These entries should contain information regarding file name, the direction that the survey was conducted, starting and ending points of each survey, distance between reference markers, and any other information that will aid in data interpretation.

2.5 - Seismic Reflection

Parsons will perform 2D seismic reflection surveys to supplement the ERI results and to provide better resolution of the location and orientation of principal faults and fractures at the site. A Geometrics Geode distributed seismic receiver utilizing 60 40-Hz geophones will be deployed. The geode system will allow seismic data acquisition using a �roll along� technique where portions of the seismic cable is moved from the back of the line to the front of the line, while keeping the geophones in the middle of the line active. Using the "roll along" technique, 96 geophones are deployed along the line with 60 of the 96 geophones active for each measurement. The seismic source will be positioned a fixed distance from the first active geophone, the seismic signal (shot) will be generated using the source, and the resultant seismic wave reflections will be measured at the 60 active geophones. After each shot is acquired, the first live geophone is turned off, the first geophone beyond the last active geophone is turned on, the source is advanced to the next shot position and another measurement is conducted. Using this roll along technique, the time required to acquire data over a long line is reduced. 

The Geode system consists of boxes that can record 24 channels each. They are connected via an Ethernet cable to a PC acting as a controller and data logger. The signals from each geophone are digitized in one of the boxes and then sent to the controller PC by the Ethernet cable. By using a distributed system, heavy seismic cables capable of transmitting 96 or more channels are not needed. This allows for more efficient data acquisition.

The seismic source to be used for the survey will be the �minibuggy� vibrator source manufactured by Industrial Vehicles International. The �minibuggy� is essentially a small vibratory seismic source mounted on an off-road vehicle. The advantages of using the �minibuggy� is it can generate a broad range of output (source) frequencies which will allow Parsons to select an optimum source frequency for the surveys. In addition, vibratory sources can be used to limit the seismic energy generated in the bands that typically generate coherent seismic noise such as ground roll. The geologic conditions that exist at CSSA, with a high velocity near surface layer, typically have problems with ground roll and so using a vibratory source to mitigate ground roll can significantly improve the quality of the data. A geophone and shot spacing of 5-10 feet will be utilized. The final selection of the spacing parameters will be based on the results of on-site testing. Geophones in front and behind the shot will be utilized to improve the results obtained from refraction statics. The maximum offset from the source to the receiver should be on the order of one and a half to two times the depth to the maximum desired target depth.

Reflection from the multiple shots along the profile will be combined using the common depth point (CDP) method. The CDP method combines seismic reflections signals from the same subsurface point. Once the seismic reflection data is collected, the data will be processed using a workstation running Promax software. The processing will be conducted by a subcontractor under the supervision of the Parsons lead geophysicist. The exact parameters and sequence of the processing steps will be evaluated using actual data, however, the nominal processing flow will consist of the following sequence:

Data Import

Pre-Correlation Spectral Whitening

Correlation

Geometry Definition

Trace Editing

Surface Consistent Deconvolution

Shot and Receiver Coherence Filtering

CDP Sort

Velocity Analysis

Iterative Refraction Statics

CDP Stacking

Bandpass Filtering

Dip Filtering

Display and Printing

Depth Conversion

Display and Printing

These steps are performed iteratively to obtain the best quality stack. The Parsons lead geophysicist will be involved in the selection of the processing parameters, and will have final approval of all the final parameters and processing steps.

Other steps typically used in seismic reflection processing, such as migration, are not effective in enhancing near surface data and will therefore not be applied.

Once final stack has been generated, reflecting horizons will be interpreted and correlated with borehole and electrical survey information to produce a final depth section along with recommendations for the location of boreholes to intersect fault features.

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