A microgravity study was conducted adjacent to the west side of the site and into the southwestern edge of the fenced compound. Microgravity surveys are used to measure small changes in the acceleration due to gravity (the gravity field) that are caused by changes in the density of material underneath the sensor. The changes may be the result of deep crevasses in the bedrock surface due to fracture zones, voids in the bedrock, etc. The purpose of the study was to further investigate the subsurface with regard to sinkholes that developed.

Results are generally comparable in that they provide estimated residual gravity in (mgals) values of the subsurface.

Electrical Resistivity Imaging

Electrical resistivity data was collected to the west and within the plant compound during the Phase II and III Investigations, using two different technologies: galvanic current transfer, and capacitive induction. These measurements were collected within the paved compound, west of the turbines, and were done instead of galvanic measurements to avoid the need to drill through the pavement to insert electrodes.

Results are generally comparable in that they provide estimated resistivity values of the subsurface.

Vertical Seismic Velocity Profiling (VSP)

Vertical seismic velocity profiling was performed around the five turbine areas as part of the Phase V Investigation.

The raw data is generally of poor quality, indicating signs of a small signal-to-noise ratio including contamination from spurious low frequency and very high frequency noise.

The signal times within the records are subsequently difficult to determine and the resulting modeled velocities are therefore suspect.

Additionally, high velocity artifacts adjacent to the boreholes and corresponding relatively low velocity artifacts away from the borehole area are common with this type of analysis.

Ground Penetrating Radar (GPR)

Ground penetrating radar data was collected as part of the risk assessment investigation in Phase IV. Data was collected with both 200 MHz and 400 MHz antennas at the site. The depth of penetration for GPR at the site is about 30 ft (9 m).

Although the result cross-section profiles indicate a significant amount of ringing and energy loss in about the upper 8 ft (2.5 m) and the signal strength beneath this layer appears weak. GPR could be used successfully around the turbine foundations to observe voids directly underneath the pavement surface that would be an imminent risk.

Multi-Channel Analysis of Surface Waves (MASW)

MASW was collected at the site during the Phase IV risk assessment investigation, the Phase V baseline investigation, and then again during the Phase VI investigation. The Phase V and VI data sets were also collected

along the same traverses with the geophone vibration sensors in as close to the same locations as was feasible.

In general, the data collected in Phase IV indicate a general shear wave velocity (Vs) of about 650 to 1300 fps (~200 to 400 m/s) within the near surface fill and potentially the top of the natural soil profile. The profiles indicate a probable change in material at about 14ft to 17 ft (~5 m) depth, and associated general shear wave velocities in the lower material that range from about 1300 fps to 2500 fps (~400 m/s to 750 m/s). The velocity profile is generally smooth and consistent both laterally and vertically. We anticipate this material represents the underlying marl, natural soil, and disintegrated rock from the in-place weathering of the underlying limestone bedrock. Another change in material shear wave velocity occurs at a depth of about 45 ft to 50 ft (~14 m to 15 m) depth, which we anticipate represents the underlying bedrock surface and velocities greater than about 3000 fps (~900 m/s).

In contrast to the Phase IV results, the data obtained in Phase V and Phase VI surrounding the turbines is in general less homogeneous and smooth, especially in the range of depths from about 13 ft to 33 ft (~4 m to 10 m). The profiles also indicate variations in the shear wave velocity that form generally horizontal zones, which seem to be indicative of increased weathering along the near horizontal bedding planes and/or variations in susceptibility to weathering within slightly different horizontally bedded lithological layers