Wind Turbine Site Characterization by the MASW Method

Approaches for Site Characterization (also see Vs30m evaluation)

As demand for wind energy grows worldwide, so is the demand for utilizing new technologies to make the construction of a wind turbine site more robust, safer, and more cost-effective. Recently, the MASW method has been applied to site characterization efforts to replace (or reinforce) the conventional drilling approach at several places in the west and the midwest United States. Park Seismic LLC has recently participated in several projects in Texas and New Mexico. The first wind turbine site characterization by the MASW method was reported by Park and Miller (2005a; 2005b) following its successful application at eighty-four (84) proposed turbine sites for the second phase construction in the Blue Canyon Wind Mill Farm (Park and Miller, 2005a) (Fig. 1) near Lawton, Oklahoma, and also at twenty selected sites near Elk River in Kansas (Park and Miller, 2005b) (Fig. 2).

Fig. 1. Location map of eighty-four (84) turbine sites in Blue Canyon Wind Mill Farm near Lawton, Oklahoma, surveyed by the MASW method (from Park and Miller, 2005a).
Fig. 2. Location map of the turbine sites near Elk River in Kansas surveyed by the MASW method (from Park and Miller, 2005b).

A wind turbine site has special characteristics that must be considered before and during the construction of the tower. For example, the heavyweight of the tower structure is built on a relatively small area on the ground (for example, 30 ft by 30 ft), and following construction, the ground volume below and around the tower will experience continuous vibration caused by the rotating blades. Because of the vibration, the dynamic properties of ground materials provided from a seismic survey, therefore, can be especially pertinent from geotechnical engineering perspectives.

On the other hand, considering the extent of ground materials related to the safety and sustainability of the tower being much more extensive than the direct area occupied by the tower, the conventional approach of drilling one or more places at the tower center may not be sufficient to ensure the overall safety and stability. A few instances of fatal crane failure caused by collapsed ground during the tower construction have been reported, so it seems this safety zone may need to be expanded even further.

The seismic investigation usually deals with the bulk-property evaluation of the ground, with the bulkiness increasing with depth. It usually performs a Vs30m survey for a 1-D profiling, or a continuous survey to generate a 2-D (and 3-D) cross-section image of the property, usually in stiffness as depicted by shear-wave velocity (Vs) information. Shear-wave velocity (Vs) is often used as a direct indicator of the shear and Young's moduli. The seismic survey is, therefore, a more thorough and appropriate approach for site characterization than conventional drilling (Fig. 3). Because a seismic survey does not need the bulky, heavy equipment that drilling does, the convenient accessibility to the site is another advantage. The overall cost is also usually some fraction of the drilling cost.

Fig. 3. Illustration showing the relative advantage of seismic survey in comparison to conventional drilling.