Geophysicists at the Kansas Geological Survey (KGS) have developed a new, non-intrusive method to quickly evaluate foundations to determine if they will support wind turbines. The method, which uses seismic waves to determine the bearing strength of the ground, is non-intrusive. Seismic energy is generated with a sledgehammer and detected by small sensors placed on the ground. The data can be quickly analyzed on a notebook computer to evaluate the proposed foundation. No drilling is required or even vehicular access, so the method is particularly applicable to quick investigations early in the planning phase.
The method, called “Multi-channel Analysis of Surface Waves (MASW)” was perfected by a research team of scientists at the KGS working for several years to make the approach practical and efficient for field surveys. Their initial tests were done on 100 proposed sites at the Blue Canyon Wind Farm in Oklahoma and near Elk River in Kansas. Since then, the procedure is catching as an efficient way to evaluate potential wind farms. Dr. Choon Park-the principal inventor of the method-has started a company to provide this service.
The field procedure is relatively simple. About two dozen vibration sensors, called geophones, are laid out in a line on the surface stretching about 100 feet. An ordinary sledgehammer is used to bang on the ground off the end of the line. The vibrations are detected by the geophones and recorded on a sensitive seismograph. Some of the waves travel along the surface, and some penetrate into the ground. The speed that the waves travel depends on the shear strength of the material, and the depth of penetration varies with the wavelength. The data is analyzed on a computer and colored plots are generated that reveal the shear strength of the ground at the surface and at various depths. Shear strength is the property that determines whether the foundation will support the vibrating motion of the wind turbine. Competent, as well as weak, materials are revealed in the plots.
The method can be used to quickly check a single site, or to efficiently map the foundation conditions to produce a 3D map of an entire wind farm. In some cases, follow-up drilling may be desirable to sample questionable areas, or in conjunction with actual construction of the wind turbine. Surface wave analysis can help avoid costly over-design or catastrophic under-design of the wind turbine foundations, at a fractional cost of extensive drilling programs.
More information is available at www.masw.com, and the Blue Canyon study is here www.kgs.ku.edu/Geophysics/OFR/2005/OFR05_22/
Services Park Seismic LLC Provides
Park Seismic LLC provides a complete field survey and reporting service for seismic investigation of wind turbine sites in a flexible and prompt manner, ranging from the most basic 1-D analysis to a complete 3-D analysis depending on the site conditions and budget availability. Field surveys may take one-half to one full day per site depending on the site accessibility, and preliminary results can be in hand by the end of the survey in the field. Multiple-site surveys can take place in much a faster and more cost-effective manner than single-site surveys. Park Seismic LLC also provides data-analysis-and-reporting services in the case that the field survey is performed by another survey company.