Kansas Geological Survey, Open-file Report 2005-22
Choon B. Park and Richard D. Miller
KGS Open-file Report 2005-22
Final Report to
Barr Engineering Company
July 5, 2005
The multichannel analysis of surface waves (MASW) method (Park et al., 1999) was applied to a total of eighty-four (84) turbine sites planned for the second phase construction in the Blue Canyon Wind Mill Farm (Figure 1) near Lawton, Oklahoma, for seismic characterization of the upper 15 m or so of ground materials. The main purposes of the project were a) to evaluate ground stiffness as deduced from shear-wave velocity (Vs), and b) to estimate the potential existence of weak zones such as voids and fractured areas for the volume of ground materials defined by 100 ft (x-width) x 100 ft (y-width) x 50 ft (z-depth) volumes.
Figure 1. Location and geologic map of all eighty four (84) turbine sites to be constructed during the second phase of wind mill farm in the Blue Canyon Wind Mill Farm near Lawton, Oklahoma (inset).
All eighty-four (84) sites were surveyed for the 2-D Vs evaluation that generated 2-D Vs maps showing both lateral and vertical change in Vs. In addition, a unique surface-wave method was also applied to each site to delineate subsurface anomalies by their surface (x and y) coordinates. This method is called "Side Scattering Analysis (SSA)" because its principles are based on the utilization of surface waves scattered from an anomaly sideways with respect to the seismic-survey lines. Multiple 2-D Vs maps were obtained because this SSA method required multiple seismic-survey lines deployed in a 2-D (x and y) format with data acquisition of each line identical to that for the 2-D Vs mapping. This increased the reliability and utility of the Vs evaluation provided by the redundant spatial sampling. Both 2-D Vs maps and SSA maps were used to interpret anomalies. As a supplementary tool a method called surface-wave imaging by attenuation (SIA) (Park et al., 1998a) was also used to help interpret anomalies by applying it to the same shot records used for 2-D Vs mapping.
For the interpretation of anomalies, any localized zone of anomalously low Vs in the 2-D Vs maps was first identified and then cross checked for possible back-scattering features on the corresponding seismic-shot records. Localized accumulations (or troughs) of amplitude in the SSA maps were also interpreted as potential anomalies. Effectiveness of SIA analysis to interpret anomalies was marginal.
A testing survey was made at one of the turbine sites on March 29 to evaluate optimum acquisition parameters. Then, production surveys were performed during the period of April 13-June 9, 2005, for a total of thirty-one (31) field days operated by a three (3)-person (on the average) crew.
Vs in the area as indicated by the 2-D Vs analysis changed from about 700 m/sec within the uppermost 5-m depth range to about 1000 m/sec at a depth range of 10 m, and then to about 1400 m/sec at deeper depths down to maximum investigation depth of approximately 20 m or so (Table 1). Within this range, however, each site showed different patterns of Vs variation in both vertical and lateral directions. A total of sixty-nine (69) anomalies were interpreted as potential voids from the 2-D Vs, SSA, and SIA analyses (Table 2). Final 2-D Vs, SSA, and SIA maps for all 84 sites are presented with interpretations of voids in separate volumes of Appendices I, II, and III, respectively.
In this report, each surveyed site is denoted by the assigned turbine-tower number. For example, the site of turbine-tower number 201 is denoted by T-201, and T-001 is the site of turbine-tower number 1. The fundamental mode of surface waves is to be denoted by M0, and higher modes are denoted by M1 (the first higher mode), M2 (the second higher mode), and so on.
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Kansas Geological Survey, Geophysics
Placed online July 13, 2006
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