Anelastic attenuation of compressional seismic waves in glacial deposits

Includes bibliographical references (pages [102]-104)The purpose of this study was to evaluate the effect of saturation and fracturing on attenuation (absorption) of compressional seismic waves in glacial deposits. The apparent seismic quality factor, Qₚ, was calculated from seismic refraction lines over a longwall mining site in Saline County, Illinois, and an unfilled portion of Settler's Hills Landfill in Kane County, Illinois. A decrease in apparent Qₚ (increase in attenuation) was observed at both sites when unsaturated glacial drift was compared to saturated glacial drift. However, a compositional change between unsaturated and saturated units made it difficult to attribute the decrease in apparent Qₚ to saturation alone as variations in grain size, composition, permeability, and porosity could all affect Qₚ. Mean apparent Qₚ values, however, were similar between both sites which suggests water content was more important than composition in seismic attenuation. Three regions were selected at the Saline County site to evaluate the effect of fracturing on attenuation. Group 1 was located over undisturbed glacial drift which exhibited the highest mean apparent Qₚ values (14.6±7.9 (1 s.d.) and 5.4±1.9 for unsaturated, sandy clay and saturated sand or gravel, respectively). Group 2 incorporated seismic lines over the edge of collapsed longwall mine panels where open fractures were present. This region had the lowest apparent Qₚ values (8.3±2.7 and 2.6±3.1 for unsaturated, sandy clay and saturated, silty sand or gravel, respectively). Group 3 included seismic lines over the center of collapsed longwall mine panels where closed fractures were prevalent. This region exhibited intermediate mean Qₚ values of 11.3±3.7 and 4.2±2.6 for unsaturated, sandy clay and saturated, silty sand or gravel, respectively. Differences between the mean apparent Qₚ values were evaluated by use of the t test. Less dramatic, but similar, changes in P-wave velocity were observed between the groups as well. P-wave velocities for saturated drift were occasionally less than 5000 ft/s as a result of low rigidity moduli or pockets of unsaturated material. From the apparent Qp changes observed at Saline County and Settler's Hills, it was concluded that small water movements in drift, enhanced by saturation or fracturing, largely absorb the P-wave energy. These results also suggest Qₚ may be a much more sensitive indicator of fracturing than P-wave velocity and may be used to identify saturated sediments when velocity data are ambiguous.M.S. (Master of Science