Investigating the Effects Fracture Systems Have on Seismic Wave Velocities at the Lajitas, Texas Seismic Station.

摘要

Refraction data collected at the Lajitas site in 1983 yielded in situ P-wave velocities of 3.5 km/s for the Santa Elena limestone. Based on accumulated data concerning P-wave velocities in homogenous limestones, the in situ P-wave velocities for the Santa Elena limestone are expected to average approximately 5.5 km/s. Fractures in the Santa Elena limestone lower the expected wave velocities. The objective of this research is to investigate the effects of macrofracture density and orientation on P-and S-wave velocity at the Lajitas site. We examine the relationship between several measurements of in situ P- SV, and Sh-wave velocities at varying azimuths and fracture orientation, porosity, and density. Thomsen's Biot-Consistent model (1985) and the Lajitas data collected in this study explain the difference between bulk rock velocities and in situ velocity measurements. Based on geologic reconnaissance and laboratory measurements the Santa Elena matrix appears isotropic and homogeneous, and fractures existing in the Santa Elena limestone are subvertical to vertical. (rh)