| Popis: |
Seismic propagation studies made in the Delaware Basin of West Texas have disclosed unusual waves traveling along near‐surface beds. The area is characterized by alternating high and low velocity layers near the surface, with a thin high velocity cap over much of the region investigated. Physical characteristics of the recorded waves have been correlated with this layering. Only those events arriving earlier than Rayleigh waves will be discussed here. Shortly after the first compressional arrival, waves are observed having apparent velocities equivalent to those of shallow refraction markers and with periods which may be predicted, assuming that waves leaving the refractors at the critical angle are reflected back into the refractors by interfaces closer to the surface. Later arriving, long period events are tentatively identified as refracted shear waves. A group of waves arriving just prior to the Rayleigh waves is observed only where the high velocity cap is present and underlain by a low velocity zone above the water table. These waves are not recorded when the source is appreciably below the low speed layer. They are considered to result from normal mode transmission of compressional waves with the low speed zone acting as a wave guide. |
