Seismic reflection techniques were used to characterize a bedrock surface buried under alluvium near a construction site on the campus of Baylor University in Waco, Texas. One of the objectives of the study was to determine if either compressional or shear seismic profiling could be used to reduce the number of engineering boreholes required to determine the bedrock depth and relief prior to building construction. The upper few meters of the alluvium is dry but the lower portion is below the water table, making the bedrock surface a difficult target for compressional waves. The compressional reflection coefficient at the water table is an order of magnitude greater than that at the bedrock surface, and the dry alluvium reduces the signal bandwidth such that the two reflections cannot be distinguished. Also, the multimode Rayleigh ground roll, traveling along the surface at about half the speed of the compressional wave, swamps the reflections. By using shear waves to profile the alluvium/bedrock interface, it was possible to avoid the water table and ground roll problems associated with compressional profiling. Walkaway survey results and analytical models presented demonstrate that shear waves do not “see” the water table, and masking of the bedrock target by the reflection at the dry/wet alluvium interface does not occur. Nor was ground roll a problem because the Love “ground roll,” traveling at a velocity almost as fast as the shear body wave, moves across the geophone spread before the return of the shallow reflections. Common depth point (CDP) and optimum offset shear profiles are presented. Uncertainty in determining the depth to bedrock from the seismic data was estimated to be 3 ft (0.9 m), which is sufficiently accurate to be useful in reducing the number of preconstruction boreholes required in the Brazos floodplain.
General expressions for the transfer function and the coherence, relating earth motion and atmospheric pressure variations observed at the surface, are derived for the case where the pressure field is a random process which is stationary in both time and space. It is found that as long as the phase velocities of the pressure field are low in comparison to seismic wave velocities, this transfer function will be a smoothed version of the Earth's response to a point pressure load applied at the surface. The form of the smoothing operator is determined by the correlation structure of the pressure field. In particular, it was found that the transfer function of an analytical model which possesses all of the observed statistical properties of the wind-generated pressure field closely resembles those associated with plane pressure waves.
Phase-matched filters are defined as a class of linear filters in which the Fourier phase of the filter is made equal to that of a given signal. An iterative technique is described which can be used to find a phase-matched filter for a particular seismic signal. The process is then applied to digital records of Rayleigh waves from a synthetic source with propagation across 55° of continental path, an earthquake in the Greenland Sea recorded in Texas, and a nuclear explosion in Novaya Zemlya recorded in New Mexico. Application of the filter allows multiple arrivals to be identified and removed and allows recovery of the complex spectrum of the primary wave train along with its apparent group-velocity dispersion curve. The amplitude spectrum of the primary signal obtained by this linear process is not contaminated by interference from multipath arrivals. The filtering process also provides significant improvement in signal-to-noise ratio, greater than a factor of four for the Greenland Sea and Novaya Zemlya events.
Sonic booms produced by aircraft moving at supersonic speeds apply moving loads to the earth's surface.In deep water, a moving underwater pressure field is observed to accompany the hyperbolic boom trace sweeping over the surface. The pressure waveform underwater near the surface is almost identical to that of the N wave in air, but it is rapidly smoothed and attenuated with depth, typically becoming one-tenth as large at a depth less than 0.6 of the N wavelength. Overpressures may exceed background noise pressures by factors of up to 100 at moderate depths for frequencies between 2 Hz and 100 Hz, but are less than 0.16% of pressures known to harm marine life in single exposures. Adequate quantitative theories for the underwater effect have been developed, and have been verified by scale-model experiments. On land, which is generally stratified, there are two major effects: the "static" deformation field traveling with the surface load, and air-coupled Rayleigh wavetrains following each N-wave transient. The latter have frequencies and amplitudes determined by the geology and the aircraft speed. The former has always been the largest effect in over 1000 seismograms recorded in field tests. Its amplitude is proportional to the sonic-boom overpressure. The maximum ground motion recorded was about 100 times the largest natural, steady seismic noise background, but was still less than 1% of the accepted seismic damage threshold for residential structures. Movement is greater in soft ground than on hard rock, and decreases rapidly with depth. Present quantitative theories for the major seismic effects agree reasonably well with the experiments. Seismic forerunner waves, which begin at least 7 sec before arrival of the sonic boom, might be exploited for automatic warnings to lessen the startle effect. Sonic booms probably cannot trigger earthquakes, but might possibly precipitate incipient avalanches or landslides in exceptional areas which are already stressed to within a few percent of instability. I. UNDERWATER RESPONSESFor an aircraft in flight over a plane surface, such as a smooth body of water, the intersection of the Mach cone with the interface is one branch of a hyperbola, whose vertex advances over the surface with the supersonic velocity of the aircraft. At the sides of the hyper-
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