Swept impact seismic technique (SIST)

In exploration seismology, summing seismic waves from a large number of small impacts is an alternative to a single explosion impulse. In this dissertation I explore several alternative strategies as means of effectively and efficiently imparting a large number of small elastic impacts into the ground. The strategies discussed here build on the Vibroseis and random impact sequences that have been developed by others.; When properly designed, the linear impact sequence (LIS) in which the impact rate changes linearly with time is an effective coding mode that results in negligible correlation noise and therefore a high signal-to-noise ratio (S/N). The simplicity in coding mode results in a fast and simple coded-impact seismic technique that can be implemented using a special but simple source and a conventional acquisition system with a slight modification. When a low-impact power source is used, the seismic pulse generated has a broad-band, high-frequency spectrum. Therefore, the technique is best suited for high-resolution, shallow reflection surveys.; Correlation noise of the LIS depends not upon the total number of impacts applied but upon the ending (or the highest) impact rate if the range of the impact rate is greater than one octave. Therefore, only a few to several hundreds of impacts are sufficient for a significant improvement of S/N.; An optimum ending impact rate depends upon the purpose of the survey, the way the target events occur in association with other events, desired improvement of S/N, and the spectral content of the signal. The optimum rate can be determined through a walk-away noise test using a conventional single-impact source. Proper control of the ending impact rate is especially important in a common-depth-point (CDP) survey to minimize any harmful effect of correlation noise on the stacked section.; Because of the simple and deterministic dependency of correlation noise on an impact parameter, the LIS is a faster, simpler, and more effective coding mode for the high-resolution data than the random impact sequence (RIS) in which the correlation noise is sensitive to several statistical parameters that are difficult to properly control such as the standard deviation, the mean impact rate, and the fluctuation of the impact rate distribution.; The correlation noise resulting from an impact sequence can be predicted based upon two different viewpoints in two different domains: the frequency domain and the time domain. The frequency-domain viewpoint relates to the shape of the power spectrum of the impact sequence that can be analyzed according to the sampling theory of discrete signal processing. The result of my analysis makes it possible to derive a nonlinear impact sequence that results in significantly less correlation noise than the LIS. It is the fractional impact sequence (FIS). The time-domain viewpoint relates to the distribution of the impact rate that can be analyzed according to the solution to a differential equation. Another nonlinear impact sequence derived from this viewpoint is the exponential impact sequence (EIS). The EIS also results in less correlation noise than the LIS.; Seismic reflection data obtained by using the LIS during a preliminary field test show an improvement in the resolution of shallow reflections when compared to data from several conventional single-impact sources.