Comparison of direct-s modes produced by different source types
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Compressional and shear body waves generated by a seismic source can be analyzed using vertical seismic profiling (VSP) data-acquisition procedures. If a goal of exploration geophysics is to study the physics and exploration applications of shear waves, it is important to know how much S-wave energy a source puts into the earth. To maximize S-wave created by a source, considerable effort has been expended to create surface sources that apply horizontally directed impulses to the earth (horizontal vibrators and horizontal impacts). In my project, radial shear (SR) and transverse shear (ST) waves generated by different types of sources and recorded by multicomponent receivers in a VSP well are examined and compared. The research question is ‘can a vertical-impact source create shear wave energy equivalent to the S-wave energy produced by standard horizontal-force shear-wave sources?’ To quantify the energy of shear-wave modes produced by different kinds of seismic sources, a VSP field test program was conducted at the Devine Test Site owned by The University of Texas at Austin. In the VSP data acquisition phase, the orientation of horizontal geophones is unknown because a borehole geophone rotates as it is lowered into a well, causing the horizontal geophones at each receiver station to be oriented in different azimuths. To study body waves, it is essential that all geophones in a vertical VSP array be oriented in a consistent azimuth. I mathematically rotated multi-component VSP sensors systems to change them from the inconsistent orientation they had at the time of data recording to a user-defined consistent-azimuth coordinate system. This rotation allowed ST and SR wave modes to be identified. After geophone rotation, direct-S wavelet amplitudes were analyzed in 90-ms windows starting at the first-break times of each arriving mode. Analysis of the rotated data showed that SR energy created by a vertical-impact source, a shot-hole explosive, and an inclined-impact source differ only slightly, and that there is essentially no difference in ST energy among these sources. Also, the signal frequency of SR and ST wave modes produced by horizontal-force shear wave sources are essentially the same as the frequency of SR and ST wave modes generated by a vertical-impact source. These test data show that vertical and horizontal vibrator sources produce shear wave modes having amplitudes 1000 times stronger than the other energy sources we tested. Considering the cost of using inclined-impact sources which is relatively expensive compared to using a vertical-impact source, and the difficulty of applying inclined-impacts in some land conditions, it is possible to obtain direct-S data of the same quality by using only a vertical-impact source or a shot-hole explosive. The arguments given above demonstrate that it is not necessary to use inclined-impact sources or horizontal vibrators to produce shear-wave data. S-wave data of the same quality produced by a horizontal-force source are provided by simple vertical-impact sources and shot-hole explosives.