COMPARISON OF VIBROSEIS AND EXPLOSIVE SOURCE METHODS FOR DEEP CRUSTAL SEISMIC-REFLECTION PROFILING IN THE BASIN AND RANGE PROVINCE

Direct comparison of low-fold, high-energy explosive and high-fold, lower-energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high-fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single-fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60-fold Vibroseis section demonstrates that low-fold explosive profiling provides a higher-quality image of the midcrust to lower crust (3-10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4-5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal-generated energy exceeding ambient noise levels down to 18-19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflections events on the 60-fold Vibroseis stack do not correlate to those on the single-fold explosion profile, suggesting that the high-fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high-fold CMP method apparently failed included complex, even time-varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal-to-noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower-energy Vibroseis section.