Background and overview of previous controlled source seismic studies

Controlled source seismic techniques offer useful tools to study the structure of the earth’s crust and upper mantle in considerable detail. The seismic reflection technique uses source–receiver offsets that are relatively small in comparison to the intended maximum depth of penetration, and the goal is to form an image of the subsurface structure through sophisticated data processing. The seismic refraction technique uses source-receiver offsets that are large in comparison to the intended maximum depth of penetration. Sophisticated interpretation methods have been developed to derive velocity models from these data. Reflection and refraction data are traditionally gathered along profiles and produce 2D results, but new instrumentation is making surveys with an element of 3-D coverage possible at a crustal scale. From the standpoint of seismic refraction and reflection surveys, coverage for the Southern Rocky Mountains and surrounding areas before the 1999 CD-ROM project was relatively poor. Reflection surveys targeting the deep crust were particularly sparse. Of the pre-1999 refraction data, several profiles are at least partly unreversed, most have only a few widely spaced shotpoints, and for most profiles, the interval between recording stations is typically >10 km. The thickest crust in the region (~50 km) does not correlate directly with the highest topography. The Southern Rocky Mountains are bisected by the Rio Grande rift whose crust thins from north to south and is at least 5 km thinner than that of adjacent regions. There is also evidence for crustal thinning across southern Wyoming.