In this study, we have produced maps of reduced travel time for various shot points in the Imperial Valley, a time-term map, and a basement depth map. The time-term map effectively integrates all the features seen on individual travel-time maps, and the basement depth map is derived from the time-term map by a simple linear relation. The basement depth map represents the depth to the top of metasedimentary basement (Vp ≈ 5.65 km/sec) in the central Imperial Valley and the top of crystalline rocks on West Mesa and in the Chocolate Mountains. We estimate the standard error in this map to be 0.6 km.
The basement depth map indicates depths of up to 6 km in the Imperial Valley and depths generally less than 3 km on West Mesa. Major zones of high gradient in depth separate the Imperial Valley from West Mesa on the west and from the Chocolate Mountains on the east. Because West Mesa and the Chocolate Mountains are underlain by different types of basement than is the Imperial Valley, these major zones of high gradient apparently mark the locations of major crustal boundaries. Most active faults have basement scarps along at least parts of their lengths. Basement highs with relief as great as 3 km occur under major geothermal areas in the Imperial Valley, presumably indicating the presence of igneous intrusions and (or) metamorphism of sedimentary rocks to a relatively higher level in the crust.