HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic history of the SRP, which represents the surface expression of this hotspot, and to understand how it affected the evolution of continental crust and mantle. An additional goal is to evaluate the geothermal potential of southern Idaho.Project HOTSPOT has completed three drill holes.(1) The Kimama site is located along the central volcanic axis of the SRP; our goal here was to sample a long-term record of basaltic volcanism in the wake of the SRP hotspot.(2) The Kimberly site is located near the margin of the plain; our goal here was to sample a record of high-temperature rhyolite volcanism associated with the underlying plume. This site was chosen to form a nominally continuous record of volcanism when paired with the Kimama site. (3) The Mountain Home site is located in the western plain; our goal here was to sample the Pliocene-Pleistocene transition in lake sediments at this site and to sample older basalts that underlie the sediments.We report here on our initial results for each site, and on some of the geophysical logging studies carried out as part of this project.

SUMMARY This paper begins with a review on how rays may be traced though isotropic inhomogeneous media separated by curved interfaces. The results are applied to the case of a layer over a half‐space in which the seismic velocity varies linearly with depth and the interface is a horizontal plane. The presence of this type of velocity inhomogeneity will affect the observed amplitudes through its influence on the discontinuity in geometrical spreading at an interface. Analytical results will be compared to an approximate method for evaluation of geometrical spreading to ensure their accuracy. We show that the focusing effects due to the presence of velocity gradients become significant in our examples only for large angles of incidence.

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