Interpretation of a Seismic Refraction Profile Across the Roosevelt Hot Springs, Utah and Vicinity
In April, 1977, a seismic refraction profile was recorded across the Milford Valley, the Roosevelt Hot Springs KGRA, and the northern Mineral Mountains in southwestern Utah. Seven shot points were used to provide multiple subsurface seismic refraction coverage along the 30 km east-west profile line. Since an inspection of power sprctrums revealed large components of 60 Hz noise on some traces, computer routines were used to low-pass filter all seismograms. Amplitude information was utilized by normalizing all traces that recorded the same blast.
Subsurface structural modeling was conducted by means of first arrival P-wave delay-time alalysis and ray tracing. Herglotz-Wiechert travel-time inversions was used for the velocity-depth distribution in the Mineral Mountains. The interpretation of the P-wave travel-times suggests that the Milford Valley fill consists of two units with a total thickness of at least 1.8 km. The surficial layer is composed of Quaternary sands and clays with a velocity of 1.8 km/s. A deeper horizon with a velocity of 4.0 km/s is interpreted to be Tertiary sediments on the basis of gravity modeling across the Milford Valley using estimated densities from velocity-density relationships. The deepest layer identified from the seismic data is interpreted as a bedrock unit of Precambrian (?) gneisses. A true velocity of 6.7 km/s for the valley basement was determined from reversed subsurface P-waves and from a sonic log obtained in a well drilled into metamorphics near the refraction line.
In the vicinity of the Roosevelt KGRA, a thin low velocity alluvial layer covers a basement igneous complex with a velocity gradient in the upper 0.1 km of basement rock may account for the absence of reflections in the record sections. Analysis of the refraction data also indicates that the main range front faulting begins at least one kilometer west of the Opal Mound fault. Amplitude decay of the first P-wave arrivals shows decreased attenuation for seismic waves that propogate across the hot springs area.
Granite velocities between 3.3 km/s and 4.0 km/s were calculated from the travel-times in the Mineral Mountains. These velocities may not be representative of the majority of the pluton because the refraction profile closely followed the east-west Hot Springs fault zone. An east dipping 5.5 km/s layer at a depth between 0.7 km and 1.5 km beneath the range corresponds to an increase in velocity indicated on a sonic log at the eastern edge of the geothermal area.