Interpretation of data from a 3.05-kilometer borehole in Precambrian crystalline rocks, Wind River Mountains, Wyoming

Data from a 3-km borehole in granitic crystalline rocks include cores, cuttings, velocity survey, formation density log, sonic log, and caliper survey. The borehole and adjacent peaks represent about 4-km exposure of granitic gneisses from a migmatite terrain typical of deeply eroded Precambrian terrains. Although the rocks are all granitic, they range in composition from quartz monzonite to quartz diorite gneiss. Mean chemical composition of these rocks is close to that of Poldervaart's shield surface rocks, the Canadian Precambrian shield, and the Finnish Precambrian shield. Compositional variations are reflected in distinct chemical variations and in density that ranges from 2.58 to 2.77 g/cm3. Mean density for all the cores is 2.70 g/cm3. Seismic velocities from the velocity survey range from 5.18 to 6.10 km/sec from the top to the bottom of the borehole. The mean velocity is 5.78 km/sec, but is 5.89 km/sec below the upper 0.5 km. Sonic log velocities, which are 6.1 to 6.2 km/sec in relatively unfractured deeper zones, are higher than interval velocities over shorter distances and reach a maximum value of 6.35 km/sec. Maximum temperature is 62°C, and maximum lithostatic pressure is 800 bars. Although pressure and composition are important factors affecting seismic velocity, fractures seem to be the most important factor at these depths. The velocity function, V = 0.16z + 5.58, is suggested for this depth range. Because these rocks are typical for exposed crystalline shields, velocities of 6.1 to 6.2 km/sec or slightly greater should be common; velocities much under 6.0 km/sec would generally be unlikely in the upper crust.