Time-term analysis of upper crustal structure in the Columbia Basin, Washington

Arrival time data from existing permanent networks and supplementary portable stations were collected during the US Geological Survey (USGS) refraction program in August 1984. Reduced travel time plots of these data and USGS record sections were used to select data from two branches of the travel time curve. Sixty-six arrival times selected in the distance range 3 to 63 km were fit to a velocity of 5.16 km/s (+-0.02 km/s). Time terms determined at 31 stations range from -0.10 to +0.15 s (+-0.05 s) and show correlations with elevation and near-surface geology. The four intercept times range from 0.15 to 0.22 s (+-0.05 s), implying an average depth of 0.5 km to the 5.16-km/s refracting horizon within the basalt. Eighty-three arrival times selected in the distance range 50 to 145 km were fit to a velocity of 6.16 km/s (+-0.02 km/s). Time terms calculated at 40 stations range from -0.6 to +0.6 s (+-0.1 s), with the four intercept times ranging from 1.4 to 2.0 s(+-0.1 s). These show a systematic regional pattern consistent with previous studies (Malone and Rohay 1983) but extended by portable-station coverage. Conversion of the time terms to basement depth is dependent upon the lateral extent of an inferred low-velocity layer beneath the basalts, which is suggested by deep-borehole logs and magnetotelluric data