As part of the VELA program for evaluating systems for detecting underground nuclear events, a three-dimensional seismic array was instrumented and operated for over a year. Three seismometers spaced 1,320 ft apart were clamped to casing in each of five abandoned oil wells near Tryon, Oklahoma, and their outputs individually recorded on magnetic tape and conventional oscillograms. By time-shifting and compositing to achieve optimum signal coherence, signal-to-noise improvement factors somewhat greater than (rad)n times the ratio for a surface detector were obtained in the signal spectrum from 4 to 10 cps, with lower improvement factors in the 1- to 4-cps range. Studies of the nature of the site noise and its attenuation characteristics with depth are presented together with autocorrelation, power-spectrum, and coherence plots. To a depth of approximately 1,300 ft, the noise decay is exponential and characteristic of the fundamental Rayleigh mode. Below this depth the pattern is more complex and may be representative of higher mode Rayleigh and body waves. Expressions were derived for the theoretical comparison of the relative directional response of two- and three-dimensional arrays.