Experiments with vertical arrays of seismometers were conducted from 1963 to 1965. Data of exceptionally high quality were obtained through the use of special 1-cps, deep-well seismometers and direct digital recording techniques. Arrays studied experimentally contained up to 7 vertical-component seismometers and extended to depths as great as 3.1 km.P-wave signals observed at depth are generally distorted because of interference between incident and surface-reflected waves. It is shown that the outputs of two or more deep-well instruments can be combined to reconstruct signal waveforms. Small additional improvements were obtained through the application of optimum multichannel filtering to vertical array outputs, but the best signal-to-noise ratios obtained were never more than 6 db above those available from single deep-well seismometers located at points of constructive signal interference.Results obtained from analysis of experimental data support the hypothesis that severe attenuation of seismic noise with depth is a characteristic only of sites where ambient noise is very intense at the surface. It is concluded that improvements in record quality which can be obtained through the application of vertical array processing at noisy sites are probably not greater than the improvements which would result from careful selection of a quiet site.