1.1 ,These test methods address compression (P) and shear (S) waves propagating in the downward direction in a nearly vertical plane. The seismic waves can be denoted as P_V or P_Z for a downward propagating compression wave and as S_VH or S_ZX for downward propagating and horizontally polarized shear wave. The S_VH or S_ZX is also referred to as an S_H wave. These test methods are limited to the determination of the interval velocities from arrival times and relative arrival times of compression (P) waves and vertically (SV) and horizontally (SH) oriented shear (S) seismic waves which are generated near surface and travel down to an array of vertically installed seismic sensors. Two methods are discussed, which include using either one or two downhole sensors (receivers).