The well seismic recording is a checkshot service which provides a depth-vs.-time calibration for the [[seismic]] reflection technique. Seismic velocities are measured at the well by recording the time required for a wavelet generated by a surface energy source to reach the [[geophone|geophones]] in a tool anchored in the well. The recorded travel times of the direct arrival are used to calibrate the [[sonic log]], which then becomes the basic seismic reference and allows a surface seismic cross section to be scaled to depth.