20 Well seismic surveying and acoustic logging between the source at the surface and the receiver in the borehole. This is illustrated in Figure 1.2 below. Figure 1.2 Illustration of the seismic raypath during a VSP or check shot (left) and a downhole (right). Reflected waves shown in purple; direct waves shown as dotted lines. Regarding the study of transmitted or direct waves, in petroleum seismic prospecting, we refer more to check shots and also to seismic coring. • In general, a check shot is carried out on the whole depth of an oil borehole (kilometric scale). It is used for seismic calibration, i.e. the geological characterization of seismic markers. Indeed, check shot results are presented in the form of graphs: time-depth, mean velocities of intervals, mean quadratic velocities as a function of the depth and the geological formations encountered in the borehole. • Seismic coring is usually carried out in a shallow borehole (hectometric or decametric scale). It is used to determine the weathering zone (WZ) parameters, particularly its thickness and velocity, which are essential parameters for static corrections. Often its acquisition follows an uphole configuration (source in borehole, such as dynamite) rather than downhole (source on surface). Geotechnics tends to focus on downhole (DH), and also on shear waves (S) in particular. For applications requiring a detailed evaluation of the Vs logging as a function of depth (modeling ground response for construction design purposes), we often favor the measurement of transmission between boreholes, enabling the establishment of a velocity profile with a metric step, referred to as crosshole.
RkJQdWJsaXNoZXIy NjA3NzQ=