Logs From P and S Vertical Seismic Profiles

Vertical seismic profiles differ from ordinary well velocity surveys, or check shots, in two aspects: the records are longer and the depth intervals between measurements are much shorter. Accurate picking of first breaks makes the plotting of P and S velocity logs possible. From the velocity logs, a vs/vp log can be calculated. Introduction A vertical seismic profile (VSP) is the seismogram obtainable by lowering a multitrace seismic array in a well, activating a seismic source near or on the surface, and recording the seismic sensors during a time equal to at least twice the travel time between the surface and the deepest mirrors.Probes presently in use are not more than a few feet long, and the VSP seismogram is obtained by clamping the probe against the walls at regularly spaced depths and activating the seismic source for each position of the probe.Fig. 1 shows the three kinds of seismic signals detected at a geophone (receiver) R placed in a well when a source S located on the surface is activated:direct wave causing the first breaks,up-going reflections, anddown-going reflections resulting from the reflection of up-going waves by mirrors located above R.Fig. 2 shows how these events line up on a VSP seismogram. The left-hand sketch shows ray paths corresponding to two locations of the probe R1 and R2, with R1 above R2. Down-going waves, first breaks, and down-going reflections reach R1 before R2. Their lineups have parallel slopes within a given velocity interval. Up-going reflections reach R2 before R1. The transit time is the same as in the case of down-going events, but the slope of the lineup on the seismogram has the opposite sign.Though field operations are basically the same, there are two main differences between VSP's and ordinary well velocity surveys, or check shots:the recording time is much longer in the case of VSP'S, andthe depth interval between successive locations of the probe is shorter in the case of VSP's 15 to 70 ft (5 to 20 m) vs. 165 to 650 ft (50 to 200 m), or even more in the case of ordinary check shots.VSP's generally are recorded in view of a study of the reflection phenomena at a given location. Good VSP's make possible the direct observation of attenuation, multiple reflections, correlation of P and S events, etc. Also, the short spacing between recording stations makes it possible to obtain transit time logs based on the accurate picking of first breaks. When both P and S events are recorded, elasticity moduli can be estimated, and a much more accurate knowledge of the lithology is made possible. It is the purpose of this paper to illustrate, through the study of an example, how interesting logs can be obtained from P and S VSP's recorded in the same well. VSP Displays and Records Figs. 3 and 4, respectively, correspond to P and S VSP's recorded in the same well. The stratigraphic series listed in Table 1, and indicated on the left side of each figure, is typical of the central part of the Paris basin in France. Depths indicated in Table 1 are exact depths in the considered well. JPT P. 1843＾