Data Description

An offset VSP experiment was carried out by an anonymous seismic contractor in the Gulf of Mexico. A plan view of the survey configuration is show in the top of Figure 4.5. The data consists of three 3-component gathers at offsets of 152 m, 610 m, and 1524 m from the well head (Figure 4.5 top). The 152 m offset gather consist of 82 receiver stations from depths of 2652 m to 3887 m at 15.2 m intervals (Figure 4.5 bottom-left). The 610 m and 1524 m offset gathers contain 95 receiver stations each between depths of 3049 m and 4489 m, again at 15.2 m intervals (Figure 4.5 bottom-right). Sample gathers are shown in Figure 4.6 with prominent modes labeled. The vertical axis is depth in meters and the horizontal axis is time from 1.2 to 3.0 seconds. The upper figure shows the vertical component primarily containing direct and reflected P-waves. A horizontal component (X) for the same shot offset is shown in the lower figure; this shot gather records reflections and transmissions from both P- and SV-waves. A P-wave velocity model is generated from well information (Figure 4.7 top-left), and an S-wave velocity model was estimated from near vertically incident PS transmission and reflection events (Figure 4.7 bottom-left). A P- to S-velocity ratio of 1.6 and 2.7 was used for the salt and sediment, respectively. Prior to migration, the data were reoriented by maximizing the P-wave energy (Ahmed, 1987). This was first done for the X- and Y-components, maximizing the energy on the X-component direct wave (Figures 4.8 and 4.9), then on the Z- and X-components, maximizing the Z-component direct wave energy. The events with the desired moveout velocity were picked and flattened. The flattened gathers were median and bandpass filtered and unflattened. Figure 4.10 shows the isolated reflected P-waves (top) and transmitted S-waves (bottom) for the 152 m offset gather.

Figure 4.5: Offshore VSP acquisition geometry. The top image shows a plan view of the relative location of the sources to the well head, denoted by $\otimes$. Since the source was on a ship there is some scatter in each offset's source location. The bottom two figures show the relative location of the receiver array to the source position for the different offsets.

Figure 4.6: Shot gather for a source at 152 m with the Z-component at the top and the X-component at the bottom. The Y-component is not shown.

Figure 4.7: Top shows P-wave migration velocity model (left) and a velocity profile (right), bottom shows similar figures for the S-wave migration velocity model. The solid lines of the velocity profiles indicate the actual velocity distribution with depth and the dashed lines represent the smoothed velocity function used for migration.

Figure 4.8: A comparison of before and after reorientation of the seismograms in the XY-plane. The 3-component geophones were rotated until the energy in a small window surrounding the direct P-wave was maximized.

Figure 4.9: The rotation angle used to reorient the X- and Y-components (top). An X- and Z- rotation was also performed (bottom). After rotation the continuity of events was improved and transverse waves were largely removed from the Z-component.

Figure 4.10: Compare with Figure 4.6. Desired events were picked, flattened, median filtered, unflattened, and bandpass filtered to produce these gathers. FK filtering could not be used because single events have a large range of velocities some of the S-wave are aliased.