--> Figure 4

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Figure 4. On a zero- or near-offset VSP, the upgoing reflected event travels down to the reflecting interface and up to the sonde containing the geophones. If the raypath had continued to the surface along the additional blue line, the event would be in two-way traveltime. The traveltime along the blue path is the first break time for zero-offset geometry. By adding this time to the trace recorded at the sonde, the VSP data is placed into pseudo two-way traveltime.