VSP acquisition geometry
Acquisition of VSP data involves a surface source that is located either close to the well head (zero-offset case) or away from the well head (offset VSP) and a geophone in the well bore. Several traces are recorded at the same geophone depth, then edited and summed. (Repeatable sources such as vibrators or air guns are used.) The geophone then is moved to a new depth location and recording is repeated. The resulting section is a profile displayed in depth and time. A comprehensive reference on VSP is the book by Hardage .
Figure 11.4-1a shows VSP acquisition geometry. Consider a few of the raypaths that are intercepted by the receivers down the borehole: the direct arrivals from shot to receivers AC and AE, reflection ABC, and refraction ABF. Each receiver location yields a trace on the VSP that is in the depth-time domain (Figure 11.4-1b). Note that trace C has both the direct arrival (1) and the reflection from the first interface (3). The direct arrival (1 on trace C) and the refraction (4 on trace F) have only downgoing paths; therefore, they are called downgoing waves. On the other hand, reflection path ABC has a final upcoming segment BC (Figure 11.4-1a) and therefore constitutes an upcoming wave (Figure 11.4-1b). Note that direct arrival 2 on trace E coincides with a reflection arrival, provided the receiver is situated on the interface that causes the reflection. At this location, the upcoming and downgoing waves coincide (arrival 2 on trace E in Figure 11.4-1b).
- Hardage (1983), Hardage, B. A., 1983, Vertical seismic profiling: Geophysical Press.