Apparent velocity is the speed calculated directly from seismic data plots, specifically the travel time plot, without adjusting for the geological structure being measured.
In seismic surveying, particularly refraction methods, geophysicists analyze the time it takes for seismic waves to travel from a source, through different subsurface layers (refractors), to receivers on the surface. The apparent velocity is derived from the slope of the segments on this travel time plot.
Here's a breakdown based on the provided information:
- Measurement Source: It is measured directly from the travel time plot.
- Correction Factor: It is uncorrected for any refractor dip relative to the surface. This is a critical distinction.
- Relationship to True Velocity: The apparent velocity is equal to the true velocity only when the refractor (the boundary between layers with different velocities) is parallel to the surface.
Apparent Velocity vs. True Velocity
Understanding the difference between apparent velocity and true velocity is key in interpreting seismic data.
| Feature | Apparent Velocity | True Velocity |
|---|---|---|
| Measurement | Derived from the slope of the travel time curve on a seismic plot. | The actual velocity of seismic waves within a specific geological layer. |
| Correction | Uncorrected for the orientation (dip) of the subsurface layer relative to the surface. | Represents the wave speed within the layer itself, independent of dip. |
| Equality | Equals True Velocity only if the refractor is parallel to the surface. | The inherent speed of waves in the material. |
| Influence of Dip | Affected by the dip (angle) of the refractor. | Not affected by the refractor's dip. |
Why is Apparent Velocity Used?
While not always the true speed, apparent velocity is the initial value obtained from analyzing the seismic data plot. It provides the first estimate of wave speeds through subsurface layers. Subsequent processing and analysis, considering factors like refractor dip (if present), are required to determine the actual or true velocities of these layers.
Implications of Refractor Dip
When the subsurface layer (refractor) is not parallel to the surface but dips upwards or downwards, the apparent velocity measured on the travel time plot will differ from the true velocity within that layer.
- Measuring over a dipping layer can yield different apparent velocities depending on whether the survey line runs updip or downdip.
- Correcting the apparent velocity for the dip is necessary to obtain the true velocity, which is essential for accurate geological interpretation and depth calculations.
In summary, apparent velocity is a provisional speed calculated from seismic data that serves as a starting point for understanding subsurface velocities, but it must be corrected for geological dip to reveal the true properties of the layers.
