NEAR SURFACE VELOCITY ANOMALIES OBSERVED

The synthetic data are sorted into common midpoint gathers and NMO corrected. The resulting data cube is displayed in Figure 3. The effect of the velocity anomaly is seen as a symmetric ``X'' pattern visible in the top panel of the data cube. This top panel is a time slice through one of the reflection events.

 

synupnmo
Figure 3 Synthetic CMP data after NMO correction. The velocity perturbation causes the distinctive ``X'' pattern visible in the time slice.

Kjartansson 1979 made plots of trace power in midpoint-offset coordinates and observed ``V'' shaped patterns due to shallow velocity or absorption anomalies in the Grand Isle data set (see also Claerbout 1993). In Figure 4, I have plotted trace power for three different synthetic data sets with velocity anomalies centered at depths of $0.1\;km$, $0.2\;km$ and $0.3\;km$. The trace power is calculated by taking the square root of the sum of the squares of the trace amplitudes. The ``X'' patterns arise because the synthetics are split spread to simulate land data acquisition. For deeper anomalies the legs of the ``X'' are closer to the midpoint axis. For shallower anomalies the legs of the ``X'' are closer to the $45^\circ$ lines representing the shot and group axes.

The qualitative observation that the trajectory of the ``X'' patterns is a function of anomaly depth gives rise to the slant stack imaging method of the next section.

 

power3
Figure 4 The trace power in midpoint-offset coordinates after NMO is displayed for velocity perturbations (a) at shallow depth, (b) at intermediate depth, and (c) at greater depth. The depth in (b) is the same as the depth in Figures 1 through 3. The shallower anomaly has energy closer to the shot and group axes.