SOURCE SIGNAL ESTIMATION AND CROSSCORRELATION

Once we have found the position of the drill bit and the velocity that best stacks its energy, we can stack the data along the appropriate moveout trajectory to obtain an estimate of the drill bit source signal. This is analogous to the signal from the accelerometer placed on the drill string by Rector et al. Crosscorrelating the data with this source signal estimate enhances drill bit energy in the data.

A plot of data after such a crosscorrelation is shown in Figure . Here the traces have been sorted by their offset from the well. There is an event at zero lag on the near offsets that can be tracked across all offsets and occurs at around 0.08 seconds on the far offsets. It is important to note that crosscorrelation preserves the moveout of the drill bit signal. Thus the same search for sources can be performed after crosscorrelation. Perhaps we will see evidence of scattering of drill bit energy off subsurface structure. This is one subject of future work.

 

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Figure 8 eaeg3

Data after crosscorrelation with an estimate of the drill bit source signal. The event at zero lag on the near offsets is the direct arrival from the drill bit. Moveout is preserved.