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Cross Correlation Window

Composite

Here is an example cross correlation in the composite mode:


This window displays the cross correlation of the section surrounding the reference point as the moving point and its window is sliding past it. The difference of depth between the moving point and the reference point is displayed in the main window status box. When you see a well defined peak, you have a good match.

X Axis

The numbers along the x-axis are the depth points in the reference window. In this example, that window is 5 meters wide, and the reference point is at 31.9, and the window runs from 29.4 to 34.4. the window.

Y Axis

The numbers along the y axis are the magnitude of the cross correlation.
The y axis only runs to -0.3 since values lower than that are of little interest.

Splicing

The cross correlation in the splicing mode is slightly different. Instead of a reference and moving point, we have two cores that, hopefully, are at corresponding depths. The upper or left core is the composite core, and the lower or right core is the splice. Here is a typical view during the splice cross correaltion:

We initiated this session with a middle mouse down action, and while holding that mouse down, sliding the mouse back and forth. When you get a well defined peak, as here:

you have a good place to put your splice.

The Cross Correlation Formula

When we cross correlate, we pre-process the data by normalizing each array of data. That is, we
  1. Remove the mean of each array, thus making their data centered around 0.
  2. We normalize by the geometric average of the energy of the two arrays used in the calculation.

In this way, the best result will always be a peak of 1.

The Little Red Bar

The little red bar that moves as you move your track marks the point of peak correlation.

General Thoughts About Cross Correlation

There are five variables for a cross correlation:
  1. Array One's origin.
  2. Array Two's origin
  3. Array Ones Window
  4. Array Two's Window
  5. The shift between the array one origin and the array two origin.
In the Composite Mode we have:
  1. Array One's origin at the reference point.
  2. Array Two's origin at the moving point
  3. Array Ones Window is set in the Composite Window
  4. Array Two's Window is the same as Array One's.
  5. The shift between the array one origin and the array two origins is the offset between the initial position of you moving point and the current position of your moving point.
So, in this mode, moving the mouse changes the offset between the reference and the moving point.

In the Splice Mode we have

  1. Array One's origin is the depth the mouse is at on the composite track.
  2. Array Two's origin is the depth the mouse is at on the splice (grab) track.
  3. Array Ones Window is set in the Splice Window
  4. Array Two's Window is set in the Splice Window
  5. The shift between the array one origin and the array two origin is 0.
So, in this mode, moving the mouse changes the origins of the cross correlation data window.