UGA Stratigraphy Lab

The data is in the strata

Cross-Correlation 1.0 Instructions

Overview

You can cross-correlate meter-scale cycles from two outcrops in three steps. First, make two data files, each listing the thicknesses of successive meter-scale cycles. Next, double-click on the Cross-Correlation program, which will ask for the names of your files, make the cross-correlation calculations, and save your results. Finally, plot your results in any program like Excel.

Making the data files

TeachText is probably the easiest way to make your data files. You can also use programs like Word or Excel, but you must save the file as a text-only file. Each of your data files should be a list of cycle thicknesses, with the oldest cycle at the TOP of the list and the youngest at the BOTTOM of the list (that is, reverse stratigraphic order). Each cycle thickness should be separated from the next by a carriage return. You can save the file with any name you wish. A data file might look something like this:

12.4
22.3
24.5
12.7
15.8
19.7
20.2
23.4

Because the cycles are in reverse stratigraphic order, 12.4 is actually the thickness of the lowermost cycle and 23.4 is the thickness of the uppermost cycle. If you forget to put both of your data sets in this order, the output is still valid, but will measure cycle offset from the top of the section, not from the bottom. If you forget to put only one of your data sets in proper order, your results will be gibberish, so watch out!

Running the program

Double-click on the Cross-Correlation program to run it. Once the program launches, it will display a dialog box for you to select one of your data files. Once you click OK, it will then display a second, identical dialog box for you to select your other data file. It does not matter which of your data files is selected first or second.

The program will display the correlation coefficient for each value of cycle offset from zero to the maximum value of cycle offset (equal to the difference in the number of cycles in the two sections). If you've entered the data correctly, with the oldest horizons at the top of your data files and the youngest at the bottom, zero cycle offset corresponds to correlating the stratigraphically lowest cycles in both sections. An offset of one means the lowest cycle in the short section correlates with the second lowest cycle in the long section. An offset of two means the lowest cycle in the short section correlates with the third cycle in the long section, and so on. At the maximum value of cycle offset, the uppermost cycles in both sections are correlated.

The program then resamples from your distribution of cycle thicknesses to calculate confidence levels on the correlation coefficient. This can take several minutes, particularly on slower computers, so be patient. When the calculations have finished you will be able to save the results window to a file and quit the program.

Viewing your results

Because the results file is a text file, you can open it in Excel or any similar program.

Compatibility

Cross-Correlation runs only on PowerPC Macs. It will not run on Windows systems, nor on pre-PowerPC Macs, such as a Centris, Quadra, Mac II, Classic, SE, Plus, etc.

In some circumstances, the program may fail because of insufficient memory. If this happens, you will be notified by an alert dialog box. You should quit the program and then increase the preferred memory by selecting the Cross-Correlation icon and selecting Get Info from the File menu.

Special note for OS X users

Cross-correlation behaves somewhat differently in OS X than in other versions. Your file listing the cycle thicknesses for the longer outcrop must be named longSection.dat (spelled exactly this way). Likewise, the data file for your shorter outcrop must be named shortSection.dat. Both of these files must be in the same folder as the application to run. If the program cannot find these files, it will tell you so. In all other respects, the OS X version runs the same as the older versions.

February 1999
Steven Holland