Processing the seismograph using ImageJ

Find the program ImageJ you installed on your computer. I suggest making a link on your desktop since it may not appear in your start menu.


ImageJ may not start very cleanly. By this I mean a black screen may appear which is associated with DOS. I found I am able to ignore this and just wait for the ImageJ program to load. If you have other programs open, minimize them. What you are looking for may appear behind other programs.

In convenient? Yes it is, however, the program was free after all.

The first time you use the program you need to make sure it is set up correctly.

Step 1: Open up one of the files you created using Paint using the File Open command.

Step 2: Click the Analyze tab and then Set Measurements. Click boxes Area and Limit to Threshold

Step 3: Click Edit Invert. Yes blue lines will now look yellow. Kind of neat!

Step 4: Click Process, then Binary, Make Binary. Now you have a black and white image. However, there appears to be a glitch in the software, you may have to go back to Step 3. Invert the image, and then invert the image back again.

Step 5: Optional. If you are working with a small screen you may want to next click Image Zoom and Zoom in.

Step 6: Make sure the little box is highlighted just below the word File. Then click on the image and carefully draw a yellow box around a line. It will look like this..I have made the yellow box easier for you to see in this image below.

Step 7: Tab Analyze then Measure (Ctrl M) If everything went correctly, you will get a Mean. Remember to look behind other windows in case the window opens behind one of the other windows. If you don't get a Mean number go back to Step 3 and invert the image again, and then invert it back again. Like I said, the software is free, not perfect.

Step 8: Drag the yellow box you made to the next line to analyze and repeat step 7.

When done your results will look like this.

What is mean?
The essential data you collect using ImageJ software is in the column labeled mean. If you select an area that is 100% black the value in the column will be 255. If you select a 100% white area, the value will be 0. Mean can be converted to a percentage value using this simple forumla:

• % of black pixels=Mean/255*100

Example:
Sample run 13 has a mean value of 172.269 which means:

• % of black pixels = 172.269/255*100%=67.55%

Why is % of black pixels not good enough?

There are several errors in the method:

1. Operator induced error by not centering the sample size correctly.
2. Pixels that are part of the page background and have nothing to do with the actual tremor.
3. Overlapped pixels that should be associated with a different segment that cause the value to be too high.
4. Lost pixels that are not within the bounds of the area that cause readings to be too low. 
5. Finally, there are the pixels associated with a line going across the page. These pixels are normally present and don't represent tremor. 

In the next few pages we will look at elimination of the background "noise".

Note: It is possible to adjust the software to try and eliminate overlapped pixels, and to recover lost pixels. However, that is not something we will cover today. A different solution to cover this problem is to greatly expand the area measured. In fact you can measure all the black pixels on the entire page. That subject we will cover.