Applied Geophysics
Gravity Data Reduction Assignment

Assigned: 13 February, 1998
Due: Try to do at at least up to Step 4 by 23 February. Do more if you can - it will be worth it. E-mail me your reduced gravity file (or tell me the filename). If you manage to get all the way through this exercise, e-mail me an example of a section you produced, and/or a Postscript file of the plots you made.

After a busy day in the field collecting gravity data, the investigator must next correct all the raw gravity readings to remove all of the expected effects of changes in latitude, elevation, tides, etc. before the fun part of interpretation starts.

This is a straightforward procedure but can be tedious and unnecessarily time consuming if done by hand (especially if you have to compute tidal corrections!). To make this process a bit less onerous, the program Xreduce was written to allow the investigator to enter raw gravity observations directly into a spread sheet type format and ask the computer to compute Simple Bouguer Anomalies.

In this exercise you will learn a few of the basic functions of Xreduce. You should also consult the manual for this program at http://gretchen.geo.rpi.edu/roecker/manuals/Xreduce/Xreduce.html for information on how this program works.

Step 1.

Starting the program.

Xreduce follows a lot of the same procedures as Xgrav, so starting up the program and getting it going are pretty similar. Once you've set up the DISPLAY properly, start the program by typing 

Xreduce

A window should appear that looks similar the the Xgrav window (with panels and a canvas).

Step 2.

Creating a Gravity Data File.

The following are actual Raw Gravity readings from a survey by the 1997 Applied Geophysics class in Canajoharie: 

	Canajoharie Field Site Raw Gravity Data

   North East Elev   Raw Grav  Station   Date   Time
     44 -74.5    0   880.232    BASE1  97 4 12 10 15
      0   0      0   880.340    BASE2  97 4 12 16 15
     50  35   -0.3   879.97     ST1    97 4 12 11 10  
    100  40   -0.4   879.909    ST2    97 4 12 11 25  
    150  45   -0.1   879.177    ST3    97 4 12 11 38  
    200  50  -0.23   879.752    ST4    97 4 12 11 50  
    250  55  -0.35   877.93     ST5    97 4 12 12 43  
    300  60  -0.15   879.387    ST6    97 4 12 12 55  
    350  70  -0.15   878.7      ST7    97 4 12 13  4  
    400  80   -0.5   878.565    ST8    97 4 12 13 11  
    500  90      1   878.072    ST10   97 4 12 13 17  
    600 100   0.65   878.033    ST12   97 4 12 13 24  
    700 110   0.95   880.097    ST14   97 4 12 13 26  
    800 120   0.75   880.285    ST16   97 4 12 13 32  
    900 130   -0.9   881.795    ST18   97 4 12 13 36  
   1000 140   -2.4   882.482    ST20   97 4 12 13 43  
   1100 150   -2.6   883.56     ST22   97 4 12 14 37  
   1200 160     -3   882.95     ST24   97 4 12 14 44  
   1300 170  -2.63   882.87     ST26   97 4 12 14 49  
   1400 180  -1.97   882.673    ST28   97 4 12 14 55  
   1500 190  -1.95   883.548    ST30   97 4 12 15  3  
   1600 200   -1.6   884.1      ST32   97 4 12 15 10  
   1700 210   -2.6   884.868    ST34   97 4 12 15 13  
   1800 220   -1.7   885.177    ST36   97 4 12 15 17  
   1900 230   -2.6   885.883    ST38   97 4 12 15 23  
   2000 240   -4.6   889.868    ST40   97 4 12 15 28  
   2100 250  -5.25   892.072    ST42   97 4 12 15 33  
   2200 260  -7.55   895.703    ST44   97 4 12 15 41

The positional coordinates of the stations (in the North, East, and Elev) columns are the relative location distances, in FEET, from the base station (location =(0,0,0)). For example, station ST42 is 2100 feet North, 250 feet East, and 5.25 feet BELOW the base station (BASE1 and BASE2).

The Raw Grav column are the readings taken right from the meter (the RPI D Meter).

The "Station" is an arbitrary identifier for the point taken.

The final columns are the date (All data from April 12, 1997) and local time (ranging from 10:15 to 16:15).

Your first task is to input this information into the program. Here is how you do it:

  1. Select the EDIT button in the upper left corner of the window. A small window ("Edit Data Choices") will appear with three option buttons.

  2. Select the "Create a NEW Gravity Datafile" button. A window will appear that looks like a form with 10 columns and 20 rows of short lines. Each row is for a particular observation. At the top is a line of labels that tells what should go in each column; thus there are 10 bits of information that must be entered for each observation.

  3. Type your observations in the appropriate blanks in the window. Note that, with the exception of station names, the values from a previous line will be copied to the next line if those entries are blank. For example, all of the readings were collected on the same date (97/05/12) so you need enter this date only on the first line.

    NOTE ON POSITION: This program expects that the first two entries will be the starting and ending Base station readings. The first base station entry should show position in longitude and latitude and the second in postion relative to the local coordinate system (typically the base station is the origin, so this is just (0,0). Also note that the order of input is East, then North, while the those in the data table above are (North, East).

  4. When you are done, or have filled up all the lines, select "OK" from the buttons at the bottom of the form. This will create a new gravity data file in memory.

  5. At this point, you might want to select "Done" from the "Edit Data Choices" window to pause in your data entry and select "WRITE DATA" to write your entries to disk. Selecting "WRITE DATA" will bring up a window that allows you to enter an output file name. Enter a name that will be easily identified as yours.

  6. You can add more entries, or edit the ones you have already entered, but selecting "Edit the Current Gravity Datafile" from the EDIT subwindow.

  7. Select "Done" from the EDIT subwindow when you are finished. Be sure to write your data to disk with "WRITE DATA" after you finish editing. Otherwise, if the program crashes, you will loose your work.

For purposes of this exercise, quit the program and restart it before doing the next step so that you will try the "READ" function.

Step 3.

Reducing a Gravity Data File.

In this step you reduce the data, which means you convert the raw gravity readings into Free Air or Bouguer Anomalies.

  1. Enter a gravity data file into memory. If you just completed Step 2 and didn't quit the program, you already have one in memory. If you wrote your data to disk and restarted Xreduce, you can read in your gravity data file using the "READ" button. In the window that appears, select the button that says "Read Gravity Data File" and type the filename on the appropriate line. Then select "OK".

    NOTE:The status of Gravity Data Files is controlled by the buttons on the second row that say "DEL GRAV", "SEL GRAV", "GRAV++" and "GRAV--". These buttons do the same operations as the "SEL DAT", "DEL DAT" buttons of Xgrav.

  2. To reduce the Gravity Data now in memory, select the "REDUCE GRAV" button. A window will pop up that allows you to change some parameters. These are:

    After you select "OK" the program will type a bunch of lines on the screen from which you started Xreduce. When it is finished, the "REDUCE GRAV" button will return to its normal color and shade.

  3. Now write out your reduced gravity data to disk using the "WRITE DATA" button. The file that is written will look like the original Gravity Data File but with two extra columns containing the Free Air and Simple Bouguer Anomaly.
NOTE: In the "WRITE DATA" window you can select which number you want to have written in the 3rd column of the output file. The default is the Raw Data, but it could be either of the two anomalies. The reason for this option is to make the result easier to access for other plotting and reduction software.

Step 4.

Plotting/Viewing a Gravity Data File.

You can make a map showing the locations of your gravity data stations using the "DRAW" button. If you have any data or map files in memory this function will draw a map of them in the canvas part of the window. You can make a hard copy of this picture by useing the PS-PREF and PS-DUMP buttons (in that order) to generate a PostScript file.

Step 5.

Selecting a Gravity Profile

Very often we want to make a 2D profile of our readings for further modeling. Xreduce makes this easy to do by allowing you to project points onto a vertical plane (hence reducing the 3D problem to a 2D problem). What you want to do first is decide that the orientation of your profile should be (many times it is perpendicular to some geologic trend), and which data point you want to include in the profile. The way you impliment your decision is to draw a box (or "Section") on the screen. All of the stations within the box will be selected for the profile, and their coordinates will be projected onto a plane that bisects the box. Here is a step by step of how you do this:

  1. After you have drawn the gravity station points on the screen, go over to the "Canvas Activity" button and select "Enter Section".

  2. The first operation will be to draw the mid-plane of the section on the screen. The endpoints of the line drawn on the screen represent the intersection of this vertical plane with the map will define the left and right ends of the profile (and, of course, the orientation as well).

    1. Select the position of the left midpoint with the left mouse button. If you make a choice but don't like it, push the middle mouse button to erase it.

    2. Hold down the left mouse button and a "rubber band" line will appear that represents the projection plane. Move to where you want the right midpoint and push the right mouse button.

    You now should have a line on the screen with a cross ("+") at each end. Now you are ready to draw in the box.

    1. Depress the left mouse button and drag is up and down or back and forth. Note that a box will appear that gets bigger and smaller as you move the mouse around.

    2. When the box is the right size, and includes all the stations you want, click the right mouse button. A box will be drawn and the letters "L" and "R" will appear to remind you which side is left and which is right.

  3. You may enter several sections if you like. To have a look at the profile of the data in a given section, choose "Select Section" from the Canvas Activity button and click on one of the corners of the section you want. A window will appear informing you how may points are in the section.

  4. The next step is to make a 2D plot of the data. To do this, select the "VEIW SECT" button at the top of the window (about in the middle). Another big window will appear that looks like the original XReduce window, but with fewer functions.

  5. Select "DRAW" in this window and a plot will appear of some quantity vs distance along the profile you choose in the previous window. There are two plotting options: Plot Distance Vs. Z and Plot Distance Vs. Value. If Plot Distance Vs. Z is selected, the plot will show distance vs elevation, while if Plot Distance Vs. Value is chosen, the plot will show distance vs. Bouguer anomaly.

  6. It will be useful to save the data in the profile window to a file for further analysis (for example, by Xgrav). To do so, select "WRITE XGRAV FILE" to make a file you can analyze with Xgrav, or select "WRITE DETAILED FILE" which gives more information about the locations of the data you selected. You can also make PostScript plots by selecting POSTSCRIPT and PS-DUMP in that order.

  7. When you are finished, select Quit to go back to the main Xreduce window.