Unit Three - Geophysical Methods
2. Gravity and Density
Lab Activity

Part 1: Determine the specific gravity of a two different minerals from your collection.

Part 2: Plot a gravity profile.

-  Suspension scale

-  Medium size beaker (or similar container)

-  Graduated cylinder

-  Thin wire

-  Answer Sheet AS10

Part 1: Determine the specific gravity of one “heavy” mineral and one “light” mineral as follows:

1. Select a heavy mineral (ex:  galena, chalcopyrite).  Use the thin wire to tie around the specimen and suspend it from the suspension scale. 
2. Record the weight.
3. Fill the beaker about 2/3 full with water and immerse the suspended specimen.
4. Record the weight of the specimen in water.
5. Calculate the specific gravity (SG) of the mineral using the formula:
   SG = (weight in air) / (weight in air) – (weight in water)
   
   Example:
   Sample weight in air = 25 grams
   Sample weight in water = 17.5 grams
   (weight in air) – (weight in water) =  25 – 17.5 = 7.5
   SG = 25 / 7.5 = 3.3
   The specific gravity of the sample is 3.3.
   
   
6. Check your result with the SG listed for the mineral in Data Sheet DS1 or Table T 7.
7. Follow the same procedure to determine the specific gravity of a ‘light’ mineral.
   
   

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   Part 2: Calculate the “residual” gravity values for the stations of the gravity survey line.  Then plot the gravity values on the profile.  Find the crest of the buried anticline and indicate the drill site.

1. Fill in the values for the Latitude Corrections on Answer Sheet, assuming a linear change going northward along the survey line.
2. Fill in the values for the Residual Gravity Values by adding or subtracting the corrections listed for the measurements at each station.
3. Plot a dot on the Gravity Profile (Answer Sheet AS10) representing each Residual Gravity Value
   Plot each dot directly above the appropriate survey station and using the scale on the right.
4. Draw a smooth curve connecting the dots to show the gravity profile along the survey line.
5. The gravity profile reflects large scale folds in sedimentary rocks beneath the unconformity.  In the subsurface below the unconformity, sketch a large anticline with the following characteristics:

   - the amplitude of the fold is 300 meters.

   - the crest of the anticlinal fold is 150 meters below the surface.

   - the crest of the anticlinal fold matches the crest of the fold in the gravity profile.

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