1. Magnetic Surveys
Explanation
Lab exercise
Resources
Vocabulary
Assessment
Appendix
2. Gravity and Density
Explanation
Lab exercise
Resources
Vocabulary
Assessment
Appendix
3. Measuring Resistivity
Explanation
Lab exercise
Resources
Vocabulary
Assessment
Appendix
Unit Three Standards

Unit Three - Geophysical Methods
3. Measuring Resitivity
Lab Exercise

 

Objectives:              

Part 1: Test and compare the electrical conductivity of several ore minerals.
Part 2:  Complete a conductivity survey to locate a hidden conductor.

Materials:    

-  Digital Multimeter or conductivity meter, w/ 9 volt battery power source
-  Ore minerals:  ilmenite, specular hematite, galena, chalcopyrite, graphite, pyrrhotite, pyrite, magnetite, arsenopyrite, chalcocite, sphalerite, native copper.  Gangue: quartz, calcite
-  Plastic terrarium
-  Stopwatch
-  Clean quartz sand (or mortar sand)
-  Handful of copper pennies
-  Salt (2 cups)
-  Large plastic pitcher
-  Water (hot tapwater)
- Answer Sheet AS12
- Teacher Supplement TS13 (photo of setup)
- Answer Sheet AS13
- Answer Sheet AS14

Procedure:              

Part 1: Test the resistivity (inverse of conductivity) of the several ore and gangue minerals in your collection.  Try to use massive varieties or large crystals so material between the electrodes is solid and continuous.

1. Turn on the multi-meter.  Place the two probes in contact with the surface of the mineral, and push the probes straight into the mineral you are testing, spaced approximately one half inch apart.

2. Start by setting of the scale of the resistivity (ohms scale) on the 20K scale (lowest resistivity minerals (ie, most conductive), and has most sensitive scale.  Observe the reading.  If there is no reading, adjust the scale to the 2,000 ohm scale, check for a reading.  Continue upward trying the 20,000, scale, then the 200,000 scale, and finally the 2,000,000 ohm scale (most resistive minerals).  That a higher reading scale is required, indicates more current is needed to transmitt the electrons through or along the surface of the minerals.  After the reading
stabilizes, write the resistivity value (in ohms) in the appropriate space on Answer Sheet AS12.

3. Compare the mineral groups by answering the following:
A.  Which mineral group has the lowest resistivity:  sulfides, oxides or native metals (copper)?  Compare by grouping the values for these mineral types.

B.  Are metallic minerals the only minerals which will conduct electricity ?

C.  Do the gangue minerals quartz or calcite conduct any electricity ?

D.  Compare minerals with the same chemical formula but different crystal shapes, such as pyrite and pyrrhotite, or magnetite and hematite.

E. What is the overall effect the same for oxides, like Fe2O3 (hematite) compared to FeTiO2 (ilmenite) ?

F.  What is the overall effect for sulfides, such as FeS2 (pyrite) compared to FeAsS2  (arsenopyrite) ?

Part 2:  Complete a resistivity survey and locate the hidden conductor.  Teacher must prepare the survey in advance as follows:

  1. Fill the plastic terrarium with approximately about 4 inches deep with mortar sand. 

  2. Bury the handful of copper pennies on the bottom and somewhere along the center line (lengthwise) of the terrarium.  Smooth the top of the sand.

  3. Mix the salt with about three quarts or so of water in a large plastic pitcher.

  4. Slowly pour the salt water solution into the terrarium corner, disturbing the sand as little as possible.  Saturate the sand with just enough till a slight layer of water starts to form on the top of the sand.

  5. Set the multi-meter on the 2,000,000 (or “2,000K”) scale.

  6. Stick the positive current probe into the sand about one half inch deep at the zero mark. Hold the black negative probe in one hand.  At the same moment you stick the negative probe in the sand at the 2 inch mark, start the timer.  (Avoid contact with the pennies with either probe.
  7. At exactly one minute, record the resistivity value in ohms in the appropriate space on Answer Sheet 13, where it says Test One (red positive probe at zero mark).

  8. Take another reading on the line by next inserting the black negative probe at the 4 inch mark.  After exactly one minute, record the reading in the appropriate space for the 4inch “Distance” on the line.
  9. Test and record the readings for the rest of the “two inch intervals”, up to 12 inches (for a total of 6 intervals). 

  10. Move the positive probe to the 2 inch mark and insert it into the sand. The first reading will have the black negative probe at the 4 inch mark, then the 6 inch, 8 inch, 10 inch and 12 inch marks. Record the data in the appropriate spaces on Answer Sheet AS13.

  11. Continue completing the test lines, each test moving the red positive probe to the next higher “Distance” value.

  12. What electrode configuration corresponds to highest conductivity (lowest resistivity) along Line One? Where would you dig a prospect pit ?  Did you make a discovery ?

 

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