Focus question

How can we measure the electrical resistance of the ground at different depths? 

Essential materials

• sharpened wooden pencil at least 15 cm (6 in) long
• electrical or masking tape
• 2 pieces of insulated wiring at least 15 cm (6 in) long
• small pocketknife
• ballpoint pen or fine tip marker
• centimeter ruler
• ohmmeter or multimeter (electronic meter with ohmmeter setting)
• optional but recommended: 2 short wires with alligator clips on each end

Main ideas and background information

• A probe is used for remote sensing (measuring something from a distance), such as measuring the electrical resistance of the soil beneath the surface of the earth. Electrical resistance is the extent to which the flow of electricity is hindered when passing through an object.
• In this activity, electrical logging is the process of obtaining a continuous reading of electrical data from a probe as it is inserted into the ground.
• For a continuous reading, the probe must remain connected to the ohmmeter as it moves down through the soil.
• The ohm is a unit used for measuring electrical resistance.
• The electrical resistance of an object is inversely proportional to the amount of current that flows through that object when a voltage is applied. If an electrical current can flow easily through something (i.e., it is a good conductor), then it has a low electrical resistance.
• Water tends to percolate downward in the ground. This means that soil near the top of the ground is drier than soil a few centimeters deeper. Wet soil is a better conductor of electricity than dry soil . Therefore it has a lower electrical resistance than dry soil. As the probe moves downward, the ohmmeter displays the changing values of the resistance.
• Salt water is a better conductor than fresh water because it contains additional charged particles called ions. Soil containing salt water has less electrical resistance than soil containing fresh water.
• Oil is not a good conductor of electricity. Soil containing oil has a relatively high electrical resistance. Oil exploration companies use this fact to find underground petroleum deposits. When the underground electrical resistance changes from low to high as the depth increases, it is possible that the probe has encountered a layer with oil in it. Note that soil forms only a relatively thin layer on the surface. When prospecting for oil, the probes are inserted deep into the rock formation below the surface. Porous rock, such as sandstone, can contain petroleum or water. The electrical resistance of the rock formation gives an indication of which fluid might be present.
• This activity simulates how electrical logging can be used to find likely locations of underground petroleum deposits. In the activity, electrical resistance is measured. Actual electrical logging in the field measures the resistivity of the rock surrounding the probe rather than the resistance. (For details on resistivity, see Digging Deeper.)

Procedural tips

• Start with a new pencil and sharpen the end just enough for a good point on the tip.
• It might be easier to tape the ends of the wires to the pencil if you remove the insulation from the ends of the wires first.
• As an option to cutting notches as shown in the activity, place a strip of masking tape along the length of the pencil. Mark one-centimeter distances on the tape and number them consecutively, beginning one centimeter from the pencil tip.
• Wires with alligator clips (optional) can be used to connect the probe wires firmly to the ohmmeter or multimeter terminals.
• Set the ohmmeter or the multimeter initially to a scale for 10K (10,000) ohms. (The symbol for ohm is Ω.) Try other scales if you do not detect a reading.
• Be sure to zero the ohmmeter for any scale you select. This is done by touching the two probes together and turning the adjustment knob until the meter reads zero ohms.
• Push the probe down into the soil slowly so that it will be easy to stop at each centimeter mark.
• Confirm your data by probing a second time near the location of the first electrical logging.
• Draw a graph or log of the relationship between the electrical resistance and the depth similar to that shown on What The Data Show .
  
  (Note: Graphs of scientific data typically have the independent variable along the horizontal axis and the dependent variable along the vertical axis. In this case, the independent variable of depth is directed downward along the vertical axis, corresponding to the direction of the probe movement. This is the same arrangement of axes in logs of actual field data.)
• Compare the shape of the electrical resistance log with the electrical resistivity log shown on The Electrical Resistivity of Materials.

Discussion

1. What did the ohmmeter measure?
   (The ohmmeter measured the electrical resistance of the soil at different depths.)

2. What is an ohm?
   (An ohm is the unit used for measuring electrical resistance. Just as we use centimeters to measure distance, we use ohms to measure electrical resistance.)

3. How did the electrical resistance readings change as the probe went farther down into the soil?
   (Responses will vary depending on actual results. In general, the resistance is very high near the surface and lower at deeper levels.)

4. Why might the electrical resistance of the soil be different at different depths?
   (The soil might contain more water at deeper levels. Under these conditions, soil is a better conductor of electricity, meaning also that the electrical resistance is lower.)

5. What do you think will happen if a much longer pencil were used?
   (Some students may speculate that resistance would continue to decrease. Others may suggest that it would approach a nonzero value as a limit. Both viewpoints can be supported by typical data. Students might also postulate that the probe could encounter a layer that is a poor conductor, such as an oil-bearing layer, resulting in higher resistance readings.)

6. How is your electrical resistance graph similar to and different from an electrical resistivity log from the field?
   (It is similar because the resistance/resistivity readings vary with depth. It is different because our graph reports resistance and the field log reports resistivity.)

7. How does this activity relate to everyday life?
   (Oil exploration companies use similar electrical logging activities to find deposits of underground petroleum.)

8. What ideas do you have for a better logging tool design?
   (Encourage students to evaluate each idea and try it out.)

Assessment

Are students able to describe how to measure the electrical resistance of the ground at different levels?

Answer: A probe connected to an ohmmeter or multimeter is used to measure the electrical resistance. The resistance of the soil at different levels can be measured by pushing the probe into the ground. This remote sensing technique is called electrical logging.

Extensions and further investigations

• Have students repeat the experiment using
  – different soil samples or soil at different locations.
  – soil with different amounts of moisture.
  – soil with a bit of vegetable oil mixed in.
  – probes of different lengths and diameters.
  – different kinds and sizes of wire.
  – different types of ohmmeters or multimeters (e.g., digital or analog).
  – different speeds for pushing the probe into the ground. 

  Note: For best results, change only one aspect at a time.

• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Any oil-related careers, especially those related to exploration

• Medicine, especially in surgical applications of remote sensing

• Manufacturing

• Electrical engineering

• Instrumentation design

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B – Physical Science: B1, B3
Standard D – Earth and Space Science: D1
Standard E – Science and Technology: E2-E6

Grades 9-12
Standard B – Physical Science: B6
Standard E – Science and Technology: E2-E6

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS2, BoS: 1a, b, c, d
KS3, Sc2: 3a, b, BoS: 1a, b, d, e
KS4, Sc2:3d, f, Sc4: 1b, BoS: 1a, b, d, e

Glossary/vocabulary

remote sensing
electrical logging
electrical resistance
probe
ohm
ion
log

Resource links

Technical overviews of several aspects of electrical logging:

J R Associates Civil and Environmental Geophysics

Downhole Methods by Geosphere Incorporated

Resistivity Logging by WELLOG