Unit 2: Geophysical Properties of the Subsurface

Compiled by Lee Slater, Rutgers University Newark (lslater@newark.rutgers.edu)


Summary

Archie (1950) defined the term petrophysics to describe the study of the physics of rocks, particularly with respect to the fluids they contain. Although originally focused on geophysical exploration, petrophysics concepts are now used to interpret near surface geophysics measurements made to address environmental and engineering problems. This unit investigates relationships between these geophysical measurements and the physical and chemical properties of soils and sediments in the Earth's near subsurface. The specific focus is on the electrical properties of soils and how they are related to the ionic concentration of the pore fluids, the water content, porosity and grain size. Field results from a geophysical survey performed in Kearny Marsh, close to Harrier Meadow, are included to illustrate how electrical conductivity of a soil measured with an electromagnetic sensor is a good proxy for pore fluid ionic concentration, in this case related to contamination from a bordering landfill.

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Learning Goals

Students will be able to :

  • Describe the basic relationships between electrical resistivity (a geophysical property) and properties of the subsurface including pore fluid ionic concentration and grain size
  • Use an Excel-based tool to determine the relative importance of surface conduction in soils as a function of pore fluid ionic concentration and identify conditions when Archie's Law breaks down
  • Infer the distribution of a leachate plume in a wetland (Kearny Marsh) that discharges from an abutting landfill using electromagnetic (EM) conductivity mapping

Context for Use

This unit is designed as a guided learning project that is best used as an introductory geophysics lab. It is the second of a sequence of 5 units and is designed to take approximately 1.5 hours, with students doing independent exploration in Excel spreadsheet. The target audience is first year or sophomore earth and environmental science students with little prior experience with near surface geophysics. However, the materials could be adapted and expanded by faculty teaching an upper level undergraduate course in geophysics. The materials needed to use this unit are all provided as spreadsheets and embedded links. This unit could also be used as an independent (of the remaining units) module to teach electrical environmental geophysics concepts.

Description and Teaching Materials

Students will:

  • Learn about the basic electrical properties of soils, including Archie's Law and the additional influence of surface conduction
  • Use an Excel spreadsheet to perform calculations on some laboratory datasets to estimate electrical properties of soils, including the electrical formation factor and the surface conductivity
  • Investigate patterns of soil contamination in a wetland resulting from a plume that discharges from a nearby landfill using measurements of electrical properties obtained from electromagnetic (EM) conductivity mapping

Basic physical and chemical properties of soils are first introduced, along with the fundamentals of electrical current flow in the Earth. Students then investigate Archie's Law, which forms the foundation for relating the electrical conductivity of a soil measured with a geophysical sensor to the ionic concentration of pore-filling fluids. They learn how Archie's Law must be modified in the presence of electrical conduction occurring in an electrical double layer that forms on the surface of soil minerals. The unit uses datasets acquired on soils by the author and his graduate students, although the user can substitute other datasets if available.

The instructor should first have students read through the student handout to get familiar with the key equations that they will work with in this unit. Then students should view the narrated introductory slideshow summarizing the electrical properties of soils. Students should then return to the instructions in the student handout and work interactively with the Excel spreadsheet. Following completion of the Excel spreadsheet, students should view the narrated slides describing how measured electrical properties of soils were used to investigate suspected contaminant plumes from landfills entering Kearny freshwater marsh. Students will need to tackle the assessment sheet after completing all the exercises described on the handout.

Materials provided:

  1. Introduction to resistivity in soils and rocks (PowerPoint Slideshow (.ppsx) PRIVATE FILE 19.2MB Oct22 21): a slideshow tutorial on electrical properties of soils
  2. Student handout, Wetlands unit 2 (Microsoft Word 2007 (.docx) PRIVATE FILE 568kB Oct22 21): a student handout, which guides students through the unit
  3. Spreadsheet, Wetlands unit 2 (Excel 2007 (.xlsx) PRIVATE FILE 44kB Oct22 21): an Excel spreadsheet for performing calculations. Solutions are available for instructors. See the "Assessment" section of this page, below.
  4. Geophysical survey of Kearny Marsh (PowerPoint Slideshow (.ppsx) 16.5MB Jul12 20): a set of slides describing application of electrical properties measurements in Kearny freshwater marsh
  5. Student worksheet, Wetlands unit 2 (Microsoft Word 2007 (.docx) PRIVATE FILE 1.4MB Oct22 21): a worksheet for students to complete, which can be used for assessment. The answer key is available for instructors. See the "Assessment" section of this page, below.

Teaching Notes and Tips

The module is designed to be completed in approximately 1.5 hours. Note that the PPSX file can be opened as a regular Powerpoint file if Powerpoint is running.

Some of the mathematical content in this module might prove challenging to some students. In such cases, it may help to have students review this tutorial on constructing a best fit line before tackling the quantitative aspects of this unit. In addition, you might consider having students team up and work in pairs.

Assessment

The Student worksheet, Wetlands unit 2 (Microsoft Word 2007 (.docx) PRIVATE FILE 1.4MB Oct22 21) is designed to help evaluate whether students have grasped the main concepts related to the electrical properties of soil introduced in this unit. Note: This file is also linked above, in the Teaching Materials section of this page.

Solutions for instructors

References and Resources

Mansoor, N., Slater, L., Artigas, F. and Auken, E., 2006. High-resolution geophysical characterization of shallow-water wetlands. Geophysics, 71(4), pp.B101-B109. This is a link to the case-study publication on the electromagnetic survey performed in Kearny freshwater marsh and used in the last part of the module. It is a relatively straightforward paper that the instructor (and some more advanced students) could follow.

Mansoor, N. and Slater, L., 2007. Aquatic electrical resistivity imaging of shallow-water wetlands. Geophysics, 72(5), pp.F211-F221. This is a link to a follow up publication on Kearny freshwater marsh where resistivity imaging was used to further investigate contaminant plumes in the marsh