Unit 3: Field Geophysical Measurements

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


Summary

Near surface geophysical measurements are performed by moving sensors across the Earth's surface. Active geophysical sensors transmit a signal into the Earth and record a returned signal that contains information on the physical and chemical properties of the Earth (see Unit 2). This unit introduces the student to the basics of geophysical data acquisition using two techniques that record variations in the electrical conductivity (see Unit 2) of the Earth: [1] electrical imaging (EI), and [2] electromagnetic (EM) conductivity mapping.

Electrical imaging is a galvanic geophysical approach whereby electrical contact with the Earth is made directly via electrodes (typically metal stakes) that are inserted into the ground. Electromagnetic conductivity mapping is a non-contact approach whereby the physics of EM induction is used to sense changes in electrical conductivity. The advantages and disadvantages of using galvanic (EI) and non-contact (EM) techniques for measuring electrical conductivity are described. Ohm's Law is introduced and students investigate how electrical resistance measurements are related to the electrical conductivity of soils. Field implementation of both EI and EM techniques is demonstrated using surveys performed in Harrier Meadow as an example. Students investigate how variations in survey configuration parameters (e.g. electrode configuration and electrode spacing in EI, frequency and coil spacing in EM) control investigation depth (how far into the ground the signals sense) and spatial resolution (what size objects can be detected). The concept of pre-modeling a geophysical survey (i.e. running some simulations of likely effectiveness of the methods before going to the field) to evaluate expected investigation depth and sensitivity is introduced. The Excel-based Scenario Evaluator for Electrical Resistivity (SEER) tool provided by the United States Geological Survey (USGS) is used to demonstrate some key concepts.

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Learning Goals

Students will be able to:

  • Apply Ohm's Law in a spreadsheet to determine how it relates electrical resistance measurements to the electrical conductivity of a soil specimen or the subsurface Earth
  • Describe how the choice of electrode array geometry influences the measured transfer resistance recorded in an electrical imaging (EI) survey
  • Explain how the choice of electrical imaging (EI) survey parameters determines depth of investigation and spatial resolution following application of an Excel-based modeling tool
  • Describe concepts that define the performance of electromagnetic (EM) conductivity mapping instruments, including factors controlling investigation depth.
  • Explain the advantages and disadvantages of galvanic (electrical imaging) and non-contact (electromagnetic) geophysics for measuring electrical conductivity variations in the subsurface

Context for Use

This unit is designed as a guided learning project that is best used as an introductory geophysics lab. It is the third of a sequence of five units and is designed to take approximately 1.5-2.0 hours, with students doing independent exploration in Excel spreadsheets. 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 investigate how variations in survey configuration parameters (e.g. electrode configuration and electrode spacing in EI, frequency and coil spacing in EM) control investigation depth (how far into the ground the signals sense) and spatial resolution (what size objects can be detected). The concept of pre-modeling a geophysical survey (i.e. running some simulations of likely effectiveness of the methods before going to the field) to evaluate expected investigation depth and sensitivity is introduced. The Excel-based Scenario Evaluator for Electrical Resistivity (SEER) tool provided by the United States Geological Survey (USGS) is used to demonstrate some key concepts. Note that SEER runs on PCs but not Macs.

Materials provided:

  1. Introduction to electrical imaging (EI) and electromagnetic (EM) conductivity mapping (PowerPoint Slideshow (.ppsx) PRIVATE FILE 71.8MB Oct22 21): a narrated slideshow providing an introduction to electrical imaging (EI) and electromagnetic (EM) conductivity mapping concepts, along with an overview of how these techniques are implemented in the field (with reference to the survey performed in Harrier Meadow)
  2. Resistivity survey setup video (MP4 Video 403.6MB Jun9 20): a video created by students from the University of South Florida describing the set up of their electrical imaging instrument
  3. Student handout, geophysical mapping, Wetlands unit 3 (Microsoft Word 2007 (.docx) PRIVATE FILE 84kB Oct22 21): a handout that leads the student through three primary activities making up the unit
  4. Spreadsheet, Wetlands unit 3 (Excel PRIVATE FILE 2.6MB Oct22 21): an Excel spreadsheet that students use to perform some calculations of basic signals and survey parameters relating to electrical imaging and electromagnetic measurements
  5. Scenario Evaluator for Electrical Resistivity (SEER), a USGS-developed Excel-based tool for simulating electrical imaging datasets that might be recorded over some common near surface Earth structures (used to explore resolution and sensitivity concepts)
  6. SEER handout, Wetlands unit 3 (Microsoft Word 2007 (.docx) PRIVATE FILE 174kB Oct22 21): a handout that describes a guided exercise to be completed with SEER
  7. Student worksheet, Wetlands unit 3 (Microsoft Word 2007 (.docx) PRIVATE FILE 96kB Oct22 21): a student worksheet where students perform or report calculations and observations related to the activities above. This worksheet can be used for assessment.

Answer keys are available for instructors. See the "Assessment" section of this page, below.

Teaching Notes and Tips

Students should first watch the narrated slideshow that provides an introduction to the electrical imaging and EM conductivity mapping techniques. Note that the PPSX file can be opened as a regular Powerpoint file if Powerpoint is running. Then they should watch the short video by University of South Florida students on setting up an electrical imaging instrument. They are then positioned to tackle Tasks 3.2.1 and 3.2.2 of the student worksheet.

Students can then turn to the task of working on the four problems in the spreadsheet. These problems focus on basic calculations to assess electrical resistivity and EM measurements. They will need to refer back to the slides in the tutorial for the equations needed to solve these problems. Students may need some help in connecting the problem with the correct equations to solve. If they struggle with solving the equations, they may benefit from reviewing this tutorial on rearranging equations to solve for a variable. After they have completed the four problems in the Excel spreadsheet, they are positioned to tackle Task 3.2.3 in the worksheet.

Finally, students turn to the extensive exercise on investigating resolution and sensitivity issues for electrical imaging surveys using the USGS Excel spreadsheet SEER. This is a PC-based macro-enabled worksheet. It will not run on a Mac as the macro-functionality of Excel on a Mac is not sufficient. It may be possible to run SEER on a Mac via a Windows emulator (not tested).

The students should first watch the four minute video that introduces SEER and is available at the USGS website for SEER. After watching the video, students should work through the two main exercises/scenarios described in the SEER handout (Microsoft Word 2007 (.docx) PRIVATE FILE 174kB Oct22 21), using the SEER Excel tool. Note that this will require the students to download the SEER spreadsheet and supporting files that are contained in the sub-directory "Response Arrays". This sub directory must sit in the directory containing the SEER spreadsheet. The Response Array directory is large (586 MB) so the instructor might want to think of other ways to make the file available to students. Once the students have worked through the activities described in the SEER handout they can tackle Task 3.2.4 in the student worksheet. Depending on the class time available and student proficiency, the instructor might want to split the class into two groups, with each group tackling just one of the two scenarios. This will reduce the class time and also provide an opportunity for students to explain results of their investigations to their peers.

Assessment

The Student worksheet, Wetlands unit 3 (Microsoft Word 2007 (.docx) PRIVATE FILE 96kB Oct22 21) is designed to help evaluate whether students have grasped the main concepts related to the resistivity and electromagnetic conductivity mapping methods introduced in this assignment. Note: This file is also linked above, in the Teaching Materials section of this page.

Solutions for instructors

References and Resources

Software:

SEER, an Excel-based Scenario Evaluator for Electrical Resistivity (for PCs only)

References:

Terry, Neil, Day-Lewis, F.D., Robinson, J.L., Slater, L.D., Halford, Keith, Binley, Andrew, Lane, J.W., and Werkema, Dale, 2017, Scenario Evaluator for Electrical Resistivity Survey Pre-modeling Tool: Groundwater, http://dx.doi.org/10.1111/gwat.12522

Terry, Neil, Day-Lewis, F.D., Robinson, J.L., Slater, L.D., Halford, K., Binley, A., Lane, J.W. Jr., and Werkema, D., 2017, The Scenario Evaluator for Electrical Resistivity (SEER) Survey Design Tool v1.0: U.S. Geological Survey Provisional Software Release, 01 May 2017, https://doi.org/10.5066/F7028PQ1