Initial Publication Date: April 20, 2021

Module Overview for Instructors

Evaluating the Health of an Urban Wetland Using Electrical Resistivity

This module introduces students to the fundamental principles and uses of electrical resistivity, with a focus on an environmental application. Students explore the characteristics and environmental setting of Harrier Meadow, a saltmarsh just outside of New York City.

Module Goals

  1. Students will explore vegetation patterns within a tidal wetland and evaluate a hypothesis that salinity patterns control the distribution of native plants
  2. Students will investigate the physical and chemical controls on electrical resistivity of soils
  3. Students will evaluate how the choice of geophysical data acquisition parameters (e.g. the electrode spacing) control the resulting geophysical images of the subsurface
  4. Students will invert electrical resistivity imaging (ERI) datasets and assess the influence of errors on the inversion results
  5. Students will explore the correlation between electrical conductivity patterns from electromagnetic surveying datasets and native plant distributions

Assessment: Assessment of student learning can be approached flexibly. Each unit of the module includes student handouts and exercises that can be used for formative assessments. In addition, each unit includes a set of multiple choice and short answer questions that can be given to students as homework, on quizzes, or on exams.


The module covers material sequentially, but the units can also often be taught as stand-alone lessons. It is also possible to use units 1, 2, and 5 without the more mathematically-intensive units 3 and 4. This approach significantly decreases the time it takes to complete the module while still highlighting the application of geophysical methods to an urban environmental question. For instructors who do not wish to use the module in its entirety, some additional options are outlined in Using IGUaNA Modules for Your Course.

Unit 1: Exploring Harrier Meadow, an Urban Wetland System

This exercise introduces the environmental setting for Harrier Meadow, a saltmarsh in the New Jersey Meadowlands. Students use Google Earth, high-resolution video acquired from an Unmanned Aerial Vehicle (UAV), and an ArcGIS Storymap to explore the association between salinity and Pickleweed, a salinity tolerant native plant that is returning to the marsh. At the end of this unit, students formulate plans to test a hypothesis for Pickleweed persistence/patterning in Harrier Meadow that foreshadows the remaining units of the module.

Unit 2: Geophysical Properties of the Subsurface

This unit investigates relationships between 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.

Unit 3: Field Geophysical Measurements

Students use Ohm's Law to investigate how electrical resistance measurements are related to the electrical conductivity of soils. Field implementation of both electrical imaging and electromagnetic techniques is demonstrated using surveys performed in Harrier Meadow as an example. Students investigate how variations in survey configuration parameters control investigation depth (how far into the ground the signals sense) and spatial resolution (what size objects can be detected).

Unit 4: The Magic of Geophysical Inversion

This unit introduces the student to the concept of geophysical inversion, which is the process of estimating the geophysical properties of the subsurface from the geophysical observations. The basic mechanics of the inversion process used to estimate spatial variations in electrical conductivity from electrical imaging (EI) datasets are introduced in a way that avoids the heavy mathematics. Students analyze a two-dimensional EI dataset acquired in Harrier Meadow using ResIPy, a graphical user interface developed for instructional use, exploring how variations in inversion settings related to the errors in the measurements result in distinctly different images.

Unit 5: Integrated Geophysical Interpretation and Comparison with Ground Truthing

In this unit, students explore spatial associations between the three-dimensional electromagnetic (EM) conductivity inversions and the visible patterns of Salicornia (Pickleweed) introduced in Unit 1. Students analyze how conductivity patterns vary with depth and explore for evidence for a relationship between electrical conductivity and Pickleweed patches based on the hypothesis introduced in Unit 1. Ultimately, students evaluate the extent to which the geophysical dataset and direct physical measurements support the hypothesis, introduced in Unit 1, accounting for the distribution of Pickleweed in Harrier Meadow.

Making the Module Work

Because this module relies heavily on the visual analysis of color imagery, it is important to keep in mind that approximately one in 12 males and one in 200 females are red-green colorblind. For these students, key elements of many of the images are indistinguishable. (You can use this color blindness simulator to see what the images look like to people with many different kinds of color vision deficiencies.) One way to accommodate students with colorblindness is to have students work in pairs or small groups to interpret the color images. Alternatively, you could provide annotated imagery to all students, with key features marked in ways that do not rely on color, so that all students have access to the same information.

To adapt all or part of the module on Evaluating the Health of an Urban Wetland Using Electrical Resistivity for your classroom you may also want to read through Using IGUaNA Modules for Your Course, which includes sample pathways through portions of IGUaNA modules.

Instructor Experiences