Learning to Use Electrical Resistivity Soundings for Interpretation of (Sediment) Stratigraphy Remke L. Van Dam - rvd@msu.edu Michigan State University, Department of Geological Sciences Summary of the activity: Combination of modeling, lab- and field measurements to develop an understanding of electrical resistivity soundings as a tool in geology and hydrology An exercise for GLG471

Audience and Other Context Enrollment: 6-12 2006 4 Undergraduate students (3 seniors, 1 junior) 2 PhD students (hydrogeology) 2007 6 Undergraduate students (4 seniors, 2 juniors) 2 MSc students (1 hydrogeology; 1 sed/strat)   Exercise is part of an Applied Geophysics course (GLG 471) First exposure to geophysics for most students in class. Required / developed skills in the activity: Current flow in layered media. Good work ethic & collaboration skills. Integrate different sources of data/information. Reporting and presentation skills. Class grade buildup: 6 homeworks 30% Exam(s) 25-40% 3-4 Projects 30-45% Total project worth: 15-25% of total grade Lab reports (30%) Presentations (10%) Final report (60%)

Learning goals Understand what influences bulk resistivity (for soils) (soil texture, water saturation, temperature, soil salinity) Learn to use basic software for forward modeling and data inversion Plan a resistivity sounding (based on available info from well logs, lab data and forward modeling exercise) Interpret a field site in terms of stratigraphy and hydrology (need to combine data from a several (inconclusive) sources) Produce short but useful progress presentations and a final report

Intro & Planning Execution Data analysis Activities and Tasks Sequence of potentially stand-alone projects and tasks (1 task / week). Students work in groups of two or three. Projects are integrated into one activity to have students participate in all steps towards successful application of a geophysical method: Intro & Planning Execution Data analysis Step (week) 1: Intro to forward modeling and inversion of sounding data. Step (week) 2: Laboratory estimation of Archie’s parameters. Step (week) 3: Design of field sounding experiment. Step (week) 4: Field data collection. Step (week) 5: Data inversion and analysis. Step (week) 6: Writing a Report. Weekly: Progress presentations

MSU Sandhill Site (non-critical for exercise) DW38 37 20 10’ 20’ 30’ 40’ 50’ DW9 DW8 Long-term ERT monitoring transect DW38 DW37 DW20 DW14 4: CSCG Used for hydrology, forestry research, and ERT. Glacial sediments, primarily clay with sand and gravel. Bedrock (sandstone/shale) of Saginaw Fm at ~18m. 3: SCS Q: what are potential issues that with this site & exercise A1: Significant lateral variation A2: Is borehole stratigraphy accurate? 2: CB

Week 1: Fwd modeling, interpretation, inversion 2-layer: glacial drift over bedrock Learning goals: Current flow in layered media Effects of θ, η, and T on ρa Discuss limitations of resistivity Get familiar with software (e.g., RES1D / DCINV / RESIST) 3-L: glacial drift over clay over bedrock 3-L: drift over saprolite over bedrock

Week 1: Fwd modeling, interpretation, inversion Apparent resistivity (Ωm) AB/2 (m) Q: How many layers minimum? A: 2 / 3 / 4 ?? Q: One location, different readings. How? Why? A1: Operational and equipment errors A2: changes in field conditions

Week 1: Fwd modeling, interpretation, inversion “blind” three-layer inversion “assisted” three-layer model RMS=8.39% RMS=9.96% Layer 2: thickness = 40m ρ > 2000 Ωm Layer 2: thickness = 5m ρ > 13,000 Ωm Ludington stratigraphy: Local fine dune sand 0-10m Coarse glacial outwash 30-50m Proglacial clay Q: Does an inversion produce the best solution? Q: Can we say how many layers are present? Q: Name different factors that influence data quality and inversion success.

Week 2: Lab-Estimation of Archie’s Parameters Learning goals: Understand relations between resistivity and texture, water saturation, temperature, and salinity. Resistivity instrument operation. Proper laboratory procedures. Each group measures relation between resistivity and other variables (θ, η, T, water salinity) for 2 samples. Need to measure geometry of test box to find resistivity in Ωm. Measurements will produce plot like to the right, which allows to obtain Archie’s parameters. 0.02 0.1 0.3

Week 3: Survey Design Important: Students will build stratigraphic models for two well logs, add resistivity values and calculate the expected sounding curves. Based on the modeling results including uncertainly, students will design a field survey with maximum resolution & minimum redundancy. Before heading out to the field have ready: # of measurements a-spacing values (or a and b) Two-tape layout: ABMN positions for each measurement Important: Need to combine knowledge gained in weeks 1 and 2 for this week’s task. Measurement # a-spacing (m) AB/2 (m) A&B pos (m) M&N pos (m) ρa (Ωm)

Week 4: Data Collection Learning goals: We will use an AGI supersting R8/IP instrument for our sounding surveys. This task will take place in groups of four students, each conducting two sounding experiments. Learning goals: Equipment handling Field procedures Safety issues Qualitative data interpretation Plotting and qualitative interpretation:

Week 5: Data inversion and interpretation Learning goals: Understand what factors influence inversion success. Discuss which of these factors you can control. We will take different approaches and discuss the pro’s and con’s: Use the expected resistivity responses (Week 3) as a starting point and invert the data. Use your qualitative interpretation (Week 4) as a starting point and invert the data. Let the software pick the number of layers and their resistivity values.

Week 6+: Writing a Report Report written in groups of 2 students.