Karst Study Using Geophysics at Bracken Bat Cave Preserve
Evelynn Mitchell, St. Marys University
Location: Texas
Abstract
South Central Texas depends on deep seated aquifers to maintain a water supply for over 5 million people. Much of this water supply is recharged through karst features in the Texas Hill Country. Understanding the features on a property helps determine the appropriate level of development, but geophysical methods have limitations on interpreting feature size. Students in this project built on previous work to examine the error of two common geophysical methods when detecting humanly accessible shallow karst features. They gained skills in site analysis using spatial software and high-resolution GPS collection, field work planning, data collection and analysis.
Student Goals
- Collect data from public sources and establish field research protocols
- Execute protocols and accurately make and record measurements and observations
- Make evidence-based arguments using findings and draw appropriate conclusions
Research Goals
- Compare geophysical methods to ground truthing methods and quantify accuracy of detecting cave passages.
- Determine the position and elevation of the minor cave passages on the Bracken Preserve in comparison to the main cave.
Context
The design of this CURE was to build research experience into a 16-week geophysics course, incorporating literature searching, site planning, and data collection, analysis and report writing skills. The class had 6 students and they were instructed to divide the work-load for the final report simulating the real-world consulting experience. The geophysics class is an upper-level elective in our program and requires at least introductory geology and college algebra to enroll. Most students also have prior experience with scientific writing, statistics and presentation skills. The class is typically Environmental Science majors and Engineering majors with Environmental Science minors.
Target Audience: Sophomores, Juniors and Seniors
CURE Duration:A full term
CURE Design
Understanding the terrain that recharges the water supply for 5 million people is important for making development decisions. Assessment of a karst landscape often relies on geophysical detection of features. However, feature size is often misrepresented by the data produced from some methods of geophysical detection. Students utilized two geophysical methods (electrical resistivity and ground penetrating radar) were to examine a known shallow karst feature and see the discrepancies first-hand. the data was compared to measurements of the feature to provide additional context. This work also contributes to a larger research project that is working to quantify the discrepancies for these two methods.
The stake holders were Bat Conservation International, the property owners of Bracken Bat Cave. The student's work allowed the organization to gain more information about another feature on their property to help manage future property development.
Core Competencies: Defining the available knowledge base about a study area, creating a field sampling protocol, collecting appropriate data for the study, data analysis and interpretation, scientific communication through a technical report.
Nature of Research: Applied Research
Tasks that Align Student and Research Goals
Student Goals ↓
Define and summarize available knowledge base about geophysics finding cave passages.
Gain experience with geophysics equipment and software.
Create a field sampling protocol for geophysics equipment.
Define and summarize available knowledge base about the study area.
Create a field sampling protocol for GPS and elevation data.
Collect appropriate data with GPR and electrical resistivity systems.
Collect appropriate data with Trimble GPS unit.
Plot survey locations on GIS Map for spatial understanding.
Download data and process it to remove noise and see results.
Produce a technical report for Bat Conservation International.
Prepare data for analysis by incorporating it into a GIS data set and producing a map.
Review data for trends and draw conclusions about spatial relationships.
Produce a technical report for Bat Conservation International.
Instructional Materials
Week 2 – Class exercise on stratigraphy and mapping.
Week 2 - Literature search: Determine what research exists (Expect to find Mitchell & Mitchell 2017 paper) on the topic and what geologic information exists for the property.
Week 3 – Lecture on GPS and exercise on using the Trimble unit on campus and transferring the data to QGIS.
Week 3 – Lecture on GPR and class practice with 2D target detection.
Week 4 – Student practice with 3D GPR gird set up, data collection.
Week 5 – Learning to process 2D and 3D GPR data.
Week 5 – Discussion of literature search results and develop GPR field plan for site.
Week 5 – 3D data collection at field site with GPR.
Week 6 – Process data from GPR survey and write up results.
Week 7 – Electrical Resistivity Theory and Data processing with Case Studies.
Week 7 – Practice for field setup with AGI R1.
Week 8 – Plan field survey with R1.
Week 8 – Collect resistivity data.
Week 9 – Process resistivity data.
Week 9 – Download and plot GPS data.
Week 9 – Verify assignments for different sections of the report.
Week 10 – Work with students on sections and collect first draft of report.
Week 11 - Review and add comments for the second draft.
Week 12 – 13- Continue to work with students to refine report sections.
Week 14 - Collection final edited report.
Assessment
Individual assignments were assessed to show that the students were gaining the skills needed to complete the project. Each skill that was introduced had an activity in which the students were required to show the skill. For this project 10 different assignments were created to ensure that students had gained the skills to perform the field surveys.
The final paper had several steps to ensure a complete report. Each student was responsible for a section of the report and had a deadline to turn their section in. The next step was for students to combine the sections into one document in Google Drive and then turn in a draft for comment. A style manual was provided as a guide and a rubric was shared through Canvas which allowed the students to see where the points would be assessed. The first draft was not graded but provided comments to allow the students to see what additional information and editing was needed to complete the report. The final submitted draft required only minor editing prior to sending the finished product to the community partner.
Instructional Staffing
I am the only geophysicist on our campus and the only instructor able to teach this course. I have been performing geophysics research with undergraduate students individually for many years as a faculty member. To facilitate a project of this type, faculty need to have experience with geophysics field work and working with undergraduates in a field setting. A working knowledge of GPS data collection and GIS software is also required.
Author Experience
Evelynn Mitchell, St. Marys University
St. Mary's University has been exploring how to add undergraduate research experiences to our classrooms for many years. I was introduced to the CURE structure in the Spring of 2022 and felt that this might be a natural extension of my personal research into my geophysics classroom. In 2016 I began work in trying to quantify the error experienced in geophysical methods when identifying karst features. Scientists who work in karst regions often discuss the "halo effect" experienced from electrical resistivity explorations, which often show void passages to be much larger than their actual measurements. The ground truthing aspect of the measurements is then performed using internal Lidar scans of the feature. However, in order to quantify the amount of error, data sets from multiple tests in different conditions need to be gathered. Because I try to provide data collection exercises in ground penetrating radar and electrical resistivity within my class, I decided to try adopting the CURE method of teaching into my Geophysics course.
Advice for Implementation
Field work provides additional challenges for classroom instruction, including site preparation. I was able to work with our community partner to have access to the property on multiple dates so that I could determine what brush cutting and land preparation would need to happen prior to the data taking days. Much of this advanced preparation I did before bringing the students to the property. Although they were determining the parameters for the data collection (spacing, size of array, etc.) I worked with the land manager to prepare spaces that would encompass the possible parameters. Due to issues that we had with one of the data taking events, I would advise that instructors stick to one geophysical method for a 16-week course and incorporate site preparation and GPS data taking into the first site visit. This will allow the students to find out if they have not planned well and be certain that they have a fall-back day if they need to retake the data. Also, incorporating a revision process into the end of the course for the paper allowed me to use the remaining classroom time to cover additional geophysical methods, so that students had exposure to all of the different techniques geophysicists use in exploring the earth.
Iteration
This course was taught for the first time in the fall of 2022. The geophysics course is typically taught on a bi-annual schedule and the next iteration will take place in 2024. In the next iteration, I intend to focus on one technology for the research aspect so that students will have more opportunity to do in person site planning and opportunity to try multiple parameters on the equipment.
Using CURE Data
The report that was produced from the 1st iteration of the project was provided to the Community Partner, Bat Conservation International. The data that was provided has helped them learn more about known karst features on the property, and where others near the first feature may exist. This will help them in making property management decisions.
Resources
Papers for faculty:
Anchuela ÓP, Casas-Sainz AM, Soriano MA, Pocoví-Juan A. 2009. Mapping subsurface karst features with GPR: results and limitations. Environ Geol. 58(2):391–399. [accessed 2022 Oct 18]. doi:10.1007/s00254-008-1603-7.
Ebraheem MO, Ibrahim HA. 2019. Detection of karst features using ground-penetrating radar: a case study from the western limestone plateau, Assiut, Egypt. Environ Earth Sci. [accessed 2022 Nov 18]. 78(18):563. doi:10.1007/s12665-019-8572-x.
Mitchell EJ, Mitchell, Joseph N. 2017. Comparison of Shallow Geophysical Cave Detection Methods to 3D Lidar Mapping. Proceedings of the 17th International Congress on Speleology. 2:122–4.
Neyamadpour A, Wan Abdullah WAT, Taib S, Neyamadpour B. 2010. Comparison of Wenner and dipole–dipole arrays in the study of an underground three-dimensional cavity. Journal of Geophysics and Engineering. 7(1):30–40 [accessed 2022 Nov 8]. doi:10.1088/1742-2132/7/1/003.
Veni G. 2014. Geophysics & Bat Guano. Probing the depths of Bracken Cave. Bat Conservation International. [accessed 2022 Sep 9]. 32(2):14–16.
Resources for Students:
EAA, Edwards Aquifer Authority. 2022. Edwards Aquifer Authority › About the Edwards Aquifer - Edwards Aquifer Authority. About the Edwards Aquifer. [accessed 2022 Nov 9]. https://www.edwardsaquifer.org/science-maps/about-the-edwards-aquifer/.Street S. 2003. SIR 3000 Manual. [accessed 2022 Oct 20]. https://www.geophysical.com/wp-content/uploads/2017/10/GSSI-SIR-3000-Manual.pdf.
TNM Download v2. appsnationalmapgov. https://apps.nationalmap.gov/downloader/#/.
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