Scholars' Mine Masters Theses Student Theses and Dissertations Fall 2020 Determining variable depth to bedrock using ERT and MASW: A geophysical investigation in St. Louis, MO Justin James Clinton Follow this and additional works at: https://scholarsmine.mst.edu/masters_theses Part of the Geophysics and Seismology Commons Department: Recommended Citation Clinton, Justin James, "Determining variable depth to bedrock using ERT and MASW: A geophysical investigation in St. Louis, MO" (2020). Masters Theses. 7963. https://scholarsmine.mst.edu/masters_theses/7963 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. DETERMINING VARIABLE DEPTH TO BEDROCK USING ERT AND MASW: A GEOPHYSICAL INVESTIGATION IN ST. LOUIS, MO by JUSTIN JAMES CLINTON A THESIS Presented to the Faculty of the Graduate School of the MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE in GEOLOGICAL ENGINEERING 2020 Approved by: Dr. David Rodgers, Advisor Dr. Evgeniy Torgashov Dr. Kelly Liu © 2020 Justin James Clinton All Rights Reserved iii ABSTRACT The research involves a geophysical study in the Maremec River Valley in St. Louis County, Missouri. The geophysical methods described in this paper can provide a relatively quick, inexpensive, and accurate means in investigating the lithologic characteristics of the subsurface. Method selection is ultimately dependent of the type of investigation and site- specific characteristics. In this investigation, Electrical Resistivity Tomography (ERT) data and Mutli-Channel Analysis of Surface Waves (MASW) data were acquired to image the bedrock beneath the Maremec River to an approximate depth of 110 ft or an elevation of 300 ft for a proposed wastewater tunnel. Two overlapping dipole-dipole arrays were used in a survey across the Maremec River to create a 1,670 ft ERT traverse. The ERT data were then processed to create a 2-D pseudo-section. Multiple MASW surveys were acquired on both sides of the river along the ERT traverse. The MASW data were processed and six shear wave velocity profiles were created. Two borings were logged on both sides of the river near the ERT traverse using material collected from a split-spoon sampler. The data from these geophysical tools were then compared and correlated with the boring data. The depth to bedrock was interpreted to be from 30 ft - 80 ft. The processed MASW data correlated fairly well with the interpreted top-of-rock with 0 ft - 15 ft. Based on the resistivity data, the bedrock can be characterized as fairly weathered beneath ERT stations 600 to 1600. A potential karst feature is present after station 1400 which extends below 300 ft elevation. From ERT stations 100 to 600, the bedrock is considered competent and less weathered. iv ACKNOWLEDGEMENTS I would like to thank Dr. Neil Anderson, Dr. David Rodgers, and Dr. Evgeniy Torgashov for making this possible and for all their guidance. I would also like to thank Michael Kleeschulte for his assistance and knowledge. Thank you to my wife, Morgann, my family, and friends for the support and encouragement to complete this degree. I would most of all like to thank the Lord God for His blessings and for giving me strength and direction to accomplish my goals. v TABLE OF CONTENTS Page ABSTRACT................................................................................................................................ iii ACKNOWLEDGEMENTS......................................................................................................iv LIST OF ILLUSTRATIONS.................................................................................................... vii LIST OF TABLES........................................................................................................................x SECTION 1. INTRODUCTION.................................................................................................................1 2. PURPOSE AND SCOPE OF STUDY...............................................................................2 3. REGIONAL GEOLOGY.....................................................................................................3 4. OTHER INVESTIGATION METHODS.......................................................................... 8 5. RESEARCH METHODOLOGY......................................................................................10 5.1. ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT)......................................10 5.1.1. Geology and Resistivity...................................................................................... 11 5.1.2. Concept of Electrical Resistivity....................................................................... 12 5.1.3. Theory of Resistivity and the Subsurface.........................................................14 5.1.4. ERT Data Acquisition.........................................................................................23 5.1.5. The Dipole-Dipole Array................................................................................... 24 5.1.6. ERT Data Processing.......................................................................................... 26 5.1.7. Interpretati on........................................................................................................ 28 5.2. MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW)............... 29 5.2.1. Wave Types. 30 vi 5.2.2. Data Acquisition................................................................................................. 35 5.2.3. Field Configuration and Parameters................................................................. 35 5.2.4. MASW Data Processing.....................................................................................39 5.2.5. Interpretation........................................................................................................ 40 6. CASE STUDY.....................................................................................................................42 6.1. OVERVIEW.................................................................................................................42 6.2. STUDY SITE.............................................................................................................. 43 6.3. GEOPHYSICAL STUDY........................................................................................... 44 6.3.1. Borehole Data...................................................................................................... 44 6.3.2. ES, SSS, SBP Data.............................................................................................. 50 6.3.3. ERT Survey.......................................................................................................... 50 6.3.4. MASW Survey.....................................................................................................51 6.4. INTERPRETATION.................................................................................................. 51 6.4.1. ERT Interpretati on...............................................................................................51 6.4.2. MASW Interpretation..........................................................................................54 7. CONCLUSION AND RECOMMENDATIONS........................................................... 60 BIBLIOGRAPHY...................................................................................................................... 62 VITA 67 vii LIST OF ILLUSTRATIONS Page Figure 3.1. Geologic provinces of Missouri............................................................................ 5 Figure 3.2. Generalized geologic map of Missouri..................................................................5 Figure 3.3. The stratigraphic column of Missouri................................................................... 6 Figure 3.4. Stratigraphic column of the Mississippian System..............................................7 Figure 5.1. Example pseudo-section....................................................................................... 11 Figure 5.2. Cylinder circuit and Ohm’s Law ........................................................................13 Figure 5.3. Current flow lines and equipotential surfaces....................................................19 Figure 5.4. Illustration of uniform equipotential surface with current flow lines in all directions (a) and 2-D cross-sections of the hemispherical surfaces at the source (b) and sink (c) electrodes..........................................................................20 Figure 5.5. Illustration of the change of equal potentials along a current flow line from source/sink .............................................................................................................. 21 Figure 5.6. Illustrates current flow lines across interfaces with different resistivity (Ph P2) ....................................................................................................................... 22 Figure 5.7. The SuperSting R8 unit (right) with 12 volt battery (left) and switchbox (middle).................................................................................................................... 24 Figure 5.8. Different types of array configurations..............................................................