Attachment_24_TM-85-832000-2020-15_PhaseII_SRT-ERT_LeadvilleWTP_12-28-2023.pdf

86-683200 2.2.4.22

United States Department of the Interior

BUREAU OF RECLAMATION

PO Box 25007 Denver, CO 80225-0007

12 October 2020

VIA ELECTRONIC MAIL

MEMORANDUM

To: Project Manager, Denver Region

Attn: 86-68190 (Nance)

From: Jared Vauk, Supervisor Engineering Geology & Geophysics Group

Subject: Transmittal of Technical Memorandum 85-832000-2020-15 – Geophysical Surveys: Phase II 2D Seismic S-wave Refraction and Electrical Resistivity Tomography, Water Treatment Plant Replacement – Leadville Mine Drainage Tunnel Project, Leadville Colorado.

Attached is a copy of the subject Technical Memorandum (TM). The TM contains results from surface-based seismic and electrical resistivity tomography field surveying and interpretation results. These geophysical survey results provide subsurface information immediately adjacent to proposed excavation cuts and retaining wall alignments, including 1) estimated depths to bedrock, 2) seismic s-wave velocity distributions, and 3) the electrical resistivity distributions related to geologic material types within the proposed excavation limits.

Please feel free to contact us if you have any questions about the subject TM or the results presented therein, jrittgers@usbr.gov telephone: 303-445-3010.

Attachment

Electronic copy recipients: (w att to each) DSDaMS@usbr.gov DamSafetyCentralFiles@usbr.gov mnance@usbr.gov, Civil Engineer (85-831900) Reclamation dhanneman@usbr.gov, Geotechnical Engineer (85-831500) Reclamation mgobla@usbr.gov, Geotechnical Engineer, (85-831300) Reclamation mhenrick@usbr.gov, Geotechnical Engineer, (85-831300) Reclamation kguerra@usbr.gov, Civil Engineer, (85-819000) Reclamation

JARED

VAUK

Digitally signed by

JARED VAUK

Date: 2020.10.14 10:30:26 -06'00' rlung@usbr.gov, Engineering Geologist, (85-832000) Reclamation jkang@usbr.gov, Civil Engineer, (85-855000) Reclamation rmarkiewicz@usbr.gov, Geophysicist, (85-832000) Reclamation dliechty@usbr.gov, Geophysicist, (85-832000) Reclamation mfield@usbr.gov, Geophysical intern, (85-832000) Reclamation jvauk@usbr.gov, Group Mgr., (85-832000) Reclamation

October 2020

Technical Memorandum 85-832000-2020-15

Geophysical Surveys: Phase II 2D Seismic S-wave Refraction and Electrical Resistivity Tomography, Water Treatment Plant Replacement, Leadville, CO

Leadville Mine Drainage Tunnel Project Upper Colorado Region

U.S. Department of the Interior

Mission Statements The Department of the Interior (DOI) conserves and manages the Nation’s natural resources and cultural heritage for the benefit and enjoyment of the American people, provides scientific and other information about natural resources and natural hazards to address societal challenges and create opportunities for the American people, and honors the Nation’s trust responsibilities or special commitments to American Indians, Alaska Natives, and affiliated island communities to help them prosper.

The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public.

Cover Photo – Example 2D seismic s-wave and p-wave tomography velocity models.

U.S. Department of the Interior October 2020

Geophysical Surveys: Phase II 2D Seismic S-wave Refraction and Electrical Resistivity Tomography, Water Treatment Plant Replacement, Leadville, CO Leadville Mine Drainage Tunnel Project Upper Colorado Region

Prepared by:

Bureau of Reclamation Technical Service Center Denver, Colorado

Acronyms and Abbreviations

1D One-Dimensional 2D Two Dimensional

ERT Electrical Resistivity Tomography

GPS Global Positioning System

P-wave Seismic Compressional-Wave

Reclamation US Bureau of Reclamation

SL Survey Lines SRT Seismic Refraction Tomography S-Wave Seismic Shear-Wave

Vp Seismic P-Wave Velocity Vs Seismic S-Wave Velocity

WTP Water Treatment Plant

XYZ Cartesian Coordinate

Technical Memorandum No. 85-832000-2020-15

Technical Approval The results, findings, and recommendations provided in this technical report of findings are consistent with the source document(s), technically sound, and consistent with current Reclamation practice.

Prepared by Justin B Rittgers, Ph.D., Geophysicist

Date

Peer Review Certification

This document has been reviewed and is believed to be in accordance with the scope of the service agreement and the standards of the profession.

Peer reviewed by Richard D. Markiewicz, Geophysicist

Date

JUSTIN

RITTGERS

Digitally signed by JUSTIN

RITTGERS

Date: 2020.10.08 10:15:11 -06'00'

RICHARD

MARKIEWICZ

Digitally signed by RICHARD MARKIEWICZ Date: 2020.10.08 11:10:29 -06'00' i

Contents Page

I. Summary

II. Background

III. Data Acquisition

A. 2D Seismic S-wave Refraction Tomography

B. 2D Electrical Resistivity Tomography

IV. Data Processing and Interpretation

A. S-wave Seismic Refraction Tomography

B. Vs30 Calculations

C. Electrical Resistivity Tomography

V. Results A. S-wave Seismic Refraction Tomography

B. Vs30 Calculations

C. 2D Electrical Resistivity Tomography

VI. References

Tables

Table Page

Table 1.— Latitude/Longitude (Y/X) coordinates in units of Colorado State Plane Central Zone 0502 US Survey Feet of start and end of each geophysical survey line/transect* Table 2.— Vs30 values (ft/s) calculated using tomography results of SL4 through

SL6

Figures

Figure Page

Figure 1.— Survey location map showing the WTP (red box) relative to the City of Leadville, CO. The inset image shows a detailed view of the WTP property ii and the geophysical survey locations for Phase II surveying (red lines and GPS waypoints) Figure 2.— Survey line location map showing the Phase I survey line locations to the north (green lines) and the Phase II survey line locations to the south relative to aerial imagery and a semi-transparent image overlay of the facility layout and topography, and the newly designed retaining wall alignments (blue polygons) as provided by project engineers. Here, Phase II SRT surveys are indicated with straight red lines, ERT surveys indicated with straight blue lines, and GPS waypoint labels indicate the start and end points for each survey line Figure 7.— SL4 s-wave velocity tomogram (top) and raypath density plot (bottom). Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the top plot Figure 8.— SL4 s-wave velocity tomogram (left) and Phase I SL1 p-wave tomogram (right). Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the left plot. SL1 p-wave tomogram is included with the same spatial scales and accurately positioned relative to SL4 s-wave tomogram for comparison to related Phase I results and interpretations Figure 9.— SL5 s-wave velocity tomogram (top) and raypath density plot (bottom). Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the top plot Figure 10.— SL6 s-wave velocity tomogram (top) and raypath density plot (bottom). Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the top plot Figure 11.— SL4: Average Vs versus depth from ground surface and down 30m.

Figure 12.— SL4: Average Vs versus depth from interpreted top of bedrock and down 30m Figure 13.— SL5: Average Vs versus depth from ground surface and down 30m.

Figure 14.— SL5: Average Vs versus depth from interpreted top of bedrock and down 30m Figure 15.— SL6: Average Vs versus depth from ground surface and down 30m.

Figure 16.— SL6: Average Vs versus depth from interpreted top of bedrock and down 30m Figure 17.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT2 Figure 18.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT3 Figure 19.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT4 iii

Appendices

Appendix Seismic Refraction Technique Electrical Resistivity Tomography Technique

Technical Memorandum No. 85-832000-2020-15

Geophysical Surveys: Phase II 2D Seismic S-wave Refraction and Electrical Resistivity Tomography, Water Treatment Plant Replacement, Leadville, CO

I. Summary

US Bureau of Reclamation (Reclamation) personnel performed three two-dimensional (2D) shear-wave (s-wave) seismic refraction tomography (SRT) profiling surveys and three 2D electrical resistivity tomography (ERT) surveys at the current Leadville Mine Drainage Tunnel Water Treatment Plant (WTP), located approximately 1.5 miles north-northeast of downtown Leadville, Colorado.

These geophysical data were collected in support of a feasibility study focusing on evaluation of geologic conditions of the underlying bedrock and overburden materials related to the slope stability of proposed excavation cut slopes.

Specifically, engineers are evaluating the mechanical properties of the overburden materials relevant to slope stability analyses for the cut slopes that will be created during excavation and construction of the proposed replacement WTP.

The immediate objectives of these various surveys included the following;

1) Assess the seismic s-wave velocity (Vs) of overburden materials and bedrock along the proposed excavation cut slopes.

2) Infer the depth to bedrock across the study site.

3) Infer the presence, depth, thickness, and lateral continuity of a suspected clay layer that was encountered during construction of the existing WTP located immediately adjacent to the proposed excavation footprint (if present within the area surveyed).

TM-85-832000-2020-15

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study

II. Background The existing WTP and facilities constructed several years ago are located approximately 1.5 miles to the north-northeast of downtown Leadville, Colorado (see Figure 1). The planned replacement WTP will help to modernize the facilities and increase capacity and treatment rates and will expand the facility’s footprint to the east of the existing WTP building (see Figure 2). The requested and actual geophysical survey lines (SL) were placed within and immediately adjacent to the newly proposed excavation footprint and cut-slopes for the WTP replacement project (see Figure 2). Phase I surveys consisted of 2D compression-wave (p-wave) seismic tomography and ERT surveys, but the proposed location of the cut slope was subsequently changed. The Phase II geophysical surveys were performed to help inform subsurface conditions in support of the current feasibility level study at the newly proposed cut slope location. Data were collected July 21st – 22nd, 2020.

Figure 1.— Survey location map showing the WTP (red box) relative to the City of Leadville, CO. The inset image shows a detailed view of the WTP property and the geophysical survey locations for Phase II surveying (red lines and GPS waypoints).

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement

Feasibility Study

Figure 2.— Survey line location map showing the Phase I survey line locations to the north (green lines) and the Phase II survey line locations to the south relative to aerial imagery and a semi-transparent image overlay of the facility layout and topography, and the newly designed retaining wall alignments (blue polygons) as provided by project engineers. Here, Phase II SRT surveys are indicated with straight red lines, ERT surveys indicated with straight blue lines, and GPS waypoint labels indicate the start and end points for each survey line.

III. Data Acquisition For this Phase II field exploration effort, two geophysical survey types were performed: 1) 2D s-wave SRT, and 2) 2D ERT. SRT and ERT surveys were performed along each of the three southern survey transects indicated in Figure 2.

Survey line numbering continues from Phase I naming convention, so Phase II survey lines consist of SL4 through SL6, and ERT2 through ERT4. Here, SL4 and ERT2 are colinear, SL5 and ERT3 are colinear, and SL6 and ERT4 are colinear. S-wave tomography was chosen for Phase II field explorations due to the potential for higher spatial resolutions compared to p-wave tomography.

Additionally, Vs values may be more useful than Vp value in slope stability analyses.

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study

Final survey line locations, orientations, and lengths were chosen in the field based on 1) actual site conditions and logistical limitations given the techniques being deployed, 2) in order to appropriately tradeoff between maximizing resolution and maximizing depths of investigation, and 3) in order to maximize data production rates. The final survey line locations were adjusted in the field in order to best avoid public roads and any potentially problematic infrastructure (e.g., metal conduits or fences) that can cause severe data quality issues if ERT surveys are run parallel and near to these types of features. All surveys were conducted by USBR geophysicist Justin B. Rittgers and supporting crew. All data analysis, modeling, and interpretations were performed by USBR geophysicist Justin B. Rittgers.

The latitude and longitude coordinates of start/end points of each survey line were recorded with a hand-held global positioning system (GPS) unit providing latitude and longitude positions with an approximate +/- 10ft accuracy (see Table 1).

Elevations were also recorded by the handheld unit but are based on a barometer reading making them too inaccurate to use in final reporting. Elevation variations along the tomography lines were recorded with a stadia rod and hand-level for accurate relative elevations along these lines.

Table 1.— Y/X coordinates of start and end of each geophysical survey line/transect in units of Colorado State Plane Central Zone 0502 US Survey Feet NAD83*

Transect ID Start LAT/LON (ft) End LAT/LON (ft)

SL4 1525307.075, 2777122.892 1525515.934, 2777021.086

SL5 1525468.943, 2777063.710 1525576.732, 2777276.688

SL6 1525396.505, 2777058.553 1525501.769, 2777268.678

ERT2 1525245.985, 2777153.221 1525515.934, 2777021.086

ERT3 1525433.850, 2776994.329 1525613.612, 2777350.048

ERT4 1525378.961, 2777023.579 1525557.971, 2777382.123

*These coordinates are only +/- ~10ft accuracy

A. 2D Seismic S-wave Refraction Tomography Figure 2 depicts the locations of the three s-wave SRT survey lines (red-solid lines) collected for this project. All refraction surveys were conducted with a 16-pound sledgehammer source with shot points generally positioned at every third geophone along each line. The SRT surveys utilized horizontally polarized s-wave geophone sensors and a similarly polarized s-wave shot plate similar to that shown on the left side of Figure 3 (example photo, not from Leadville WTP project site).

Feasibility Study

Figure 3.— Example photos showing typical horizontally-polarized seismic s-wave shot plate (left), and vertically-polarized seismic p-wave shot plate (right), used with sledge-hammer or weight-drop impactive seismic sources.

B. 2D Electrical Resistivity Tomography Figure 2 also depicts the location of the three ERT survey lines (ERT2 through ERT4) performed at the project site (see Figure 4 for example photo of a 2D ERT survey setup- photo not from Leadville WTP project site). All three ERT surveys were conducted using a Guideline Geo ABEM LS-2 resistivity survey system that was configured to collect resistivity data using either 61 electrodes (ERT2) or 81 electrodes (ERT3 and ERT4), with an electrode spacing of nominally 5ft (total survey line lengths of ~300ft and ~400ft, respectively).1

A “Dipole-Dipole” survey data collection routine (dipole geometries for a given data point/measurement) was used in order to optimize data collection rates while maintaining adequate lateral and vertical resolutions and sensitivities to spatial variations in bulk electrical conductivity. The planned excavation cut slopes and the corresponding required depth of investigation are relatively shallow across the

1 Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Government.

Feasibility Study entire site (nominally 30-40 feet depth of investigation was required), and so the chosen survey configuration was deemed appropriate for the project objectives.

Figure 4.— Example photo of a linear ERT survey line.

IV. Data Processing and Interpretation Standard pre-processing and modeling were conducted for all resistivity and seismic datasets. Below is a brief description of the general steps performed for each data type. Because of the lack of precision in absolute elevation values recorded in the field with a hand-held GPS, accurate relative variations in elevation were recorded in the field with a stadia-rod and hand-level along each survey line.

Feasibility Study

A. S-wave Seismic Refraction Tomography Figure 5 shows an example 48-channel s-wave refraction shot gather record, after applying appropriate frequency-domain filtering and time/offset-dependent amplitude gains. Each vertical “wiggle-trace” is a geophone sensor recording of amplitude (horizontal axis) versus time (vertical axis). In Figure 5, the source location is indicated with the red star between channels 36 and 37. Both positive and negative polarity records (corresponding to hammer impacts on opposite sides of the shear wave source-plate) are plotted for each channel in green and red, respectively. The stacked signals used in final data analysis are plotted in black. Collecting both positive and negative polarities at each shot point allows the data processor to stack opposite polarity traces in order to enhance the signal-to-noise ratio of coherent s-wave arrivals (indicated with red lines in Figure

5) and diminish faster p-wave energy (indicated with blue lines in Figure 5) arriving prior to s-wave energy.

After picking the first arrivals of the refracted wave, this s-wave shot along with all the other shot gathers on this survey line are arranged and used as the data input for inversion (only the first arrival picks for each unique source-receiver pair are used for inverse modeling, and not the entire seismic trace). A multiple iteration inversion process is implemented to create a seismic tomogram. Seismic tomograms provide an image of the seismic velocity distribution within the subsurface below a survey line (see Figure 7 through Figure 10). These refraction tomography figures also provide a raypath density coverage map that displays each raypath “Fresnel zone” associated with each source-receiver pair (bottom plots on each figure). The density of the raypath coverage reveals the relative distributions of sensitivity for a given survey line/tomogram (e.g., areas of high confidence in recovered seismic velocities - See Appendix A for more details).

It is worth noting here, that while the raypath density coverage plots visually suggest the presence of “pockets” of little-to-no coverage along the tops of the tomograms (immediately below ground surface), this is not the case, and is simply the result of elevated raypath counts along primary or pronounced refractors (fast layers at depth). There is actually very good constraint on the near-surface velocities due to the geometry of receivers and density of shot points implemented for this project.

B. Vs30 Calculations In addition to providing 2D s-wave velocity distributions beneath each survey transect, Vs30 values have been calculated using the SRT results. Specifically, Vs30 values have been calculated using the average Vs versus depth calculated from 2D s-wave velocity tomography models. In each case, Vs30 values were

Feasibility Study calculated for depth intervals extending both from the average ground surface elevation and down 30 meters (~100ft), and from the interpreted top of bedrock and down and additional 30 meters (~100ft) for each 2D tomogram. Depth to bedrock is variable across the project site but was assumed to be the following at each relevant transect location: ~25ft BGS near the center of SL4, and ~40ft below ground surface beneath both SL5 and SL6. Vs30 values calculated for each seismic transect are presented in Table 2.

For the sake of calculating Vs30 from tomography results, s-wave velocities obtained for each relevant seismic line were first averaged using a simple geometric average as a function of depth (see line-plots in Figure 11 through Figure 16). These depth-averaged s-wave velocities, along with the vertical separation between recovered tomography parameters were then used in the following equation:

𝑉𝑉𝑉𝑉30 =

� ℎ𝑖𝑖

𝑉𝑉𝑖𝑖

𝑛𝑛 𝑖𝑖=0

Here, the number 30 represents the total ~30m depth interval across which the Vs30 calculation is made, ℎ𝑖𝑖 is the thickness of the ith layer/interval, 𝑉𝑉𝑖𝑖 is the modeled s-wave velocity of the ith layer/interval, and 𝑛𝑛 is the total number of layers across the total ~30m depth interval. This equation can be viewed as 30m divided by the sum of all individually calculated seismic s-wave transmission times for all individual depth intervals (denominator) within the upper-most 30m of bedrock.

Feasibility Study

Figure 5.— Figure shows an example shot record for s-wave refraction data. Each trace is plotted as a variable area “wiggle.” For this shot record, the seismic source was located between channels 36 and 37. Here, two opposite polarity shots (+ is green/- is red) are recorded at each shot point and reverse-stacked (stacked data plotted in black) to help amplify the shear-component of refracted energy (indicated with red lines) while minimizing (stacking out) contamination of faster [converted] p-waves (indicated with blue lines) that are generally characterized by having the same polarity regardless of the source impact direction.

C. Electrical Resistivity Tomography ERT surveys generally consist of a series of measurements that are each recorded using two current injection electrodes (Electrodes “A” and “B” form a source dipole) and two measurement electrodes (Electrodes M and N form a receiver dipole). Each unique source/receiver dipole pair is referred to as a quadrapole measurement, which constitutes an ERT data point/measurement. For each measurement/quadrapole:

• the cartesian coordinate of each electrode (XYZ) is recorded

• the injected electrical current injected between A and B electrodes is recorded

• the measured voltage between M and N electrodes is recorded

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study

These data values allow for the calculation of an “apparent resistivity” value for each measurement. This apparent resistivity value, along with geometry of each measurement is the final input data for the iterative inverse ERT modelling process. The inversion process seeks to minimize the difference between the recorded and recovered data while obtaining some “meaningful” or “geologically reasonable” model (e.g., smooth model with reasonable model parameter values expected for a given site’s geology/conditions). An example taken from ERT3 is presented in Figure 6.

Figure 6.— Example of ERT data (apparent resistivity values) plotted as a pseudo-section (top), the associated forward-modeled apparent resistivity data (center), and the associated recovered Earth resistivity model of “true” resistivity values (bottom).

V. Results In order to aid the reader in understanding tomography results, cardinal directions are annotated at the start/end of each tomogram on all figures presented in this report. These cardinal directions can be used in concurrence with Figure 2 (line location map) to help the reader orient cross-sections. Finally, start/end coordinates for each survey line are provided in Table 1.

Feasibility Study

A. S-wave Seismic Refraction Tomography The quality of SRT data collected along each survey transect is good to excellent, and a high level of confidence is placed on the ERT results presented here. The main reasons for this qualitative assessment include the following:

• Overall data quality associated with refraction surveys (e.g. first arrivals of energy relative to background noise levels) was good to excellent

• Relatively low RMS errors (e.g., data misfits) for each inversion (all <3ms)

• Overall corroboration between recovered s-wave tomogram velocity distributions/patterns (e.g., low-velocity and high-velocity zones recovered in the same regions of the subsurface)

• Overall corroboration between field observations, corroboration with other collocated data types/models, and the trends seen in recovered s-wave velocity models

Figure 7 through Figure 10 present the results of seismic refraction tomography surveys conducted for this project. These figures can be thought of as a 2D vertical view of the subsurface beneath each seismic survey line, where cool colors (e.g., blues and greens) represent low-velocity materials, while warm colors (e.g. yellows and reds) represent faster velocity materials such as more compacted overburden and bedrock. S-wave velocity tomograms are all presented with the same color-scale of 500-3,100ft/s in order to aid comparisons and interpretations. On each tomogram, interpreted top of weathered bedrock is indicated with a dashed-black line, and the interpreted top of competent bedrock is indicated with a solid black line.

These interpretations are strictly based on the results of seismic tomography modeling, and no ground-truthing data are available for validation of interpreted top of weathered or competent bedrock. Here, the vertical gradient in seismic velocity distributions and the raypath density plots were mainly used to help guide the interpreted interfaces. However, there is the possibility that the true locations of these geologic interfaces vary from the provided interpretations.

Feasibility Study

Figure 7.— SL4 s-wave velocity tomogram (top) and raypath density plot (bottom).

Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the top plot.

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study

Figure 8.— SL4 s-wave velocity tomogram (left) and Phase I SL1 p-wave tomogram (right). Interpreted top of weathered bedrock is indicated with the dashed black line and the interpreted top of competent bedrock is indicated with the solid black line on the left plot.

SL1 p-wave tomogram is included with the same spatial scales and accurately positioned relative to SL4 s-wave tomogram for comparison to related Phase I results and interpretations.

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Figure 9.— SL5 s-wave velocity tomogram (top) and raypath density plot (bottom).

Interpreted top of weathered bedrock is indicated with the dashed black line and

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Figure 10.— SL6 s-wave velocity tomogram (top) and raypath density plot (bottom).

Interpreted top of weathered bedrock is indicated with the dashed black line and

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study

B. Vs30 Calculations Vs30 values were calculated from 2D s-wave SRT tomography results obtained for each of the three survey transects. Where applicable, Vs30 values were calculated for depth intervals corresponding to the ground surface and down 30m (~100ft), and also from the estimated/interpreted top of bedrock and down 30m (e.g., the upper-most 30m of bedrock). Figure 11 through Figure 16 and Table 2 present the resulting Vs30 values calculated for each of the three s-wave SRT surveys:

Table 2.— Vs30 values (ft/s) calculated using tomography results of SL4 through SL6

Transect ID Vs30 (ft/s) from Ground

Surface Vs30 (ft/s) from Interpreted Top of Bedrock

SL4 1235 1733

SL5 1306 2215

SL6 1263 2208

Averages 1268 2052

Figure 11.— SL4: Average Vs versus depth from ground surface and down 30m.

Figure 12.— SL4: Average Vs versus depth from interpreted top of bedrock and down 30m.

Figure 13.— SL5: Average Vs versus depth from ground surface and down 30m.

Figure 14.— SL5: Average Vs versus depth from interpreted top of bedrock and down 30m.

Figure 15.— SL6: Average Vs versus depth from ground surface and down 30m.

Figure 16.— SL6: Average Vs versus depth from interpreted top of bedrock and down 30m.

Feasibility Study

C. 2D Electrical Resistivity Tomography The quality of ERT data collected along each survey transect good to excellent, and a high level of confidence is placed on the ERT results presented here. The main reasons for this qualitative assessment include the following:

• Low contact resistances were achieved for all electrodes, allowing for high current injection levels and excellent electrical coupling with the ground

• Low data collection repeatability errors of nominally 1% or less for most data points: all measurements were recorded using a minimum of 2 waveform cycles, and two repeats (so at least 4 repeated measurements of voltage per current injection), allowing for statistical assessment of data quality and averaging of recorded signals

• Consistency in recovered resistivity values between the three surveys, and recovered values that are consistent with the expected geologic conditions on site

The final ERT models presented here were generated by applying data weighting based on RMS errors of repeated measurements, and by removing suspect outlier data points from the inversion workflow (i.e., to avoid noisy data negatively influencing the results). The results of the ERT surveys are presented in Figure 17 through Figure 19. In these figures, the final 2D Earth resistivity model obtained via inverse modeling is plotted using a logarithmic color-scale. The same color-scale and limits are used for each figure, for ease of comparison.

Generally, the resistivity values are fairly high throughout each tomogram, and are consistent with dry to partially saturated sands, gravels, and weathered igneous and metamorphic rock-types. These higher resistivity values indicate that little to no clay is present within the limits of the survey. The transition to slightly lower resistivity values with depth could indicate the presence of increased saturation at depth. This transition to lower resistivity values can be observed in each ERT tomogram at nominally 20ft below ground surface. Subsurface explorations such as test-pitting or drilling would be required to verify these interpretations.

Figure 17.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT2.

Figure 18.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT3.

Figure 19.— Final 2D Earth resistivity model obtained via inverse modeling for data collected along survey transect ERT4.

Feasibility Study

VI. References

[1] Redpath, B., 1973, Seismic Refraction Exploration for Engineering Site Investigations, AD-768 710, Explosive Excavation Research Laboratory, Livermore, California.

[2] Hopkins, R., 1998, Casitas Dam Borrow Area Seismic Refraction Survey, TM No. D8330-98- 019, Reclamation Technical Service Center, Denver, Colorado.

Appendix A

Seismic Refraction Technique

Feasibility Study – Appendix A

A-1

Refraction Concept

The seismic refraction method is based on contrasts in seismic velocity between two layers (Snell's Law of Refraction). When a compressional or shear wave meets a contact between materials exhibiting different velocities, the wave is reflected off the contact or refracted along the contact boundary. In general, the denser or more structurally rigid a rock body, the faster a body wave will travel through it.

The s-wave refraction method is essentially identical to its compressional wave counterpart, but offers higher resolution and sensitivity to air/water filled voids and/or fractured/jointed zones within the subsurface. This particular survey used a 20 pound sledge hammer impact on a metal shear-wave plate (weight-coupled “shoe” with a 45-degree plate mounted on either side). S-waves are generated at the ground surface, travel down to a contact between the overlying slower velocity rock (or combination of sediment and alluvial materials) and the underlying higher velocity rock. These s-waves are then refracted at this interface, travel along the contact at the faster underlying material’s velocity (head waves), and simultaneously travel back to the surface where they are recorded by geophones. Figure A-1 illustrates the seismic refraction in terms of ray paths and the corresponding first arrival.

Feasibility Study – Appendix A

A-2

Figure A-1.— Raypath Seismic Refraction Concept & travel time plot. [1]2

Figure A-2 illustrates the seismic refraction method in terms of wave fronts at different points in time. As time progresses, the wave front propagates spherically in all directions away from the source. When the wave front contacts the boundary between different velocities of rock, some of the energy is reflected back to the surface, and some of the energy continues downward into to the higher velocity rock. The wave front accelerates in the higher velocity rock passing ahead of the initial and reflected wave in the slower velocity rock above it.

As the wave continues through the high velocity rock it creates a “wake” at the boundary between the upper and lower rock layers. This “wake”, seen in dark crimson in Figure A-2 is the refracted wave front. It propagates upward through the low velocity rock to the surface where geophones sense its arrival ahead of the surface waves and reflected waves.

2 Bracketed numbers indicate a numbered reference at the end of this report section.

Feasibility Study – Appendix A

A-3

Figure A-2.— Wave front Seismic Refraction Concept.

Ray Path Geometry

The rays shown above demonstrate wave direction more clearly than a wavefront itself. A ray is perpendicular to a wavefront and points in the direction of propagation of a given compression or shear wave. Refracted rays are bent at the contact boundary between two rock layers and travel along the top of the underlying layer, moving at the higher velocity of the underlying layer. The angle at which a ray is refracted along the contact is given by Snell’s Law:

sin iV 1 = sin rV 2 where i and r are incident and refracted ray angles, and V1 and V2 are their layer velocities respectively. When angle r is equal to 90 degrees, the bent, or refracted ray, will travel along the contact interface at the velocity of the denser underlying layer and is called the critically refracted ray.

As it travels along the contact, secondary waves are continually created in the overlying rock layer at the critical angle:

ic = sin −1

(V 1 / V 2)

At a certain distance away from the source, known as the critical distance, this secondary wave front, created by the critically refracted energy, reaches the surface geophones faster than direct arrivals along the ground surface. This can be

A-4

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study – Appendix A seen as a change in slope of the velocity curve seen in the first arrival plot in Figure A-1.

Rays transmitted into the underlying faster layer will continue downward until they intersect with another acoustic impedance contact and will again be reflected and refracted. For this reason, there are limits to the optimal depth of investigation of a refraction survey. In general, the depth of exploration is one fourth to one half the length of the seismic line depending on the ratio of velocity contrast between two rock layers. For example, a velocity contrast of 1.8:8.0 (top and bottom layers respectively) allows for steeply dipping rays (small incident angles from perpendicular, approx. 13 degrees) to be refracted over short distances of seismic spread. However, a velocity contrast of 1:1.5 (top and bottom layers respectively) requires much shallower dipping rays (large incident angles from perpendicular) around 42 degrees for rays to be critically refracted. This shallow angle results in the need for much longer seismic survey lines, and increases the difficulty and cost of a given survey. [2]

Appendix B

Electrical Resistivity Tomography Technique

– Appendix B

B-1

ERT Technique

An ERT measurement is executed by directly passing electrical current through the subsurface with two electrodes (A and B) and measuring the resulting voltage on the surface with two other electrodes (M and N) located between the current electrodes. In a Wenner array (one electrode geometry configuration of many ERT survey types), all electrodes are spaced equidistantly, as seen in Figure B-1. Spacing between electrodes can range from inches to hundreds of feet apart. Depth of investigation and resolution of models are directly related to the electrode spacing.

Electrodes themselves are 16 inch long stainless steel rods, about a ½ inch in diameter. These rods are inserted into the ground 6 to 12 inches deep.

The direct current resistivity concept is based on changes in voltage observed at the ground surface caused by varying electrical current flow at different locations in the subsurface. These changes in current flow are caused by the presence of relatively higher or lower resistivity anomalies surrounded by a fairly uniform half-space, or subsurface.

The current passes from the source, A electrode, into the subsurface finding the path of least resistance. Flowing through lower resistivity anomalies and flowing around higher resistivity anomalies, it then passes back up to the surface sink created by the B electrode (see Figure B-1).

The M and N electrodes measure the voltage at the ground surface. By using the equation V=IR, the instrument uses the electric current applied and the voltage measured to identify the resulting resistance. Resistance is automatically converted into an apparent resistivity value computed by the instrument based on the geometry of the given measurement (by convention, half-space geometry is applied for surface measurements, and whole spaces are used for down hole surveys).

ρa=2пaV/I or ρa=2пaR ρa is the apparent resistivity, п is 3.14, a is the distance between each electrode, V is voltage, I is current, and R is resistance. This equation is specific to a Wenner Array survey where A-M = M-N = N-B. Finally, the calculated apparent resistivity values, along with the associated electrode geometry for each value, is used as the input into inverse modelling software. The inverse ERT modelling process generally involves an iterative process that seeks an earth conductivity model that describes/explains the data collected in the field, and tries to minimize the difference between that recorded field data and the forward-calculated data (using finite difference and/or finite element modelling of related physics to predict expected data) while recovering a reasonable

Geophysical Surveys: Phase II, Leadville Mine Tunnel WTP Replacement Feasibility Study – Appendix B

B-2 conductivity model (e.g., smooth model with reasonable resistivity values expected for a given site/material). Figure B-2 provides a general overview of the inverse modelling process.

Figure B-1.— ERT Electrical Current Flow and charge accumulation schematic.

– Appendix B

B-3

Figure B-2— Overview of inverse modeling process.

VI. References