B08_Attach_22_MEVE_218998_FIO_Final_Utility_Locate_Report_2023_11_10.pdf

November 10th, 2023

Yeh and Associates, Inc.

2000 Clay Street, Suite 200 Denver, CO 80211

Attn: Sam Sherwood Ofc: 303-781-9590 Email: ssherwood@yeh-eng.com

Re: Geophysical Investigation Report No. 2 of Utility Mapping

Ground Penetrating Radar (GPR) Chapin Mesa Historic Buildings and Houses Mesa Verde National Park, Colorado Olson Job No. 7355A

Ms. Sherwood, We are pleased to submit the report regarding the results of our subsurface utility mapping investigation that was performed at the site of the Chapin Mesa Historic Buildings and Houses in Mesa Verde National Park, Colorado. The purpose of this investigation was to locate and identify subsurface utilities using Ground Penetrating Radar (GPR) testing. The fieldwork was performed by Ms. Callia Cortese, Project Geologist, and Mr. Julian Vazquez, E.I.T. Staff Engineer, on the week of August 7th-11th, 2023, and October 17th-18th, 2023 with on-site assistance from Ms.

Sherwood.

EXECUTIVE SUMMARY

Investigation Overview. Olson Engineering served as a subconsultant to Yeh and Associates in this project for their work in regards to restoration of the 8 historic staff housing buildings and the Park’s Museum in Mesa Verde National Park. The investigation was focused on identifying each of the four subsurface utilities (gas, sewer, electric, water) surrounding each of the 8 historic employee housing buildings and the northern wall of the park museum, for aid in the habilitation of the staff buildings and construction of a swale along the north museum wall.

GPR Survey Results. GPR was conducted surrounding the 9 building areas to identify utilities and assist in the rehabilitation of the historic buildings. GPS points were collected, marking visible surface utility clues such as manholes, meters, gas tanks, etc. The GPR data was analyzed in the office to produce a subsurface utility map of gas, electric, water, and sewer lines around each building for the Park Service that can be used in the future. Maps of the utilities are included herein and PDF files can be provided if requested.

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PROJECT BACKGROUND AND INVESTIGATION OVERVIEW

The area of interest is shown below in Figure 1, which are the Historic Houses (Buildings 1-8) and the northern wall of the Park’s Museum, where gas, electric, sewer, and water utilities were identified and located as reported herein. The surveys were conducted both along the paved road that connects the buildings and on the lots around the buildings. Extreme precautions were taken to protect the endangered vegetation around the park, which were marked with a rare plant flag and through indications as to such vegetation locations by the park rangers.

To evaluate subsurface utilities of interest, Olson Engineering performed GPR surrounding the historic buildings. Olson Engineering personnel inspected the area for utility locations by performing a grid of GPR scans around those locations. All data was geolocated using GPS units on the test systems. GPS was also used to locate individual surface utility clues such as manholes, gas tanks, and electric vaults to help determine where each subsurface utility line was located around the 9 buildings.

Figure 1. Map view of the locations for the historic buildings and houses surveyed with GPR in Mesa Verde National Park.

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GROUND PENETRATRING RADAR (GPR) METHOD AND SURVEY OVERVIEW

Ground penetrating radar (GPR) is a high frequency electromagnetic geophysical method that can quickly detect both metallic and non-metallic targets in the subsurface. GPR performance is highly site dependent, working well on lower conductivity sandy soils and performing poorly on higher conductivity clayey soils which retain a lot of moisture. GPR works by generating a high frequency electromagnetic wave in a transmitter antenna (Tx) which propagates away from the antenna and into the subsurface. As the electromagnetic wave interacts with materials with different electrical permittivity then some energy is transmitted, and some is reflected from the interface between the different permittivities. The reflected energy that arrives back at the GPR unit is received by the receiving antenna (Rx). The data that is recorded by the GPR unit is amplitude versus two-way travel time (TWTT). A compact target such as a utility line will generate a hyperbolic response as the GPR passes across and over it. Each line of data is saved as a B-Scan, which is amplitude versus TWTT along the line profile. Several parallel lines can be processed into a three-dimensional (3D) volume. Time or depth slices can be extracted from this 3D volume to create a map view of the subsurface. GPR operates in the frequency range of approximately 10 MHz to 3 GHz. Lower frequencies can penetrate deeper, but also have lower spatial resolution.

For the type of subsurface utility survey requested here a GPR antenna in the frequency range of 400 MHz to 800 MHz is most commonly used.

For this project we utilized an ImpulseRadar PinPointR which is a real-time-sampling (RTS) GPR system that operates at two frequencies simultaneously. The ImpulseRadar PinPointR operates at both 400 and 800 MHz and is shown in Figure 2 below. The 400 MHz antenna can detect targets to a depth of approximately 15 ft in ideal soil conditions, while the 800 MHz antenna can resolve smaller and shallower targets down to about 8 ft in ideal soil conditions. Both antennas are controlled with the same ruggedized tablet interface and software.

The GPR data was collected around the 8 historic buildings and the northern wall of the museum. In areas of suspected dense utility locations, a tighter GPR scan grid was performed. A GPS system was mounted on the PinPointR to track the location of the scans.

Assumptions and Limitations

Collection of GPR data was limited by terrain, such as densely vegetated areas, steep/rocky terrain, the presence of rare plant species, or stairs. GPR data could not be collected where the Emlid GPS system could not communicate with satellites due to the view of the sky being blocked

- this occurs where the GPS unit on the PinPointR is covered by dense tree canopy or overhanging roofs. This GPS blockage was common around Building #3, and therefore increases the uncertainty of the water line in this area. Although surface utility clues such as electric transformers and manholes allowed for higher accuracy of subsurface utility identification, any surface utilities that are grouped in one area or any subsurface utility lines that did not stay relatively straight required some amount of assumption and increases uncertainty. Lines with higher amounts of uncertainty are dashed on the utility maps. It is important to note that GPR is not able to distinguish different types of utilities from each other, and rather shows a hyperbolic response when the wave energy reflects from a highly conductive material. Also note that a secondary gas line for Buildings #2 and #8 was not able to be identified due to the lack of surface utility clues.

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Figure 2. Picture of the ImpulseRadar PinPointR with GPS attached that was used on-site.

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GROUND PENETRATING RADAR (GPR) INVESTIGATION RESULTS

The GPR surveys were performed around the nine buildings. The scans were conducted in a gridded pattern as features are detected by crossing them perpendicularly with the PinPointR GPR unit. The GPR testing identified features up to 4-5 feet deep due to the soil’s properties. In Figure 3, an example of the GPR scan is shown where utilities were found and the effective depth of the data from the survey is about 3.5 to 5 ft.

Figure 3. Screenshot of a GPR scan after processing where the y-axis is depth in feet and the x-axis is linear distance in feet. Utilities are identified with a pink dot and the effective depth of the GPR scan (~3.5 ft) is marked in red (likely bedrock) where a strong reflection is detected, and reflective noise is prominent following this feature.

After mapping the utilities in the field with GPR scans, identified utilities were plotted together at our office using Geolitix software along with a satellite image of the Mesa Verde National Park. These identified utilities, in hand with the surface utility points, were used to create the maps of nine buildings with located gas, sewer, water, and electric utilities. These utility maps are presented below in Figures 4-9. Table I presents the approximate depth to utilities per building as requested by the National Park Service. The approximate depths were calculated using utility GPR points crossed by a specific utility path, for each utility line respectively. Note that the GPR scan lines were selected in the field in consultation with Ms. Sherwood to cross the likely subsurface utility line locations based on visible above ground utility features. The lines were then sketched in to the utility maps in Figures 4-9 based on the GPS locations of identified subsurface features in the GPR scans that corresponded to apparent utilities.

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Figure 4. Map of Buildings 8 (right) and 7 (left), showing subsurface utilities.

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Figure 5. Map of Buildings 6 (left) and 5 (right), showing subsurface utilities.

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Figure 6. Map of Buildings 4 (left) and 3 (right), showing subsurface utilities.

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Figure 7. Map of Building 2, showing subsurface utilities.

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Figure 8. Map of the Sewer Pump station (lower left) and Building 1 (right), showing subsurface utilities.

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Figure 9. Map of the GPR performed along the northern wall of the Museum Building, showing subsurface utilities, to aid in the construction of a swale.

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Table I. Approximate range and average depth to utilities for each of the eight staff housing buildings and the museum.

Building Utility Approx. Depth Range (ft) Approx. Depth (ft)

Building #1 Gas 0.8 - 1.63 1.2

Electric 0.76 – 2.18 1.4

Sewer 1.44 - 2.31 1.8

Water 0.8 – 2.54 1.5

Building #2 Gas 0.94 - 1.39 1.1

Electric 0.94 - 2.13 1.4

Sewer 0.66 - 2.49 1.5

Water 0.57 - 2.12 1.1

Building #3 Gas 1.00 - 2.56 1.5

Electric 0.65 – 2.47 1.3

Sewer 1.35 - 2.04 1.7

Water 0.91 – 2.29 1.6

Building #4 Gas 1.16 - 2.04 1.6

Electric 0.57 - 2.29 1.7

Sewer 0.66 - 1.61 1.3

Water 0.53 – 1.69 1.1

Building #5 Gas 1.09 – 2.21 1.8

Electric 0.91 – 2.21 1.6

Sewer 1.00 - 2.90 1.9

Water 0.65 - 1.86 1.1

Building #6 Gas 0.74 – 2.04 1.4

Electric 1.43 - 2.81 2.1

Sewer 1.17 - 2.42 1.9

Water 0.65 - 1.86 1.1

Building #7 Gas 1.09 - 2.04 1.6

Electric 1.18 - 2.21 1.5

Sewer 1.07 - 3.07 1.7

Water 0.85 - 2.54 1.5

Building #8 Gas 2.22 – 3.33 2.8

Electric 0.74 - 2.56 1.4

Sewer 1.07 - 3.07 1.7

Water 0.85 - 2.54 1.5

Museum Gas NA NA

Electric 1.44 – 1.58 1.5

Sewer NA NA

Water 0.90 - 3.46 1.9

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CLOSURE

If you have any questions regarding the field procedures, analysis, or the interpretive results presented herein, or develop additional information that would impact the findings of this report, please do not hesitate to contact us. Due to the limitations of GPR surveys, if precise depths and destructive confirmation of utility locations is desired, we recommend that utility location test holes be performed (typically with soil vacuum methods). We appreciate working with Yeh & Associates and the support you provided to us for this project.

Respectfully submitted, Olson Engineering, Inc.

Callia J. Cortese Project Geologist

Larry D. Olson, PE 11-10-2023 Chief Engineer

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