town of hicksville earth resistivity report

Report of Earth Resistivity Survey Village of Hicksville Wellfield Survey Hicksville, Defiance County, Ohio

September 10, 2019

Prepared for:

Prepared by:

Xenon Geosciences, Inc. 52 W Broadway Street Danville, Indiana 46122

XGI Report 19‐05.R1

Report of Earth Resistivity Survey of 5 Village of Hicksville Wellfield Survey Hicksville, Defiance County, Ohio


Contents 1. Introduction ......................................................................................................................................... 3

2. Survey Location ................................................................................................................................... 3

3. Site Conditions ..................................................................................................................................... 3

4. Data Acquisition ................................................................................................................................... 3

5. Data processing .................................................................................................................................... 3

6. Results .................................................................................................................................................. 4

7. Interpretation ....................................................................................................................................... 5

8. Recommendations ............................................................................................................................... 5

9. Limitations ........................................................................................................................................... 5

Attachments

Figure 1: Site Location Map

Figure 2: Resistivity Profile Locations

Figure 3: Resistivity Profiles

Figure 4: Comparison of Soil Boring Logs and Earth Resistivity

Figure 5: Interpreted Resistivity Profiles



1. Introduction

Xenon Geosciences, Inc. (“Xenon”) performed an earth resistivity survey in an area being evaluated as a future wellfield site. The purpose of this work was to evaluate aquifer extents and thicknesses with the setback area shown in Figure 1. This work was performed in accordance with Xenon proposal number P190319 and approved by Jones & Henry Engineers, Limited (“JHE”)..

2. Survey Location

Figure 2 shows the locations of the three resistivity profiles (“Lines”) performed in the survey area. The profile locations were modified from those shown in the proposal to assure maximum depth of coverage and to provide coverage throughout the setback area. Line 2 was shortened on the west side due to the presence of a bean field.

3. Site Conditions

Field work was performed on July 24 and 25, 2019. The survey was free of obstacles and the area had been cleared of vegetation at the time of the survey. There was less than 0.10 inches of precipitation in the three days leading up to the field work. A drilling crew was performing test borings at the same time as the geophysical survey. JHE provided the spatial coordinates and draft boring logs for the test borings.

4. Data Acquisition

Earth resistivity data were collected with a Sting/Swift system rented from Exploration Instruments in Austin, Texas. For Lines 1 and 3, 112 electrodes were deployed at 20-foot spacings, resulting in 2,220-foot arrays. Line 2 was collected using 56 electrodes at 20-foot spacing resulting in an 1,100-foot array. As shown in Figure 2, the resistivity arrays extended beyond the 300-foot setback area so that the maximum depth of investigation could be obtained within the setback area as possible (see Figure 3).

Data were collected using a dipole-dipole array and a command file generated by Xenon Geosciences using SuperSting Administrator software by AGI. Electrodes were deployed along vinyl tapes that extended across the survey area (Figure 2). GPS1 coordinates were recorded at numerous locations along each profile.

5. Data processing

The ER data were loaded into EarthImager software from AGI. EarthImager was used to perform inversion modeling and to build a resistivity model for each profile. The software iteratively calculates an apparent resistivity psuedosection to closely match the measured resistivity dataset, and measures the quality of fit by least squares error calculations. The psuedosection can be considered a geophysical model of subsurface resistivity distributions that would produce the resistivity values that were measured in the field.

Data were reviewed and edited as necessary to remove spurious measurements. Inversion parameters were as shown in Table 1. On Lines 1 and 3, the data recorded on electrode #95 were found to be particularly noisy and were removed prior to inversion. The three data sets were inverted in EarthImager using the parameters shown in Table 2.

1 Global positioning system using Wide Area Augmentation System.



Table 1: Data editing parameters.Minimum voltage 0.2 millivolts

Minimum absolute value of measurement 0.005 Ohms

Maximum repeat error 3 %

Minimum apparent resistivity 1 Ohm‐meter

Maximum apparent resistivity 10,000 Ohm‐meter

Maximum reciprocal measurement error 5 %

Remove negative resistivities Yes

Table 2: Inversion modeling parameters.Minimum resistivity of starting model 1 Ohm‐meter

Maximum resistivity of starting model 100,000 Ohm‐meter

Maximum number of iterations (stop criteria) 8

Maximum root mean square error (stop criteria) 3 %

Maximum error reduction (stop criteria) 5 %

Smoothness factor 10

Damping factor 10

Estimated Noise 3 %

Remove negative resistivities Yes

The two resulting inverted files were gridded in Surfer using kriging. The grids were generated using ~10-foot grid increments with the median gradient value recorded for each grid cell. Color relief maps were generated for the two individual grid files, resulting in the three psuedosections shown in Figure 3.

6. Results

The results of the ER surveys are presented in Figure 4. The images are shown in color scale according to the electrical resistivity values determined by inversion. Distance along the profile is shown on the horizontal axis and depth on the vertical axis.

Although the measurements collected for lower numbered electrodes were of very good quality, the quality of the resistivity data was poor in Lines 1 and 3 for the segments containing high-numbered electrodes. Most all of the data collected in Line 1 was of high quality because the electrode locations of poor quality (electrodes 67-112 / station 1340–2240) were mostly outside the setback area. Unfortunately, the eastern portion of Line 3 (electrodes 57 -2240 / station 1280–2240) contains poor quality within the setback area and the psuedosection above station 1140 is less reliably indicative of subsurface conditions.

Xenon compared the earth resistivity data with the soil boring logs provided by JHE (Figure 4). Because drilling was being performed during the resistivity survey, the profile lines did not pass directly through the boring locations so as to avoid conflict with the drilling crew. Xenon projected the boring logs onto the resistivity sections at the locations shown in Figure 3. Pseudo-resistivity logs were generated from the resistivity sections at the projected locations of the borings. Soil boring SB-3 could not be used because it was located at the end of Line 3 where there the depth of penetration tapers off nearly to zero (see Figure 3). shows the soil boring logs plotted with the pseudo resistivity logs.



7. Interpretation

The following interpretations are based on our review of the earth resistivity profiles as shown on Figure 5.

Fine-grained deposits (CL, CL/CM) will often correlate with the lowest resistivity zones due to their high porosity and electrical properties of their constituent clay minerals. Clay deposits can exhibit lower resistivities than water, so water-saturated sands and gravels can have higher resistivities. The sand minerals will have much higher resistivity than clay minerals and the resistivity of the sand unit is primarily due to its water content. Soil boring logs indicate the sand units are over-consolidated, suggesting lower porosity and consequently higher resistivity of the sands in comparison to sand deposits that are not over-consolidated.

The draft soil boring logs were used as a primary means of interpreting lithologic boundaries (see Figure 5). Boring SB-1 and SB-2 on Line 1, and SB-2 on Line 2 show the top of the sand correlates with the 95 Ω•m contour. Boring SB-1 on Line 3 does not correlate as well possibly because of the thickening of the fine (low resistivity) deposits within stationing 0 – 500 on Line 3; this suggests that geologic conditions invalidate the projection of the boring to Line 3. Because the vertical coverage of resistivity profiles diminishes at the ends of the profiles, there is no resistivity data for comparison to SB-3.

The base of the sand unit is more difficult to distinguish, likely because the weathered shale bedrock has a low resistivity similar to the sand units. Deeper, presumably less-weathered shale bedrock is interpreted to correlate with higher resistivities deeper in the sections as shown on Figure 5. Note that bedrock may be present in the zones between the lines showing the interpreted base of the sand and the lines showing the interpreted top of bedrock.

8. Recommendations

Our interpretation improves with the constraints of known conditions. More informed interpretations may be possible if additional information regarding the present-day site conditions becomes available. Xenon recommends further interpretation of these datasets as site conditions are documented.

9. Limitations

Like all geophysical methods, magnetic methods cannot completely resolve all subsurface conditions. The causes of these limitations are numerous and include the non-unique features that can result in similar magnetic anomalies, positioning errors inherent in the method(s) used to determine measurement locations, the presence of magnetic and electromagnetic noise, and the possible presence of natural or man-made conditions that are unknown but result in anomaly values similar to those assumed as indicators of the presence of underground structures. Because of these inherent limitations, Xenon does not guarantee that the geophysical data and/or interpretations presented herein has detected all subsurface conditions of interest, or that the interpreted identities, locations, or depths are exact or in fact correct.

Xenon Geosciences project 19-05Prepared for Jones & Henry Engineers, Ltd.September 7, 2019

Figure 1:Site Location Map

Potential Wellfield InvestigationVillage of Hicksville, Ohio

Xenon Geosciences, Inc.geology & geophysics52 W Broadway StreetDanville, IN 46122(317) 745-0002

NOTESSite location shown by blue line.USGS digital topographic map via ExpertGPS.

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Approximate setback area

beanfield -noaccess


Figure 2:Resistivity Profile Locations


NOTESMap shows locations of earth resistivity profiles recorded on July 24 and 25, 2019. Refer to report for details.Coordinates in State Plane 1983 - Ohio North (feet).Dots show electrode locations. Stationing (dots) was determined using 300-ft. vinyl tapes. Spatial locations at

various locations along the tapes were measured from GPS WAAS.Boring locations provided by Jones & Henry.Approximate setback boundary was estimated by Xenon Geosciences and is provided for general reference only.Basemap from USGS digitial orthophoto quadrangle via ExpertGPS.


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Figure 3:Resistivity Profiles


NOTESEarth resistivity profiles recorded on July 24 and 25, 2019. Refer to report for details.Boring locations were projected onto sections by Xenon Geosciences using coordinates privided by Jones & Henry (see Figure 2).Horizontal stationing in feet as measured using 300-ft. vinyl tapes. Depths determined from inversion software and are approximate.Approximate setback boundaries were estimated by Xenon Geosciences and is provided for general reference only.


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Typical geologic column prepared by Xenon from boring logs provided by JHE.

Thinner, green segments show fine-grained tills.

Wider, yellow segments show sand units.

Curve to right shows n-values.


Figure 4: Comparison of Soil Boring Logsand Earth Resistivity



VISUAL COMPARISON OF SOIL BORINGS AND EARTH RESISTIVITY MEASUREMENTSSoil lithology shown with fine-grained (CL, CL/ML) material in narrower green pattern and sand (SM, SP/SM)

as wider yellow pattern. Lithology from draft boring logs prepared by TTL Associates, Inc. and provided by JHE.Resistivity shown as orange bars. Maximum bar width occurs at the top of Line 1 / SB-1 at a value of 272 Ω-m.Black line curve shows n-values as reported on draft boring logs.Depth in feet as shown.

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Figure 5:Interpreted Resistivity ProfilesPotential Wellfield InvestigationVillage of Hicksville, Ohio

NOTESEarth resistivity profiles recorded on July 24 and 25, 2019. Refer to report for details.Boring locations were projected onto sections by Xenon Geosciences using coordinates privided by Jones & Henry (see Figure 2).Horizontal stationing in feet as measured using 300-ft. vinyl tapes. Depths determined from inversion software and are approximate.Approximate setback boundaries were estimated by Xenon Geosciences and is provided for general reference only.Refer to Figure 1 for notes on inset map.


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Blue areas are interpreted as highly conductive silty clay (CL) deposits.

Red circles show areas interpreted as locally thickened sand units.

SB-2

Interpretedshale bedrock

possible bedrockinterpreted bedrock

interpreted bedrock

Interpreted bedrock Interpreted bedrock

Bedrock may be present in zonesbetween interpreted lines showingbottom of sand and top of bedrock.

Dotted line indicateszone of lower confidencedue to poor data quality

town of hicksville earth resistivity report

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