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Open-File Report 97-679: EM Induction and DC Resistivity surveys near the Norman, Oklahoma Landfill

EM Induction Surveys

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Electrical conductivity is the property of a material that facilitates the flow of electrical current. Electromagnetic induction instruments, models EM31-D and EM34-3, manufactured by Geonics Limited (Mississauga, Ontario, Canada; McNeill, 1980) were used to measure the apparent electrical conductivity of the ground near the Norman Landfill. Apparent conductivity is equivalent to true conductivity only when conductivity is uniform throughout the subsurface. The conductivity of the alluvium and bedrock is controlled by the matrix (sediment or rock), porosity, pore space saturation, and conductivity of pore solutions (water or leachate).

See Lucius and Bisdorf (1995) for detailed descriptions and locations of EM conductivity measurements made near the landfill cells. The main utility of the conductivity measurements is to map relative changes in ground conductivity rather than to determine the actual values of conductivity. The separation and orientation of the instrument coils determine the depths of investigation and sensitivity. When the coils are vertical (horizontal magnetic dipoles, HMD) the instruments are most sensitive to near-surface materials to a depth of about of the coil spacing. When the coils are horizontal (vertical magnetic dipoles, VMD) the instruments are most sensitive at depths of about the coil spacing, and detect changes down to about 1.5 times the coil spacing.

The accompanying figures show the gridded EM apparent conductivity as color-scale image maps. Hotter shades (toward red) indicate higher conductivity. The station locations are indicated by the black dots within the shaded areas. It is evident in each figure, that the highest conductivities occur within 200 meters of the landfill mounds, suggesting that the conductive part of the leachate plume probably does not extend beyond this limit. Variations in intensity near the landfill also suggests that the distribution is not equal over the area. Away from the landfill, there appears to be little variation in the ground conductivity, except for increases with depth. The EM34-3 is very sensitive to coil misalignment when used in the VMD (horizontal coil) mode. Inaccurate readings due to coil misalignment are probably the main cause of the blotchy appearance of EM34-3 VMD image maps.

Assuming the alluvium is 10 to 14 meters thick, the conductivity values in all the image maps, except for the EM34- 20m VMD map, are dominantly derived from the saturated and unsaturated alluvium. Only in the EM34-20m VMD map do electric currents in the Hennessey Formation below the alluvium contribute substantially to the measurements. The somewhat more uniform appearance of the data in the EM34-20m VMD map then suggests that the leachate has not substantially penetrated into the shale below the alluvium. The EM method used could not resolve the vertical distribution of conductive material with precision. But the data suggest that a conductive layer, presumably contaminated ground water, is in the saturated zone and may not extend into the Hennessey Formation beneath the alluvium.

REFERENCES

Lucius, J.E. and Bisdorf, R.J., 1995, Results of Geophysical Investigation Near the Norman, Oklahoma, Municipal Landfill, 1995: U.S. Geological Survey Open-File Report 95-825, 125 p.

McNeill, J.D., 1980, Electromagnetic Terrain Conductivity Measurements at Low Induction Numbers: Mississauga, Ontario, Canada, Geonics Ltd. Technical Note TN-6, 15 p.

IMAGES

• Locations of EM stations along test lines 1 and 2, and perimeter
  
• Locations of EM stations on grid
  
• Image map of EM31-D HMD apparent conductivity for Norman, OK landfill area
  
• Image map of EM34-3 10-m HMD apparent conductivity for Norman, OK landfill area
  
• Image map of EM34-3 20-m HMD apparent conductivity for Norman, OK landfill area
  
• Image map of EM31-D VMD apparent conductivity for Norman, OK landfill area
  
• Image map of EM34-3 10-m VMD apparent conductivity for Norman, OK landfill area
  
• Image map of EM34-3 20-m VMD apparent conductivity for Norman, OK landfill area
  

DC Resistivity Surveys

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In 1995 and 1996 the U.S. Geological Survey made 61 dc electrical resistivity soundings using the Schlumberger array at the Norman Landfill, near Norman, Oklahoma (Lucius and Bisdorf, 1995 and Bisdorf, 1996). The location map shows the sounding locations, the outline of the landfill, the slough and sewage outfall, and the north bank of the Canadian River (as of Feb. 1995). These soundings were made to determine if surface electrical techniques could delineate the horizontal and vertical extent of the conductive portion of the contaminant plume believed to be migrating from the landfill. The site is generally clear of cultural features except for the area between the slough and the landfill that has piles of debris including metallic objects. Sounding expansion was limited to a maximum of 2000' between the current electrodes (1000' AB/2). With the exception of some long (100-200 ft) sections of old metallic gas pipe, neither the metallic debris nor other cultural features affected the Schlumberger soundings.

DC RESISTIVITY SOUNDING

Schlumberger sounding is a geophysical technique that uses variations in the electrical resistivity of earth materials to help detect buried geologic structures. Dc resistivity (the inverse of conductivity) is a fundamental rock property that varies due to rock type, clay content, porosity and the quantity and quality of the water contained in the rock. Resistivity is normally expressed in ohm-m. Within a given rock type, the resistivity of the rock is primarily dependent on the quality and quantity of water and the amount of clay present. Generally speaking, higher clay content and/or poorer quality (higher TDS and/or chlorides) ground water lowers the rock resistivity.

Schlumberger sounding uses a symmetric electrode array to vertically explore the subsurface. The name Schlumberger derives from Conrad Schlumberger, an early proponent of the array geometry. Schlumberger soundings are processed by computer modeling of the sounding data as a series of horizontal layers (Zohdy, 1989 and Zohdy and Bisdorf, 1989). More detailed explanations of processing and automatic interpretation procedure can be found in Bisdorf (1985) and Zohdy and others (1993). A series of individual soundings can be combined to generate either a geoelectrical cross section or a map view of interpreted resistivity. Cross sections, which can be thought of as vertical slices through the ground, similar to a road cut show lateral as well as vertical variations of resistivity. Maps of interpreted resistivity show areal distributions at a particular depth or elevation and can be thought of as horizontal slices through the earth.

RESISTIVITY CROSS SECTIONS

Resistivity cross sections are generated from individual sounding interpretations. Each sounding interpretation is sampled in a manner to approximate a continuous vertical distribution of resistivity with depth (Bisdorf, 1982). This vertical data is then horizontally interpolated to create a grid. Color values are assigned based on the interpolated resistivity values and the desired contour levels. Triangles on the upper surface of the cross section designate the sounding locations. Topographic information, input as sounding elevations, is represented by connecting the surface location of the soundings by straight lines. The cross sections are displayed to an elevation of 260 m.

Figure 1 shows the geoelectrical cross section as shown on the East-West cross-section map. This cross section is vertically exaggerated 4 times and extends from sounding 31 on the northeast across the landfill to sounding 8 at the Canadian River. The high resistivities (>300 ohm-m) under soundings 1 through 8 represent dry sand. The intermediate resistivities (45-300 ohm-m) represent sand saturated with relatively good quality water. Low resistivities (<45 ohm-m) represent either fine grained materials such as mudstone, shales or clays, or represent sand saturated with poor quality (high TDS) water. Under soundings 30 to 6 a low resistivity layer exists from elevations 329 m to 320 m. This low resistivity layer is interpreted as the conductive portion of the contaminate plume. The thickness of this layer cannot be accurately determined because the underlying Hennessey group is fine grained and hence also a low resistivity unit and the two therefore cannot be differentiated. Unpublished penetrometer readings put the elevation of the Hennessey at about 319 meters (Christenson, 1996, electronic communication) near sounding 6. Using this elevation as a lower limit, the leachate plume can be up to 9 m thick.

Figures 2 through 8 are additional geoelectrical cross sections. Their locations can be seen on North-South cross-section map and the East-West cross-section map The interpretation of resistivity ranges as described above will generally apply to these cross sections.

• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6
• Figure 7
• Figure 8

Maps of interpreted resistivity at a particular elevation are generated by sampling the sounding interpretations at depths determined by the difference between the surface elevation at that sounding and the desired elevation. The resultant sampled interpreted resistivities and the corresponding location values are gridded using a minimum curvature gridding algorithm (Webring, 1981). To prevent possible interpolated resistivities of less than zero, the logarithm of the resistivities is used for gridding. Color values are assigned based on the grid value and the desired logarithmically spaced contour levels. Since these maps are raster (pixel) based, a bicubic interpolating program is used to increase the size of the resultant image. An interpolating program is used to resample the grid, as opposed to simply gridding the data at the desired final interval, because the minimum curvature algorithm generates undesirable results if the data are over sampled. The Kolor-map and section program (Zohdy, 1993) uses similar procedures and provides a discussion of the nuances of resistivity map generation.

Figure 9 shows a map of interpreted resistivity for an elevation of 330 meters. Over most of the surveyed area, with the exception of the landfill proper, this elevation is within 2 to 5 m. of the surface. The black dots represent sounding locations. The outline of the landfill, the slough, and the north bank of the Canadian River are presented for reference The map shows an area of high resistivity (>300 ohm-m) about 150 m southwest of the slough. These high resistivities represent the dry sand present in the dune area near the river. Areas of lower resistivity (7 to 45 ohm-m) form coherent zones or trends and represent fine grained deposits. These may have been zones of quiet water or over bank deposits that allowed the fine grained materials to settle. These areas are generally above the water table and water quality is not thought to be a predominant factor in determining their resistivity. The low resistivities under the landfill cap are in areas that were probably excavated and filled with trash. These lower resistivities may be caused by removal of the original sand subsequent deposits of trash. It is also possible that there is a rise in the water table under the landfill and these resistivities are water quality related.

Figure 10 shows a map of interpreted resistivity for an elevation of 328 meters. This elevation was chosen because it shows some shallow conductive features that are not considered to be related to the main portion of the contaminate plume. The low resistivity anomalies just north of the word slough and just north of the northern part of the base of the landfill are interesting in that they don't have the same source of contamination, if indeed they are contaminated, as the main conductive anomaly centered on the landfill. The conductive anomaly just north of the word slough is associated with an area were the trucks from the asphalt company wash the beds of their trucks. The runoff from the trucks may be the cause of this conductive feature.

Figure 11 shows a map of interpreted resistivity for an elevation of 326 meters. This elevation was chosen because it falls near the middle of low resistivity anomaly seen on the cross section in Figure 1. The lowest resistivities occur under and just west of the large cell of the landfill and are interpreted to represent the conductive portion of the leachate plume as indicated by the specific conductance values previously discussed. The general shape of the contours to the west and south of the landfill resemble the potentiometric surface (Christenson, 1996, electronic communication), implying that the low resistivity feature is caused by the ground water and not clays. This resemblance doesn't change until about 75 m west of the slough, indicating that there is still some contamination to that point. The gradual increase in the resistivities away from the landfill indicates that the contaminant plume is either becoming less conductive (i.e. less contaminated) or thinner.

Figure 12 shows a map of interpreted resistivity for an elevation of 320 meters. This elevation was chosen because it is just above the top of the Hennessey formation. At this depth the pattern of the contaminate plume can still be seen, implying that the unconsolidated materials are contaminated to the top of the Hennessey.

Figure 13 shows a map of interpreted resistivity for an elevation of 305 meters. This elevation was chosen because it is entirely in the Hennessey formation and displays a consistent low resistivity pattern without any pervasive trends.

REFERENCES

Bisdorf, R.J., 1982, Schlumberger sounding investigations in the Date Creek Basin, Arizona: U.S. Geological Survey Open-File Report 82-953, 55 p.

Bisdorf, R.J., 1985, Electrical techniques for engineering applications: Bulletin of the Association of Engineering Geologists, v. XXII, no. 4, p. 421-433.

Bisdorf, R.J., 1996, Schlumberger soundings at the Norman landfill, Norman, Oklahoma: U.S. Geological Survey Open-File Report 96-668.

Lucius, J. E., and Bisdorf, R. J., 1995, Results of geophysical investigations near the Norman, Oklahoma, municipal landfill, 1995: U.S. Geological Survey Open-File report 95-825, 125 p.

Webring, Michael, 1981, MINC: A gridding program based on minimum curvature: U.S. Geological Survey Open-File Report 81-1224, 12 p.

Zohdy, A.A.R., 1989, A new method for the automatic interpretation of Schlumberger and Wenner sounding curves: Geophysics, v. 54, p. 245-253.

Zohdy, A.A.R., 1993, Program Kolor-Map & Section, Amiga Version: U.S. Geological Survey Open-File Report 93-585, 113 p.

Zohdy, A.A.R., and Bisdorf, R.J., 1989, Programs for the automatic processing and interpretation of Schlumberger sounding curves in QuickBASIC 4.0: U.S. Geological Survey Open-File Report 89- 137 A&B, 64 p., + disk.

Zohdy, A.A.R., Bisdorf, R.J., and Martin, Peter, 1993, A study of sea-water intrusion using Schlumberger soundings near Oxnard, California: U.S. Geological Survey Open-File Report 93-524, 139 p.