Geophysical Survey for Buried Materials

Neals Dump Site

September, 1998

1 INTRODUCTION

1.1 LOCATION AND DESCRIPTION

A geophysical survey was conducted at Neals Dump Site, located south of the Town of Spencer, Indiana. Spencer is situated about 15 miles northwest of Bloomington, Indiana onState Road 46 and about 35 miles southwest of Indianapolis. Neals Dump Site is located some10 miles south of Spencer, just west of Pottersville Road, adjacent to a small group of homes(Figure 1.1).

The site occupies approximately 1 acre which consists of trash dumped in a shallow depression on the southern side of a steep ravine that empties to the west. This area is 50 to-200 feet north of several residential properties. Access to the area has been limited by a tall galvanized steel fence, which is irregular in shape.

Originally overgrown with trees and brush, this area was cleared before the geophysical investigation. Pieces of sheet metal, old appliances and other easily moved metallic trash were moved to 4 locations within the fenced compound. Several clusters of monitor wells arealso present on the site; wires were seen connecting sampling devices in several wells to acentral location near the gate on the southwest side. Several electrical capacitors were observed in a small opening near the center of the compound (Figure 1.2).

The property possesses an undulating topography with an overall slope to the west and is covered with a veneer of small to pea-sized gravel. Occasional small trees and some brush have taken root in this surface.

1.2 PURPOSE

Geosphere was contracted by Psara Technologies to detect and accurately map any buried metal debris including capacitors using geophysical methods. These methods included electromagnetics (EM) and magnetometry. The EMS 1 instrument is capable of measuring ground conductivity changes and detecting accumulations of metallic objects. A magnetometer is designed to measure local changes in the earth's magnetic field which maybe caused by buried ferrous objects. Both methods can detect buried metal objects to depths of approximately 20 to 40 feet, depending on the size of the metal accumulations. The survey was conducted 31 August 1998.

2 GEOPHYSICAL METHODS

2.1 GENERAL DESCRIPlION

Two different geophysical methods were used at Neals Dump Site south of Spencer, Indiana:

l ) Electromagnetics (EMS 1 )

2) Magnetometry (MAG).

A description for each method is given below as it applies to the site.

2.2 EM TECHNIQUE

EM was used to locate conductivity and inphase anomalies that might represent buried landfill wastes and metallic materials. Buried metallic materials could represent buried capacitors,paint cans and other metallic containers that may contain unwanted residues.

The Geonics EM31 instrument was used because it:

1) provides conductivity and inphase (metal) information to 20+ feet depth
2) is capable of resolving lateral features such as buried capacitors and other containers

3) can be easily outputted to a digital recorder for near-continuous profile coverage.

The EM induction method determines electrical properties of earth materials by inducing electrical currents in the ground and measuring the secondary magnetic field produced by these currents. An alternating current is generated in a wire loop or coil above the ground'ssurface; both the primary magnetic field (produced by the transmitter coil in the instrument) and the secondary field (produced by currents in the earth) induce a corresponding alternating current in the receiver coil of the instrument. The coils are kept at a fixed distance and orientation to simplify handing of the data (Appendix A).

After compensating for the primary field (which can be computed from the relative positionsand orientations of both coils), both the magnitude and relative phase of the secondary field are measured. These measurements are then converted to components of inphase and 90 degrees out-of-phase with the transmitted field. The out-of-phase (or quadrature-phase) component, using certain simplifying assumptions, is converted to a measure of apparent ground conductivity. This apparent conductivity conversion assumes a homogeneous,isotropic earth. In practice, this value is an estimate of the average conductivity of the ground in the proximity of the instrument, to a depth of investigation

A

(approximately 20 feet for the EM31) which is dependent on the coil spacing (3.66 meters), orientation, operating frequency, and the conductivity distribution of the earth. The inphase output of the EM31 is a semi-quantitative signal representing the metallic nature of nearby targets; in essence it is very similar to a large metal detector. Small (less important) targetsare effectively filtered out of the signal. For this reason, the inphase signal can be a good measure of large targets such as steel drums and tanks.

Data quality of the conductivity (quadrature) signal may be degraded by the presence of cultural interference such as that caused by utility lines, steel fences, and large metallic objects whose high conductivity values may overwhelm the conductivity of the ground itself. Often, very high metallic responses will cause negative values in the quadrature data. Both the quadrature and inphase data can be recorded in analog or digital form on two channel, battery-powered recorders.

At this site, both conductivity and inphase data were recorded digitally on an Omni Datalogger unit in units of milliSiemens/meter (mS/m) and parts per thousand (ppt), respectively.

2.3 MAGNETOMETER TECHNIQUE

The magnetometer geophysical method was used to locate iron anomalies which mayindicate the presence of buried steel capacitors and other containers. The gradiometer configuration of a fluxgate magnetometer was employed for this purpose.

A magnetometer is designed to measure the intensity of the earth's magnetic field. Variations or disturbances in this relatively uniform field may be caused by the natural distribution ofiron oxides within the underlying soil and rock or caused by the presence of buried iron orsteel objects. Localized distortions of the earth's magnetic field cause higher and lower readings on different sides of the buried object. The greater the mass of the target or combined mass of many targets in contact with one another, the greater is the induced distortion or magnetic anomaly in the earth's field. Another factor which will influence the response of a magnetometer is the permanent magnetism of that individual target which mayact in conjunction with or against the induced magnetism of the object. Further, the target's shape, orientation in the earth's field, and its state of deterioration affect its magnetic responsiveness. Accordingly, the magnetic responses of similar buried objects may vary overa wide range, making quantitative analysis of the data difficult, yet semi-quantitativein ormation may be used to assess the general nature of any buried iron materials. A magnetometer does not respond to nonferrous metals such as aluminum, copper, tin, and brass(Appendix A).

The earth's magnetic field intensity changes hourly and daily with sunspots and ionospheric conditions. With time, these variations produce unwanted noise and can seriously affect local magnetic measurements. Older field techniques included a base station, to which changes in the instrument's readings were frequently compared and used in correcting the data taken in the interval between base-station readings. A more efficient way to minimize these diurnal effects is to use two sensors (mounted vertically with one another), measuring changes in themagnetic field simultaneously. This technique records the gradient field, completely anceling out any large uniform, diurnal variations. This instrument is called a "radiometermagnetometer and may be used in discrete station or continuous profile measurement modes.The continuous profile mode provides more data and greater flexibility.

Magnetic data signals may be recorded using either analog or digital methods. Analog methods permit real-time evaluation of the data as it is collected; digital methods provide the advantage of recording large data sets for ease in entering the data into a computer for further processing and plotting. The magnetometer may also be used in a "sweeping" mode (like ametal detector) to confirm EM31 anomaly zones and penetrate dense undergrowth areas.Caches of drums and other steel objects have been detected as great as 30 to 40 feet.

At this site, gradient magnetic data were recorded in digital format on an OmniData logger inunits of nanoteslas per foot.

3 DATA ACQUISITION

2.1 SURVEY AREA AND COVERAGE

Upon arrival at the site, Geosphere expanded a staked reference grid (previously installed) consisting of colored flags at 10-foot intervals along lines every 50 feet over the area within the fence. Following this effort, red spray paint was used to mark additional grid nodes on 10 foot centers between the flagged 50-foot gridlines. In some locations, additional effort wasneeded to push through or over fallen trees, limbs and brush. In the following discussions, reference to the cardinal directions are relative to grid north.

The grid coordinates (lOOE/lOON) were assigned to a wooden reference stake 75 feet east of the site gate-post. The remaining coordinates were assigned in a standard Cartesian system layout (see Figure 1.2).

The entire gridded area was surveyed with both EM and gradio meter magnetometer instruments.

3.2 DATA ACQUISITION

3.2.1 EM

EM31 data were collected at 2.5-foot intervals on east-west survey lines spaced at 5-foot increments along the completed grid system (Figure 3.1); this orientation maximized the amount of information obtained across the suspected burial (north-south) axis. Along each line, both quadrature and inphase information were digitally recorded on an OmniData logger system at the sampling interval of 2.5 feet. EM data were periodically downloaded to a portable field computer for preliminary processing and incorporation into gridding, contouring and plotting software (Surfer for Windows).

The EM data were collected with the Geonics EM 1 conductivity instrument. The EM 1 converts the quadrature-phase reading directly into apparent ground conductivity inmilliSiemens/meter (mS/m), also called millimhos/meter. The inphase data are given in unitsof parts per thousand (ppt). The EM31 instrument has a nominal depth of investigation ofabout 20 feet.

3.2.2 Magnetic Measurements

The magnetic measurements were also collected at 2.5-foot intervals on east-west lines spaced at 5-foot increments coincident with the EM3 1 readings (Figure 3.1). The sensor was kept at least 2 feet above the ground surface to minimize unwanted effects from surfacetrash (of which they was much). Data were periodically downloaded to a portable field computer for preliminary processing and incorporation into gridding, contouring, and plotting software (Surfer).

Magnetic readings were measured using a FERAL Colgate magnetometer. The magnetometerwas used in the gradio meter mode, i.e., data were obtained as vertical magnetic gradients by reading the difference between two vertically mounted fluxgate sensors at the end of a short boom. This precluded the necessity of using a base station (as required for total field magnetic measurements). These data measure magnetic anomalies in units of nanoteslas perfoot. The investigative depth of the instrument for individual capacitor-sized steel objects is estimated at approximately 4 to 8 feet and 20 to 30 feet for 50 to 100 capacitors in pits or tenches.

4 RESULTS AND INTERPRETATION

4.1 DATA ANALYSIS AND RESULTING MAPS

Both the EM and Mag data were processed, gridded and contoured using the computer program Surfer for Windows. Color contour maps were then combined with a base map that was constructed from our observations/measurements taken on site.

Figures 4.1 and 4.2 present the EM31 conductivity and inphase metal contour results, respectively. Figure 4.3 gives the magnetic gradient results. Interpretations are provided as labels on these maps.

4.2 ANOMALY DESCRIPI IONS

4.2.1 Conductivity Features

Analysis of Figure 4.1 reveals a fairly sharp contrast in normal EM soil conductivities (1530mS/m) and negative conductivity values. Negative (gray-colored) contours are indicative of buried metallic materials; the irregular area in the central portion of the site corresponds tomounds of metallic debris visible at the ground surface. No evidence of a conductive plume or soil contamination is visible. The linear negative pattern running between the southwest andnorthern portions of the site was caused by a series of buried cables used for controlling automated sampling devices in some of the monitor wells. High values around the perimeter are associated with the nearby steel fence and should be discounted. Areas labeled "SM" indicate zones where surface metal trash was piled during clearing activities.

Inphase Metal results (Figure 4.2) show a similar distribution pattern of surface and buried metallic objects as seen in Figure 4.1. The different color scheme does highlight changes in metallic character within the central zone of surface and buried metal as well as several locations on this area's western edge and an unexplained anomaly centered at coordinates50E/ 1 32N.

4.2.2 Magnetic Features

Figure 4.3 provides a plot of the distribution of magnetic anomalies over the survey area. Analysis again indicates that the distribution of (iron) metallic materials is very similar to thatviewed in Figures 4.1 and 4.2. The major difference is the occurrence of notable anomaliesnear the clusters of wells at the four monitor well locations. The magnetometer is moresensitive to smaller vertical steel objects than the EM instrument. As with the EM results, the magnetic gradient contours show a relatively restricted zone of surface/buried metal in the central portion of the site, between gridlines 40N to 195N and 120E to l90E.Small unexplained anomalies are again found near coordinates 50E/132N as seen in the EM data.

Considering the amount of debris found scattered over the surface of the site, the relativelycontained occurrence of buried (or mounded) material is unusual. This pattern along with relatively low magnetic amplitudes suggests that metallic materials were dumped in shallow depressions near the center of the site without the bother of digging pits or trenches.