Classification of Magnetic Susceptibility Anomalies and Their Relevance to UXO Detection. Introduction

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1 Classification of Magnetic Susceptibility Anomalies and Their Relevance to UXO Detection Janet E. Simms, US Army Engineer Research and Development Center, Vicksburg, MS Remke L. van Dam and Jan M.H. Hendrickx, New Mexico Tech, Socorro, NM Introduction The attention given the natural geologic background of a site and its influence on geophysical sensors has grown considerably over the past several years, primarily because of the governmentsupported unexploded ordnance (UXO) technology demonstrations and government mandated UXO cleanup activities. Explosive and ordnance detection (EOD) teams have encountered difficulties at sites thought to be conducive for geophysical surveying, as well as at geologically challenging sites. Geophysical anomalies thought to be UXO have been dug to reveal rusty soils, hot rocks, animal burrows, and unexplained occurrences. No longer can a site be considered typical and its geoenvironmental setting ignored. The physical aspects and geologic makeup of a site must be evaluated prior to initiating a survey to ensure proper sensor selection for an area. The primary geophysical methods used for UXO detection are magnetometry and electromagnetics (EM). To understand how the geologic environment affects these sensors, it is necessary to understand what physical property or properties the sensors respond to and how these properties are manifested within the geo-environment. The constitutive properties relevant to magnetometry and EM methods are magnetic susceptibility, electrical conductivity, and dielectric permittivity. It becomes evident how challenging a UXO geophysical survey can be after viewing the range of values these parameters can exhibit for different soil types and moisture conditions (Table 1). The time domain EM induction sensors (TDEM) are primarily influenced by the magnetic susceptibility, whereas the frequency domain EM induction sensors (FDEM) are influenced more by the electrical conductivity but also magnetic susceptibility. Ground penetrating radar (GPR) instruments are affected by all three parameters, dielectric permittivity, electrical conductivity, and magnetic susceptibility. The electrical conductivity of a soil or rock is controlled by the pore fluid, moisture content, pore connectivity, and ions in solution. As the latter three factors increase, so does the electrical conductivity. Clay-enriched soils, particularly those containing montmorillonite, have the capacity to absorb significant amounts of water, thus increasing the electrical conductivity. As the electrical conductivity increases, the detection capabilities of FDEM and GPR sensors decrease. The three properties collectively affect the power attenuation of a GPR sensor. Figure 1 shows how the attenuation varies for different parameter values. Magnetic susceptibility influences all sensors used for UXO detection and discrimination. This paper concentrates on magnetic soil occurrences and their influence on UXO detection. General Soil Variability The magnetic susceptibility of a soil generally does not exhibit extreme variations in an area where the soil was formed under similar geologic processes and subjected to the same weathering conditions. However, in regions where the soil was formed by the same geologic process but exposed to different cultural and environmental factors, measurable differences in magnetic susceptibility have been recorded. Recent studies by van Dam et al. (2004, 2005) has revealed magnetic susceptibility variations correlated to annual precipitation; in industrialized areas an increase in soil magnetic susceptibility has been measured as a result of Fe emissions by manufacturing or processing plants (Kapička et al., 2003);

2 measurements of magnetic susceptibility in agricultural fields has found differences in the plough rows (). Soils formed from sedimentary material typically have a lower susceptibility than those composed of metamorphic material, which have a lower susceptibility than igneous source material (Dearing, 1999). The susceptibility strength of soils is generally greater than the parent material. This aspect is attributed to the weathering process and the heavier magnetic minerals settling and concentrating during deposition. The magnetic susceptibility of a site typically has a unimodal distribution with a narrow peak (Figure 2) (Butler, 2003). Table 2 provides a list of measured susceptibility values for soils having different geologic environments; also included for comparison are data acquired on rock samples from Kaho olawe, Hawaii. Note that the Kaho olawe soils have a greater strength of the same magnitude or an order of magnitude higher than rocks from the same area. Classification of Magnetic Susceptibility Anomalies The important consideration in a UXO survey is the measurable contrast between the ordnance and the geologic background. Ordnance are comprised primarily of metal and have a large magnetic susceptibility. The majority of sites encountered are geologically quiet, having a significant contrast between UXO and background for targets within the size limit and depth detection range of the sensor. Sites have been encountered though where localized or site-wide geologic features are present that compromise the use of geophysical methods for UXO detection. Magnetic susceptibility anomalies can be classified based on their size and extent and source into four categories: localized, feature-related, underlying geology, and environmental. A localized anomaly has a small or traceable extent. Sources of localized magnetic susceptibility anomalies include lightening strikes, hot rocks, animal burrows, and roadbeds or trails. Figure 3 is an example of a magnetic gradient survey (Schwartz, personal communication, 2004) that shows a strong magnetic response. The area of the test pit was excavated to a depth of 0.9 m (3 ft) with the only thing encountered being gravel-size rock. The pit was backfilled and resurveyed with survey results similar to the original data. The test pit was excavated again, this time to a depth of 1.5 m (5 ft) and the open pit surveyed. Gravel-size rock was encountered during the second excavation with measured gradient magnitudes similar to that of the original and first excavation survey. A final survey was conducted after the test pit was backfilled with results similar to the other surveys. It was concluded that the gravel uncovered during the excavations had a high magnetic susceptibility and part of an old roadbed. The feature-related classification includes drainage areas and topographic features where the heavier magnetic minerals can accumulate. The shallow drainage area identified on Jefferson Proving Ground (JPG) (Butler et al., 1999) that is related to a large geologic magnetic anomaly provides a good example of a feature-related magnetic susceptibility anomaly. Figure 4 (upper plot) shows an inset of the total magnetic field anomaly and contour plot of magnetic susceptibility data collected on a 6.1 m (20 ft) grid within a 61-m 61-m (200-ft 200-ft) survey area. The magnetic susceptibility data were acquired using a Bartington MS2D sensor. A north-south profile of survey line K and the elevation profile are shown in the lower plot of Figure 4. Also included on this figure are magnetic susceptibility measurements acquired at depth and with the Geonics EM-38 terrain conductivity meter. Note that both the Bartington and EM-38 curves exhibit peaks near the same point along the slopes of the topographic depression. The weathering and transportation processes of water flowing through the drainage ditch could have concentrated the heavier magnetic minerals within the soil. The elevation of the susceptibility peaks may represent the lowest point of the ditch depression in the past. It is likely that this process will continue and, if water levels in the ditch become high enough, erode the present concentration of magnetic minerals and redeposit them.

3 The large-scale geology of a site is an important consideration when assessing the likely success of a UXO detection survey. Both the local soil and underlying bedrock are capable of having magnetic susceptibility values high enough to interfere with or mask the detection of UXO. Figure 5 is a plot of the relative total magnetic field (TMF) acquired over a 2-m 2-m area at JPG. Its location is approximately 30 m (100 ft) southwest of the eastern end of the east-west trending geologic anomaly (refer to Figure 4). The large increase in magnetic field strength from northeast to southwest over the 2- m plot is caused solely by the natural distribution of magnetic minerals in the soil. The data plots shown in Figure 6 emphasize the influence of highly magnetic bedrock on a time domain EM sensor. The data were collected in Kaho o lawe, Hawaii (Pasion, personal communication, 2004) where the bedrock is a tholeitic basalt comprised of up to 20 percent magnetite. In a low noise geologic environment, environmental influences decay rapidly and are only present in the very early time channels. However, as seen in Figure 6, influence of the magnetic bedrock is measurable even in the later time channels. Soil and bedrock susceptibility values of these magnitudes (see Table 2) can cause difficulties in resolving the presence of UXO. Natural, cyclic processes also influence the distribution of magnetic minerals within the nearsurface soils. Recent studies by van Dam et al. (2004, 2005) in tropical regions suggest a correlation of magnetic susceptibility strength with annual precipitation. The data in Figure 7 are the average of 137 measurements acquired in 11 sample pits representing nine soils series on the Hawaiian Islands of Oahua, Kaho olawe, and Hawaii. These data indicate an inverse relationship where the measured soil magnetic susceptibility decreases as annual precipitation increases. In this geographical region, a distinct change is noted at about 1500-mm mean annual rainfall. At precipitation rates less than 1500 mm/yr, significant leaching of the magnetic minerals in the upper soil layer occurs. Based on frequency-dependent susceptibility data, van Dam et al. (2005) also postulate that transformation from a primary to secondary magnetic mineral assemblage occurs during the weathering process. Summary It is important to evaluate the geo-environmental setting of a site to identify the characteristics that can interfere with the detection of UXO. The magnetic susceptibility of the soil affects all geophysical methods used for UXO detection. On the majority of sites contaminated with UXO, there generally is a significant susceptibility contrast between the soil and target. However, concentrations of magnetic minerals can be found on different spatial scales and have been classified as localized, featurerelated, underlying geology, and environmental. Evaluating a site relative to the type of soil, parent material, topographic signatures, and climate can aid in identifying areas within a site that could be problematic in detecting UXO.

4 References Butler, D.K., Llopis, J.L., and Simms, J.E. 1999, Phenomenological investigations of the Jefferson Proving Ground UXO Technology Demonstrations, Technical Report GL-99-7, US Army Engineer Waterways Experiment Station, Vicksburg, MS. Butler, D.K. 2003, Implications of magnetic backgrounds for unexploded ordnance detection, Journal of Applied Geophysics, 54, pp Dearing, J. 1999, Environmental magnetic susceptibility using the Bartington MS2 System, Chi Publishing, Kenilworth, UK. Kapička,A., Jordanova, N., Petrovský, E., and Podrazský, V. 2003, Magnetic study of weakly contaminated forest soils, Water, Air, and Soil Pollution, 148, pp Van Dam, R.L., Hendrickx, J.M.H., Harrison, B., Borchers, B., Norman, D.I., Ndur, S.A., Jasper, C., Niemeyer, P. Nartey, R., Vega, D., Calvo, L. and Simms, J.E., 2004, Spatial variability of magnetic soil properties, In: Detection and remediation technologies for mines and minelike targets VIII, Proceedings of the SPIE, 5415, Van Dam, R.L., Harrison, Hendrickx, J.M.H., B.J., Borchers, B., North, R.E., Simms, J.E., Jasper, C., Smith, C.W. and Li, Y. 2005, Variability of magnetic soil properties in Hawaii, Proceedings of the SPIE, March 2005.

5 Table 1.: Representative Values of Geophysical Parameters for Different Soil Types and Moisture Conditions USDA Soil Type Electrical Conductivity ms/m Volume Magnetic Susceptibility 10-5 SI Relative Dielectric Permittivity Sand 0.1 to to to 5 Loamy sand 1 to to to 10 Sandy loam 1 to to to 15 Loam 1 to to to 20 Sandy clay loam 1 to to to 20 Clayey loam 1 to to to 30 Sandy clay 1 to to to 25 Silt 1 to to to 30 Silty loam 1 to to to 20 Silty clay loam 1 to to to 25 Silty clay 1 to to to 35 Clay 2 to to 25 5 to 40 Moisture State Dry 0.01 to 1 NA 3 to 5 Moist 1 to 100 NA 5 to 30 Wet <1000 NA 20 to 40

6 σ = 1 ms/m σ = 10 ms/m σ = 25 ms/m σ = 50 ms/m σ = 100 ms/m σ = 500 ms/m Figure 1.: Influence of geophysical parameter values on GPR attenuation.

7 30 Number of Occurrences x x x x x x x x10-3 Magnetic Susceptibility (SI) 1.6x x10-3 Figure 2.: Example of unimodal distribution of magnetic susceptibility data over a site

8 Table 2.: Magnetic Susceptibility of Soils and Rocks in Different Geologic Regions Volume Magnetic Susceptibility Location x 10-5 SI Aberdeen Proving Ground, MD UXO Standardized Test Site Former Fort Ord, CA ODDS Jefferson Proving Ground, IN 40-Acre Site Yuma Proving Ground, AZ UXO Standardized Test Site Kaho olawe Island, HI Seagull Site Lua Kakika Site ROCKS Kaho olawe, HI 0.4 to to 68 7 to to to to to 905, Avg to 249, Avg to 243, Avg to 265, Avg to 420, Avg 319 Comment Geology: fluvial over bank deposit Soil: silt or clay with small amount of sand Geology: dune Soil: sand Geology: weathered glacial depositssoil: sandy silt Geology: alluvial fan complexes and alluvial plains Soil: gravelly silty sand Geology: volcanic Soil: tholeiitic basalt parent rock with up to 20% magnetite Orange-brown, not vesicular, rough surface Orange-brown, flattened, vesicular on one side Orange-brown, flattened, vesicular Orange-brown, not vesicular, spherical Orange-brown, not vesicular, spherical

9 Open Pit 2nd excavation (5 ft) Initial Survey Back-filled after 1 st excavation (3 ft) nt/m Back-filled after 2nd excavation Figure 3.: Example of localized anomaly caused by hot rocks.

L K 5 M Volume Magnetic Susceptibility, SI Units 0.0020 0.0018 0.0016 0.0014 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0 K13 Geonics Bartington MS2 Depth 0.1 m Depth 0.

10 L K 5 M Volume Magnetic Susceptibility, SI Units K13 Geonics Bartington MS2 Depth 0.1 m Depth 0.5 m Depth 1 m Elevation, m K7 K6 K4 K Elevation, m S K-Line Distance, ft N Figure 4.: Magnetic susceptibility anomaly (upper plot) caused by drainage feature on JPG. Lower plot is the magnetic susceptibility profile along the K survey line.

11 nt Figure 5.: Natural variation in the soil magnetic field strength on JPG.

12 Channel 5 90 Channel mv mv EM63 Figure 6.: TDEM response showing influence of magnetic bedrock on later time channels.

13 40 lf (10-6 m 3 /kg) Waianae-Oahu Kanapou-Kaho'olaw e Kohala-Haw aii Mauna Kea-Haw aii Mean annual precipitation (mm/y) Figure 7.: Variation in magnetic susceptibility with annual precipitation in a tropical region.

ANALYSIS OF LOCALIZED HIGH MAGNETIC SUSCEPTIBILITY ZONES AT JEFFERSON PROVING GROUND, INDIANA

ANALYSIS OF LOCALIZED HIGH MAGNETIC SUSCEPTIBILITY ZONES AT JEFFERSON PROVING GROUND, INDIANA Ryan E. North*, Eric W. Smith, and Jose L. Llopis U. S. Army Engineer Research and Development Center Geotechnical