• The Utility Of Horizontal Component Measurements In Random-Walk TEM Surveys

  • UXO Classification using characteristic modes of the broadband electromagnetic induction response

  • Model-Based UXO Classification Based On Static 3-Component TEM Measurements

  • Imaging UXO Using Electrical Impedance Tomography

  • Possibilities for UXO classification using characteristic modes of the broad band electromagnetic in

  • Physics based characterization of UXO from multicomponent TEM data

  • Fast TEM for UXO mapping at Gambell, Saint Lawrence Island, Alaska

  • Target Prioritization In TEM Surveys For Sub-Surface UXO Investigations Using Response Amplitude, De

The Utility Of Horizontal Component Measurements In Random-Walk TEM Surveys

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Laboratory tests and field examples of the horizontal field component measurements in shallow transient electromagnetic (TEM) surveys show the utility of these data in target characterization in reconnaissance, random-walk surveys for unexploded ordnance (UXO) and underground utilities. For example, prior work has shown that the Hx component (which we define as the horizontal field component in the direction of travel of the measurement system) is often useful in distinguishing small 3-D targets from linear features (such as buried pipelines or power lines). The current work expands on this capability, which is particularly important in UXO projects in which random transects are evaluated to determine a statistical estimate of density and distribution of potential ordnance and explosives. Anomalies from pipelines or power lines can skew the statistical evaluation toward over-estimating the number of potential UXO in a given area. The current research provides examples of the additional information that is available in the horizontal components of TEM surveys, particularly in the early time after transmitter turnoff. For example, the Hy component data are useful in estimating the orientation of linear features such as pipes and power lines with respect to the survey lines.

UXO Classification using characteristic modes of the broadband electromagnetic induction response

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Electromagnetic induction methods are effective in locating unexploded ordnance (UXO). However, the induction EM instruments that are used for UXO detection generally have limited bandwidths and provide little, if any, information for UXO classification. It is well known that the broadband induction EM response from confined conductors (such as UXO) can be parameterized in the time-domain as a series of damped exponential decay curves, and in the frequency domain as a set of discrete real first order poles and their residues. Characteristic decay time or its equivalent real pole has been shown to be a function of characteristic target dimensions, target conductivity, and relative magnetic permeability. Therefore, parameterization of the broadband EM response in terms of these characteristic modes provides a basis for the classification of UXO anomalies.

In this paper we have used a numerical method (Prony) to analyze TEM decay curves to obtain a set of exponential decay time-constants and their corresponding residues. Using a commercially available field data acquisition system, we have acquired fast transient TEM data from UXO. We show that these data can be analyzed and displayed in a way that is simple to understand and useful for classifying the TEM response.

Model-Based UXO Classification Based On Static 3-Component TEM Measurements

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The dipole model characterized by an anisotropic polarizability tensor is well accepted as a valid model describing the EM behavior of electrically small, highly conductive metal objects such as UXO. The model has been used successfully to characterize and classify EM anomalies acquired with single-gate TEM metal detectors such as the EM-61. Successful inversion of EM-61 anomalies using this dipole model depends on measurements at many points within a small radius of the target. Since these measurements are acquired dynamically, survey specifications in terms of line-spacing and survey speed must be tight in order to be assured of acquiring a sufficient number of independent data to robustly parameterize the target for classification.

An alternate method of characterizing an anomaly is to reacquire it and to take more precise data by locating the antenna array at a few discrete stations in a pattern referenced to the center of the observed anomaly. In this paper, we describe experiments in UXO characterization using a multi-gate 3-component fast TEM (NanoTEM) system. With this system, three orthogonal receiver antennas simultaneously acquire 31-gate TEM transients. The 3-component data triple the number of independent data measurements supplied at each field point. Using this system, we have acquired data sets using two methodologies. In the first methodology, we take measurements with a 3-component cart system at 5 locations centered on the anomaly peak, thus acquiring 15 31-gate transients for use in the dipole inversion. In the second methodology, we use an array of flat-lying loops arranged to illuminate in 3 orthogonal directions and measure the target's polarization response over a range of angles. Both data sets assure that the UXO has been polarized in its 3 principal directions. The dipole model simultaneously models both time and spatial components of the measured fields and reports a three-dimensional target position, spatial attitude, and polarizability parameters (i.e., the "beta" parameters) as a function of time. Results from characterization of various UXO and non-UXO targets models buried locally in Tucson and from the NRL Baseline Ordnance Classification Test site at Blossom Pt will be used to illustrate the technique.

Imaging UXO Using Electrical Impedance Tomography

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This paper reports the results of tests where electrical impedance tomography (EIT) was evaluated as a tool for detecting and locating buried unexploded ordnance (UXO). The method relies on the electrolytic polarization induced at the boundary between soil and buried metal. This induced polarization (IP) produces a measurable phase delay between the electric current imposed on the subsurface and the resulting voltage distribution. If natural sources of induced polarization are small compared to those due to buried metal objects, then tomographs of impedance phase may be used to indicate where metal-soil polarization may be present.

Three controlled tests were performed at a field site containing inert UXO buried in known locations. These tests produced a phase anomaly of about 20 milliradians that closely matched the known location of buried UXO objects. A fourth uncontrolled or blind test was performed under a building without prior knowledge of UXO presence. That test yielded phase anomalies as high as 75 milliradians. Limited excavation was performed at some of these anomalies but only a small amount (a few tens of grams) of metal was recovered. More extensive excavations are too costly until the building is razed.

Possibilities for UXO classification using characteristic modes of the broad band electromagnetic induction resistivity

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The measurement of the broadband induction electromagnetic response in the form of a complex function of frequency (FEM) or, alternatively, as a transient function of time (TEM) has been applied in geophysical exploration for 30 years or more. Broadband EM methods are used in exploration in two different ways: 1) to perform "soundings" wherein the objective is to map the earth conductivity as a function of depth, and 2) for "inductive prospecting" wherein the broadband response permits the detection and characterization of large highly conductive "ore bodies" at great depth. Until recently, broadband induction EM methods were not routinely applied for shallow exploration problems. The principles of the induction EM method require that as the geometric scale of the problem decreases, there must be a corresponding broadening of the frequency range of interest in FEM systems or, equivalently, shortening of the time interval of interest in TEM systems. At present only a few field instruments are available with the requisite bandwidth for effective application to sounding or prospecting in the shallow subsurface (< 30 m).

Elementary induction EM principles are also applied in metal detectors and as such they have enjoyed a long and successful history in applications such as utilities location and in the location of metallic mines. Metal detectors are typically optimized for detecting very small objects located within a few 10's of cm from the surface. Recently, however, new induction EM instruments have been developed specifically for shallow metal detection and site characterization. These instruments have significantly increased the depth of detection for shallow-buried metallic objects and, at least in one case, they provide measurements at more than one frequency or time delay. These instruments are now widely applied for detecting and mapping shallow-buried metal objects including UXO.

The potential for using the characteristics of the broadband induction EM response measured in the proximity of buried metallic objects to discriminate target types is generally recognized. In the context of UXO detection with TEM, McNeill et. al., concluded that ". . ., given "a priori" knowledge of the decay characteristics of UXO that are expected in a survey area, the evidence presented in this paper suggests that it might be possible to separate out various types of UXO (a) from each other and (b) from exploded ordnance and other trash metal." Moreover, there is ongoing research and development directed toward developing instruments and techniques for object detection and classification with broadband induction EM.

In cooperation with Earth Tech, Zonge Engineering has been investigating how a fast TEM system might be applied for UXO characterization. In this paper, we explore how broadband induction EM responses (i.e., TEM transients, or FEM spectra) can provide a basis for UXO classification. In that regard, the next section will review briefly some important characteristics of the inductive EM response of confined conducting and permeable objects. These characteristics, long recognized by exploration geophysicists, provide a basis for expanding an FEM spectrum or TEM transient as a series of characteristic modal functions whose parameters contain information about the conductivity and size characteristics of the target. One method for the decomposition of the EM response into these characteristic modes, Prony's method, is discussed briefly and is applied to both synthetic and real TEM transients. Finally, we present data acquired with a prototype antenna system consisting of a horizontal transmitting antenna and a 3-axis receiving antenna. With this antenna system, data were acquired using a Zonge 3-channel NanoTEMTM system.

Physics based characterization of UXO from multicomponent TEM data

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Newly developed transient electromagnetic (TEM) equipment collects four-dimensional data with three dB/dt components and time variation. Transient data from three receiver loops are recorded at 31 delay times, ranging from 1 to 1900 usec, and transients are recorded 32 times per second, generating high-density data sets. The equipment is flexible, transmitter and receiver loops can be reconfigured to optimize survey results based on expected target characteristics. With such large data sets and variable equipment configuration, modelling plays an important role. Inversion to dipolar models extracts physically meaningful target parameters from high-density TEM data.

Patches of TEM data near target anomalies are parameterized with the widely used anisotropic dipole model. Three orthogonal magnetic dipoles are used to represent target polarizability along three axes. Spatial variation of receiver loop dB/dt across the data patch is projected onto three, generally tilted target axes, compressing data from hundreds of measurement positions and multiple loop orientations into three polarizability values for each transient delay time. Time-dependent target properties are parameterized by fitting a transient-shape model to the polarizability transient for each of the three target axes.

Targets can be characterized by these physics-based model parameters. Transient shape is influenced by target size, shape, conductivity and permeability. Polarizability magnitude is proportional to target volume. Ratios of target-axis polarizability are indicative of target symmetry. We present application of our modelling routines to multi-component data obtained from a demonstration survey at NRL UXO test site located at the Army Research Lab facility at Blossom Point, MD.

Fast TEM for UXO mapping at Gambell, Saint Lawrence Island, Alaska

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A fast-sampling, Transient Electromagnetic (TEM) system was assembled for mapping probable UXO submerged in a shallow fresh-water lake near Gambell, Saint Lawrence Island, Alaska. The survey was designed to generate optimal data given the survey area, UXO characteristics, and the required depth of exploration. Since the search area was a fresh-water lake in an arctic climate, the survey was conducted in early May. The lake ice was still sufficiently thick to provide safe access, but most of the snow cover had melted, making it easy to move equipment across the search area. A continuous-sampling TEM system that records averaged data every three seconds was mounted on a sled and towed at a slow walk by the equipment operator. A real-time differential GPS unit recorded the sled's location every five seconds.

Using this system, a total of 426,000 line-feet of profile with an average sample interval of 7.5 ft were surveyed over a period of 14 days. At each sample point, vertical-component transient data were recorded at 26 delay times, ranging from 7 to 570 microseconds. In this paper, we describe in detail the design of the antenna system and the survey. Area maps of the TEM response at several gates are presented. Using the maps and transient profile plots, target locations were picked. Analysis of the transient decay curves observed over the targets provides additional information about target size and conductivity.

Target Prioritization In TEM Surveys For Sub-Surface UXO Investigations Using Response Amplitude, Decay Curve Slope, Signal To Noise Ratio, And Spatial Match Filtering

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To reduce the number of false-positives in identifying UXOs and related debris for ongoing Site Investigation, a target prioritization routine was developed based on the likelihood of a metallic source. Numerous parameters were evaluated statistically using over 2000 existing intrusive investigation results. Those showing the best ability to discriminate between metallic and non-metallic sources were incorporated into the scheme, including peak amplitude response, power-law decay slope, signal to noise ratio, and spatial match filter response. Parameters were assigned increasing numeric values based on likelihood of a metallic source, and summed to produce a target rank. Subsequent application to over 4000 new intrusive investigations revealed that the highest ranked 25% of targets were nearly five times more likely to yield a metallic source when investigated than the lowest ranked 25%. These results show that target selection routines in UXO investigations would benefit from using more parameters than just magnitude response, allowing more aggressive target identification and reduced costs by lowering the number of false positives that are investigated. Proposed refinements may further increase predictive capabilities.

Possibilities for UXO classification using characteristic modes of the broad band electromagnetic induction resistivity

Download Zonge document

The measurement of the broadband induction electromagnetic response in the form of a complex function of frequency (FEM) or, alternatively, as a transient function of time (TEM) has been applied in geophysical exploration for 30 years or more. Broadband EM methods are used in exploration in two different ways: 1) to perform "soundings" wherein the objective is to map the earth conductivity as a function of depth, and 2) for "inductive prospecting" wherein the broadband response permits the detection and characterization of large highly conductive "ore bodies" at great depth. Until recently, broadband induction EM methods were not routinely applied for shallow exploration problems. The principles of the induction EM method require that as the geometric scale of the problem decreases, there must be a corresponding broadening of the frequency range of interest in FEM systems or, equivalently, shortening of the time interval of interest in TEM systems. At present only a few field instruments are available with the requisite bandwidth for effective application to sounding or prospecting in the shallow subsurface (< 30 m).

Elementary induction EM principles are also applied in metal detectors and as such they have enjoyed a long and successful history in applications such as utilities location and in the location of metallic mines. Metal detectors are typically optimized for detecting very small objects located within a few 10's of cm from the surface. Recently, however, new induction EM instruments have been developed specifically for shallow metal detection and site characterization. These instruments have significantly increased the depth of detection for shallow-buried metallic objects and, at least in one case, they provide measurements at more than one frequency or time delay. These instruments are now widely applied for detecting and mapping shallow-buried metal objects including UXO.

The potential for using the characteristics of the broadband induction EM response measured in the proximity of buried metallic objects to discriminate target types is generally recognized. In the context of UXO detection with TEM, McNeill et. al., concluded that ". . ., given "a priori" knowledge of the decay characteristics of UXO that are expected in a survey area, the evidence presented in this paper suggests that it might be possible to separate out various types of UXO (a) from each other and (b) from exploded ordnance and other trash metal." Moreover, there is ongoing research and development directed toward developing instruments and techniques for object detection and classification with broadband induction EM.

In cooperation with Earth Tech, Zonge Engineering has been investigating how a fast TEM system might be applied for UXO characterization. In this paper, we explore how broadband induction EM responses (i.e., TEM transients, or FEM spectra) can provide a basis for UXO classification. In that regard, the next section will review briefly some important characteristics of the inductive EM response of confined conducting and permeable objects. These characteristics, long recognized by exploration geophysicists, provide a basis for expanding an FEM spectrum or TEM transient as a series of characteristic modal functions whose parameters contain information about the conductivity and size characteristics of the target. One method for the decomposition of the EM response into these characteristic modes, Prony's method, is discussed briefly and is applied to both synthetic and real TEM transients. Finally, we present data acquired with a prototype antenna system consisting of a horizontal transmitting antenna and a 3-axis receiving antenna. With this antenna system, data were acquired using a Zonge 3-channel NanoTEMTM system.