Leonard R. Pasion

A Discrimination Algorithm for UXO Using Time Domain Electromagnetics

An assumption is made that the Time Domain Electromagnetic (TEM) response of a buried axisymmetric metallic object can be modelled as the sum of two dipoles centered at the midpoint of the body. The strength of the dipoles depends upon the relative orientation between the object and the source field, and also upon the shape and physical properties of the body. Upon termination of the source field, each dipole is assumed to decay as k(t+α)−βe−t∕γ. The parameters k, α, β and γ depend upon the conductivity, permeability, size and shape of the object, and these can be extracted from the measurements by using a nonlinear parametric inversion algorithm. Investigations carried out using an analytic solution for a sphere and laboratory measurements of steel and aluminum rectangular prisms, suggest the following two-step methodology: (1) The value of β is first used as a diagnostic to assess whether the metallic object is non-magnetic or magnetic, (2) the ratios of k1∕k2 and β1∕β2 are then diagnostic indicators as...

Evaluating the effects of magnetic soils on TEM measurements for UXO detection

The task of discriminating UXO from non-UXO items are more difficult when sensor data are contaminated with geological noise originating from magnetic soils. In particular, magnetic material affects the decay curve characteristics in electromagnetic surveys and this can adversely affect inversion calculations that try to estimate parameters of the UXO or determine whether the item is UXO or scrap. This talk focuses on the effects magnetic soils and magnetic viscosity have on time domain and frequency domain electromagnetic sensor data. By forward modelling of a 1-D layered earth model we investigate the sensitivity of surveys to magnetic soils, and attempt to determine the relative strengths of magnetic soils and buried metallic objects. In particular, we consider soil susceptibility models appropriate for Kaho’olawe Island, Hawaii, where the UXO clearance project has been hampered by the presence of highly magnetic basaltic soils. Finally, we investigate how perturbations in signal due to magnetic soil affect the ability to recover parameters of the dipole model presented in Pasion and Oldenburg (2001a).

Unexploded ordnance discrimination using magnetic and electromagnetic sensors: Case study from a former military site

In a study at a military range with the objective to discriminate potentially hazardous 4.2-inch mortars from nonhazardous shrapnel, range, and cultural debris, six different discrimination techniques were tested using data from an array of magnetometers, a time-domain electromagnetic induction (EMI) cart, an array of time-domain sensors, and a time-domain EMI cart with a wider measurement bandwidth. Discrimination was achieved using rule-based or statistical classification of feature vectors extracted from dipole or polarization tensor models fit to detected anomalies. For magnetics, the ranking by moment yielded better discrimination results than that of apparent remanence from relatively large remanent magnetizations of several of the seeded items. The magnetometer results produced very accurate depths and fewer failed fits attributable to noisy data or model insuffi-ciency. The EMI-based methods were more effective than the magnetometer for intrinsic discrimination ability. The higher signal-to-noise ...

Joint and Cooperative Inversion of Magnetic and Time Domain Electromagnetic Data for the Characterization of UXO

Magnetics and electromagnetic surveys are the primary techniques used for UXO remediation projects. Magnetometry is a valuable geophysical tool for UXO detection due to ease of data acquisition and its ability to detect relatively deep targets. However, magnetics data can have large false alarm rates due to geological noise, and there is an inherent non-uniqueness when trying to determine the orientation, size and shape of a target. Electromagnetic surveys, on the other hand, are relatively immune to geologic noise and are more diagnostic for target shape and size but have a reduced depth of investigation. In this paper we aim to improve discrimination ability by developing an interpretation method that takes advantage of the strengths of both techniques. We consider two different approaches to the problem: (1) Interpreting the data sets cooperatively, and (2) Interpreting the data sets jointly. For cooperative inversion information from the inversion of one data set is used as a constraint for inverting another data set. In joint inversion, target model parameters common to the forward solution of both types of data are identified and the model parameters from all the survey data are recovered simultaneously. We compare the confidence with which we can discriminate UXO from non-UXO targets when applying these different approaches to results from individual inversions. In this paper we focus on the details of the joint and cooperative inversion methodologies. Examples of the application of the methodology to TEM and magnetics data sets collected at the former Fort Ord in California are presented. This work is funded in part by the U.S. Army Engineer Research and Development Center and the Army Research Office.

EXAMPLES OF THE EFFECT OF MAGNETIC SOIL ENVIRONMENTS ON TIME DOMAIN ELECTROMAGNETIC DATA

During September 2004 a field study was carried out on the Hawa iian Island of Kaho’olawe to explore various aspects of the effect of magnetic soils on time domain electromagnetic (TEM) measurements. This field work was in support of two Strategic Environmental Research and Development Program funded research projects (UX1355 and UX1414) whose goals are to inv estigate the source and spatial variability of magnetic soil anomalies, to create a methodology for mode lling the response of magnetic soils, and to develop TEM data collection techniques that can better di scriminate between the response of magnetic soils and unexploded ordnance (UXO). Detailed electromagn etic surveys were carried out at a test site on the island. The data from the surveys verify the commonly hel d b lief that magnetic variations in the soil can complicate the identification of UXO. However, the data a lso show that short wavelength variations in the TEM response due to micro-topographic variations and co il orientation effects can generate responses that could mask a UXO and/or result in a false positive. An ove rview of the surveys, a discussion of preliminary results and some practical recommendations fo r urveying in magnetic soil environments will be presented.

Cooperative Inversion of Time Domain Electromagnetic and Magnetometer Data for the Discrimination of Unexploded Ordnance

Magnetic and electromagnetic induction (EMI) surveys are the primary techniques used for unexploded ordnance (UXO) remediation projects. Magnetometry is a valuable geophysical tool for UXO detection because of the ease of data acquisition and its ability to detect relatively deep targets. However, magnetic data can have large false alarm rates caused by geological noise, and there is an inherent non-uniqueness when trying to determine the orientation, size and shape of a target. EMI surveys, on the other hand, are relatively immune to geologic noise and are more diagnostic for target shape and size but have a reduced depth of investigation. We aim to improve discrimination ability by developing an interpretation method that takes advantage of the strengths of both techniques. We consider cooperative inversion, where information from the inversion of one type of data is used as a constraint for inverting another. We compare the confidence with which we can discriminate UXO from non-UXO targets when inverting the data sets cooperatively, to results from individual inversions. Examples are given of the application of the methodology to time domain electromagnetic induction (TEM) and magnetic data sets collected at the Yuma Proving Ground UXO Standardized Test Site calibration grid and the Former Camp Sibert.

Locating And Determining Dimensionality Of Uxos Using Time Domain Electromagnetic Induction

We assume that the Time Domain Electromagnetic (TEM) response of a buried axisymmetric metallic object can be modelled as the sum of two dipoles centered at the midpoint of the body. The strength of the dipoles depends upon the relative orientation between the object and the source field, and also upon the shape and physical properties of the body. Upon termination of the source field, each dipole is assumed to decay as . The parameters , , and depend upon the conductivity, permeability, size and shape of the object, and these can be extracted from the measurements by using a nonlinear parametric inversion algorithm. Investigations carried out using an analytic solution for a sphere and laboratory measurements of steel and aluminum rectangular prisms, suggest the following two-step methodology: (1) The value of is first used as a diagnostic to assess whether the metallic object is non-magnetic or magnetic, (2) the ratios of and are then diagnostic indicators as to whether the geometry is plate-like (uninteresting) or rodlike (a high candidate for being a UXO). Results from the application of this algorithm to a TEM field data set acquired at the United States Army Corps of Engineers Environmental Research and Development Centre UXO Test Site have successfully identified a UXO to be magnetic and rod-like.

Computing Transient Electromagnetic Responses of a Metallic Object Using a Spheroidal Excitation Approach

The model-based spheroidal excitation approach is extended to accurately compute the transient electromagnetic response of a highly conducting and permeable object. The complete formulation is presented in the convolution form. The method is applicable to data from various sensors that measure the transient field or voltage in either the on- or off-time regime and for an arbitrary transmitter waveform. The technique is tested against state-of-the-art time-domain electromagnetic induction sensor data. The results show good agreements between modeled and measured data. In addition, numerical studies demonstrate the importance of accounting for the transmitter waveform.

Nonlinear Inversion for Multiple Objects in Transient Electromagnetic Induction Sensing of Unexploded Ordnance: Technique and Applications

We develop an inversion technique to process overlapping data that arise from closely spaced targets. In contrast to a usual single-object inversion model, a multiobject problem is more challenging because of the increased number of parameters to be found and because of the additional nonlinearity and nonuniqueness. Our solution strategy is to break down the full problem into a sequence of smaller problems so that optimization is conducted in a lower dimensional model space. In the numerical implementation, a set of nonlinear model parameters, e.g., the locations of the underlying sources, is sought while the set of linear model parameters, i.e., their polarization tensors, are updated accordingly in a nested manner. This is an explicit separable nonlinear optimization technique that we cast. We employ a joint diagonalization to find an average principal direction among multiple magnetic polarizability tensors. Since the principal directions are more sensitive to the inaccuracies in the estimated polarization tensor, we suggest a subsequent procedure to optimize the two sets of parameters: orientation and principal polarizations of objects. For initialization, we propose a selected multistart nonlinear algorithm for source localizations that paves an efficient way to find a good initial guess of model parameters and makes the nonlinear inversion effectively automated. We report the new applications of the technique to the test-stand and field data acquired with next-generation sensor systems of the TEMTADS and MetalMapper and study the issue of the spatial resolution of overlapping anomalies through inversions and using the metric defined as the total uncertainty of the polarizabilities.

Assessing the Quality of Electromagnetic Data for the Discrimination of UXO Using Figures of Merit

The need for assessing data quality in unexploded ordnance (UXO) remediation problems arises from two sources. In the planning stage it is essential that the data are acquired in sufficient numbers and with sufficient accuracy to answer the detection or discrimination problem of relevance. At the interpretation stage it is critical to objectively assess whether the data are of sufficient quality to warrant subsequent processing, inversion, and classification. Faced with this practical challenge of defining data quality we propose a Figure of Merit (FOM). FOM is a reliability indicator derived from quantities that affect the quality of data, such as anomaly coverage, line spacing, station spacing, instrument noise, survey location errors, etc. The FOM can also include informative features of the inversion, such as the variance of key model parameters, and thus it depends on the inverse model to be applied. Anomalies associated with higher values of FOM should have increased reliability in classification. A...