H.F. Morrison

A new approach to modeling the electromagnetic response of conductive media

We introduce a new and potentially useful method for computing electromagnetic (EM) responses of arbitrary conductivity distributions in the earth. The diffusive EM field is known to have a unique integral representation in terms of a fictitious wave field that satisfies a wave equation. We show that this integral transform can be extended to include vector fields. Our algorithm takes advantage of this relationship between the wave field and the actual EM field. Specifically, numerical computation is carried out for the wave field, and the result is transformed back to the EM field in the time domain. The proposed approach has been successfully demonstrated using two‐dimensional (2‐D) models. The appropriate TE‐mode diffusion equation in the time domain for the electric field is initially transformed into a scalar wave equation in an imaginary q domain, where q is a time‐like variable. The corresponding scalar wave field is computed numerically using an explicit q‐stepping technique. Standard finite‐diffe...

A numerical solution for the electromagnetic scattering by a two‐dimensional inhomogeneity

A numerical solution for electromagnetic scattering from a two‐dimensional earth model of arbitrary conductivity distribution has been developed and compared with analog model results. A frequency‐domain variational integral is Fourier transformed in the strike direction, and a solution is obtained using the finite‐element method for each of a finite number of harmonics or wavenumbers in transform space. The solution is obtained in terms of the secondary electric fields. Principally due to the inaccuracy associated with numerical derivatives of electric fields, the secondary magnetic field is computed by integrating over the scattering currents in harmonic space and is then inverse Fourier transformed.

UXO detection and identification based on intrinsic target polarizabilities: A case history

Electromagnetic induction data parameterized in time dependent object intrinsic polarizabilities allow discrimination of unexploded ordnance (UXO) from false targets (scrap metal). Data from a cart-mounted system designed for discrimination of UXO with 20 mm to 155 mm diameters are used. Discrimination of UXO from irregular scrap metal is based on the principal dipole polarizabilities of a target. A near-intact UXO displays a single major polarizability coincident with the long axis of the object and two equal smaller transverse polarizabilities, whereas metal scraps have distinct polarizability signatures that rarely mimic those of elongated symmetric bodies. Based on a training data set of known targets, object identification was made by estimating the probability that an object is a single UXO. Our test survey took place on a military base where both 4.2-inch mortar shells and scrap metal were present. The results show that we detected and discriminated correctly all 4.2-inch mortars, and in that process we added 7%, and 17%, respectively, of dry holes (digging scrap) to the total number of excavations in two different survey modes. We also demonstrated a mode of operation that might be more cost effective than the current practice.

Experience with the EM-60 electromagnetic system for geothermal exploration in Nevada

Lawrence Berkeley Laboratory (LBL) conducted controlled-source electromagnetic (EM) surveys at three geothermal prospects in northern Nevada. Over 40 soundings were made in Panther Canyon (Grass Valley), near Winnemucca; Soda Lakes, near Fallon; and McCoy, west of Austin, to test and demonstrate the applicability of LBLs EM-60 system to geothermal exploration. The EM-60 is a frequency-domain system using three-component magnetic detection. Typically, +-65 A is applied to an 100-m-diameter four-turn horizontal loop, generating a dipole moment >10/sup 6/ MKS over the frequency range 10/sup -3/ to 10/sup -3/ Hz. With such a source loop, soundings were made, at transmitter-receiver separations of up to 4 km, providing a maximum depth of penetration of 4 km.

A Multisensor system for the detection and characterization of UXOMM-0437

A prototype active electromagnetic system has been developed for detecting and characterizing UXO. The system employs two orthogonal vertical loop transmitters and a pair of horizontal loop transmitters spaced apart vertically by 0.7 m. Eight vertical field detectors are deployed in the plane of each of the horizontal loops and are arranged to measure offset vertical gradients of the fields. The location and orientation of the three principal polarizabilities of a target can be recovered from a single position of the transmitter-receiver system. Further characterization of the target is obtained from the broadband response. The system employs a bipolar half sine pulse train current waveform and the detectors are dB/dt induction coils designed to minimize the transient response of the primary field pulse. The target transient is recovered in a 40 mu sec to 1.0 msec window. The ground response imposes an early time limit on the time window and system/ambient noise limits the late time response. Nevertheless for practical transmitter moments and optimum receivers the size and the ratio of conductivity to permeability can be accurately recovered. The prototype system has successfully recovered the depths and polarizabilities of ellipsoidal test targets.

UXO Discrimination Using a Multiple-component AEM System

Summary Electromagnetic induction data transformed into time dependent object intrinsic polarizabilities allow discrimination of unexploded ordnance (UXO) from false targets (scrap metal). Discrimination of UXO from irregular scrap metal is based on the principal dipole polarizabilities of a target. A near-intact UXO displays a single major polarizability coincident with the long axis of the object and two equal smaller transverse polarizabilities, whereas metal scraps have distinct polarizability signatures that rarely mimic those of elongated symmetrical bodies. Moreover, UXO have unique polarizability signatures, and thus distinctions can be made among various UXO targets. We have developed several approaches for discriminating UXO from scrap metal, based on these object intrinsic polarizabilities as a function of time and a small training data set of known targets. Ground truth from one survey area confirmed that our approach was highly successful we identified all UXO and added relatively few additional “dry” holes (excavations that unearth scrap metal) to the total number of excavation points while maintaining the highest conservatism in determining “stop digging point”.

Design and performance of a hand-held UXO discriminator

Summary Because hand-held systems have the advantage of being lightweight, compact, portable, and deployable under most site conditions, they are particularly useful in areas of dense vegetation or challenging terrain. In heavily wooded areas or areas with steep or uneven terrain, hand-held sensors may be the only suitable device for unexploded ordnance (UXO) detection and discrimination because it can be carried through spaces that the operator could walk through or at least approach. Based on these considerations, we designed and built a sensor package in a shape of a 14-in (0.35 m) cube. This time-domain electromagnetic handheld prototype incorporates the key features of the cartmounted system – (a) three orthogonal transmitters and ten pairs of receivers, and (b) difference or gradient measurements that significantly reduce the ambient and motion noise, and greatly enhance the sensitivity to the gradients of the target. Electromagnetic induction data transformed into time dependent object intrinsic polarizabilities allow discrimination of UXO from false targets (scrap metal). A near-intact UXO displays a single major polarizability coincident with the long axis of the object and two equal smaller transverse polarizabilities, whereas metal scraps have distinct polarizability signatures that rarely mimic those of elongated symmetrical bodies. Our prototype results have demonstrated that the same discrimination capabilities afforded by the cart-mounted system are available in the hand-held unit albeit with a slightly reduced depth of detection. The hand-held UXO discriminator is able to discriminate small (20 mm) objects at a depth of 0.45 m and large (105 and 155 mm) objects at the depth of 0.85 m. Large objects are detected down to 1.15 m.

Hand-held UXO Discriminator

With prior funding (UX-1225, MM-0437, and MM-0838), we have successfully designed and built a cart-mounted Berkeley UXO Discriminator (BUD) and demonstrated its performance at various test sites (e.g., Gasperikova et al., 2007, 2009). It is a multi-transmitter multi-receiver active electromagnetic system that is able to discriminate UXO from scrap at a single measurement position, hence eliminates equirement of a very accurate sensor location. The cart-mounted system comprises of three orthogonal transmitters and eight pairs of differenced receivers (Smith et al., 2007). Receiver coils are located on ymmetry lines through the center of the system and see identical fields during the on-time of the pulse in all of the transmitter coils. They can then be wired in opposition to produce zero output during the n-ime of the pulses in three orthogonal transmitters. Moreover, this configuration dramatically reduces noise in the measurements by canceling the background electromagnetic fields (these fields are uniform ver the scale of the receiver array and are consequently nulled by the differencing operation), and by canceling the noise contributed by the tilt of the receivers in the Earths magnetic field, and therefore reatly enhances receivers sensitivity to the gradients of the target.

A multisensor system for detection and characterization of UXO(MM-0437) - Demonstration Report

DEMONSTRATION REPORT A MULTISENSOR SYSTEM FOR THE DETECTION AND CHARACTERIZATION OF UXO MM-0437 SITE LOCATION: U.S. ARMY YUMA PROVING GROUND DEMONSTRATOR: LAWRENCE BERKELEY NATIONAL LABORATORY ONE CYCLOTRON ROAD, MS: 90R1116 BERKELEY, CA 94720 p.o.c. Erika Gasperikova, egasperikova@lbl.gov, 510-486-4930 TECHNOLOGY TYPE/PLATFORM: BUD/CART JUNE 2006