DETECTION AND CHARACTERIZATION OF SUBSURFACE CONDUCTORS

Examples from the Berkeley Unexploded Ordnance Discrimination (BUD) Project

H Frank Morrison Erika Gasperikova

Torquil Smith Alex Becker

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Unexploded ordnance (UXO, sometimes abbreviated as UO), unexploded bombs (UXBs), or explosive remnants of war (ERW) are explosive weapons (bombs, shells, grenades, land mines, naval mines, cluster munition, etc.) that did not explode when they were employed and still pose a risk of detonation, sometimes many decades after they were used or discarded. UXO does not always originate from wars; areas such as military training grounds can also hold significant number of UXO, even after the area has been abandoned. UXO from World War I continue to be a hazard, with poisonous gas filled munitions still a problem. When unwanted munitions are found, they are sometimes destroyed in controlled explosions, but accidental detonation of even very old explosives also occurs, sometimes with fatal results. In addition to the obvious danger of explosion, buried UXO can cause environmental contamination. In some heavily used military training areas, munitions-related chemicals such as explosives and perchlorate (a component of pyrotechnics and rocket fuel) can enter soil and groundwater.

According to US Environmental Protection Agency documents released in late 2002, UXO at 16,000 domestic inactive military ranges within the United States pose an "imminent and substantial" public health risk and could require the largest environmental cleanup ever, at a cost of at least US$14 billion. Some individual ranges cover 500 square miles (1,300 km2), and, taken together, the ranges comprise an area the size of Florida.

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UXO cleanup in the US involves over 10 million acres (40,000 km2) of land and 1,400 different sites. Estimated cleanup costs are tens of billions of dollars. It costs roughly $1,000 to demolish a UXO on site. Other costs include surveying and mapping, removing vegetation from the site, transportation, and personnel to manually detect UXOs with metal detectors. Searching for UXOs is tedious work and often 100 holes are dug to every 1 UXO found. Other methods of finding UXOs include digital geophysics detection with land and airborne systems.

There are areas in the millions of acres that have underwater UXO. The US Army Corps of engineers has identified 400 underwater formerly used Defense sites. These sites all have munitions or UXO in less than 40 m of water. A size-depth plot typical for these UXO from DiMarco et al. (2010) was used as a design guide for the first MBUD system and is reproduced here as Figure 1. A compilation of polarizability calculations from a variety of targets for BUD and MBUD is shown by the dashed line in Figure 1. This line indicates that the BUD/MBUD systems can effectively classify virtually all targets down to about 1.2 m.

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Figure 1

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All subsurface conductor detectors rely on detecting the secondary magnetic fields arising from magnetization, or induced Faraday currents, caused by inducing magnetic fields from an external source.

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MX

Vertical loop

BZ BX

Induced dipole moment

Induced current

X

Induced Faraday currents in a non-ferrous conductor

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Induced magnetization moment, MDC , and induced Faraday moment, MAC

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TRANSMITTERS

Waveform:

Psedo random

T Sinusoidal

Square wave

T

b

a

Half sine

Triangle

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Induced moment, M, = k.F(t) where k is a function of the T-R geometry and inducing step-function field strength

R

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Current in transmitter (primary magnetic field , BP)

Receiver coil emf from

Expanded secondary field transient in 2 msec off-time

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Receiver coil emf from 𝜕𝜕𝑃

𝜕𝜕

plus secondary field from target.

INDUCED MOMENTS THAT HAVE ANALYTICAL FORMULAS

Black: magnetic moments - MDC

Red: Faraday (electromagnetic) moments - MAC

oblate ellipsoid prolate ellipsoid sphere

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MDC (black) and MAC (red) for some regular shapes 14 The Berkeley Course in Applied Geophysics

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PRINCIPAL DIPOLE MOMENTS AND POLARIZABILITY

Mx My

Mz Any body can be approximately represented by three orthogonal dipole moments, the Principal Dipole Moments (PDM) [Am2]

These PDMs change with frequency or time

The PDMs are functions of the size, shape, conductivity and permeability of the body

The polarizability, P, is defined as the dipole moment divided by the inducing magnetic field

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CIRCUIT SYSTEM ELEMENTS

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RESPONSE FROM THREE ORTHOGONAL TRANSMITTERS

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Horizontal loop Vertical loop

MULTIPLE TARGET POLARIZATION

a)

b)

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Solutions for polarizabilities using a three-component transmitter and a co-located three component receiver at multiple locations on a grid

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Estimated polarizability

9-INCH SPHEROID

True polarizability

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Angle = 45°, z = 0.6 m Horizontal, z = 0.7 m

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9-INCH STEEL SPHEROID

ALUMINUM

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9-INCH METAL SPHEROID

STEEL

MAGNETIC SPHERE B and dB/dt

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A

t

system noise

ground response

dB/dt

SYSTEM OPTIMIZATION

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T R

0.2 m

0.2 m

τ = 104

ρ = 10 Οηµ.m

L

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Berkeley UXO Discriminator (BUD) 1 m

1 m

BUD

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TEAM: (from left to right) Erika Gasperikova, Frank Morrison, Torquil Smith, Alessandro Ratti, Robin Lafever, Jim Greer, Alex Becker, Harold Yaver, Larry Doolittle, and in absence Jean-Francois Beche FIELD CREW: K. Kappler, R. Haught, M. Szajbler, A. Morales, P. Cook

2004 “Project of the Year” Award from the Strategic Environmental Research and Development Program (SERDP) 2007 R&D100 Award

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Transmitter coils

Receiver coils

HAND-HELD UXO DISCRIMINATOR

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IMPROVISED EXPLOSIVE DEVICE (IED) ROAD CLEARANCE SYSTEM

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MARINE BUD (MBUD) CONFIGURATION

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.75m

GROUND RESPONSE

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DISCRIMINATION

True Estimated

Z 43 cm 41.2 cm

X 0 cm 1.8 cm

Y 0 cm - 1.0 cm

φ 0º ± 1° 3º

Non - UXO UXO

True Estimated

Z 75 cm 74.0 cm

X 0 cm -1.8 cm

Y 0 cm 0.4 cm

φ 0º -0.5º

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16 Gage 15 cm Loop 12# Shotput

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BUD RESULTS

20 mm Projectile

Vertical, z = 0.43 m Horizontal, z = 0.45 m

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Vertical, z = 0.9 m Horizontal, z = 1.30 m

155 mm Projectile

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BUD RESULTS

20 mm projectile - 56 cm deep

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IED RESULTS

Scrap metal --- 56 cm deep

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IED RESULTS

US Quarter -- 36 cm deep

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IED RESULTS

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D cell -- 56 cm deep

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IED RESULTS

I(w)

Induced currents

VERTICAL CURRENT LOOP ANTENNA

a a Capacitive electrode

x

h

d

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I(w)

Induced currents

VERTICAL CURRENT LOOP ANTENNA

Capacitive Electrode pair

x

l h

EX

EX anomaly

A

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Ex V/m

+x

Ex

+z

d

My

E0 Px

By

By nT

Vertical Loop Transmitter

By Sensor Ex Dipole Sensor

By Ex h

px

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Ex V/m

+x Ex

+z

d

My

E0

By

By nT

Vertical Loop Transmitter

By Sensor Ex Dipole Sensor

By

Ex

h

px

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Ex V/m

+x Ex

+z

d

My

E0

By nT

Vertical Loop Transmitter

By By Sensor

By

Ex Dipole Sensor Ex

h

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