Empirical Investigation of the Factors
Influencing Marine Applications of EMI
MR-2409
Thomas Bell, SAIC
Daniel Steinhurst, Nova Research
Carl Friedrichs, VIMS
Brief to the Scientific Advisory Board
September 11, 2013
1
Performers
Drs. Thomas Bell, Dean Keiswetter and Bruce Barrow
Science Applications International Corporation
Specialists in applications of electromagnetic induction to UXO
detection and classification
Dr. Daniel Steinhurst and Mr. Glenn Harbaugh
Nova Research
Specialists in UXO detection and classification technology
Drs. Carl Friedrichs and Grace Cartwright
Virginia Institute of Marine Science
Specialists in sediments and marine technology
2
Problem Statement
●
●
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There is a significant munitions contamination problem in
U.S. coastal and inland waters
SERDP/ESTCP-developed advanced EMI sensor arrays
can reliably detect and classify buried munitions on land
under operational conditions
The marine environment introduces complexities in the
response of these sensor systems which can adversely
affect performance

There are significant gaps in our understanding of the effects of
the marine environment on relevant EMI signals and noise
3
Technical Objective
?
●
Determine how to implement EMI-based classification in
marine environments

Complexities will constrain applications
4
Technical Background: Conventional EMI
●
●
EMI sensors measure
magnetic fields from
currents induced in target
Signals well understood
and characterized

●
Classification is based on
target response library
matching
Noise sources (MR-200508)



Uncompensated variations in sensor background response
Atmospheric EM fluctuations (lightning, radio, etc.)
Sensor motion effects (fluctuations in soil response & changing
geomagnetic flux)
5
Technical Background: Marine Environment
●
Conducting host medium
complicates marine EMI





●
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Seawater return
Signal attenuation and
distortion by seawater
Electric field (current
channeling) signals
Sediment conductivity
Air-sea interface effects
Noise effects modified by marine environment
Model calculations substantially more involved

Complicates estimation of target parameters for classification
6
Technical Background: Seawater Response
●
Primary field excites electric
currents in seawater, sensor
sees induced magnetic field

●
Model studies in earlier SERDP
projects (MR-1632, -2228) have
drawn conflicting conclusions
regarding significance
Interface effects (surface &
bottom) modulate background
response

Seawater response comparable
to TEM background levels
There is little relevant data on
sediment properties
105m projectile (50 cm range) and
background signals from Camp
Beale demonstration compared
with calculated seawater response
7
Technical Background: Signal Distortion
●
EMI classification is based on
signal library matching

●
Uncompensated signal distortion
can compromise classification
performance
Marine environment modifies
EMI signals


Signal includes contributions
from electric field effects, plus
attenuation and phase shifts due
to EM diffusion in seawater
Can complicate estimation of
target features
Seawater distortion examples
steel bar in water
air
Measured FDEM response
San Filipo & Won, MR-1321
Calculated TEM distortion
8
Technical Background: Noise Effects
●
●
Classification limited by noise
Noise modified by marine
environment, net impact unclear



Sensor motion effects are reduced
by smoother motion of towed
platform in water (but may be
amplified by high conductivity of
bottom sediments)
Atmospheric noise is reduced by
seawater attenuation
Additional noise can be introduced
by surface wave modulation of
seawater response
Smoother sensor trajectory
with underwater tow
in water
on land
9
Technical Background (Summary)
●
Theory difficult for all but simplest geometries


●
Few controlled tests to measure relevant factors and
effects

●
Model studies have drawn conflicting conclusions (MR-1632 vs.
MR-2228)
Effects appear to be neither insignificant nor overwhelming
MR-1321: In-air & in-water measurements with GEM-3
(frequency domain) sensor - observed signal distortion due to
propagation and/or electric field (current channeling) effects at
frequencies above a few kHz
Little data on sediment properties relevant to EMI


Electrical conductivity, magnetic susceptibility
Spatial variability
10
Technical Approach
Task 1
Task 2
Tank Measurements
of Seawater Effects
Measurements of
Sediment EM
Properties
Task 4
Modeling and
Analysis
Task 3
Field Tests of EMI in
Marine Environment
Task 5
Go/NoGo Decision. Proceed to field tests if results
of tank tests and modeling of seawater and sediment
effects indicate that EMI classification should be
possible in typical marine environments.
Reporting and Performance
Assessment
11
Tank Measurements
●
Controlled measurements of salt water response, signal
distortion, boundary effects, etc.

●
Effects of target surface condition on electric field (current
channeling) signal components
Identify sensor/target geometries for which target feature
extraction is practicable



Large molded polyethylene
storage tank
Salt/water mixtures to simulate
marine environments
NRL TEM sensors and
standard test items (insulated
& bare metal)
12
Field Measurements
●
York River Estuary





●
VIMS research vessels and
instrumentation

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Tidal range ~ 1 m
Depth 0 to 20 m
Salinity 0 to 25 o/oo
Muddy and sandy areas
Well characterized
R/V Elis Olsson
VIMS
Dockside EMI measurements
to evaluate range of R/V hull
influence
NRL EMI sensors
13
Field Tests (year 1): Sediment Properties
●
●
EM properties of bottom sediments at potential test sites
Box cores from summer 2014 VIMS cruises


Local sampling to address spatial variability
Topside measurements of electrical resistivity and magnetic
susceptibility vs. depth
14
Field Tests (year 2): In Situ EMI Effects
●
Space-time variability of seawater effects in a natural
environment


Thermohaline stratification, fine structure, internal waves
Surface waves, bottom sediments, tides & currents
• TEM profiles (with and
without reference object)
• ADCP current profiles
• Niskin water samples for
suspended sediments
• Conductivity-temperaturedepth (CTD) profiles
• Bottom samples
15
Physics-Based Models for Field Test
Analysis and Interpretation
●
●
Standard EM models for seawater signals (from
geophysics and NDT applications)
Dipole models for target signals


●
Linearized solutions for air/sea interaction effects

●
Magnetic and electric polarizabilities, bare metal and insulated
surface
Attenuation and phase shift due to conducting host medium
Wave amplitude << skin depth
Diffusion models for sediment effects
16
Performance Assessment
●
Validate EM models for marine applications

●
Signal distortion

●

Effects of spatial variation of bottom return
Reduction in atmospheric noise & sensor motion noise
Implications for system design


●
Impact on classification performance & processing implications
Noise levels

●
Simplifying assumptions & parameter values
Tx coil size & Rx coil configuration
Tow height above bottom
System response curves for targets of interest
17
Year 1 Project Plan
1. Tank Measurements
2. Sediment Properties
3. Field Tests
4. Modeling and Analysis
5. Reporting and Assessment
Total
$294.0K
71.5K
10.0K
154.6K
51.9K
$582.0K
18
Overall Project Plan
Year 1
Year 2
Y3
Task 1. Tank Measurements
of Seawater Effects
Task 2. Measurements of
Sediment Properties
Task 3. Field Tests
Task 4. Modeling and
Analysis
Task 5. Reporting and
Performance Assessment
Go/NoGo Decision. Proceed to field tests if salt water environment per se does
not compromise the utility of advanced TEM sensors for target classification.
19
Project Funding
$K
SERDP
Year 1
582.0
Year 2
573.7
Year 3
77.5
Total
1,233.2
20
Deliverables
●
●
●
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Basic information on, and understanding of effects of the
marine environments on EMI classification
Validated models for relevant EM phenomena in marine
environments
Peer reviewed articles
2 graduate students supporting field work
21
Backup Slides
22
Reviewer Comments
Comment (SERDP Program Office): The proposed distribution of effort
among the data collection, analysis, and measurement site selection tasks
seems to be heavily weighted toward the analysis tasks. Please revisit the
proposal tasks and justify in detail the distribution of effort in the revised
proposal.
Response: We have revised the distribution of effort among the data
collection, analysis and measurement site selection tasks. Additional weight
has been placed on the data collection by increasing the involvement of VIMS
personnel in the tank tests and adding a sediment characterization task. The
weight assigned to the analysis tasks has been correspondingly reduced.
The revised year 1 project plan is compared with the original on the next slide.
23
Year 1 Project Plan
1. Tank Measurements
2. Sediment Properties
3. Field Tests
4. Modeling and Analysis
5. Reporting and Assessment
Original
Revised
$242.1K
--10.0K
261.2K
40.9K
$554.2K
$294.0K
71.5K
10.0K
154.6K
51.9K
$582.0K
24
Reviewer Comments
Comment (Reviewer 15106): My biggest concern is that the experiments will
measure the limits of the sensors used and not the effects being
considered…will [the experimental results] be generalizable to an improved
sensor?....Will the models be validated against the controlled tests done for
SERDP project MR-1321 by Geophex?…It seems like the models should apply
to both time and frequency domain systems....
Response: Our intent is to focus on the fundamental physics of EMI in the
marine environment, so the results will be generalizable to improved sensors.
The models do apply to both time and frequency domain systems. The
connection is through the duality expressed by Fourier Transforms. We will
validate the models against both (time domain) test data collected during this
project and the frequency domain data collected in MR-1321.
25
Reviewer Comments
Comment (Reviewer 15103): There is not much information about the models
to be used in this project work. At least reasonable amount of details should be
provided about the physics-based models and the data analysis techniques
which are mentioned to be used for EMI data inversion…the models to be used
for EMI data inversion must be stated more clearly in the proposal.
Response: The basic model is for the field of an alternating current loop in a
layered conducting medium. Derivations can be found in the literature for
various applications, including geophysics (Wait and Spies, IEEE Trans.
Antennas Propagat., 1972), nondestructive testing (Dodd and Deeds, J. Appl.
Phys., 1968) and mine detection (Das, IEEE Trans. Geosci. Remote Sens.,
2006). Transient systems are modeled by using Fourier transforms to express
the transmit current waveform in terms of simple harmonic components. For
each frequency component, modification of the field at the target by the
seawater is calculated using this standard model. A similar calculation applies
26
Response to Reviewer 15103, continued
for the return field by the principle of electromagnetic reciprocity. Application of
inverse Fourier transforms converts back to the time domain.
The target response has two parts. The first is the magnetic dipole induced by
the primary magnetic field. The magnetic polarizability tensor relates the dipole
moment to the primary field, as in terrestrial applications. We have extensive
libraries of magnetic polarizabilities for targets of interest. More generally,
published results from SERDP projects MR-1225 and 1595 have shown that
polarizabilities for typical targets can be approximated by the response of
suitably chosen spherical targets. Such response is calculated using models
found in standard textbooks such as Smythe’s Static and Dynamic Electricity or
Grant and West’s Interpretation Theory in Applied Geophysics. The second part
is an electric dipole induced by the electric field at the target. This is the socalled current channeling response. It is not present in normal terrestrial
applications. Results from SERDP project MR-1321 indicate that it is strongly
dependent on whether the target’s surface is conducting or insulating. Having
to account for this component would significantly complicate classification,
27
Response to Reviewer 15103, continued
because then we would need an electric polarizability library in addition to our
standard magnetic polarizability library, and there would be an unknown degree
of mixing between the two components depending on the target’s surface
condition.
Effects of planar boundaries (flat air/sea interface, uniformly layered sediments)
can be calculated using the standard model. More complicated geometries
quickly become intractable and approximations are required. We should be
able to use linearized theory to accommodate small undulations on a boundary
(e.g surface waves) provided that the electromagnetic skin depth is large
compared to the amplitude of the undulations at frequencies of interest.
Background contributions from gently sloping sediment layers can be
approximated using the standard model assuming locally plane boundaries
between the seawater and sediment layers.
28
Equipment, Techniques
29
York River
Estuary
Sediments
Examples of seabed data
collected in York River
Estuary by Friedrichs/
Cartwright lab (from Kraatz
dissertation, in prep.)
30
(k)
Sediment Analyses
●
(x 10-5 SI units)
Magnetic susceptibility

Acquire Bartington MS2 core logging
loop sensor for magnetic susceptibility
Example MS2 core
logger data from Van der
Land (2011)
Sediment
Units
(normalized)
31
●
Resistivity profiles

Fabricate Wenner
resistivity probe
(following Andrews, 1981)
Sediment resistivity
= 2 p a V/I
Depth below seabed (cm)
Sediment Analyses
Upper
estuary
Estuary
mouth
Middle
estuary
Resistivity (Ohm-meters)
Test of Wenner probe on
York River Estuary cores
(Kineke, Valesco,
Friedrichs, unpub. 2000)
32
Transition Plan
●
Follow-on ESTCP project to demonstrate towed EMI
array for underwater munitions detection and
classification

Advanced TEM sensors adapted to existing Marine Towed Array
platform (MR-1322, -200324)
33