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Empirical Investigation of the Factors Influencing Marine Applications of EMI

MR-2409

Thomas Bell, SAICDaniel Steinhurst, Nova Research

Carl Friedrichs, VIMS

Brief to the Scientific Advisory BoardSeptember 11, 2013

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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

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Problem Statement

● 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

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Technical Objective

● Determine how to implement EMI-based classification in marine environments Complexities will constrain applications

?

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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)

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Technical Background: Marine Environment

● Conducting host medium complicates marine EMI 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

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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 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

Seawater response comparable to TEM background levels

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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

Measured FDEM responseSan Filipo & Won, MR-1321

Calculated TEM distortion

steel bar in water

air

Seawater distortion examples

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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

on land

in water

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Technical Background (Summary)

● Theory difficult for all but simplest geometries Model studies have drawn conflicting conclusions (MR-1632 vs.

MR-2228) Effects appear to be neither insignificant nor overwhelming

● Few controlled tests to measure relevant factors and effects 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

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Technical Approach

Task 1

Tank Measurements

of Seawater Effects

Task 2Measurements of

Sediment EM Properties

Task 4Modeling and

Analysis

Task 3Field Tests of EMI in Marine Environment

Task 5Reporting and Performance

AssessmentGo/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.

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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)

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Field Measurements

● York River Estuary Tidal range ~ 1 m Depth 0 to 20 m Salinity 0 to 25 o/oo

Muddy and sandy areas Well characterized

● VIMS research vessels and instrumentation Dockside EMI measurements

to evaluate range of R/V hull influence

● NRL EMI sensors

VIMS

R/V Elis Olsson

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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

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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-temperature-

depth (CTD) profiles• Bottom samples

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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 Magnetic and electric polarizabilities, bare metal and insulated

surface Attenuation and phase shift due to conducting host medium

● Linearized solutions for air/sea interaction effects Wave amplitude << skin depth

● Diffusion models for sediment effects

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Performance Assessment

● Validate EM models for marine applications Simplifying assumptions & parameter values

● Signal distortion Impact on classification performance & processing implications

● Noise levels Effects of spatial variation of bottom return Reduction in atmospheric noise & sensor motion noise

● Implications for system design Tx coil size & Rx coil configuration Tow height above bottom

● System response curves for targets of interest

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Year 1 Project Plan

1. Tank Measurements $294.0K

2. Sediment Properties 71.5K

3. Field Tests10.0K

4. Modeling and Analysis 154.6K

5. Reporting and Assessment 51.9K

Total $582.0K

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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.

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Project Funding

$K SERDP

Year 1 582.0

Year 2 573.7

Year 3 77.5

Total 1,233.2

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Deliverables

● 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

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Backup Slides

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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.

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Year 1 Project Plan

Original Revised

1. Tank Measurements $242.1K $294.0K

2. Sediment Properties --- 71.5K

3. Field Tests10.0K 10.0K

4. Modeling and Analysis 261.2K 154.6K

5. Reporting and Assessment 40.9K 51.9K

$554.2K $582.0K

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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.

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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

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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 so-called 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,

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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.

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Equipment, Techniques

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York River Estuary

Sediments

Examples of seabed data collected in York River Estuary by Friedrichs/ Cartwright lab (from Kraatz dissertation, in prep.)

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Sediment Analyses

● Magnetic susceptibility Acquire Bartington MS2 core logging

loop sensor for magnetic susceptibility

Sediment Units

(k)

(normalized)

(x 10-5 SI units)

Example MS2 core logger data from Van der Land (2011)

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Sediment Analyses

● Resistivity profiles Fabricate Wenner

resistivity probe (following Andrews, 1981)

Sediment resistivity= 2 p a V/I

Resistivity (Ohm-meters)

Dep

th b

elow

sea

bed

(cm

)

Test of Wenner probe on York River Estuary cores (Kineke, Valesco, Friedrichs, unpub. 2000)

Estuary mouth

Middleestuary

Upperestuary

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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)