Empirical Investigation of the Factors I nfluencing M arine A pplications of EMI MR-2409

1. 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

2. 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

3. 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

4. Technical Objective • Determine how to implement EMI-based classification in marine environments • Complexities will constrain applications ?

5. 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)

6. 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

7. Technical Background: Seawater Response Seawater response comparable to TEM background levels • 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

8. Technical Background: Signal Distortion Seawater distortion examples • 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 steel bar in water air Measured FDEM response San Filipo & Won, MR-1321 Calculated TEM distortion

9. Technical Background: Noise Effects Smoother sensor trajectory with underwater tow • 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 in water on land

10. 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

11. Technical Approach Task 1 Tank Measurements of Seawater Effects Task 2 Measurements of Sediment EM Properties Task 4 Modeling and Analysis Task 3 Field Tests of EMI in Marine Environment Task 5 Reporting and Performance Assessment 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.

12. 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)

13. 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 measurementsto evaluate range of R/V hull influence • NRL EMI sensors R/V Elis Olsson VIMS

14. 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

15. 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

16. 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

17. 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

18. Year 1 Project Plan 1. Tank Measurements $294.0K 2. Sediment Properties 71.5K 3. Field Tests 10.0K 4. Modeling and Analysis 154.6K 5. Reporting and Assessment 51.9K Total $582.0K

19. Overall Project Plan 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.

20. Project Funding

21. 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

22. Backup Slides

23. 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.

24. Year 1 Project Plan Original Revised 1. Tank Measurements $242.1K $294.0K 2. Sediment Properties --- 71.5K 3. Field Tests 10.0K 10.0K 4. Modeling and Analysis 261.2K 154.6K 5. Reporting and Assessment 40.9K 51.9K $554.2K $582.0K

25. 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.

26. 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

27. 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’sStatic and Dynamic Electricityor 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,

28. 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.

29. Equipment, Techniques

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

31. Sediment Analyses (k) (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)

32. Sediment Analyses • Resistivity profiles • Fabricate Wenner resistivity probe (following Andrews, 1981) Depth below seabed (cm) Upper estuary Middle estuary Estuary mouth Resistivity (Ohm-meters) Sediment resistivity = 2 p a V/I Test of Wenner probe on York River Estuary cores (Kineke, Valesco, Friedrichs, unpub. 2000)

33. 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)