Multi-Sensor Measurements for the Detection of Buried Targets

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Waymond R. Scott, Jr. and James H. McClellan

School of Electrical and Computer EngineeringGeorgia Institute of Technology

Atlanta, GA 30332-0250

June 28, 2006

Multi-Sensor Measurements for the Detection of Buried Targets

Scott, Georgia Tech MURI Review 2006 2

Outline

• Introduction• Deep Targets (Tunnel/Buried Structure)

− Bi-static Seismic Experiments− Multi-static GPR Experiments− Comparison

• Multi-static GPR Issues• Accomplishments and Plans

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Multi-Sensor Cooperation/Adaptation

GPR

EMI

Seismic

Imaging

SigProc

Imaging

Features

Features

Features

DecisionProcess

ExploitCorrelation & Sensitivity

Feedback

Feedback

Controls

Controls

Controls

Controls

Controls

ControlsFrequency BandwidthFocusingRemote ImagingMeasurement Spacing

Frequency BandwidthMeasurement Spacing

Frequency BandwidthArray Elements UsedMeasurement Spacing


Experimental Test Bed

• Develop a set of experiments to investigate the potential of multi-modal processing− Shallow Targets \ Landmines (Three sensors/six modes)

• EMI• GPR

• Bi-Static• Multi-Static

• Seismic• Unfocused• Focused• Remote Detection

− Deep Targets \ Tunnel or Buried Structure (Two sensors/four modes)

• GPR• Bi-Static• Multi-Static

• Seismic• Bi-Static• Multi-Static

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Outline





Joint Seismic and Electromagnetic Tunnel Detection

Aero-acousticSource

Ground Contacting Sensor

Tunnel

AirEarth

Tunnel

AirEarth

TX2 TX1 RX1 RX2

Multi-Static GPRBi-Static Seismic

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Joint Seismic and Electromagnetic Tunnel Detection

48cm 48cm

TTRRRR

Aero-Acoustic Source

Ground Contacting

Sensor

Multi-Static Array

Resistive-Vee

Antennas

Multi-Static GPRBi-Static Seismic


Experimental ModelScale model for a shallow Tunnel

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Outline





Configuration of the Seismic System

• Ground Contacting Source− Mono-static

• Source ringing• Near coupling

− Bi-static• Source ringing• Rayleigh coupling

• Aero-Acoustic Source− Almost no ringing due to soil

shaker interactions− Direct coupling to sensor is

far less

Air Pressure


AirEarth Ground

Motion

Ground Contacting SensorAir

Earth

Ground Contacting Source

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Seismic SystemComponents• Aero-Acoustic Source

− 5” Speaker• Sensor

− PC Accelerometer

Data acquisition• The array is scanned over 1.8m x

1.8m scan region• The scan region is gridded into 46

x 46 points (∆x = ∆y = 4cm)• Per point

− 4s Chirp − 100 Hz to 8 kHz

Diagram of Bi-Static Seismic

Aero-acousticSource


Tunnel

AirEarth


Practical Ground Contacting Sensor

ADXL103 Accelerometer

ICP Preamplifier

Pipe Insulation Foam Spring

Sorbothane Sensor Foot

19mm O.D. X 46 cm Long Aluminum Tube

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Possible Implementation of a Seismic Ground Contacting Sensor

3 by 10 Element Array of Ground-

Contacting Sensors

Source Platform

Sensor Platforms


Seismic: Raw Data

Scale Model of Tunnel10cm DIA.58 cm deep

3600 Hz center Frequency

35 dB Scale

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Seismic: Migrated Images


3600 Hz center Frequency

20 dB Scale


Outline




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Resistive-Vee Antenna• Why resistive-vee antenna?

− Performs well in bistatic GPRs− Radiates clean, short pulses− Has a low radar cross section

• Reduces multiple reflections between the antenna and the ground

− Geometry suitable in array applications

• Antenna design− Optimized resistive loading profile− Optimized arm shape− Frequency: up to 8GHz Microwave

absorber

Chip resistors

Double-Y balun

FoamSMA panel mount


Multistatic GPR System

Components• Antenna array (2 Tx’s and 4 Rx’s)

− Placed in a dielectric frame− Provides 8 bistatic apertures – aperture sizes are

12cm to 96cm with 12cm increment

Data acquisition• The array is scanned over 1.8m x 1.8m scan

region• The scan region is gridded into 91 x 91 points (∆x

= ∆y = 2cm)• Per point

− GPR obtains 8 bistatic responses at 401 frequency points

− 60MHz to 8.06GHz (20MHz increment)

Calibration• Free space response (FREE)

− Free space is simulated by pointing the array toward absorber-padded corner

− Subtraction removes direct coupling in the system• Through response (THRU)

− Antenna ports are connected by a 5ft cable− Division removes distortion in the cables feeding

the antenna

Diagram of Multi-Static GPR

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Multi-Static GPR: Raw Data

T1 R1 (12cm) T1 R2 (24cm) T1 R3 (36cm) T1 R4 (48cm)



2.5 GHz Center Frequency


Multi-Static GPR: Raw Data





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Multi-Static GPR: Migrated Images



Sum of All Pairs



25 dB Scale


Multi-Static GPR: Migrated ImagesScale Model of Tunnel

10cm DIA.58 cm deep




Sum of All Pairs

25 dB Scale

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Multi-Static GPR: Migrated ImagesScale Model of Tunnel

10cm DIA.58 cm deep


Sum over all Pairs

25 dB Scale


Outline




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Comparison of Seismic and GPR Data : Migrated Images

GPRSeismic

20 dB Scale 25 dB Scale


Comparison of Seismic and GPR Data

GPRSeismic

3-D Iso-images of the Migrated Data

Tunnel

Tank Wall Tank Wall?

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Seismic and GPR Data

Combined Seismic and GPR

Tunnel

Tank Wall

Tunnel Estimates Using Radon Transform


Comments

• The scale model of the tunnel is clearly visible in both the Seismic and GPR data− Clutter is different for the two sensors− Weak parts of the images are at different locations− Combining the Seismic and GPR images is probably beneficial

• The combined multi-static image has less clutter than the images for individual pairs

• Migration clearly improved the images• Radon processing clearly found the tunnel

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Outline





Free Space Target Measurements

Migrated Image

Picture of Target

0 dB

-10

-5

-15

-20

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Multistatic Inversion Issues

Back-Projected Images of 1” Metal Sphere

Input pulse: Diff. Gaussian; f0 = 2.5GHz

T1-R1 T2-R4

• Image alignment between antenna pairs has been the most problematic issue with the multi-static imaging even for the targets in air− Why?

• Complex phase center for the antenna?• Complex scattering from the targets?


Antenna Phase Center• Investigation of the antenna phase center

− Experimentally from antenna elements in array• Targets in air• Ground bounce

− Experimentally from antenna elements• Matched pair measurements• Field measurements

− Numerically• Matched pair models• Field measurements

• The antenna is poorly characterized by a single phase center• The antenna is approximately characterized by a phase center that varies with

frequency• The antenna phase characteristics are not very sensitive to the target depth in

the range of interest

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Antenna Phase Center• Phase center calculated from numerical model for the antenna


Antenna Phase Center Effects• Made a numerical model to investigate how

the antenna effects the images− MOM model for the antenna and scatterer− 8 bi-static spacings: 12, 24, 36, … 96 cm− Scan region: 1.8 by 1.8 m with 2 cm spacing− Scatterer 4cm long wire: 25 cm below tips of

antenna− Frequency: 40 MHz to 8.04 GHZ in 40 MHZ

steps• Made images using a frequency domain

beamforming algorithm− BF-FZ Beamfomed using the far-zone phase

of the antennas vs angle and frequency instead of a simple phase center

− BF-P4 Beamfomed using a frequency independent phase center: 4 cm from the antenna tip

− BF-P13 Beamfomed using a frequency independent phase center: 13 cm from the antenna tip

T R

S

4mm Long WireScatterer (z=-42.15cm)

x

y

z

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Resolution: Single Frequency Images: S=12cm

2 G

Hz

8 G

Hz

Cross-Range Range

BF-FZ

BF-P13

BF-P4


Alignment: Moveout Graphs of the Eight Pairs

Each trace is the real part of the depth image for a single bi-static antenna pair

S = 12, 24, 36, … 96

Incr

easi

ng T

-R s

paci

ng

-z, -z,

BF-FZ BF-P13

Traces line up for all eight pairs

Traces do not line up

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Comments

• The complex phase center of the antennas clearly affect the alignment of the images− The correction of the phase in the BF-FZ algorithm clearly greatly

improved the alignment− A simpler algorithm that used the frequency dependent phase

center would probably work almost as well


Accomplishments• Three-sensor experiment to study multimodal processing for landmines

− Developed new metal detector and a radar− Investigated three burial scenarios− Showed responses for all the sensors over a variety of targets− Demonstrated possible feature for multimodal/cooperative processing

• Seismic experiments, models, and processing − Developed array to be used with the optimal maneuver algorithm− Demonstrated reverse-time focusing and corresponding enhancement of mine

signature− Demonstrated imaging on numerical and experimental data from a clean and a

cluttered environment• Multi-static radar

− Demonstrated radar operation with and without clutter objects for four scenarios− Investigated pre-stack migration imaging of multi-static data

• Two-sensor experiment for tunnels and buried structures− Investigated seismic wave interactions− Developed new seismic probing system with easily movable sensors.− Made combined seismic and multi-static GPR measurements on a scale more of a

tunnel

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Plans• Three-sensor experiment for landmines

− More burial scenarios based on inputs from the signal processors?− Real time?

• Seismic experiments, models, and processing− Finish work on reverse-time focusing− Multi-static array for tunnel experiment

• Multi-static radar− Improved antennas?− Continue investigating pre-stack migration imaging of multi-static data

• Two-sensor experiment for tunnels and buried structures− More burial scenarios based on inputs from the signal processors− Scale?

• Continue to work and share data with the signal processors

Multi-Sensor Measurements for the Detection of Buried Targets

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