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Imaging Waves for Bathymetric Retrievals

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Imaging Waves for Bathymetric Retrievals

Dr. Steven P AndersonSenior Principal ScientistEnvironmental Intelligence Group Areté AssociatesArlington, VA

Presentation to KHOAJune 18, 2013

Streak Tube Imaging LIDAR (STIL)

Areté “RenderWorld” scene of open ocean

- “Areté Associates is an advanced science and engineering company that provides innovative solutions to the most challenging technical problems faced by the United States.”
- Founded in 1976 and employee-owned.
- 320 employees at seven locations.

- Core competencies:
- First-principles physical modeling of signatures, environments, and sensors.
- Ruggedized sensor development.
- Comprehensive field experiment design and execution.
- Delivering operational products to end users.

Approved for Public Release, Distribution Unlimited.

A shore mounted Bathymetric Radar will provide persistent monitoring of water depths

- Multi-beam sonar is widely utilized for precision bathymetry;
- However, swath width/area coverage is limited in shallow water

- Lidar effective for shallow-water bathymetry at large coverage rates

AIR

L (WAVE LENGTH)

- Ocean surface waves are powered by gravity
- When water surface is displaced up (or down), gravity acts to drive the wave forward
- Our own personal experience tells us that waves slow down as they approach a beach where they eventually break
- Dispersion Relationship:
- the mathematical relationship between
- wave speed - c
- wave period - T
- wave length – L
- And water depth - H.

H (DEPTH)

GRAVITY

C (WAVE SPEED)

WATER

Wikipedia.com

water depth

vectorcurrents

gravity

Linear wave dispersion assumes:small amplitude wavesuniform currentsdepth constant

H = ∞

Frequency1/T

Wavenumber1/L

If we can observe the wavenumber-frequency relationship, we can invert and solve for depth and currents directly.

shallow-water

- Capability developed during World War II.
- Uses a single image taken by aircraft.
- Assumes:
- monochromatic waves (single frequency)
- linear wave dispersion.

Williams 1947

Measure Wavelength in Deepwater

L=260ft

A

B

Measure Wavelength near-shore

L=180ft

Williams 1947

H= 26ft

H= ∞

A

B

T=7s

L=180ft

L=260ftdeepwater

Williams 1947

Measure the wavelength and period information directly

- Areté developed the Airborne Remote Optical Spotlight System (AROSS) to collect time series imager of ocean waves
- Spot-dwell EO imager – digital camera(s) with navigation control system to maintain pointing at virtual target on the water

See Dugan et al (2001 JGR)

US ONR funded

From Duck, N.C. USACE Field Research Facility

Effectively separates space & time

Raw imagery (camera coordinates)

Registered & mapped imagery (ground coordinates)

dashed linedeepwater dispersion

solid lineobserved dispersion

Fourier Transform

Example image stack (data cube)

2-D slice through the 3-D spectrum for shoaling waves

Note the broadening of the observed dispersion relationship associated with wave shoaling

AROSS data from Corps of Engineers' Field Research Facility in Duck, NC

See (Dugan et al 2001;Piotrowski and Dugan 2002).

AROSS Retrievals

Note: Algorithm assumes linear wave dispersion

Comparison to Ground-truthLARC Survey

5-10% RMS Errors

Lighter Amphibious Resupply Cargo

Bias: < 0.25m

Accuracy:

- 1m RMS for depths 2-10m
- 10% of water depth>10m-15m

- 128 m for depths 2-6m
- 256 m for depths 6-15m

See Dugan et al (2001 JGR)

US ONR funded

Duck, N.C. USACE Field Research Facility

Areté'sFuruno radar at the Diablo Canyon

- Advantages:
- All weather
- day-night
- unlimited dwell

- Challenges:
- Maritime Radars designed to minimize “clutter”…. thus low, signal to noise ratio for wave imaging
- Limited resolution, especially at range
- High elevation needed to maximize range

X-band radar sees waves as modulations in the radar cross section

Figure from Borge et al 2004, JPO

- Challenge:
- Develop an advanced algorithm to improve accuracy and resolution over existing capability and deliver a near-realtime solution.

- Goal:
- Resolution | 2-3 times water depth
- Accuracy | 0.25 m (or 2.5% water depth>10m)

- Approach:
- Leverage new understanding of nonlinearities in the wave dynamics.
- Use state-of-the-art computers to accelerate computations

- Technical Challenges:
- Develop an universal solution
- Wave field is dynamic; conditions never exactly repeatable
- Individual radar systems have their own specifications

- Creating an meaningful “error-metric”
- Provide an indication of confidence along with depth measurement

- Develop an universal solution

Develop a new bathymetry algorithm that:

- Better matches data towave kinematics
- Provides higher spatial resolution
- Yields more accurate results

- Phase I.
- Collect radar data in operational relevant location
- Demonstration depth retrievals using existing capabilities
- Develop and demonstrate new algorithm that accounts for non-linear wave dynamics to provide higher resolution and more accurate results

- Phase II.
- Implement and test “error-metric”
- Refractor software and optimize for speed
- Design and fabricate a prototype system
- Field test prototype system

- One Island on the Bay located on the Chesapeake Bay Bridge-Tunnel.

First suggestion of estimating bathymetry by using wave imaging radars

Wave Monitoring System WaMoS

Bar And Swash Imaging Radar (BASIR)

Radar Inlet Observing System (RIOS):

OceanWaves GmbH, Germany